Sch2010ac

Sch2010ac
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Palaeogeography, Palaeoclimatology, Palaeoecology 299 (2011) 175–196
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Late Quaternary paleoenvironmental records from the western Lena Delta,
Arctic Siberia
Lutz Schirrmeister a,⁎, Guido Grosse b, Moritz Schnelle a, Margret Fuchs c, Matthias Krbetschek d,
Mathias Ulrich a, Viktor Kunitsky e, Mikhail Grigoriev e, Andrei Andreev f, Frank Kienast g, Hanno Meyer a,
Olga Babiy e, Irina Klimova e, Anatoly Bobrov h, Sebastian Wetterich a, Georg Schwamborn a
a
Department of Periglacial Research, Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, D-14473 Potsdam, Germany
Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK-99775, USA
c
TU Bergakademie Freiberg, Institute of Geology, Bernhard-von-Cotta-Strasse 2, D-09596 Freiberg, Germany
d
Saxon Academy of Science, Section Quaternary Geochronology, Leipziger Str. 23, D-09596 Freiberg, Germany
e
Permafrost Institute Yakutsk, Russian Academy of Science, Siberian Branch, ul. Merzlotnaya, 36, RUS-677010 Yakutsk, Russia
f
Institute of Geology and Mineralogy, University of Cologne, Zuelpicher Str. 49a, D-50674 Cologne, Germany
g
Senckenberg Research Institute, Quaternary Palaeontology Research Station, Am Jakobskirchhof 4, D-99423 Weimar, Germany
h
Moscow State University, Faculty of Soil, Vorobievy Gory, RUS-119899 Moscow, Russia
b
a r t i c l e
i n f o
Article history:
Received 22 July 2010
Received in revised form 22 October 2010
Accepted 31 October 2010
Available online 4 November 2010
Keywords:
Periglacial
Permafrost
Cryolithology
Geochronology
Paleoecology
Paleogeography
Laptev Sea shelf
Western Beringia
a b s t r a c t
The three main Lena Delta terraces were formed during different stages of the late Quaternary. While only the
first floodplain terrace is connected with active deltaic processes, the second and third terraces, which
dominate the western part of the delta, are erosional remnants of arctic paleolandscapes affected by
periglacial processes. The landscape dynamics of the second and the third terraces, and their relationship to
each other, are of particular importance in any effort to elucidate the late Quaternary paleoenvironment of
western Beringia.
Multidisciplinary studies of permafrost deposits on the second terrace were carried out at several sites of the
Arga Complex, named after the largest delta island, Arga–Muora–Sise. The frozen sediments predominantly
consist of fluvial sands several tens of meters thick, radiocarbon-dated from N 52 to 16 kyr BP. These sands
were deposited under changing fluvial conditions in a dynamic system of shifting river channels, and have
been additionally modified by synsedimentary and postsedimentary cryogenesis. Later thermokarst processes
affected this late Pleistocene fluvial landscape during the Lateglacial and the Holocene. In addition, eolian
activity reworked the fluvial sands on exposed surfaces at least since the Lateglacial, resulting in dune
formation in some areas. Contrary to the Arga Complex, the third terrace is mainly composed of polygenetic
alluvial and proluvial ice-rich permafrost sequences (Ice Complex deposits) radiocarbon-dated from 50 to
17 kyr BP which cover older fluvial sand units luminescence-dated to about 100–50 kyr BP. Paleoecological
records reflect tundra-steppe conditions that varied locally, depending on landscape dynamics, during the
Marine Isotope Stage (MIS) 4 and 3 periods, and a persistent change to shrub and arctic tundra during
Lateglacial and Holocene periods.
The study results indicate a continuous fluvial sedimentation environment for the Laptev Sea shelf in the
region of the second Lena Delta terrace during the late Pleistocene, and confirm the presence of a dynamic
channel system of the paleo-Lena River that flowed at the same time as the nearby subaerial Ice Complex
deposits were being formed.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Arctic river deltas are fragile environmental systems that are
situated at the interface between the mainland and the Arctic Ocean.
The Lena Delta in North Siberia is the largest Arctic river delta
⁎ Corresponding author.
E-mail address: [email protected] (L. Schirrmeister).
0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2010.10.045
(Walker, 1998; Fig. 1A). The geology of this region has been studied by
Russian researchers since about 1960 (e.g. Galabala, 1987; Grigoriev,
1966, 1993; Gusev, 1961; Ivanov, 1972; Kolpakov, 1983; Kunitsky,
1989; Lungersgauzen, 1961; Saks and Strelkov, 1960). Recently,
geological and paleoenvironmental research in the Lena Delta have
been continued under Russian–German science collaborations (e.g.
Andreev et al., 2004; Krbetschek et al., 2002; Pavlova and Dorozhkina,
2000; Schirrmeister et al., 2003; Schwamborn et al., 2002a,b,c;
Wetterich et al., 2008). Schwamborn et al. (2002a) provided a
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Fig. 1. Study area maps showing (A) the position of the study area on the circum-arctic permafrost map, (B) an overview map with locations mentioned in the text: Islands: 1 — Ebe–Basyn–
Sise, 2 — Khardang–Sise, 3 — Turakh–Sise, 4 — Arga–Muora–Sise, 5 — Dzhangilakh–Sise, 6 — Kurungnakh–Sise, 7 — Bykovsky Peninsula; Channels: a — Olenyekskaya, b — Arynskaya,
c — Tumatskaya, and (C) the study area in the western Lena Delta with exposure positions (see also SOM-1).
comprehensive picture of the delta architecture, its three main
geomorphological terraces, and their genesis. However, one of the
still-debated questions concerns the formation of the second terrace
in the Lena Delta, the so-called Arga Complex (named after the largest
delta island, Arga–Muora–Sise), and its connection to the southern
adjoining islands of the third terrace. The Arga Complex was built up
of extended and thick sand deposits located in the western part of the
delta. The Arga Complex not only differs from the first terrace with its
Late Holocene to modern deltaic accumulation, but is also clearly
distinguished from the third terrace which is a relic of a late
Pleistocene Ice Complex formed on an accumulation plain and predating the delta development. The surface morphology of the second
terrace is largely characterized by NNW-SSE-oriented thermokarst
depressions often containing lakes (Fig. 1B, C) resembling similar
structures on other Arctic plains such as Alaska's North Slope (Hinkel
et al., 2005). The dimensions of these thermokarst features cannot be
related to current ground ice conditions of the mostly sandy facies of
the second terrace but are likely linked to fluvial depressions which
originated in a paleo-Lena River bed (Schwamborn et al., 2002a). Still,
it was not clear whether the sands of the Arga Complex were facially
and stratigraphically correlated with sandy deposits found below the
Ice Complex deposits of the third terrace along the Olenyekskaya
Channel (Fig. 2). Previous hypotheses describing Arga Complex
formation include marine (Ivanov, 1972), lagoonal, limnic-alluvial,
alluvial-aeolian (Gusev, 1961; Lungersgauzen, 1961), glaciofluvial
(Grosswald, 1998), or fluvio-nival (Galabala, 1987; Kunitsky, 1989)
conditions. More recent hypotheses favor fluvial formation by ancient
Lena River branches (Grigoriev, 1993; Schwamborn et al., 2002c).
New data on composition and structure of late Quaternary
deposits were acquired within the cooperative Russian–German
scientific “System Laptev Sea 2000” project from 1998 to 2004
(Grigoriev et al., 2003; Rachold and Grigoriev, 1999, 2000, 2001). To
3
Fig. 2. Schematic cross-section of Quaternary deposits in the western part of the Lena Delta (according to Galabala, 1987). Note: The aQ3–4
III and aQIII signatures including large
syngenetic ice wedges on Khardang–Sise Island (third Lena Delta terrace) corresponding to the Ice Complex Unit of the third Lena Delta terrace; a.r.l. — above the river level.
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improve understanding of the late Quaternary history of the Lena Delta
a field expedition was realized in summer 2005 to focus on periglacial
landscape dynamics in the western Lena Delta (Schirrmeister et al.,
2007).
The aim of this paper is to synthesize new field observations and
analytical results with previous datasets in order to explain the
formation of the second terrace (Arga Complex) and its geological
relationship to the third terrace, as seen in the context of late
Quaternary paleolandscape dynamics in the modern Lena Delta region
and broader supraregional paleoenvironmental developments on the
Laptev Sea shelf.
2. Study area
Three geomorphological terraces occur in the Lena Delta: (1) the
first terrace (0–10 m above sea level; a.s.l.) is a late Holocene to
modern delta floodplain, mainly stretching along the main river
channels in the central and eastern parts of the delta; (2) the second
terrace (20–30 masl) is of older – presumably late Pleistocene fluvial –
origin, and is located in the northwestern part of the delta area;
(3) the third terrace (30–55 masl) is an erosional remnant of a late
Pleistocene accumulation plain north of the Chekanovsky Ridge that
covers parts of the southern and southwestern delta areas.
The western Lena Delta is bordered to the east by the Tumatskaya
Channel, to the south by the Chekanovsky Ridge, to the west by Kuba
Bay, and to the north by the Laptev Sea. Study sites are located on
several delta islands, including Ebe–Basyn–Sise, Khardang–Sise,
Dzhangylakh–Sise, and Kurungnakh–Sise (Fig. 1B, C). These islands
are separated by the Bulukurskaya, Olenyekskaya, and Arynskaya
channels and smaller river branches (Fig. 1B). Arga–Muora–Sise
Island, the largest island of the western Lena Delta, is located about
20 km to the north of these studied sites. According to Galabala
(1987), the sandy Muorinsky Suite on Arga–Muora–Sise Island is
completely covered by Turakhsky Suite sands. These widely-distributed sands were stratigraphically correlated with sand horizons
exposed in the lower horizons of the third terrace at the Olenyekskaya
Channel (Fig. 2).
The separate and elevated position of the Arga Complex is
explained by relative tectonic uplift during the late Quaternary (Are
and Reimnitz, 2000; Drachev et al., 1998). Nevertheless, the
stratigraphic relationship between the Arga Complex that forms the
second terrace and the sandy sequences covered by Ice Complex
deposits of the third terrace has not yet been sufficiently explained.
Age determinations of sandy deposits at the bluffs of Lake Nikolay on
Arga–Muora–Sise show that the sandy deposits at depths of about 1–
4 m below the surface were formed between 14.5 and 10.9 kyr BP
(Krbetschek et al., 2002; Schwamborn et al., 2002a), whereas the
Lower Sand Unit of the third terrace below the Ice Complex Unit along
the Olenyekskaya Channel was luminescence-dated to between 100
and 60 kyr (Schirrmeister et al., 2003; Schwamborn et al., 2002c).
177
plastic bags. Additionally, core bits were sampled at ca. 1 m intervals
for ice content measurements. Gravimetric ice contents were
calculated using the ratio of wet to dry sample weight.
To distinguish various sediment types, basic grain-size parameters
were measured with a Laser Particle analyzer (Coulter LS 200). Total
carbon (TC), total organic carbon (TOC), and total nitrogen (TN)
contents were determined with a Carbon–Nitrogen–Sulfur (CNS)
analyzer (Elementar Vario EL III). In addition, the mass-specific
magnetic susceptibility (MS) was analyzed using a Bartington MS2
instrument equipped with the MS2B sensor. The values are expressed
in SI units (10− 8 m3kg− 1). Stable carbon isotope ratios (δ13C) of TOC
were measured with a Finnigan DELTA S mass spectrometer. The
values are expressed in delta per mil notation (δ‰) relative to the
Vienna Pee Dee Belemnite (VPDB) Standard and the analyses were
accurate to ±0.2‰.
3.2. Geochronology
3. Methods and materials
Optical stimulated luminescence (OSL) and radiocarbon accelerator mass spectrometry (AMS) methods were used to determine the
depositional ages of sands and the age of peat and fossil plant remains.
For OSL analysis, frozen samples were drilled with a batterypowered hand-drilling machine. A modified drill head, opaque plastic
cylinders, and opaque plastic bags were used to protect samples from
sunlight exposure. Parallel to each OSL sample, sediment was taken
for radioisotope analyses using HP-Ge γ-spectrometry. OSL sample
preparation (quartz, 100–160 μm) and age determination were
carried out in the Luminescence Laboratory of the Saxon Academy
of Science (Inst. of Appl. Physics, TU Freiberg, Germany). To determine
the paleodose of each sample, 20–40 aliquots were measured with a
Risø DA15 OSL/TL Reader. The measurement procedure followed the
single-aliquot regenerative-dose (SAR) protocol of Murray and Wintle
(2000). A detailed description of the entire OSL dating procedure,
including the statistical data treatments, is given in Schirrmeister et al.
(2009). All estimated parameters (paleodose, dose rate, radiation
absorption correction, and error analyses) were processed for age
calculation with the ADELE software (Kulig, 2005). Previous Infrared
Stimulated Optical Luminescence (IRSL) age measurements on
potassium feldspar grains were calculated using a multiple aliquot
additive (MAAD) protocol (Krbetschek et al., 2002). High errors
indicate insufficient bleaching. Therefore, such IRSL ages should be
regarded as maximum estimates. We can exclude with confidence the
possibility of age underestimation (IRSL-fading) due to the applied
dose determination procedure (MAAD, including laboratory fading
tests) and the low sediment storage temperatures (permafrost
environment).
For radiocarbon dating, small plant fragments like grass roots,
leaves, and twigs were separated from the sediment under a stereo
microscope and analyzed by AMS at the Leibniz Laboratory for
Radiometric Dating and Stable Isotope Research (Kiel, Germany). The
Leibniz Laboratory AMS procedures are described in detail by Grootes
et al. (2004) and Nadeau et al. (1997, 1998).
3.1. Cryolithology and sedimentology
3.3. Mineralogy
During field work in summer 2005, numerous profiles were
excavated in river bank exposures on the second and third Lena Delta
terraces. The exposures were described, sketched, and photographed.
A total of 240 frozen sediment samples weighing up to 1 kg each were
collected. Ground ice samples (70) were collected separately for
stable water isotope analysis. In order to obtain a longer sediment
profile from the Arga Complex, an 11.4 m long core (Tur-2) was
drilled in front of an exposed sediment section (Tur-1) at the river
bank of the Arynskaya Channel (Fig. 1C). Each core segment (20–
30 cm long) was cleaned, described, photographed, and sampled at
10 cm intervals. Sample segments 5–10 cm long were packed in
The 63–125 μm and 125–250 μm subfractions of heavy and light
minerals were analyzed. The grains were separated using a sodium
metatungstate density solution (2.89 g cm− 3). Polarized light was
used to identify 300–400 grains of each fraction on microscope slides.
The presence of certain mineral types was calculated and expressed as
grain percentages.
3.4. Stable water isotopes of ground ice
Hydrogen and oxygen isotopes (δD, δ18O) in ground ice were
measured with a Finnigan MAT Delta-S mass spectrometer, using
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equilibration techniques. Values are given as per mil difference from
Vienna Standard Mean Ocean Water (VSMOW) Standard, with
internal 1σ errors of less than 0.8‰ and 0.1‰ for δD and δ18O,
respectively (Meyer et al., 2000). The values are plotted in δ18O–δD
diagrams in relation to the Global Meteoric Water Line (GMWL)
(Craig, 1961). In general, the most negative δ18O and δD values reflect
the coldest temperatures. Slope and intercept in a δ18O–δD diagram
indicate the source of ocean water evaporated from different regions,
and the possible participation of secondary evaporation processes
(Dansgaard, 1964). In addition, the deuterium excess (d = δD–8δ18O)
is an indicator of non-equilibrium fractionation processes (Dansgaard,
1964).
3.5. Paleoecology
Pollen samples were prepared with a standard HF acid technique
(Berglund and Ralska-Jasiewiczowa, 1986). About 200–300 pollen
grains were counted in each sample with 400× magnification. The
pollen percentages are calculated in relation to the total sum of
terrestrial pollen. The spore percentages are expressed in relation to
the sum of pollen and spores. The relative abundances of reworked
Tertiary spores and re-deposited Quaternary pollen are based on the
sum of in situ and re-deposited pollen, and the percentages of algae
are based on the sum of pollen and algae. The Tilia/TiliaGraph/TGView
software (Grimm, 2004) was used for final calculation and graphical
presentation of pollen and spore assemblages.
For plant macrofossil studies between 100 and 300 g of dry
minerogenous sediments or between 20 and 80 g of dried peat were
used. The samples were soaked in water and sieved through sieves
with 0.25 mm minimum mesh size. The residue was dried. Plant
remains in each sample were identified using a binocular and a
reference plant collection (IQW, 2009).
Testate amoebae were separated from 1 g of sediments with a
500 μm sieve and then concentrated with a centrifuge (Bobrov et al.,
2004). A drop of the suspension was placed on a slide and fixed with
glycerol. Five subsamples were analyzed at 200–400× magnification
under a light microscope.
4. Results
4.1. Cryolithology and sedimentology
According to cryolithological and sedimentological field and
laboratory data, seven sediment units (Units A–G) were classified for
the Arga Complex which forms the second terrace of the Lena Delta.
Sediments of the third terrace were defined as Lower Sand, Peat, and Ice
Complex units (Schirrmeister et al., 2003; Wetterich et al., 2008).
Detailed descriptions of various locations are given in this section.
4.1.1. The second Lena Delta terrace at the Arynskaya Channel
One drill core (Tur-2) and two exposures (Tur-1, Ebe-4) were
studied on both banks of the Arynskaya Channel (Fig. 1C, SOM-1).
The 11.4 m long Tur-2 core (Fig. 3) was extracted next to the Tur-1
exposure, starting one meter above the river level (a.r.l.). The
lowermost part of the core at − 9.91 to −8.82 m below the river
level (b.r.l.) was characterized by medium-grained sand with
numerous small black patches. A lighter, mica-bearing horizon
containing a 2 mm thick ice vein was found between − 8.82 and
−8.27 m b.r.l. In higher sediments (−8.27 to −4.36 m b.r.l.), the
color changed to spotty orange-brownish due to iron oxide impregnations. In addition, plant detritus interbeds and twig fragments were
visible. At − 4.31 m b.r.l., a second ice vein occurred. At − 4.726 to
−0.01 m b.r.l. the core sequence consisted of grayish, bedded, fine-,
medium-, and coarse-grained sand characterized by a massive
cryostructure. The next decimeters (−0.01–0.42 m b.r.l.) contained
a thin vertical ice vein. The uppermost meter consisted of unfrozen
modern river sand. The gravimetric ice content varied between 20 and
40 wt.% with only two ice-rich layers.
The Tur-1 profile (Fig. 3) was excavated on the 6 m high bank of
Arynskaya Channel next to the Tur-2 coring location. From 0 to 1.0 m
a.r.l., the profile showed cross-bedded fine- to medium-grained sands,
characterized by a massive cryostructure. Further up (1.0–1.5 m a.r.l.),
fine-grained sand with small diagonally-arranged ice veins occurs.
The uppermost frozen part (1.5–4.2 m a.r.l.) features a cryoturbated
sandy soil characterized by brownish iron oxide impregnations and
humus bands. Its upper boundary corresponds to the permafrost
table. The ice veins are comparable to similar structures found on the
south bank of Nikolay Lake (exposure D-1) 40 km to the north
(Schwamborn et al., 2002c). An unfrozen peaty layer at 4.2–4.5 m a.r.l.
containing a large piece of wood covers the frozen sequence. The
following unfrozen horizon (4.5–4.7 m a.r.l.) is characterized by
alternating thin (2–5 mm) brownish and gray laminae, possibly
originating from repeated eolian covering of soil layers.
For sediment characteristics, stratigraphical classification, and
correlation, several typical parameters were compared for most of the
studied exposures (Fig. 4). Comparable sediment layers were labeled as
correlated units (Units A–G). In the combined Tur-2/Tur-1 core and
exposure sequence, the lowermost approximately forty centimeters of
Unit A at −9.85 to −9.48 m b.r.l. consist of well-sorted mediumgrained sand (mean: 200–300 μm) that is almost organic-free
(TOC: b0.1–0.2 wt.%), with a high MS (130–580 SI). Unit B1, the section
immediately above (−7.83 m b.r.l.), is characterized by silty finegrained sand (mean: 120–200 μm) with an upwards gradually-finer
mean diameter and a rising TOC content of up to 0.5 wt.%. The MS
changes between 20 and 160 SI Unit C, the following ~100 cm thick
well-laminated horizon up to −6.78 m b.r.l., consists of less- to
medium-sorted silty fine sand and sandy silt and contains significantly
higher organic carbon (TOC: 0.5–1.6 wt.%) and lower magnetic mineral
content (MS: 20–40 SI). The organic matter content was high enough to
measure TN contents and to calculate C/N ratios between 17 and 22 and
δ13C values of −25.3 to −27.3‰. This horizon, radiocarbon-dated to
about 52 kyr BP, probably accumulated under the shallow still-water
conditions of an oxbow lake. Unit B2, the next horizon, is about 270 cm
thick and is granulometrically similar to Unit B1 but contains layers with
higher TOC (0.2–1.3 wt.%) characterized by δ13C values of −25.5 to
−26.7‰. With clear changes in several sediment parameters at
−4.38 m b.r.l., the well-sorted medium-grained sand of Unit D, nearly
free of organic carbon and with a lower MS of 10–40 SI, completes the
Tur-2 core sequence up to the beach level. Several thin layers with
different sediment parameters reflect short-term changes in fluvial
accumulation. Unit D continues up to 1.4 m a.r.l. in the lowermost part of
the following Tur-1 exposure sequence. Although grain-size characteristics do not significantly change for the well-sorted medium-grained
sand further up, a separate Unit E was classified because of the
occurrence of ice veins and higher ice contents as well as cryosol
patterns, peat inclusion, and very low MS (5–10 SI). These features
probably reflect postsedimentary cryogenic and pedogenic impacts. The
30 cm thick peaty horizon (Unit F) covering the permafrost table is
characterized by a TOC value between 10 and 20 wt.%, C/N ratios of 19–
23, and heavier δ13C values of −27.8 to −29.44‰. The entire Tur-2/Tur1 sequence is completed by the 60 cm thick Unit G, composed of wellsorted medium-grained sand (mean: ~270 μm) with low TOC contents
(0.1–0.3 wt.%) and a low MS of 10–20 SI.
A second profile (Ebe-4) was excavated in the opposite 7 m high
bank of the Arynskaya Channel (Figs. 1C and 3). Only the lowest three
meters of the approximately 5 m high profile was frozen. Between 2.0
and 3.0 m a.r.l., spotty, yellowish-gray-to-brownish sand occurred
without any visible sediment or ice structure (ice content 20–24 wt.%).
This horizon was covered by about 10–15 cm of weakly-bedded finegrained sand with plant detritus. Angular, dark-brown, frozen peat
fragments were incorporated into sandy frozen sediments between 3.1
and 4.0 m a.r.l. More peat inclusions, less angular than those found
Fig. 3. Scheme, cryolithological description, and age determinations of two exposures (Ebe-4 and Tur-1 profiles) and a core (Tur-2) at the southern boundary of the second Lena Delta terrace (Arga Complex) studied at the Arynskaya Channel:
profile Ebe-4 (Ebe–Basyn–Sise Island, left bank), and profile Tur-1 and core Tur-2 (Turakh–Sise Island, right bank). Radiocarbon ages are marked in blue while OSL ages are marked in red.
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Fig. 5. Additional Arga Complex exposures: (A) Photograph and exposure scheme of T-021 dug in a terrace of a thermokarst lake on Turakh–Sise Island; (B) The LD98-D1 exposure on
the southwest bank of Lake Nikolay, Arga–Muora–Sise Island (according to Schwamborn et al., 2002c).
below, were visible between 4.0 and 5.0 m a.r.l. within frozen, yellowish
sand. This peaty-sandy layer featured 2 and 5 cm wide striped ice-sand
veins as well as a sand-filled frost crack. In addition, two horizons
showing cryoturbation structures occurred at 4.0 and 4.8 m a.r.l. Two
buried soils about 1 m thick were observed above the permafrost table.
The lower part of these soils was characterized by distinct dark bands.
The Ebe-4 exposure has a stratigraphical composition similar to that
of the Tur-1 exposure (Fig. 4). Well-sorted medium-grained sand
(mean: ~300 μm) exposed between 1.9 and 3.1 m a.r.l. is almost free of
organic material with an MS of 16–25 SI, corresponding to the sediment
parameters of Unit D (see earlier discussion). The quite ice-rich horizon
up to the permafrost table at 5 m a.r.l. is characterized by a high TOC of
5–11 wt.%, C/N of 20–27, and δ13C values of −28.1 to −28.5‰. Because
of the sediment characteristics and the occurrence of ice veins and
cryoturbance patterns, this horizon is defined as Unit E. The topmost
layer of the Ebe-4 exposure was classified as cover Unit G. It is about
two meters thick and consists of well-sorted medium-grained sand
(mean: 240–350 μm) with low organic content (TOC: b0.1–0.6 wt.%)
and an MS of 9–26 SI.
4.1.2. The second Lena Delta terrace on Turakh–Sise and Arga–Muora–
Sise islands
The T-021 exposure was excavated on Turakh–Sise Island at about
3 masl into a terrace level, often surrounding the oriented thermo-
karst lakes on the Arga Complex (Figs. 1B and 5A). The permafrost
boundary was found 1.25 m below the surface (b.s.). The lowermost
gray, frozen sands (ice content 15–20 wt.%) are penetrated by orangebrown-colored cracks with orientations similar to those of the ice
veins described in the previous exposures. Above the permafrost
boundary, alternate-bedded, gray-to-orange sand is covered by 25 cm
of sand exhibiting dark-brown iron-oxide impregnation. At 0.50–
0.75 m b.s. a layer with horizontal, orange-brown bands was visible.
Between 0.25 and 0.5 m b.s., the well-bedded sand contains more
organic matter and was spotty-orange to gray in color. The uppermost
part of the profile consists of gray and light-yellowish sand with roots.
Finally, a layer of dune sand about 0.5 m thick had accumulated at the
surface.
The lowest part of the T-021 exposure (Fig. 1C) between 1.1 and
2.0 m b.s, is composed of well-sorted fine- to medium-grained sand
(mean: 275–300 μm) with low MS (5–13 SI) and low TOC values (b0.1–
0.17 wt.%). Together with the observed crack pattern (Fig. 5A) this
horizon is similar to Unit E, which is exposed in all other profiles of the
Arga Complex area. According to sediment parameters, the next
unfrozen horizon between 0.35 and 1.0 m b.s. is similar to Unit D, but
because of the bedding structure and location near the lake shore it is
regarded as a separate lacustrine sediment, Unit H. The uppermost
30 cm thick layer with TOC values between 0.4 and 1.8 wt.%, C/N values
of 8.4–12.3, and a δ13C value of −26.7‰ is labeled as cover Unit G.
Fig. 4. Sediment data from exposures of the second Lena Delta terrace (Arga Complex) on Arga–Muora–Sise and Ebe–Basyn–Sise islands (please note the different scales for TOC of
the Tur-1 and Ebe-4 exposures); b.s. — below surface.
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Table 1
Radiocarbon AMS dating results from the core Tur-2 sequence, and the Tur-1, Ebe-4, and Kha-2 exposures. Calibration was accomplished using CALIB rev 4.3 software (Stuiver et al., 1998).
Lab. no.
Sample
Description
Depth
(m b.s.)
Height
(masl)
Radiocarbon ages
(years BP)
Calibrated ages
(cal. BP), σ 95%
KIA29866
KIA29865
KIA31040
KIA31039
KIA29249
KIA31041
KIA31042
KIA31043
KIA31044
KIA31045
KIA29250
KIA29251
KIA29252
KIA29867
KIA29253
KIA29862
KIA31036
KIA29863
KIA31037
KIA29864
KIA31038
KIA31035
KIA31064
KIA31034
KIA31033
KIA31032
KIA31031
KIA31030
KIA31029
Tur-1-14
Tur-1-10
Tur-1-1
Tur-1-5
Tur-1-3
Tur-1-S-15/S-16
Tur-2-1
Tur-2-3
Tur-2-4/2-5
Tur-2-7 to 2-11
Tur-2-15
Tur-2-30
Tur-2-50
Tur-2-76
Tur-2-99
Ebe-4-2
Ebe-4-7
Ebe-4-9
Ebe-4-13
Ebe-4-15
Ebe-4-17
Kha-2-32
Kha-2-28
Kha-2-24
Kha-2-18
Kha-2-13
Kha-2-7
Kha-2-3
Kha-2-1
Plant remains
Driftwood
Plant remains
Plant remains
Peat inclusion
Plant remains
Plant remains
Plant remains
Plant remains
Plant remains
Plant remains
Plant remains
Plant remains
Twigs
Plant remains
Plant remains
Peat moss
Peat
Peat moss
Plant remains
Peat moss
Plant remains
Twigs
Peat inclusion
Plant remains
Peat
Peat moss
Peat inclusion
Peat inclusion
0.2
1.0
4
1.9
2.5
4.35
1.04
1.29
1.4
1.53 to 2.10
2.34
4.88
6.84
8.97
10.82
0.75
2.45
2.85
3.65
4.2
4.85
5.18
4.38
1.38
3.58
2.88
1.03
0.477
0.22
0.1
− 0.03 to − 0.52
− 0.91
− 3.36
− 5.32
− 7.45
− 9.30
6.01
4.31
3.91
3.11
2.56
1.91
16.5
14.5
10.1
8.6
6.6
5.0
4.2
3.1
310 + 25/−20
695 ± 35
6630 ± 70
9105 ± 50
10,775 ± 45
15,390 + 150/−140
29,280 ± 610
15,510 ± 190
15,980 + 300/−280
18,490 + 260/−250
26,670 + 350/−330
40,020 + 2100/−1660
46,960 + 2580/−1950
52,130 + 2770/−2050
N52,110
565 ± 30
2910 ± 35
3685 ± 25
4825 ± 40
12,335 ± 55
12,640 ± 90
20,100 ± 100
24,890 ± 160
28,050 ± 190
29,770 ± 250
N50,020
N52,090
N49,030
N43,550
443–350
686–629
7624–7940
10,401–10,186
13,001–12,771
19,052–17,840
An exposure (LD98-D1) located in the center of Arga–Muora–
Sise Island on Nikolay Lake's southern bank (23–27 masl) studied
by Schwamborn et al. (1999, 2002c) (Fig. 1B, C) indicates sediments
with horizontal lamination on an mm- to cm-scale. The sediments
contain 15–20 wt.% ice and a system of already-described
connected ice veins (Fig. 5B). The deposits consists of well-sorted
fine and medium-grained sand (mean grain size: 200–300 μm)
almost free of organic carbon (TOC: 0–0.3 wt.%) and with an MS of
10–60 SI. These parameters correspond very well with the characteristics of Unit D.
4.1.3. The second Lena Delta terrace in transition to the third Lena Delta
terrace on Ebe–Basyn–Sise Island
The northern area of Ebe–Basyn–Sise Island, where the lateral
contact between Arga Complex and Ice Complex was assumed, was
studied in three profiles (Ebe-2, Ebe-3, and Ebe-5) (Fig. 1C, Table 1).
The Ebe-2 exposure (SOM-2 A) was excavated at about 18 m a.r.l on
the western slope of a hill located in the transition area between the
second and the third terraces. The lower frozen part of the 1 m deep
profile featured grayish fine-grained sand (ice content ca. 19 wt.%).
Therein, a 5 cm thick horizontal ice vein composed of horizontal striped
and vertical needle-like structures was observed. Above the permafrost
table a 15 cm thick well-bedded gray sand and a brownish-gray, nonbedded, silty-fine sand containing vertical grass roots were exposed.
Further up, a 25 cm thick rooted and cryoturbated brownish soil
horizon was covered by 5 cm thick gray, dry fine sand, probably of
eolian origin.
The 0.6 m deep Ebe-3 profile was excavated at about 16 masl in the
same area into a northern hill slope. The small pit (SOM-2 A) exposed
frozen grayish-brown fine sand with concentric rings of iron oxide
impregnations between 0.3 and 0.6 m depth (ice content 25–30 wt.%).
Above the permafrost table, grayish fine sand containing small
brownish bands and a cryoturbated brown horizon formed the
modern soil layer.
Two subprofiles of the Ebe-5 exposure were excavated into the
6 m high cliff on the left bank of a small channel flowing parallel to the
19,238–17,875
19,949–18,216
22,845–21,093
642–585
3084–2951
4092–3924
5556–5471
15,431–14,534
15,626–14,293
Utyan–Uyesya Channel (Fig. 1C), which exposed unfrozen sands and
frozen sands with diagonal ice veins (SOM-2 B). Undulate-bedded,
fine- to medium-grained, gray and frozen sand was exposed 3 m a.r.l.
Just below the permafrost table a 5 cm thin ice vein was connected to
a 0.3 m wide diagonal ice vein. The thin ice vein was horizontally
striped and contained many gas bubbles. Well-bedded, fine- to
medium-grained sand occurred above the permafrost table, and was
sampled up to 5 m a.r.l.
The Ebe-2, Ebe-3, and Ebe-5 exposures located several kilometers
to the southwest on Ebe–Basyn–Sise Island (Fig. 1C) are composed of
well-sorted fine- to medium-grained sand with similar MS values of
50–80 SI and low TOC values of 0.17–0.35 wt.%. Only the cryosol
horizon of Ebe-2 contains more organic carbon (0.5 to 0.7 wt.%). These
characteristics correspond widely to Unit D (Fig. 4).
4.1.4. The third Lena Delta terrace on Khardang–Sise, Ebe–Basyn–Sise,
and Kurungnakh–Sise islands
An exposure on the northwest bluff of Khardang–Sise Island
(Fig. 1B, SOM-1) was studied in order to understand the stratigraphical relationship between the second terrace sands of the Arga
Complex and the Lower Sand Unit of the third terrace. The 20 m high
bluff was excavated in three sections (Kha-1, -2, and -3). Several subprofiles (Kha-2 A–E) were exposed in thermokarst mounds. The
identified permafrost deposits consist of four different units (Fig. 6).
The lower part is the Lower Sand Unit that extends from the beach to
5 m a.r.l., followed by a 1–2 m thick peat horizon, a peaty-sandy
transition zone (Peat Unit) about 1.5 m thick, and the Ice Complex
Unit up to 12 m thick on top. Large ice wedges typical of Ice Complex
exposures could not be excavated. However, thermokarst mounds
indicate their presence.
The lower part of the cliff (Fig. 6, Kha-1) is mostly composed of
horizontally-laminated and cross-bedded medium- to fine-grained
frozen sands with silty interbeds (Lower Sand Unit). Only the
lowermost 2.5 m could be excavated and sampled. The ice content
was 24–32 wt.%. In addition, cracks filled with small ice crystals
occurred within the frozen sandy deposits.
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183
Fig. 6. Scheme and cryolithological description of one exposure sequence (Kha profiles) of the third Lena Delta terrace on Khardang–Sise Island.
A Peat Unit extends between 5 and 7 m a.r.l. along the studied cliff
section (Fig. 6, Kha-2 A). The lowermost layer consists of peat
fragments in fine-laminated, grayish-green, fine-grained sand (ice
content 25–55 wt.%). A thin layer above is sandier and shows
slumping structures. Further up, a 0.5 m layer of frozen moss peat
(ice content 105 wt.%) reflects autochthonous accumulation. The peat
horizon is overlain by grayish-brown cross-bedded sand. Even further
up, strongly-disturbed sediment structures probably indicate refrozen
modern slump material; therefore, this material was not sampled.
Between 6.5 and 7.5 m a.r.l. (Fig. 6, Kha-2 B), lattice-like
intersecting ice veins (up to 15 cm wide) were exposed consisting
of parallel and alternating ice and sand bands. A larger peat inclusion
was covered by grayish fine sand containing the ice veins. Above the
ice wedges, yellowish-gray silty fine sand showed sloping parallel
structures. The lower part of the Ice Complex Unit was exposed
between 7.5 and 8.5 m a.r.l. (Fig. 6, Kha-2C). This subprofile consisted
of two layers featuring ice-rich dark grayish-brown silty fine sand
with banded cryostructures (ice content 38–44 wt.%) alternating with
grayish-brown sand with ripple-bedding (ice content 26 wt.%). The
layers contained weakly-developed cryoturbated paleosol horizons.
Similar deposits were observed up to 12 m a.r.l. (Fig. 6, Kha-2 D). Icerich grayish sands (0.1–0.3 m thick) containing twig fragments, ice
bands, and lens-like broken cryostructures alternated with light-gray to
brownish sand layers (about 0.2 m thick). Between 14.5 and 17 m a.r.l.,
the uppermost Kha-2 E subprofile consists of several alternating layers
of ice-rich, ice-banded silty fine sand. Several lens-like reticulated
cryostructures and ice-poor paleosol horizons with peat inclusions can
also be observed. The ice content ranged between 50 and 115 wt.%.
A composite ice-sand wedge about 0.5–1 m wide consisting of
alternating 0.5–2 cm wide sand and ice stripes was studied in
exposure Kha-3 (Fig. 6) about 300 m north of exposure Kha-2 and
sampled for isotope studies at different levels. The ice wedge
penetrates into sandy deposits. The lowermost part consists of
yellowish-gray fine sand. Further up, the ice wedge traverses the
already-described Peat Unit. The higher part of the section consists of
grayish fine sand with numerous brownish and blackish spots and
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Fig. 7. Sediment data from exposures of the third Lena Delta terrace on (A) Khardang–Sise Island and (B) Nagym on Ebe–Basyn–Sise Island.
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small twig fragments. Ice belts concavely bent towards the ice wedge
reflect syncryogenetic ice wedge formation.
On Khardang–Sise Island, the Lower Sand Unit, exposed in the
Kha-1 section (up to 4 m a.r.l), is composed of almost organic-free
well- to medium-sorted fine- to medium-grained sand (mean: 190–
235 μm) with an MS of 20–90 SI (Fig. 7A). The sedimentological
parameters are similar to those described earlier for Unit D. The peaty
segment exposed between 3 and 6 m a.r.l. in section Kha-2A is
characterized by poorly-sorted fine-grained sand to sandy silt (mean:
63–230 μm) with MS between 20 and 70 SI. Relatively high TOC
values of 1–8.6 wt.%, C/N values of 9–29, and δ13C values of − 26.5 to
−29.9‰ reflect peat components at different stages of decomposition
in these deposits. These deposits are not comparable to the alreadyclassified Units A–G and are therefore labeled as an additional Peat
Unit (Fig. 7A). The sediments of section Kha-3 are also classified in the
Peat Unit. The Ice Complex Unit further up consists of poorly- to verypoorly-sorted silty fine-grained sand with MS values between 20 and
55 SI. The TOC content ranges between 0.3 and 3.2 wt.% with δ13C
values from −25.0 to −26.5‰. The quite narrow C/N ratio of 7.6–12.1
reflects more highly decomposed organic matter. These features are
caused by the existence of buried cryosol sequences within the Ice
Complex sequence.
For completeness, several updated datasets from previouslystudied exposures from Nagym on Ebe–Basyn–Sise Island and Buor
Khaya on Kurungnakh–Sise Island (Fig. 1B, C) that belong to the third
Lena Delta terrace are included in this study (Fig. 8). These exposures
showed a similar stratigraphic composition of sand deposits covered
by Ice Complex deposits (Schirrmeister et al., 2003; Schwamborn
et al., 2002c; Wetterich et al., 2008).
Finally, the already-mentioned exposure at the Nagym location
(Fig. 1C) has a lithostratigraphical composition generally similar to
that of the Khardang sequence (Fig. 7B). The well-sorted mediumgrained Lower Sand Unit is characterized by low TOC values of 0.2–
0.4 wt.%, single plant detritus layers (TOC: 1.0–5.4 wt.%), δ13C values
of −24.1 to − 26.1‰, and MS values of 14–50 SI (Fig. 7B). The
covering Ice Complex Unit is composed of less-sorted fine- to
medium-grained sand slightly coarser than that seen in Ice Complex
deposits at Khardang–Sise Island (Fig. 7A). According to the
geochronological results, this Ice Complex sequence is N47.4–
44.2 kyr BP old (Schirrmeister et al., 2003) and belongs to the MIS 3
stage.
4.2. Geochronology
4.2.1. Age of the second Lena Delta terrace
Radiocarbon and OSL dating was carried out from the Tur-2/Tur-1
deposits and the Ebe-4 exposure (Figs. 1C and 3). In addition,
previously-published IRSL ages from the LD98-D1 exposure are
reviewed (Krbetschek et al., 2002; Schwamborn et al., 2002c).
The age–height correlation is generally good for the studied
sequences on both sides of the Arynskaya Channel (Fig. 3, Table 1).
The Tur-2 core was radiocarbon-dated to between N52 kyr BP and
15.5 ± 0.2 kyr BP. One age inversion occurs in the uppermost part,
probably caused by reworking processes at the beach level. The
subsequent Tur-1 exposure continues the radiocarbon age sequence
between about 15.4 ± 0.15 kyr BP and 0.3 ± 0.02 kyr BP. One age
discrepancy at 1.3 m (6.6 ± 0.07 kyr BP) was probably caused by
contamination with Holocene plant matter in frost cracks. The
radiocarbon age sequence of the Ebe-4 exposure also covers the
Lateglacial to late Holocene period with a gap between about 12.3 and
4.8 kyr BP.
The OSL ages of the Tur-2 core and the Tur-1 exposure also exhibit
a good age–height correlation between 37 ± 6 kyr and 6 ± 1 kyr
(Fig. 3, Table 2). Whereas OSL and radiocarbon ages correlate well in
Tur-1, the older OSL ages in Tur-2 tend to be only half of the
radiocarbon ages. The age difference in older parts can be as much as
185
20 kyr. Nevertheless, when summarizing the geochronological datasets we can narrow down the formation age of the cored sand
sequence to the Middle to Late Weichselian period, while frozen sands
in the lower segments of the Tur-1 and Ebe-4 exposures were formed
during the Lateglacial period between 12.6 and 10.7 kyr BP. This is in
good agreement with IRSL feldspar ages of about 12–13.5 kyr BP for
the sedimentologically and cryolithologically similar LD98-D1 exposure at Nikolay Lake (Fig. 5B, Tables 1 and 2). The Lateglacial
sediments at the Arynskaya Channel are overlain by frozen Early to
Middle-Holocene deposits, and finally covered by Late Holocene
unfrozen sediments.
4.2.2. Age of the third Lena Delta terrace
The exposures at Khardang–Sise Island were also dated by
radiocarbon and OSL methods (Fig. 8, Tables 1 and 2). The Lower
Sand Unit of profile Kha-1 was OSL dated to about 23–22 kyr BP. The
Peat Unit of profile Kha-2 between 3 and 7 m a.r.l. shows infinite
radiocarbon ages of N52.1 to N43.5 kyr BP. The overlying Ice Complex
Unit was formed between 30 and 20 kyr BP. A strong discrepancy
between radiocarbon and OSL ages is apparent when the results of
these methods are compared. However, it should be noted that
subprofile Kha-1 was only dated using OSL, whereas the radiocarbon
ages refer to a set of subprofiles in the slightly-different Kha-2
sections (Fig. 8).
New OSL (quartz) age determinations on third terrace from the
Lower Sand Unit in the Nagym (Ebe–Basyn–Sise Island) and Buor
Khaya (Kurungnakh–Sise Island) exposures at the Olenyekskaya
Channel (Table 3) were added in order to obtain a complete regional
geochronological dataset. Samples were previously taken in 1998 and
described in former publications (Krbetschek et al., 2002; Schirrmeister et al., 2003; Schwamborn et al., 2002c). These horizons were
dated between 36 ± 5 kyr and 20 ± 3 kyr for the Nagym section and
54 ± 9 kyr and 30 ± 5 kyr for the Buor Khaya section (Fig. 8). The good
age–height correlation within the new OSL ages is only marred by the
youngest OSL age in the Nagym section. OSL ages from both sections
are comparable to OSL results from the Khardang section. Nevertheless, OSL ages from both sites are significantly younger than the
former IRSL age determinations on feldspars (Nagym 70–50 kyr; Buor
Khaya 90–60 kyr; Fig. 8; Krbetschek et al., 2002). In these cases, the
high errors indicate insufficient bleaching; such IRSL ages must be
considered maximum estimates only (Krbetschek et al., 2002).
Numerous radiocarbon ages of the overlying Ice Complex deposits
contradict ages younger than 43 kyr and indicate ages for the
underlying Lower Sand Unit of N57 to N43 kyr BP in the Nagym
section and 49 to 37 kyr BP in Buor Khaya (Schirrmeister et al., 2003;
Wetterich et al., 2008). We should realize that the application of
different geochronological methods to permafrost deposits can result
in large age differences although the specific method-related
chronologies seem to show consistency within and between investigated sections.
4.3. Mineralogical analysis
About 60 samples from all exposures were studied for their heavy
and light mineral compositions. Most of the studied heavy mineral
grains of the fine fraction (63–125 μm) are not or are only weakly
rounded. The garnet grains always show angular shapes. Intergrowths
of several minerals, e.g. feldspars with pyroxene or ilmenite, garnet
with zircon or epidote, epidote with chlorite, and sphene with rutile
are common. Rhombohedral pyroxenes consist of hypersthene. The
monocline diopsides are variously colored. Augites seldom occur.
Titanium minerals (ilmenite, sphene) are often transformed to
leucoxene. Newly-formed siderite and rounded, probably reworked,
iron hydroxides occur. These features allow us to infer the existence of
relatively short transport paths and source areas with metamorphic
basement. Mineral grains were partly affected by weathering and
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187
Table 2
OSL (quartz) dating results (new) and IRSL (feldspar) results (*Krbetschek et al., 2002) (De: equivalent dose; SD: standard deviation; Drate: dose rate).
OSL sample
Corresponding sediment sample
Turakh–Sise Island (Arynskaya Channel)
Tur-OSL-1
Tur-2-20
Tur-OSL-5
Tur-2-30
Tur-OSL-6
Tur-2-46
Tur-OSL-7
Tur-2-84
Height
(masl)
Description
Aliquots
(no.)
− 2.18
− 3.48
− 4.98
− 8.23
Fine- to medium-grained sand
Fine- to medium-grained sand
Medium-grained sand, organic-rich
Fine- to medium-grained sand,
peat inclusions
Fine- to medium-grained sand,
peat inclusions
Fine- to medium-grained sand,
organic patches
Fine- to medium-grained sand
28
14
7
27
Tur-OSL-4
Tur-1-6
4.08
Tur-OSL-3
Tur-1-3
2.88
Tur-OSL-2
Tur-1-15
Nikolay Lake (Arga–Muora–Sise Island)
ARG 5
ARG 4
ARG 3
ARG 1
Khardang–Sise Island (Arynskaya Channel)
Kha-OSL-2
Kha-OSL-1
Nagym (Ebe–Basyn–Sise Island)
Ole-5
Ole-4
Ole-3
Buor Khaya (Kurungnakh–Sise Island)
Ole-8b
Ole-8
0.88
27.9
27.3
26.6
25.1
4.0
Horizontally-bedded
Horizontally-bedded
Horizontally-bedded
Horizontally-bedded
fine-grained
fine-grained
fine-grained
fine-grained
De ± SD
(Gy)
Drate
(mGy/y)
OSL age
(kyr)
19.4 ± 4.0
32.4 ± 3.9
44.7 ± 8.7
84.0 ± 13.2
2.2
1.8
1.9
2.3
9±2
b 18
24 ± 6
37 ± 6
31
12.6 ± 1.2
2.1
6±1
12
25.8 ± 2.2
2.0
13 ± 1
29
23.4 ± 2.3
2.1
11 ± 1
3.23 ± 0.23
3.24 ± 0.22
3.04 ± 0.17
3.74 ± 0.11
13.1 ± 1.1 *
12.0 ± 1.1 *
13.3 ± 1.5 *
13.4 ± 1.1 *
sand
sand
sand
sand
42 ± 2.0
39 ± 2.4
40 ± 3.9
50 ± 2.6
Fine- and medium-grained-sand,
fine-bedded
Fine- and medium-grained-sand,
fine-bedded
31
51.6 ± 5.2
2.2
23 ± 3
24
49.8 ± 3.3
2.3
22 ± 2
9.0
7.8
5.8
Fine-grained sand
Fine-grained sand
Fine-grained sand, silty interbeds
26
25
34
72.2 ± 14.3
98.9 ± 8.8
39.3 ± 3.4
2.6
2.7
2.0
28 ± 7
36 ± 5
20 ± 3
7.2
Fine-grained sand, silty and
medium-grained interbeds
Fine-grained sand, partly cryoturbated
15
69.6 ± 7.1
2.3
30 ± 5
18
110.1 ± 11.3
2.0
54 ± 9
2.0
5.7
pedogenic processes. Because of the similar heavy mineral associations in the fine and the coarse fractions, the presentation of analytical
results is focused on the more-representative finer fraction. The only
difference is the grain roundness. In samples from Ebe–Basyn–Sise
Island the coarser grains are only weakly- or non-rounded. In contrast,
the coarser grains from Turakh–Sise Island and Khardang–Sise Island
exposures are mostly sub-rounded but seldom well-rounded or nonrounded.
On the Arga Complex, the mineralogical composition differs
significantly for the segment of Units A, B1, C, and B2 in the Tur-2
core down to a depth of − 4.38 m b.r.l., which is characterized by
higher mica contents in the heavy (0.3–1.2.9%) as well as the light
(0.3–3.5%) fractions (SOM-3, Table 3); this higher mica content is
correlated with the high MS of these units (Fig. 4). Unit A is
additionally marked by the highest zircon and garnet contents
(Table 3). Variations in the mineral composition of both the covering
Unit D and Unit E are on a similar scale. This is also true for samples
from the T-021 exposure on Turakh–Sise Island and Ebe-4, Ebe-2, Ebe3, and Ebe-5 on Ebe–Basyn–Sise Island (SOM-3). The mineralogical
similarities of Units D, E, F, and G probably reflect similar sediment
sources for these Arga Complex deposits.
In third terrace deposits there are no significant differences
between the heavy mineral signatures from the Lower Sand Unit
and the Ice Complex Unit on Khardang–Sise Island, or from the
Nagym site at the southern rim of Ebe–Sise Island (SOM-3, Table 3).
Smaller variations could be caused by different accumulation
conditions and postsedimentary cryogenic and pedogenic modifications. However, clear differences are evident between the Lower
Sand Unit and the covering Ice Complex Unit at the Buor Khaya site
on Kurungnakh–Sise Island. This is especially reflected in higher
garnet and epidote contents as well as lower pyroxene and
amphibole contents of the Ice Complex deposits (Table 3), a
distribution that is similar to brook sediments from Chekanovsky
Ridge (Schwamborn et al., 2002c). Therefore, the nearby Chekanovsky
Ridge is considered to be the source area for the Ice Complex
deposits on Kurungnakh–Sise Island. However, the heavy mineral
composition of the Lower Sand Unit at Kurungnakh–Sise Island is
comparable to all the other samples mentioned earlier. The source
areas for all deposits with similar heavy mineral signatures should
be similar, and should correspond to the integral heavy mineral
signature of the modern Lena River deposits (Schwamborn et al.,
2002c). Therefore, the Lena River catchment is considered to be the
sediment source for deposits of the second and partly of the third
Lena Delta terraces.
4.4. Stable water isotopes of ground ice
In various types of ground ice the stable isotope composition
(δ18O, δD, d-excess) has been determined for studying ice differentiation, stratigraphical correlation, and postcryogenic alteration.
Furthermore, paleoenvironmental interpretation concerning winter
precipitation that fed ice wedges has been undertaken. The
corresponding sample positions are shown in the exposure schemes
(Figs. 3, 5 and 6, SOM-2).
All the ice veins of the Tur-1, Ebe-2, Ebe-4, Ebe-5, and LD-98 D-1
exposures in the Arga Complex have similar isotope signatures that
are heavier (less negative) than that of the Khardang ice wedges
(Table 4) and correlate very well (R2 = 0.98) with a slope of 7.2 and an
intercept of −10.3, close to the GWML (Fig. 9A). This isotope
signature reflects a similar genesis of the ice veins in Unit E of the
Arga Complex. Texture ice values from the Tur-2 core correlate well
(R2 = 0.99) with a slope of 9.3 and an intercept of 32.8 below the
Fig. 8. Radiocarbon and OSL ages of the exposures at Khardang–Sise (Kha), Ebe Basyn Sise (Nagym), and Kurungnakh–Sise (Bkh) islands. *OSL samples were taken in 1998
(Krbetschek et al., 2002) from different profiles; due to erosion of river banks, these positions might not be identical with positions sampled in 2000. Radiocarbon ages are marked in
blue while OSL and IRSL ages are marked in red.
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Table 3
Heavy mineral compositions (in grain percent) of the classified sediment units in the combined Tur-2/Tur-1 sequence and in the other exposures (mean, min–max); the Ice Complex
Unit on Kurungnakh Island is marked in gray.
Tur-2, Unit A
Tur-2, Unit B1
Tur-2, Unit C
Tur-2, Unit B2
Tur-1, -2, Unit D
Tur-1, Units E, F, and G
Ebe-2
Ebe-4
Ebe-5
Arga, D1
Tur-021
Kha-2, Ice Complex Unit
Kha-1, Lower Sand Unit
Nagym, Ice Complex Unit
Nagym, Lower Sand Unit
Buor Khaya, Ice Complex Unit
Buor Khaya, Lower Sand Unit
Ilmenite
Epidote
Pyroxene
Amphibol
Garnet
Zircon
Apatite
Sphene
21.8
14.0−29.4
10.1
7.3−14.2
8.0
3.9−13.1
7.7
5.2−14.9
12.3
6.7−17.7
10.9
7.6−13.8
11.0
9.7−12.5
13.6
9.7−17.9
9.7
8.7−10.6
13
12.3−13.6
6.6
5.3−8.0
6.0
4.5−7.1
10.0
6.6−14.9
6.6
4.8−10.2
5.0
2.1−7.5
2.5
0.8−5.7
12.0
6.8−16.9
15.8
11.3−20.7
20.6
17.2−24.8
15.0
12.1−19.9
19.6
15.2−22.9
14.5
10.6−23,3.
13.9
10.6−17.1
15.9
12.3−18.2
12.0
10.4−13.6
14.1
12.7−15.3
11.8
9.0−15.0
10.5
9.0−12.7
15.9
13.0−20.9
13.8
11.1−15.3
16.4
15.2−19.3
13.1
11.7−14.2
30.8
25.6−36.5
10.7
8.8−12.6
21.7
15.0−27.2
18.9
11.0−22.0
19.4
16.2−20.9
20.4
16.2−24.5
23.0
17.4−28.2
26.8
23.2−30.2
22.1
19.5−24.5
24.2
20.3−23.7
22.1
21.2−23.1
24.6
16.7−33.0
28.5
25.5−32.0
18.2
11.1−21.4
18.7
16.5−19.8
11.4
7.6−17.6
13.1
11.1−15.4
6.7
1.9−9.8
21.8
17.6−29.9
40.0
28.5−44.3
42.4
38.7−45.7
49.4
42.7−60.9
43.1
37.9−48,4
41.9
38.2−45.2
39.7
35.2−43.2
41.9
40.8−44.4
45.4
41.1−51,9
44.9
41.8−48.0
32.3
31.6−33
48.4
45.5−51.6
42.2
31.6−47.9
44.7
37.1−49.7
41.0
28.4−50.8
48.2
42.4−52.1
10.3
2.9−15.6
33,2
30.2−38.2
14.7
10.1−16.9
8.3
5.6−10.5
6.8
3.1−10.3
8.2
4.2−13.8
11.2
8.1−14.9
10.9
7.9−13.8
9.0
7.5−10.9
10.5
7.8−13.2
8.5
6.8−11.3
12.4
9.8−16.0
5.9
4.0−9.2
8.5
3.6−13.4
6.4
5.9−6.9
14.0
10.2−17.9
10.6
6.1−15.7
34.3
31.2−36.9
15.1
14.9−26.0
5.1
3.6−7.3
1.6
1.1−2.1
1.5
0.8−2.8
1.0
0.3−1.8
1.3
0.4−2.7
1.0
0.6−1.3
1.9
1.1−2.5
1.3
0.4−2.4
1.6
1.1−2.1
0.9
0.0−1.8
0.1
−
0.9
0.3−2.3
0.9
0.3−1.5
2.3
0.7−3.5
1.4
0.3−2.7
1.1
0.7−1.7
1.1
0.3−1.5
2.7
2.3−4.7
3.0
1.7−4.4
3.8
3.1−4.8
2.3
1.3−3.5
2.5
1.1−6.8
2.3
1.5−3.0
3.6
2.8−4.4
2.5
1.2−3.5
2.2
1.8−2.5
4.1
3.0−6.2
1.1
0.7−2.7
3.4
2.3−4.6
2.9
1.8−4.4
2.2
0.3−4.6
3.8
2.0−6.8
5.5
3.9−7.3
1.9
0.8−3.9
6.3
5.2−7.2
5.1
3.2−7.1
4.1
2.0−6.8
5.5
3.1−9.7
5.5
3.7−7.2
5.2
3.7−6.1
5.2
4.1−7.2
4.2
3.1−5.2
5.8
5.6−6.0
0.9
0.0−2.0
4.8
3.5−5.7
5.0
3.9−11.7
3.6
3.3−4.0
6.1
3.6−9.0
4.8
3.2−7.4
8.9
5.1−12.1
4.1
1.8−5.9
GMWL over a wide range (Table 4). The heaviest isotope composition
was measured in texture ice of the Tur-1 and Ebe-4 exposures.
The lightest (most negative) stable isotope composition was
measured on the third terrace in the composite sand-ice wedge of
section Kha-3 from the Ice Complex Unit of Khardang–Sise Island
(Fig. 9A, Table 4). The data correlate very well (R2 = 0.98), with a
n
3
5
4
9
13
4
3
5
3
3
3
8
3
4
6
5
5
slope of 6.6 and an intercept of −39.5, and plot below the GMWL. The
data from small ice veins in nearby section Kha-2 B fit this dataset
well. The d-excess of the Khardang ground ice is rather low (4.0 to
−1.2‰). The isotope signature is similar to those of Ice Complex ice
wedges from the Nagym site (Schirrmeister et al., 2003) and from
Kurungnakh–Sise Island (Wetterich et al., 2008). Texture ice samples
Table 4
Stable isotope signatures in various ground ice samples from the second and third terraces of the Lena Delta (mean, max to min).
Ground ice sample
N
δ18O
(VSMOW, ‰)
δD
(VSMOW, ‰)
d-excess
(‰)
Kha-3 ice wedge
10
− 30.1
(− 27.4 to
− 27.4
(− 26.3 to
− 22.9
(− 18.9 to
− 24.5
(− 21.0 to
− 22.0
(− 20.8 to
− 23.0
(− 21.0 to
− 22.2
(− 21.9 to
− 27.04
− 25.6
(− 23.9 to
− 23.70
− 18.3
(− 17.5 to
− 25.1
(− 21.9 to
− 18.4
(− 18.2 to
− 239.3
(− 222.2 to
− 217.9
(− 211.8 to
− 175.9
(− 144.6 to
− 188.0
(− 162.1 to
− 170.3
(− 164.2 to
− 173.8
(− 158.5 to
− 170.1
(− 167.9 to
− 205.5
− 195.0
(− 181.4 to
− 193.3
− 138.9
(− 136.9 to
− 201.09
(− 170.7 to
− 144.91
(− 144.2 to
1.2
(4.0 to − 3.5)
1.0
(2.2 to − 1.2)
7.0
(10.0 to 3.9)
8.2
(10.2 to 5.8)
6.0
(12.7 to 2.4)
9.9
(10.3 to 9.4)
7.7
(7.9 to 7.4)
10.8
9.8
(20.5 to − 1.4)
− 3.6
5.5
(8.7 to 2.6)
− 0.2
(4.4 to − 1.7)
2.1
(1.8 to 2.3)
Kha-2 ice wedge
3
Tur-1 ice wedge
14
D1 ice wedge
4
Ebe-4 ice wedge
3
Ebe-5 ice wedge
2
Ebe-2 ice wedge
3
Kha-1 crack
Kha-2 texture ice
1
14
Kha-3 texture ice
Tur-1 texture ice
1
3
Tur-2 texture ice
6
Ebe-4 texture ice
2
− 32.0)
− 27.9)
− 25.3)
− 26.1)
− 23.9)
− 24.9)
− 22.5)
− 26.94)
− 18.5)
− 26.3)
− 18.5)
− 252.3)
− 221.0)
− 192.6)
− 198.3)
− 178.9)
− 189.1)
− 172.0)
− 216.5)
− 142.6)
− 212.1)
− 145.6)
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189
Fig. 9. δD–δ18O cross plot for various ground ice samples from the second and third Lena Delta terraces showing data from (A) ice wedges and (B) texture ice.
from the Ice Complex Unit on Khardang–Sise Island scatter for δ18O
between − 23.9 and −26.9‰ and for δD between −181.4 and
−216.5‰, both above and below the GMWL without any clear
correlation (Fig. 9B).
In general, both types of ground ice show the same pattern of
lighter isotope composition (Fig. 9) in late Pleistocene ground ice
reflecting formation under colder conditions, and heavier isotope
composition in Lateglacial and Holocene ground ice that formed
under warmer conditions.
4.5. Paleoecology
4.5.1. Pollen records
On the Arga Complex, pollen are almost absent in the Tur-2 core
sequence. Only single grains of Poaceae and reworked Pinaceae were
found in a few samples except for deposits between −5 and −6 m
depth. Their spectra are dominated by Poaceae pollen with a few
Artemisia, Asteraceae, Cyperaceae, Brassicaceae, and some other
herbs. Higher amounts of reworked coniferous taxa (Pinus s/g
Haploxylon, Larix, Picea, and Abies) are also notable. Coprophilous
fungi spores (Sporormiella, Sordaria, and Podospora) are common.
Generally, these spectra are similar to the Ice Complex pollen
assemblages from Khardang–Sise Island (pollen zone III, PZ-III,
Fig. 10B) and reflect similar vegetation conditions.
The sediments from the Tur-1 exposure above the Tur-2 core are
rich in pollen (Fig. 10A). The oldest pollen spectrum (PZ-I),
radiocarbon-dated to ca 10.8 kyr BP, is dominated by Betula sect.
Nanae, Cyperaceae, and Poaceae pollen, pointing to the rather warm
and wet local environment typical at the end of the Allerød. The
similarly-dated terrestrial pollen records from the Laptev Sea region
are also characterized by higher contents of birch pollen in the betterprotected inland areas (e.g. Andreev et al., 2003; Grosse et al., 2007;
Makeyev et al., 2003; Pisaric et al., 2001). In the following PZ-II, only a
few Encalypta and Lycopodium spores as well as reworked ancient
Pinaceae were found, reflecting unfavorable vegetation conditions
during the Younger Dryas cold interval. The PZ-III above (ca 9.1 kyr BP)
is characterized by a very high pollen concentration dominated by
Betula, Alnus fruticosa, Ericales, Cyperaceae, and Poaceae, reflecting
shrub tundra during the early Holocene. Paleoclimate reconstructions
conducted on pollen and chironomid records from nearby Nikolay Lake
show that temperatures in this part of the Arctic were up to 4 °C higher
at that time than they are nowadays (Andreev et al., 2004). The pollen
spectrum of PZ-IV (ca 0.7 kyr BP) is dominated by Salix and Cyperaceae
pollen, reflecting the modern willow- and sedge/grass-dominated
tundra vegetation. Pollen concentration is quite low. Tree and shrub
pollen (mostly Picea, Larix, and Pinus) in this spectrum are of longdistance wind-transported origin. The uppermost PZ-V, dated ca
0.3 kyr BP, is characterized by a very low pollen concentration and
poor pollen preservation and could not be used for the environmental
reconstructions.
On Khardang–Sise Island (third terrace), only single Poaceae and
Cyperaceae pollen and a few reworked ancient coniferous pollen were
found in the Lower Sand Unit (Fig. 10B, PZ-I, 0–1.6 m a.r.l.). The Peat
Unit (subprofile Kha-2 A), radiocarbon-dated to an infinite age
(Table 2), is dominated by Cyperaceae pollen with a few Poaceae
(PZ-II, 3.1–8.3 m a.r.l.). There are rather numerous well-preserved
pollen grains of Pinus s/g Haploxylon and Larix with few other tree and
shrub pollen types. High amounts of reworked ancient coniferous taxa
are also notable in this zone. We assume that the sediments
accumulated during the early MIS 3 Interstadial. Although tree pollen
are numerous in PZ-II it is very unlikely that trees like larch or Siberian
pine (Pinus sibirica) or shrubs like stone pine (Pinus pumila) could
grow in the area. The permanent presence of relatively thermophilic
coniferous trees like Picea, Abies, and especially Tsuga in the pollen
spectra, points to the reworked character of the tree fraction. The tree
taxa are probably present due to long-distance transport by the Lena
River from the southern Yakutian hinterland. Numerous Cyperaceae
pollen dominating in PZ-II reflect a relatively wet local environment with
sedge vegetation during the MIS 3. The Ice Complex Unit, radiocarbondated to between 30 and 20 kyr BP (subprofiles Kha-2 B–E), is
dominated by Poaceae pollen with some Cyperaceae, Artemisia, and a
few other herbs (PZ-III, 8.3–16.5 m a.r.l.). High amounts of reworked
ancient coniferous pollen (mostly Pinus s/g Haploxylon and Larix) are
also notable for this zone. Pollen assemblages reflect an open,
predominantly grass-sedge steppe-like environment. Coprophilous
fungi spores (Sporormiella, Sordaria, and Podospora) are numerous in
the lower part of the zone, indirectly pointing to the presence of grazing
mammals in the area.
Generally, the lower sediments from the Nagym site are
characterized by predominantly low pollen concentration and
numerous reworked pollen. Therefore, these data are not suitable
for paleoenvironmental interpretations. The Lower Sand Unit and
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191
Fig. 11. Paleogeographic scheme of the late Quaternary landscape dynamics in the western Lena Delta area. (A) Early Weichselian , (B) Middle to late Weichselian, (C) Lateglacial, and
(D) Holocene landscapes.
lower segments of the Ice Complex Unit contain mostly reworked
pollen of Abies, Pinus, Picea, and Larix. The studied pollen spectra are
very similar to PZ-I of Khardang section 1 (Fig. 10B). The upper
segment of the Ice Complex Unit contains the typical MIS 3 pollen
association dominated by Cyperaceae and Poaceae with numerous
Artemisia and Caryophyllaceae and single Salix, Fabaceae, and
Valeriana pollen. The Holocene cover deposits on top of the Ice
Complex, radiocarbon-dated to about 5 kyr, are dominated by pollen
of Betula sect. nana and Alnus fruticosa and a few Cyperaceae, Poaceae,
and Artemisia pollen.
Fig. 10. Pollen records from the western Lena Delta: (A) Pollen diagram from the Lateglacial to Holocene Tur-1 exposure on Turakh–Sise Island; (B) Pollen diagram of the Ice Complex
Kha-2 exposure on Khardang–Sise Island (R— reworked; A — algae; Z— zoological remains).
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4.5.2. Plant macrofossil records
Plant macrofossil studies of third terrace sequences were conducted at the Nagym section in a continuation of similar studies at
Kurungnakh–Sise Island (Wetterich et al., 2008). Both sequences
contained well-preserved plant remains, which fluctuated in abundance and diversity depending on taphonomical conditions. Altogether, 62 plant taxa could be identified in the Nagym section (SOM-4). The
sharp sedimentological boundary between the Lower Sand and the Ice
Complex units is not clearly detectable in the plant macrofossil record
(SOM-5), indicating that the change in the hydrological system
resulting in altered sedimentation did not result in general changes
of the vegetation. For example, plants typical of the Arctic, characteristically found in snow beds and tundra wetlands, and arctic pioneer
vegetation, as well as species from steppes and Kobresia mats that
occur at dry places in the Arctic, were found in both units (SOM-5).
The main difference between these units was a higher percentage of
thermophilous plants in the early Weichselian fluvial sands, many of
them typical of freshwater and littoral habitats likely correlated with
their subaquatic deposition. The littoral plants indicate water level
fluctuations, likely correlated with strong seasonal paleo-Lena River
discharge variations, which are characteristic of arctic rivers. Some
littorals, such as Rumex maritimus, Tripleurospermum hookeri, and
Ranunculus sceleratus, indicate fluctuating salt concentration, which
might signify a high rate of evaporation. Possibly, the remains of some
southern extralimital plants were introduced by the paleo-Lena River
from the Yakutian hinterland. For example, we have found Rubus cf.
idaeus, Fragaria sp., and Stellaria longifolia, which are restricted to the
forest zone in Yakutia. Together with Chamadaphne calyculata and
Duschekia fruticosa, they also might indicate the proximity of
subarctic shrubbery during certain times of Lower Sand Unit
deposition.
In the Ice Complex Unit, the percentage of plants occurring in dry
places, e.g. in steppes and Kobresia mats, increases distinctly. Such
vegetation existed on accumulation plains formed under the arid
cold-stage Siberian Arctic climate. The plant macrofossil record is
similar to other late Pleistocene cold-stage assemblages in the Laptev
Sea region (Kienast et al., 2005; Wetterich et al., 2008).
4.5.3. Testate amoebae records
Testate amoebae (Testacea) are shell-bearing rhizopods that
reflect varying subsurface conditions, e.g. moisture and temperature,
in soils and small shallow water bodies. The preservation of these
small thin shells indicates in situ (non-reworked) accumulation
conditions. In total, 51 samples have been analyzed; 92 species and
intraspecific taxa are identified in 26 samples (SOM-6) and classified
into four ecological groups (SOM-7). No shells were found in 25
samples; this may reflect the reworked character of the sediment
(fragile shells were destroyed), or alternatively such deposits were
never exposed long-term at a paleo-surface and/or inhabited by
testate amoebae.
In the Arga Complex deposits Testacea are absent in most of the 10
samples analyzed from the Tur-2 corer. Only at −5.32 m b.r.l. do
single finds of the soil species Schoenbornia humicola reflect
pedogenesis. This sample is one of the few Tur-2 samples that contain
some pollen grains. More variable species and subspecies (16 taxa)
occurred in the Tur-1 exposure where the xerophilous species
Trigonopyxis arcula found in Unit E at 2.88 m a.r.l. indicates a
temporarily dry condition of a generally boggy stage, with water
depth of about 30 cm (Bobrov et al., 1999). Sphagnobiotic species of
genera Heleopera and Nebela dominate at 4.18 m a.r.l. However, the
hydrophilic species Phryganella hemisphaerica, and hydrophilous
Centropyxis elongata and C. cassis, indicating oligo-mesotrophic
boggy conditions, were also found there. Testacea (10 taxa) were
found in only one sample from the Ebe-4 exposure at 4.80 m a.r.l.
Three species of the genus Arcella, especially the hydrophilic Arcella
discoides and two hydrobiotic species of genus Difflugia, and the
absence of soil eurybiotic and sphagnobiotic species reflect aquatic
conditions.
At the third terrace, the Peat Unit of the Kha-2 exposure exhibits the
highest Testacea abundance (up to 25 taxa). This wet and boggy habitat
is evidenced by sphagnobiotic species of genus Heleopera and Nebela,
hydrophilous species of genera Difflugia and Lagenodifflugia, and by
hygrophilous Centropyxis aculeata, C. discoides, and C. platystoma.
Decreasing numbers of hydrophilous species of genus Difflugia and
the partial absence of the obligatory hydrobiotic genus Lagenodifflugia
indicate alternating moisture conditions. Well-preserved shells of
eurybiotic soil species and hygrophilous Difflugia lucida found at
8.7 m a.r.l. indicate drier soil conditions. The Testacea assemblages
reflect a generally wet soil environment, with occasional dry periods.
This conclusion is in good accordance with the dominance of sedge
pollen in the Peat Unit.
The predominances of hygrophilous species from the sphagnobiotic
group (Centropyxis aculeata, C. discoides, C. ecornis, C. gibba, Cyclopyxis
arcelloides, Heleopera petricola, and H. sylvatica), and of hygro-hydrophilous species of genus Difflugia (D. lucida and D. minuta) in exposure
Kha-3 also point to boggy conditions. Calceophilic C. plagiostoma,
C. plagiostoma f. longa, and C. plagiostoma major were also found at this
site.
The Lower Sand Unit of the Nagym section (Fig. 1C) at 0.7 m a.r.l.
contains eurybiotic soil species of the genera Centropyxis and
Cyclopyxis, pointing to an initial stage of soil formation. Eurybiotic,
sphagnobiotic, and hydrobiotic soil species (5 taxa), including single
shells of hygrophilous species from the genus Difflugia and the
eurybiotic Centropyxis sylvatica, dominate at 7 m a.r.l. The absolute
dominance of Trinema lineare indicates a water depth of 25–30 cm
which is optimal for the growth of the named species (Bobrov et al.,
1999), reflecting boggy conditions. Two subspecies with sizes smaller
than the form typica indicate unfavorable soil conditions for testate
amoebas (Bobrov and Mazey, 2004). Several sandy samples from this
unit do not contain any shells, indicating poor preservation of
Testacea under fluvial accumulation conditions.
In the Ice Complex Unit of the Nagym exposure, some samples
containing the sphagnobiotic species Heleopera petricola and Difflugia
lucida characterize boggy conditions, whereas other deposits containing hydrobiotic Difflugia penardi and D. pulex were accumulated under
aquatic conditions. In addition, the domination of some deposits by
eurybiotic soil species e.g. Schoenbornia humicola indicates pedogenesis. The uppermost Ice Complex sample (22.6 m a.r.l.) contains
hydrophilous species of the genus Difflugia and hygrophilous
Phryganella hemisphaerica, reflecting a very shallow water depth of
0–5 cm (Bobrov et al., 1999). Sphagnobiotic Heleopera petricola and
Euglypha compressa f. glabra, with fragile shells that can be easily
destroyed and are therefore very rarely preserved, are present in a
buried surface horizon of a wet sphagnum bog. Finally, the Holocene
cover deposit of the Nagym exposure radiocarbon-dated to about
4.8 kyr BP contains the species Centropyxis ecornis v. minima and
Phryganella hemisphaerica, which prefer wet habitats.
In general, soil species are most abundant in the Ice Complex Unit,
together with hygrophilous and sphagnobiotic taxa (SOM-6), while
Testaceae associations in the Lower Sand Unit are dominated by
hydrophilic and hydrobiotic taxa.
5. Discussion and interpretation
Based on our studies, the Arga Complex is composed of several
sand units of predominantly fluvial origin that were subsequently
affected by subaerial periglacial processes such as frost cracking,
eolian reworking, cryosol formation, and thermokarst. The entire sand
sequence exposed in the Tur-2 core is interpreted as deposits of a late
Pleistocene paleo-Lena River system, which flowed further to the
northwest than the modern main Lena River channels (Fig. 11A),
crossing the exposed Laptev Shelf. An extended (braided) river
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system was formed on this shelf plain when the Lena River left its
narrow valley (the “Lena pipe” or “Lensky truba”, Alekseev and
Drouchits, 2004) between the Chekanovsky and the Kharaulakh
ridges.
The combined Tur-2/Tur-1 sequence on Turakh–Sise Island
represents an almost continuous period of late Pleistocene sedimentation by the Lena River when the thick sand deposits of the Arga
Complex were accumulated. Changes in granulometry, MS, TOC, and
mica content indicate the fluvial dynamics of river runoff, sedimentation area, and source material (e.g. flow channels, river banks, sand
bars, and a shallow oxbow lake).
The oldest studied sand deposits of the Arga Complex, radiocarbondated to N50 kyr BP, consist of well-sorted weakly-bedded mediumgrained sand with the highest MS values (Unit A), corresponding to a
river channel with high stream velocity during the MIS 4 period
(Fig. 11A).
A decrease in runoff and a shallower water depth are probably the
reason for smaller grain sizes and higher accumulation of organic
remains in the segments of Units B1, C, and B2. This reflects conditions
in an oxbow lake during the early MIS 3 (Fig. 11A). Changes in grainsize parameters were caused by seasonally or annually variable river
discharge. The continuing relatively high mica and magnetic mineral
contents were contributed by specific source materials, and were
enriched because of lower stream velocity. Based on similar
sedimentological parameters and geochronological results (Table 5),
we assume that the accumulation areas of the Lower Sand Unit of the
third terrace and Units A to C of the Arga Complex belonged to the
same river system but are from different local facies. We correlate
the sand deposits exposed between 0 and 15 m a.r.l. along the
Olenyekskaya Channel and its tributaries with sand deposits of the
Arga Complex, which today are located deeper than 6 m b.r.l. These
deposits were accumulated in a fluvial runoff system that was more
than one hundred kilometers wide. Only later was the primary
consistent landscape development divided into two orientations in
different territories (Table 5).
The thickest and most widespread deposits of the Arga Complex are
the relatively coarse-grained, well-sorted, and organic-free sands of
Unit D. A periglacial river landscape with a network of continuouslyflowing river branches (Fig. 11B) is assumed from the late MIS 3 until
the late MIS 2 period (40–15 kyr BP). Vandenberghe and Woo (2002)
described such a multichannel periglacial runoff system as anastomosing rivers, which are characterized by individual channels in stable
positions, in contrast to braided rivers whose channels migrate
irregularly and can run dry.
Comparing the geochronological data of Arga Complex sand
deposits on the second terrace and Ice Complex deposits on the
third terrace, we have ample evidence to conclude that both deposits
were partially formed during the same period (Fig. 11B). Plant
Table 5
Stratigraphical correlation of the second terrace sequences (Arga Complex) and of the
third terrace sequences.
Period
(kyr)
2nd terrace
3rd terrace
Arga Complex
Khardang–Sise
Nagym site
Kurungnakh–Sise
0.7 to
0.3
10 to
3
20 to
12
40 to
20
50 to
40
Units F and G
Peat and dune
Unit E
Cryosol
Unit D
Fluvial sand
Unit D
Fluvial sand
Unit B1, C, B2
Fluvial,
alternating
Unit A
Fluvial
–
–
–
–
Holocene cover
–
–
Holocene cover,
Alas deposits
–
Ice Complex Unit
Ice Complex Unit
Peaty soil zone
Peaty soil
Ice Complex
Unit
Peaty soil
Lower Sand Unit
Fluvial
Lower Sand Unit
Fluvial
Lower Sand Unit
Fluvial
N 50
193
macrofossil studies from the Nagym section (SOM-4) as well as from
the Buor Khaya site on Kurungnakh–Sise Island (Wetterich et al.,
2008) indicate no sharp vegetation shift between the Lower Sand Unit
and the overlying Ice Complex Unit. During this period, paleogeographical changes were not directly driven by climate but by regional
neotectonic movements in the Arctic rift zone. In addition, we must
distinguish between the formation of Ice Complex deposits on Ebe–
Sise, Kurungnakh–Sise, and Khardang–Sise islands. The latter deposits
are younger and finer-grained (Fig. 7A) than either of the others, and
the Ice Complex deposits of Kurungnakh–Sise Island are significantly
different in their heavy mineral content (Table 3). Therefore, we
assume that the Ice Complexes of Ebe–Sise and Khardang–Sise islands
were formed as Lena Delta floodplain sediments during the MIS 3 and
MIS 2 periods, respectively. During this time, the Lena River flowed
further northwest of this site. The Ice Complex deposits exposed
further east along the Olenyekskaya Channel were formed by
proluvial sediments from Chekanovsky Ridge which accumulated on
a flat foreland plain in front of the mountain range (Fig. 11B) as
already described by Schirrmeister et al. (2003), Schwamborn et al.
(2002c), and Wetterich et al. (2008). Various accumulation processes
(e.g. proluvial, alluvial, nival, and eolian) during different periods and
in different landscapes (Schirrmeister et al., 2010) formed the
polygenetic Ice Complex of the third terrace.
Later, the Ice Complex area was eroded by the Lena River as it
formed large meanders 20–30 km in size, which are still visible today
at the northern rims of Khardang–Sise, Dzhangilakh–Sise, and
Kurungnakh–Sise islands (Figs. 1B and 11C). This erosive event
could not be dated directly but we assume it occurred during the
strong Lateglacial landscape transformation, when thermokarst
processes (Romanovskii et al., 2000) and the postglacial shelf
transgression (Bauch et al., 2001) started to transform the regional
hydrological system of the shelf landscape. In addition, neotectonic
events (Are and Reimnitz, 2000) probably affected the environmental
dynamics.
The Arga Complex Unit E, characterized by the specific, thin,
lattice-like arranged ice veins and weakly-developed cryosol horizons, consists of deposits similar to those of Unit D, which were later
(15–12 kyr BP) affected by cryogenesis and pedogenesis. A rich
assemblage of testate amoebae reflects subaerial soil formation. The
relative heavy isotope signature of ice wedges in Unit E reflects
warmer winter temperatures that are presumably characteristic of the
temperate Lateglacial Bølling–Allerød Interstadial. Frost cracking and
soil formation proceeded more or less synchronously during the
Lateglacial to Early Holocene transition period, when the landscape
was affected by further strong transformation.
Areas of the second and the third terraces were strongly affected
by permafrost degradation, which caused large thermokarst depressions as well as numerous small thermokarst valleys to form.
According to studies from Nikolay Lake on Arga–Muora–Sise Island,
thermokarst processes started around 10 cal kyr BP (Andreev et al.,
2004; Schwamborn et al., 2002a). In general, thermokarst lake growth
in Ice Complex areas in the Laptev Sea coastal lowlands must have
happened very rapidly as thermokarst deposits had already appeared
by 11.5 cal yr BP (Grosse et al., 2007; Schirrmeister et al., 2002).
Today, the existence of peat soils within thermokarst depressions on
the Arga Complex (Ulrich et al., 2009) and on top of the studied
exposures indicates surface stabilization without significant thermokarst processes.
The question remains: Why has the Arga Complex been preserved
in its separate shape, and not further influenced by river and delta
activities? Late Pleistocene neotectonic activities perhaps resulted in a
relative uplift of the area and in a reorganization of the river system
with a new westward runoff of the Lena River along the Olenyekskaya
Channel (Fig. 11D). Similar and stronger tectonically-caused reorganization of the middle and lower Lena River systems was assumed by
Alekseev and Drouchits (2004) for the middle Pleistocene. If this is
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true, the uplift disturbed the thermal permafrost balance and
supported the ongoing thermokarst processes. Perhaps the formation
of the numerous NNE-SSW-oriented lakes (Morgenstern et al., 2008)
was partly caused by the existence of former river branch courses and
unfrozen zones (taliks) below the river bottom.
Rather good internal data consistency exists for both the AMS and
OSL dating methods (Figs. 3 and 6). However, the AMS and OSL age
sequences differ significantly between each other, and OSL ages are
almost always younger. Similar problems of permafrost dating using
two different methods were already presented by Winterfeld et al.
(under review) for permafrost sequences from the western Laptev
Sea. An age difference of a factor of two is a systematic error that, thus
far, cannot be explained. Potential reasons concern different physical
effects for dating with correspondingly different uncertainties, as well
as permafrost-specific cryogenic processes which perhaps mobilized
and relocated sediment or affected mineral crystal parameters as ice
grew within pore spaces. Systematic tests of factors and processes
potentially impacting AMS and OSL dating methods for permafrost
samples are needed.
Finally, the landscape dynamics of this region can be discussed
within a larger regional context. About 90 kyr ago the easternmost
boundary of the Eurasian Ice Sheet was located in the Byrranga
Mountains on the Taymyr Peninsula and in the Putorana Plateau in
Central Siberia (Svendsen et al., 2004), which are about 700 km and
1200 km away from the Lena Delta study sites, respectively. Because
of these large distances, a direct glacio-fluvial influence of the
Eurasian Ice Sheet is very unlikely. During the Middle and Late
Weichselian periods the eastern margin of the Eurasian Ice Sheet was
even further west, and thus even farther away from our study region.
We assume that the landscape dynamics of the Lena catchment
probably exerted a very large influence on changes in the hydrological
regime in our study region. The late Pleistocene glaciation of the
Verkhoyansky Mountains was expressed by several strong mountain
glacier advances during the early Weichselian (MIS 4) period, and
minor advances during the Middle and Late Weichselian (Arkhipov
et al., 1986; Kind, 1975; Kolpakov, 1979; Popp et al., 2006; Stauch and
Gualtieri, 2008; Stauch and Lehmkuhl, 2010; Stauch et al., 2007).
These glacier dynamics, as well as paleoprecipitation patterns in the
large Lena catchment, likely also controlled the dynamics of the Lena
River in its lower reaches.
6. Conclusions
Several stages of landscape and paleoenvironmental dynamics
could be classified for the late Pleistocene and Holocene of the
western Lena Delta. The Arga Complex is composed of several sand
units reflecting various fluvial facies of the paleo-Lena (e.g. river
channels, sand bars, and oxbow lakes); later cryogenic processes such
as freezing, frost cracking, cryoturbation, and epigenetic ice wedge
growth were superimposed upon it.
The MIS 4 glacial period in the Laptev Sea coastal lowland is
characterized by a fluvial landscape. This is evidenced by the Lena
Delta record and by similar sand deposits below the Ice Complex
further west at Cape Mamontov Klyk (Schirrmeister et al., 2008;
Winterfeld et al., under review) and at Bykovsky Peninsula further
east (Grosse et al., 2007; Schirrmeister et al., 2002).
During the MIS 3 Interstadial the river landscape shifted further
north, and the sand sequences in front of the Chekanovsky Ridge
were covered by subaerial-formed ice-rich deposits (i.e. Ice Complexes) that accumulated in a polygonal landscape. Fluvial sand
deposits of the Arga Complex accumulated synchronously with, and
in relatively close proximity to Ice Complex deposits. The possible
reasons for shifts of the Lena River hydrological regime include
changes in the runoff patterns due to higher fluvial spring and
summer runoff during the interstadial time, or seismotectonic
events, or both together.
Both fluvial and subaerial accumulation continued during the MIS
2 glacial period. However, Ice Complex deposits from this period are
not well preserved along the Olenyekskaya Channel, probably due to
erosion by the reactivated western main delta channel and thermokarst processes in the subsequent Holocene period.
During the MIS 4 and 3 periods the study area was dominated by
tundra steppe with locally varying environmental conditions. Changes
in paleoecological records also reflect landscape dynamics, i.e.
sediment accumulation patterns. A strong thermokarst-related
landscape transformation during the Lateglacial and the Holocene is
supported by ground ice and paleoecological records reflecting a
general warming trend and the occurrence of shrub tundra during the
Lateglacial to Middle Holocene and of arctic tundra during the late
Holocene.
In general, shifts in the runoff system recorded in paleofluvial
sediments of the Lena Delta area were caused by late Pleistocene
tectonic and paleoclimatic events, which influenced the geomorphological relief and water runoff patterns and volume.
Supplementary materials related to this article can be found online
at doi:10.1016/j.palaeo.2010.10.045.
Acknowledgements
This study was conducted within the framework of the Russian–
German “Laptev Sea System” scientific collaboration. The plant
macrofossil studies were supported by the German Research
Foundation (DFG) within the KI 849 project. We thank all Russian
and German colleagues who helped us with Siberian fieldwork in
2005. The analytical work in the laboratories at the Alfred Wegener
Institute Potsdam was greatly supported by Ute Bastian and Lutz
Schönicke. We also thank Prof. P. Grootes and his colleagues at the
Leibniz Laboratory for Radiometric Dating and Stable Isotope Research
(Kiel, Germany) for their excellent radiocarbon analyses. Finally, the
paper benefited by valuable comments and English language
correction from Dr. Candace O'Connor (University of Alaska,
Fairbanks).
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