Limnological studies in the Dmitrii Laptev Strait region

Introduction

Polygonal tundra landscapes are characteristic features of the Northeast Sibe-rian permafrost zone. Their formation is caused by ice wedge growth and lead to typical polygonal pattern (Figure 5.2-1). Depending on their position in the relief and the stage of development, polygons are often occupied by shallow waters, so-called polygon ponds. Frozen sediments of polygons are already used in late Quaternary palaeoenvironmental reconstructions (e.g. Schirrmeis-ter et al., 2002a, b, 2003; WetSchirrmeis-terich et al., 2005, 2008), but modern reference data on polygon development, pond hydrology and flora and fauna are still rare.

The main idea of our limnological fieldwork in summer 2007 in the Dmitrii Lap-tev Strait region was the record and monitoring of abiotic parameters such as climate conditions, temperature fluctuations, vegetation, ionic and stable isotope composition in the ponds in relation to bioindicators such as pollen, diatoms, chironomids, rhizopods and ostracods. The investigation of the modern condi-tions in the ponds allows the quantification of influencing parameters, which control the modern occurrence of these indicator organisms. In future, results of the study can be useful for interpretations of fossil data from sediment cores and outcrops and also for quantitative palaeoenvironmental reconstructions of the region using several palaeo-bioindicators.

Figure 5.2-1: Polygonal patterned tundra landscape in the East Siberian lowlands with ponds and initial thermokarst lakes (Photograph during helicopter flight by Frank Kienast, Senckenberg Weimar)

Pond types and study sites

The relief in the study region is characterised enormous thermokarst depres-sions (alases) which are bordered by Yedoma hills (up to 40 m a.s.l.). These hills represent unthawed remains of Late Pleistocene Ice Complex deposits.

Other common geomorphological features are thermoerosional valleys (logs) and distinct river valleys.

To infer the influence of polygon development on the life conditions of indicator organisms, we sampled ponds according to different stages in several geomor-phological units.

In July 2007, limnological investigations were performed in the southern part of Bol’shoy Lyakhovsky Island (73°N, 141°E) in 15 waters (Figure 5.2-2a).

The studied ponds are situated on the floodplain of the Zimov’e and Vankina Rivers, in alas bottom, on different slope levels connecting alases and Yedoma hills and on the tops of Yedoma hills. Ponds on river floodplains are of in-trapolygonal genesis i.e. the waters occur in the low-centred polygons between the rim-covered ice wedges (Figure 5-3a). In alases and on the top and on the slope of Yedoma hills, the ponds are located interpolygonal, i.e. between high-centred polygons in small scale depressions over the thawed ice wedges (Fig-ure 5.2-3b, c).

The second study area was located at the coast south of the Dmitrii-Laptev-Strait at the location Oyogos Yar (72°N, 143°E) (Figure 5.2-2b). In August 2007, 16 lakes were investigated in this region which is also mainly structured by the above described geomorphological combination of alas depression and Ye-doma hills. The sampled ponds in the alas depression and on YeYe-doma slopes and tops belong to the above described interpolygonal type. (Figure 5.2-3d-f).

Additionally, we sampled the margin of one thermokarst lake and one thermo-erosion valley (log) with slowly floating water.

In September two ponds and one lake at the bottom of a mountainous valley near Tiksi were sampled.

Figure 5.2-2:Location of the sampled polygon ponds (a) on Bol’shoy Lyakhovsky Island and (b) on Oyogos Yar. Map compiled by H. Lantuit, AWI Potsdam using satellite images: (a) LANDSAT 7 ETM+ 29.06.2001 and (b) SPOT 24.07.2007.

Figure 5.2-3a-f: Different types of polygon ponds on Bol’shoy Lyakhovsky Island and on Oyogos Yar.

Material and methods

Investigations on properties of water chemistry and physics in the waters were undertaken in order to describe the recent life conditions for organisms. Our investigations included the estimation of water and size. We quantified pH, elec-trical conductivity (EC) and temperature using a WTWpocket meter. Still in the field, the determination of oxygen concentrations, total hardness, alkalinity and acidity was performed on means of titrimetric test kits (Viscolor).

For hydrochemical analyses in lab the pond water was sampled above the sediment surface from each site. Samples for cation analyses (15 ml) were acidified with 200 μl HNO3, whereas samples for anion analysis and residue samples were cool stored. Before conservation, samples for cation and anion analyses were filtered by a cellulose-acetate filtration set (pore size 0.45 μm).

Additionally, precipitation and pond water samples for δ¹⁸O and δD isotope analyses (30 ml) were preserved without any conservation.

Surface sediments of the ponds were sampled for sedimentological and botani-cal, zoological and paleontological analyses at the centre of the ponds. For these purposes studies on pollen, diatoms, chironomids, rhizopods and ostra-cods are planned. Live ostraostra-cods were caught in surface sediment samples

from different pond zones using an exhaustor system (Viehberg, 2002) and pre-served in 70 % alcohol. Further taxonomical work using soft body characteris-tics will provide the first description of modern ostracod assemblages from the study area.

In July, one pond (Lap-01) on Bol’shoy Lyakhovsky Island on the Yedoma top, and in August one pond (Lap-16) on Oyogos Yar in an alas depression were selected as monitoring sites. Here, we performed continuous temperature measurements at three levels using temperature logger (MinidanTemp 0.1, ESYS). The loggers were placed in the centre of the pond at the pond bottom, below the water surface and in the air (Figure 5.2-4).

Figure 5.2-4: Temperature monitoring in two ponds; in July on Bol’shoy Lyakhovsky Island (Lap-01) and in August on Oyogos Yar (Lap-16). Three levels (above the pond sediments: in blue; below the water surface: in green; above the water surface: in yellow) were instrumented with temperature loggers in order to obtain daily and seasonal temperature amplitudes during the summer.

Additionally, every four/eight days repeated hydrochemical measurements and sampling of water, sediments and ostracods were performed in order to obtain seasonal dynamics of the studied parameters and proxy as well as their rela-tionships among each other.

Results

The studied polygon ponds in both study regions belong to the interpolygonal type according to their location in alas depressions, and Yedoma slopes and tops (Chapter 6.4-1, 6.4-2). The size of the polygon ponds reaches from 2 x 5 m up to 20 x 100 m whereas the shape often delineates the position of the thawing underlying ice wedge system (Figure 5.2-3). The observed intrapolygon ponds on floodplains extend from 5 x 7 m to 19 x 19 m and represent water filled low centre polygons (Figure 5.2-3). The water depth in such waters is low and do not exceed 0.7 m.

The pond substrate is mostly represented by silty or sandy material and rich in more or less decomposed plant detritus. The composition of mineral and or-ganic components results in a general muddy substrate. Freshly flooded parts are underlain by tundra soil. The studied ponds are mostly covered by sub-merse mosses and/or marsh plants. Genuine water plants are generally lacking.

Most abundant vegetation is composed of Arctophila, Eriophorum and Arcta-grostis species at the pond margins on Yedoma sites, and Carex sedges, Erio-phorum species and Sphagnum moss at ponds in alas depressions (for details see Chapter 5.3).

Results of the finger-print hydrochemistry during the fieldwork are presented in Figure 5.2-5 and in Chapter 6.4-3. In general, ponds studied on B. Lyakhovsky Island are characterised by neutral to slightly alkaline pH (pH 6.5 to 8.7; pHmean

= 7.7) and higher EC (48 to 245 µS/cm; ECmean = 148 µS/cm) as compared to ponds on Oyogos Yar, where neutral to slightly acidic (pH 6 to 7; pHmean = 6.5) waters with very low EC (17 to 175 µS/cm; ECmean = 72 µS/cm) occur (Figure 5.2-5a, b). Except of two outlier, the acidity (Figure 5.2-5c) of the waters varies mostly between 0.2 and 0.6 mmol/l in both regions and amounts to a mean value of Acimean = 0.4 mmol/l. The alkalinity (Figure 5.2-5d) in ponds on Bol’shoy Lyakhovsky Island ranges from 0.4 to 2.3 mmol/l and shows general higher val-ues (Alkmean = 1.3 mmol/l) as compared to ponds on Oyogos Yar which ranges from 0.2 to 1.4 mmol/l (Alkmean = 0.8 mmol/l).

Trends over the monitored period in July on Bol’shoy Lyakhovsky Island (Lap-01) and in August on Oyogos Yar (Lap-16) are obvious by increasing EC and alkalinity in Lap-01 (Figure 5.2-5b, d). These parameters remained unchanged in Lap-16. Acidity and pH do not show distinct gradients over the monitored pe-riods in both regions. The observed increases in July are most likely caused by high evaporation during sunny days and no precipitation, which results in low P/E ratio. In contrast, the August was rainy that leads to higher P/E ratio and unchanged ionic concentrations in the ponds.

The temperature monitoring was performed in July on Bol’shoy Lyakhovsky Is-land in a 0.6 m deep interpolygon pond with a 0.4 m deep thawed zone below on Yedoma top (Lap-01) and in August on Oyogos Yar in a 0.65 m deep inter-polygon pond with a 0.2 m deep thawed zone below in an alas depression. The data reflect the dependence of pond water temperatures from two main influ-ences: air temperature and active layer depth below the ponds (Figure 5.2-6).

Figure 5.2-5: Ranges of hydrochemical parameters measured during the summer 2008 on Bol’shoy Lyakhovsky Island (July), on Oyogos Yar (August) and near Tiksi (September): (a) pH as dots; (b) electrical conductivity (EC) as diamonds; (c) acidity as stars and (d) alkalinity as triangles. The repeated measured monitoring ponds Lap-01 on Bol’shoy Lyakhovsky Island and Lap-16 on Oyogos Yar are marked by red symbols. One-time measured ponds are marked by black symbols.

Pond Lap-01 bottom (Tbottom) and surface (Tsurface) water temperatures are co-varying in lower amplitude with air temperature variations and shows the same variations. Daily Tair amplitudes reach up to about 20 °C at the end of July with absolute Tair maxima of about 25 °C and Tair minima of about -1 °C at the be-ginning of July. The highest daily amplitude in Tsurface and Tbottom amounts to about 9 °C in the beginning of July, whereas absolute maxima of about 17 °C occur at the end of July and minima of about 3 °C at the beginning (Figure 5.2-6).

The Lap-01 temperature record and its daily variations are mainly controlled by air temperature. The underlying permafrost table is here of minor importance due to its relatively high thickness.

In pond Lap-16 only Tsurface co-varies with Tair and follows its daily variations, whereas Tbottom remains stable at about 3.6 °C, what is most likely controlled by an active layer depth of 0.2 m. Obviously, the close permafrost table affect T

bot-tom by cooling more than the warming influence of Tbottom. Daily Tair amplitudes reach up to about 23 °C at the middle of August with absolute Tair maxima of about 28 °C and Tair minima of about -1 °C at the end auf August. The highest daily amplitude in Tsurface amounts to about 17 °C in the end auf August with ab-solute maxima of about 17 °C and minima of about 0 °C (Figure 5.2-6).

Figure 5.2-6:Daily temperature variations and means in two ponds; in July on B. Lyakhovsky Island (Lap-01) and in August on Oyogos Yar (Lap-16). Three levels are figured out (Tbottom

above the pond sediments: in blue; Tsurface below the water surface: in green; Tair above the wa-ter surface: in yellow).

Outlook

Pollen, diatoms, chironomids, rhizopods and ostracods will be investigated to illuminate their relationship to environmental factors such as temperature, pH and conductivity. This information will later be applied to fossil assemblages, obtained from sediment cores and permafrost deposits, in order to infer quanti-tative environmental changes via organism-environment transfer-functions.

In laboratory, water samples will be analysed for element content by means of an ICP-OES and anion content by Ion Chromatography. Furthermore, analyses

of δ¹⁸O and δD isotopes on water and precipitation samples will be performed in order to compare these data with isotope values in calcareous ostracod valves.

The understanding of the recent relationship between isotope ratios in waters and in ostracod valves will lead to an interpretation tool for palaeoenvironmental information preserved in fossil ostracods. For the same purpose trace element analyses (e.g. Ca, Mg, Sr) in waters and ostracod valves will be undertaken.

On surface sediment samples analyses of nitrogen, organic and total carbon contents by CN-Analyzer as well as grain-size distribution by laser granulometry will be carried out in order to characterize the sedimentological setting of the investigated ponds.