Dräger, N., Plessen, B., Kienel, U., Słowiński, M., Ramisch, A., Tjallingii, R., Pinkerneil, S., Brauer, A. (2019 online first): Hypolimnetic oxygen conditions influence varve preservation and δ¹³C of sediment organic matter in Lake Tiefer See, NE Germany. - Journal of Paleolimnology. | doi:10.1007/s10933-019-00084-2

Abstract:
Stable carbon isotopes of sediment organic matter (δ¹³C_OM) are widely applied in paleoenvironmental studies. Interpretations of δ¹³C_OM, however, remain challenging and factors that influence δ¹³C_OM may not apply across all lakes. Common explanations for stratigraphic shifts in δ¹³C_OM include changes in lake productivity or changes in inputs of allochthonous OM. We investigated the influence of different oxygen conditions (oxic versus anoxic) on the δ¹³C_OM values in the sediments of Lake Tiefer See. We analysed (1) a long sediment core from the deepest part of the lake, (2) two short, sediment–water interface cores from shallower water depths, and (3) OM in the water column, i.e. from sediment traps. Fresh OM throughout the entire water column showed a relatively constant δ¹³C_OM value of approximately − 30.5‰. Similar values, about − 31‰, were obtained for well-varved sediments in both the long and short, sediment–water interface cores. In contrast, δ¹³C_OM values from non-varved sediments in all cores were significantly less negative (− 29‰). The δ¹³C_OM values in the sediment–water interface cores from different water depths differ for sediments of the same age, if oxygen conditions at the time of deposition were different at these sites, as suggested by the state of varve preservation. Sediments deposited from AD 1924 to 1980 at 62 m water depth are varved and exhibit δ¹³C_OM values around − 31‰, whereas sediments of the same age in the core from 35 m water depth are not varved and show less negative δ¹³C_OM values of about − 29‰. The relation between varve occurrence and δ¹³C_OM values suggests that δ¹³C_OM is associated with oxygen conditions because varve preservation depends on hypolimnetic anoxia. A mechanism that likely influences δ¹³C_OM is selective degradation of OM under oxic conditions, such that organic components with more negative δ¹³C_OM are preferably decomposed, leading to less negative δ¹³C_OM values in the remaining, undegraded OM pool. Greater decomposition of OM in non-varved sediments is supported by lower TOC concentrations in these deposits (~ 5%) compared to well-varved sediments (~ 15%). Even in lakes that display small variations in productivity and terrestrial OM input through time, large spatial and temporal differences in hypolimnetic oxygen concentrations may be an important factor controlling sediment δ¹³C_OM.

Ben Dor, Y., Neugebauer, I., Enzel, Y., Schwab, M. J., Tjallingii, R., Erel, Y., Brauer, A. (2019): Varves of the Dead Sea sedimentary record. - Quaternary Science Reviews, 215, pp. 173-184. | doi:10.1016/j.quascirev.2019.04.011

Highlights:

• The Dead Sea is the only deep hypersaline lake with varved sediments.
• The chronological and paleoclimatic value of Dead Sea varves is demonstrated.
• The interpretation of alternating aragonite-detritus laminae as varves is evaluated.
• Lake monitoring confirms the interpretation of laminated halite as varves.
• Distinguishing between annual and non-annual laminations requires micro-facies analyses.

Abstract:

The sedimentary record of the Dead Sea provides an exceptional high-resolution archive of past climate changes in the drought-sensitive eastern Mediterranean-Levant, a key region for the development of humankind at the boundary of global climate belts. Moreover, it is the only deep hypersaline lake known to have deposited long sequences of finely laminated, annually deposited sediments (i.e. varves) of varied compositions, including aragonite, gypsum, halite and clastic sediments. Vast efforts have been made over the years to decipher the environmental information stored in these evaporitic-clastic sequences spanning from the Pleistocene Lake Amora to the Holocene Dead Sea. A general characterisation of sediment facies has been derived from exposed sediment sections, as well as from shallow- and deep-water sediment cores. During high lake stands and episodes of positive water budget, mostly during glacial times, alternating aragonite and detritus laminae (‘aad’ facies) were accumulated, whereas during low lake stands and droughts, prevailing during interglacials, laminated detritus (‘ld’ facies) and laminated halite (‘lh’ facies) dominate the sequence. In this paper, we (i) review the three types of laminated sediments of the Dead Sea sedimentary record (‘aad’, ‘ld’ and ‘lh’ facies), (ii) discuss their modes of formation, deposition and accumulation, and their interpretation as varves, and (iii) illustrate how Dead Sea varves are utilized for palaeoclimate reconstructions and for establishing floating chronologies.