From lacustrine carbonates to paleohydrology - how robust are our proxies?
Final Report Abstract
Lake Van, the largest alkaline lake in the world, belongs to a canon of closed basins famous for their sensitive reactions to environmental change. Isotopic composition of lacustrine carbonates often provides the key to the local paleohydrological conditions. However, in case of ICDP PALEOVAN data dom oxygen isotope record of the bulk carbonate fraction was random and inconsistent with other environmental proxies. This conundrum sparked the motivation for a proxy-validation study. We have performed a systematic documentation of the different carbonate polymorphs (primary aragonite and calcite, and early diagenetic dolomite) present in the Lake Van profile, covering the last 150 ka. This fantastically simple and elegant work not only solved part of the Lake Van carbonate-conundrum, but also has wide implications for, and way beyond lacustrine carbonate studies in general. It highlights the interpretational dangers of overlooking the complex nature of lacustrine carbonates and shows clear evidence of early diagenetic, low temperature (<10°C), deep-water, non-marine dolomite, not related to evaporative conditions. This finding calls for re-evaluation of some interpretative approaches that per default relate the presence of dolomite to increased aridity, and opens new conceptual venues for solving the ‘dolomite problem’. Next we presented a conceptual model describing the spatial and temporal variation of the precipitation of different polymorphs (aragonite and calcite) in Lake Van surface water. Understanding the processes governing carbonate precipitation in this particular lake is obviously important for the interpretation of the hydrological conditions on local/regional scale. However, this study uses Lake Van only as an example of basins with more than one carbonate phase and encourages careful inspection of sedimentary material prior to analyses and thinking ‘out of the box’ of outdated assumptions when interpreting mixed carbonate records. Finally a close-up on biogenic carbonates (valves of benthic crustaceans – ostracods) from the Lake Van profile validates their application as salinity indicators. Salinity of closed lakes is usually a function of lake level changes and a good measure or precipitation/evaporation changes. Still, only few lacustrine proxies reliably record salinity changes. Ostracods are a multiproxy organism themselves, providing ecological and geochemical information on their habitat. We have coupled carefully performed time-consuming and tedious morphological analyses of ostracod valves coupled with stable isotope measurements of oxygen and carbon (d18O and d13C). The result demonstrate that the isotopic composition of ostracod valves from deep, closed, alkaline lakes faithfully reflects the conditions of their microhabitat, but may subdue the basin-wide signal, while the morphology of their shells is very sensitive towards changes in water chemistry. Apart from proposing a novel salinity proxy (ostracod valve morphology), this study brings together, and accounts for, all Lake Van carbonate data and is – as such – the most comprehensive lacustrine carbonate record ever compiled. The project successfully reached its goal assessing the robustness of carbonatebased proxies.
Publications
- 2018. Controls on cyclic formation of Quaternary early diagenetic dolomite. Geophysical Research Letters
McCormack, J., Bontognali, T., Immenhauser A. and Kwiecien, O.
(See online at https://doi.org/10.1002/2018GL077344) - 2019. Ostracods as ecological and isotopic indicators of lake water salinity changes: The Lake Van example. Biogeoscience
McCormack, J., Viehberg, F., Akdemir, D., Immenhauser A. and Kwiecien, O.
(See online at https://doi.org/10.5194/bg-16-2095-2019) - 2019. Refining the interpretation of lacustrine carbonate isotope records: Implications of the mineralogy-specific Lake Van case study. Chemical Geology
McCormack, J., Nehrke, G., Jöns, N., Immenhauser A. and Kwiecien, O.
(See online at https://doi.org/10.1016/j.chemgeo.2019.03.014)
