Dinocyst records from deep cores reveal a reversed salinity gradient in the Caspian Sea at 8.5–4.0 cal ka BP

Abstract

Highlights Dinocysts grouped by salinity, providing long-term water level/flow reconstructions. Dinocysts show a higher highstand after the Mangyshlak lowstand than previously seen. A South to North water flow gradient at 8.5-4 cal ka BP, reversed from the current one. Water level influenced by precipitation over Karakum and perhaps indirectly monsoon. A major turnover in dinocyst assemblages in all three sequences at c. 4 cal. ka BP. AbstractUnderstanding the long-term environmental forcings driving Caspian Sea (CS) water levels is of utmost importance, not only owing to its large size, or to the surrounding developing economies but also to improve global climate models and forecasts. However, Late Quaternary CS level changes and their amplitude are mostly documented from incomplete coastal sediment records. Because of the CS idiosyncrasies, that behaves neither as a sea nor as a lake, the methods used to reconstruct water levels in the global ocean or in freshwater lakes do not always apply.Here, we propose a first step toward the use of dinoflagellate cysts records to reconstruct qualitative changes in water mass, focusing on new and published deepwater sedimentary sequences from the south and middle CS basins. Trends in water level changes are reconstructed on the relative proportions of dinocyst assemblages with different levels of brackishness.2 A higher highstand than previously seen is reconstructed post-Mangyshlak lowstand. A reverse water flow gradient from S to N, not previously detected, is identified at 8.5-8 to 4 cal ka BP. A major turnover in dinocyst assemblage is found at 4 cal ka BP. While the Volga River is the main source of water nowadays, we propose that the source of water to maintain the 8.5-8 to 4 cal ka BP highstand is the now-disconnected drainage basin of the Amu-Darya. The CS was at that time most likely strongly influenced by low latitude climates, with more precipitation over the Karakum and, perhaps, even indirect monsoonal influence.

Hydroclimatic changes on multiple timescales since 7800 y BP in the winter precipitation–dominated Central Asia

“…Leroy et al ( 18 ) reported a sea level maximum in the southeast corner of the Caspian Sea (CS) during the early–middle Holocene. The water flow gradient was from south CS to north CS during the mid-Holocene, which reversed in the late Holocene ( 19 ), implying a drying trend in the WCA during the Holocene. Nevertheless, the driving forces of CS lake level changes during the Holocene are complicated because of the impacts of evaporation, meltwaters from high-altitude glaciers, and supplemental waters from several rivers.…”