Lake level fluctuation controls the formation, types, and abundance of coated grains in hypersaline lakes: a case study from Urmia Lake (NW Iran)

The hypersaline Urmia Lake in NW Iran offers unique sedimentary environments sensitive to climate and environmental shifts, fostering coated grain formation and serving as a vital indicator of paleoenvironmental conditions. This study characterizes coated grains within a 25-m sediment core dating back to ~50 cal ka bp, assessing their significance through morphology, internal structures, mineralogy, and geochemistry. Coated grains in Urmia Lake exhibit concentric laminations, primarily calcite and aragonite, revealing alternating light carbonate-rich and dark organic-rich laminations. These reflect seasonal and long-term variations in water chemistry and biogenic production. Dry season algal blooms contribute to lamination, highlighting the interplay between seasonal climate fluctuations and the consequent lake water enrichment in calcium, carbonate, and bicarbonate ions. The diversity and abundance of coated grains indicate three main lake level fluctuation stages in the last ~50 cal ka: a lowering stage with dominant coated grains, a low lake level with dominant terrigenous fragments and minerals, and a high lake level with prominent Artemia urmiana fecal pellets. The role of the brine shrimp A. urmiana in coated grain formation involves absorbing calcium, carbonate, and bicarbonate ions and inhibiting coated grain formation during high lake levels while providing nuclei during lake lowering. An in-depth investigation of coated grains provides a chemical and biological formation framework, highlighting three main episodes in the lake's history.