Coated Grains in Urmia Lake, NW Iran: Insights into Depositional Processes and Environmental Change

Urmia Lake in NW Iran, renowned for its hypersaline characteristics, provides distinctive sedimentary environments that respond to climate and environmental shifts, fostering the development of coated grains. The ۲۵ m sedimentary sequence from the northern lake consists of biogenic mud (micrite) and sand, mainly comprising brine shrimp Artemia fecal pellets, with occasional shell fragments, terrigenous minerals, evaporites and coated grains. Coated grains, characterized by a distinct nucleus surrounded by a cortex, typically form during lake-level decrease induced by high-energy, shallow-water conditions from wind-induced waves. This study characterizes coated grains, evaluating their paleoenvironmental significance through morphology, internal structures, mineralogy, and geochemistry, utilizing binocular, polarized, SEM-EDS microscopic observation and XRD analysis. Results reveal distinct concentric laminations in various colors, predominantly composed of carbonate minerals, mainly calcite and aragonite. SEM images illustrate alternating patterns of light carbonate-rich and dark organic-rich laminations. Coated grains are classified into five types: superficial, regular, compound, broken, and patchy. Morphological features are influenced by various nuclei sources, including fecal pellets, detrital grains, shell fragments, and a minor presence of gypsum. The paleoenvironmental history of Urmia Lake unfolds through distinct episodes spanning approximately ۵۰ kyr BP. During MIS ۳, a gradual lake level lowering is marked by an abundance of coated grains. In MIS ۲, the lake reaches its lowest point, potentially exposing the lake floor with brown fecal pellets, volcanic lithics, and an increased prevalence of sulfate minerals. The subsequent period, MIS ۱, witnesses a rapid rise in lake level to a high stand, with deposits rich in fresh cream pellets and an absence of sulfate minerals.

کلیدواژه ها:

Hypersaline ، Urmia Lake ، Coated Grains ، Paleoenvironment ، SEM