Heterogeneous long-term and seasonal brine evolution, and Artemia fecal pellet controls on Urmia Lake (NW Iran) salt-crust formation and mineralogy

Abstract

Urmia Lake resides as a substantial hypersaline lake characterized by notable fluctuations in water salinity, brine composition, and water level over long-term, annual, and seasonal intervals. Extremely rapid water elevation fall (> 7 m) in the last three decades has caused the formation of a salt crust on the lake floor. A manmade stone causeway divided the lake into two relatively deeper northern parts with minimal water inputs and a shallower southern part with maximal river inflows. Restricted water flow through the narrow water passage of the causeway leads to complex salinity processes, brine evolution, and salt-crust formation in Urmia Lake. This research analyzes the ionic composition of lake-sediment and salt-crust pore water, the mineralogy of salt crusts, and the ionic composition of both surface and deep lake waters during both the wet and dry seasons of 2019. The findings indicate that the northern and southern parts of the lake undergo stratification during wet seasons due to significant freshwater input, whereas they become homogenized during dry seasons through progressive evaporative concentration and water mixing. The spatial and temporal variations in the lake brine type (primary Na-Mg-Cl) and ionic composition contribute to the formation of a halite salt crust (NaCl > 97%) with heterogeneous mineralogy and thickness. In Urmia Lake, the variable thickness and mineralogy of the exposed marginal salt crust suggest rapid salt-crust reorganization by annual and seasonal deposition and dissolution processes. Conversely, the submerged central salt crust, with continuous thickening and constant mineralogy, remains unaffected by seasonal variations in brine type and dissolution processes. It is noteworthy to mention that Artemia (a brine shrimp) controls the mineralogy of the lake salt crust through the deposition of calcium and carbonate ions in the form of biochemical fecal pellets.