Sediment flux and early diagenesis inferred from high-resolution XRF-CS data and iron and arsenic sulfides during the last 30 kyr in Lake Urmia, Iran: Implications for studies of hypersaline lakes

We analyzed by high resolution X-ray fluorescence core scanning (XRF-CS) the elemental composition of a 12.5 m long composite sediment core extracted from the hypersaline Lake Urmia (NW Iran). The main objective of the study was to reconstruct on a high temporal scale, the changes in sediment origin/inflow and water levels of Lake Urmia over the last 30,000 years. Combined with previously obtained results on 14C accelerated mass spectrometry (AMS) chronology, grain size and distribution of major minerals, albeit with a much lower temporal resolution than the XRF-CS data, this multi-proxy approach allowed recognition of specific hydrological changes and associated sedimentary processes in the Lake Urmia basin, including physical and chemical weathering, depositional fluxes and carbonate crystallization processes in lake water, and early diagenesis in anoxic bottom sediments. The high natural logarithm ratios (ln-ratios) of K/Ti, Ca/Ti and Sr/Ca, and low ln-ratios of Rb/Sr reveal increasing chemical weathering in the catchment area in conjunction with increasing authigenic carbonate precipitation in the lake. These findings reflect wet conditions, high lake levels and less saline water during the following intervals (i) 29.8–20.2, (ii) 15.2–13.3, (iii) 11.8–5.6 and (iv) 4.1–2.3 cal kBP. In contrast, the increasing ln-ratios of Rb/Sr associated with decreasing ln-ratios of K/Ti, Ca/Ti and Sr/Ca indicates enhanced erosion in the catchment area linked to drier conditions, lower lake levels and more saline conditions predominating at around 30–29.8 cal kBP, 20.2–15.2, 13.3–11.8, and 5.6–4.1 cal kBP. Based on the As-XRF-CS pattern and magnetic parameters we identified iron and naturally-occurring arsenic sulfides rarely reported from saline lakes, acting as specific indicators of anoxic conditions in the lake bottom environment. Moreover, XRF-CS data revealed short-lived periods with increasing chemical weathering in the catchment area at (i) 28.6–28.3, (ii) 27.3–27.1, (iii) 4.1–3.6 and (iv) 3.4–3.3 cal kBP, as well as increasing erosion at (i) 27.0–26.8, (ii) 26.3–26.0, (iii) 25.6–25.3 and (iv) 24.6–24.4 cal kBP. Overall, this study confirms the previously reported findings concerning the sediments of Lake Urmia as well as completing these, both by providing more details on the environment for the periods already identified and by highlighting events of short duration. In particular, the results reported here permit better definition of the processes of chemical