UNRAVELING THE HYDROCLIMATIC HISTORY OF LAKE URMIA, IRAN FROM THE EARLY HOLOCENE THROUGH THE PENULTIMATE GLACIATION

An approximately 200 kyr record of hydroclimatic variability is reported for Lake Urmia, a hypersaline, terminal basin in northwestern Iran. Oxygen and carbon isotopes, measured on aragonitic fecal pellets of brine shrimp, show distinct climatic periods that are tentatively correlated with Marine Isotope Stages 7, 6, 5e, 5c, 4 and 3. Glacial periods (i.e., MIS 6 and 4) are characterized by dry conditions denoted by high δ¹⁸O values indicative of evaporative concentration of the lake water. Interglacial/Interstadial intervals (MIS 7, 5e, 5c, 3) are characterized by wetter conditions denoted by low δ¹⁸O values associated with more frequent precipitation. Interglacial and glacial isotope values do overlap, although this may partly be due to the chronology. Isotope values correlative with MIS 3 fall almost entirely within this “overlap” zone. Correlations of the isotope record with MIS 5a, 2 and the early Holocene are less robust due to complications presented by changes in the pellet composition from aragonite to mixtures of gypsum and low-Mg calcite. The cause of these lithologic changes is unknown but is essential for interpreting certain sections of the record. In addition to the calcite, these intervals are marked by a shift from grey clays and fecal sands to red clays. Many levels lack sufficient carbonate for isotope analysis. Comparison with the vegetation record indicates that these periods are associated with high values of Hippophaë rhamnoides (a salt and cold tolerant shrub) and decreased aquatic pollen. Combined the evidence suggests that during certain periods of the lake’s history (principally during the Last Glacial Maximum and the penultimate glacial maximum), water levels dropped sufficiently to create a facies change from open water precipitates to mud flats that were occasionally or seasonally inundated. The shift to low-Mg calcite is likely due to detrital influx of calcite from outcrops within the catchment. Details of the δ¹⁸O record are similar to those of the Soreq Cave speleothem record in Israel.