Multi-proxy records from Urmia Lake sediments (NW Iran) show different modes of composition at low lake levels during the late Pleistocene-Holocene

Large hypersaline Urmia Lake (NW Iran) has been undergoing a dramatic shrinking during the last decades and has become a playa now. In a sediment study, several lake records are examined to assess the sedimentary variation preceding the drastic modern change. The longest core, which is studied in greater detail, has been retrieved close to the northern margin (۲۵ m), it stretches back near to ۵۰ kyr BP. Other cores come from close to the central-eastern and the south-eastern shorelines (up to ۸.۴ m long). All cores have an overlying salt crust ranging between a few decimeters and near to three meters. Sediment records are dominated by biogenic micritic mud; partly layers of silty sand (i.e., aragonitic fecal pellets and diagenetic gypsum) or silty mud are intercalated. Core comparison based on lithostratigraphy and XRD (X-ray diffractometry) of bulk and clay mineral composition highlight that (i) the late Pleistocene lake level low stand is marked by a pronounced peak occurrence of gypsum in the northern and dolomite in the southern lake. In contrast, the modern-day lake level low stand is marked by halite accumulation. (ii) Mineral phases vary within and across sediment units, (iii) the clay composition is pointing to basin-wide particle dispersal and mixing, and (iv) the bulk composition reflects proximal (feldspar-clay-dominated) versus distal (aragonite-dominated) sites. (v) Sedimentation rates are higher in the southern lake, where much of the fluvial freight enters the lake. (vi) Hypersaline conditions existed already in late Pleistocene time as reflected in a time-continuous halite signal.

کلیدواژه ها:

x-ray diffractometry ، hypersaline ، gypsum ، clay