Artemia fecal pellet is an important paleoenvironment indicator in hypersaline lakes : A case study Urmia Lake (NW İran)

Abstract

Hypersaline lakes are dynamic sedimentary environments responding change in a complex pattern. The Urmia Lake in NW Iran (Azerbaijan area), is the second-largest hypersaline lake in the world. The lake is a hybrid terrigenous-biochemical sedimentary environment that completely shifted to an almost pure chemical sedimentary environment due to an extreme rapid water level fall in the last decades. Artemia urmiana, as a saline water shrimp, is the only animal living in Urmia Lake. Artemia fecal pellets are the major bio-chemical component of the lake sediments, recording the lake's hydro-chemical variation through time. For paleoclimate reconstruction of the Turkish-Iranian Plateau in the Late Pleistocene-Holocene, based on Artemia fecal pellets characteristics, we studied a 25 m long sediment core collected from the northwestern part of Urmia Lake in 2017. This research uses morphological, mineralogical, textural, biochemical, geochronological, and stable isotopic characteristics of Artemia fecal pellets along with the lithostratigraphy of the core to reconstruct paleoclimate and paleogeography of the lake in the Late Pleistocene-Holocene. In Urmia Lake sediments, fecal pellets are capsule shaped grains in silt to medium sand size. The frequency, size, shape, colour and surface morphology of fecal pellets were studied using a binocular, polarizing and scanning electron microscopes on 140 sediment sub-samples. Sub-samples were collected according to facies variations along the studied core. Correlations of sedimentary units with fecal pellets variations along the core we classified five lake level stages based on fecal pellets frequency and characteristics variation: Lake-level falling stage (2500-1472 cm depth) is mainly composed of coated grains, reworked shell fragments and diagenetic sulfate minerals with a minor amount of brownish broken fecal pellets; Lake-level low stand stage with lake floor exposure (1471-816 cm depth) has black, orange, and brown pellets, rounded diagenetic gypsum minerals with volcanic lithics and glasses admixed; Lake-level rising stage (815-611 cm depth) is dominated by sulfate minerals and the absence of pellets; Lake-level high stand (610-320 cm depth) is dominated by fresh cream-coloured pellets; and lake-level falling towards drying out stage (319-0 cm depth) is composed of evaporite minerals as a salt crust on lake floor. The radiocarbon ages of fecal pellets indicate that the 25 m lake sediment cover ~ last 50 kyr with some glacial and interglacial periods. The age-depth model indicates different sedimentation rate in different depth which are consistent with our identified sedimentary facies. The oxygen and carbon isotopic data variation along the core exhibit deeper part of the core is more dry and cold climate (glacial period) and the upper core is more humid climate (interglacial period). Overall, the stable oxygen and carbon isotopic composition are in agreement with determined sediment facies and lake level fluctuations.