Seasonal/Interannual Variation and Controlling Factors for Oxygen and Carbon Isotopes of Ostracod Shells Collected From a Time-Series Sediment Trap in Lake Qinghai

Zhangdong Jin,Jin Wang,Xiangzhong Li,Fei Zhang,Chenyu Jin

doi:10.3389/feart.2021.727330

Abstract

Because the shell substance of an ostracod is derived entirely from the water body where it lives, its chemical compositions are sensitive to aquatic environment and thus have been used to reconstruct past climatic and environmental changes. However, there is controversy about the controlling factors for the different compositions of ostracod shells from various water bodies. In this study, seasonal and interannual variations in daily flux and stable oxygen-carbon isotopic compositions (δ18O, δ13C) for two species of ostracod shells (Limnocythere inopinata and Eucypris mareotica) and their controlling factors are discussed, based on ostracod shell samples collected from a time-series sediment trap from July 2010 through September 2012 and from surface sediments in Lake Qinghai, which were correlated with the state-of-the-art sensing data of the lake water. The results show that the daily flux of L. inopinata shells is an order of magnitude higher than that of E. mareotica. The δ18O and δ13C of both L. inopinata and E. mareotica shells have distinctly interannual and seasonal variations, with species differences. Interannual differences of δ18O for the two species of ostracod shells directly reflect the systematic differences of the summer water temperature between 2010 and 2012. We propose that seasonal variations of both δ18O and δ13C for the two species are affected by the precipitation of authigenic carbonates in microenvironment induced by high water temperature in summers, highlighting their environmental implications in Lake Qinghai.

Highlights

As small, bivalved crustaceans in various water bodies, ostracod shells consist of low magnesium (Mg) calcite, which is derived almost entirely from the water body where they live, and are often well preserved in sediments
Other studies have shown that the isotopic composition of ostracod shells in Lake Qinghai reflects summer bottom-water conditions, with species difference (Henderson et al, 2003; Liu et al, 2009), whereas their δ13C is affected by the δ13C of dissolved inorganic carbon (DIC) and can reflect lake water salinity change (Li et al, 2012; Li and Liu, 2014)
The decreased salinity can be attributed to the removal of Mg2+ and Ca2+ by the precipitation of authigenic carbonates from the water column due to high temperature, because the Lake Qinghai water was supersaturated with respect to both calcite and dolomite (Jin et al, 2010; Jin et al, 2013a)

Summary

Introduction

As small, bivalved crustaceans in various water bodies, ostracod shells consist of low magnesium (Mg) calcite, which is derived almost entirely from the water body where they live, and are often well preserved in sediments. In addition to the water chemistry, there are other factors controlling the variations in both δ18O and δ13C of lacustrine ostracod shells, such as water temperature, authigenic carbonate precipitation, and/or isotopic fractionation between the shell and water, for δ13C (e.g., Mckenzie, 1985; Herczeg and Fairbanks, 1987; Heaton et al, 1995; von Grafenstein et al, 1999; Keatings et al, 2002; Decrouy et al, 2011; Devriendt et al, 2017). Other studies have shown that the isotopic composition of ostracod shells in Lake Qinghai reflects summer bottom-water conditions, with species difference (Henderson et al, 2003; Liu et al, 2009), whereas their δ13C is affected by the δ13C of dissolved inorganic carbon (DIC) and can reflect lake water salinity change (Li et al, 2012; Li and Liu, 2014). There is currently little information on the seasonal variations in δ18O and δ13C of lacustrine ostracod shells in Lake Qinghai

Methods

Results

Conclusion