Evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas: The modern Qinghai Lake, Qinghai–Tibet Plateau

Abstract

The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in arid or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (< 60 μm). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai–Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the lakes and ponds shows a clear response to lake water δ¹⁸O values, and these vary with water salinity. The relationship between lake water δ¹⁸O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small ‘vital offsets’ of ostracods. Finally, the effect of water depth on temperature leads to increasing δ¹⁸O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface.

For arid, high-altitude Lake Qinghai, our results suggest that variations in the δ¹⁸O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive δ¹⁸O values of ostracod and bulk carbonate.

Introduction

The isotopic composition of oxygen and carbon in the lacustrine carbonate of closed lakes is widely used as a proxy for past climatic and environmental changes (i.e., Fritz et al., 1975, Gasse et al., 1987, Talbot, 1990, Holmes, 1996, Lawrence and Gedzelman, 1996, Holmes and Chivas, 2002, Hammarlund et al., 2003, Sampei et al., 2005, Yang et al., 2005, Bright et al., 2006, Kieniewicz and Smith, 2007, Stevens and Dean, 2008, Christopher et al., 2008). However, the paleoclimatic interpretation of the oxygen isotopic composition of lacustrine carbonate (ostracod, bulk carbonate, and fine-grained carbonate) can be difficult because the isotopic signals in lake carbonates may be affected by multiple factors, such as the oxygen isotopic composition of source water, temperature (i.e., Lister, 1988, Lister et al., 1991, Chivas et al., 1993, Haskell et al., 1996, Smith et al., 1997, Xia et al., 1997, Schwalb et al., 1999, Sampei et al., 2005). Investigation of the oxygen-isotope distribution in carbonate from modern lakes in different locations can help us to understand the significance of isotopic signals as proxies for the paleoenvironmental reconstruction of lakes in a given region.

For example, Lake Qinghai, located on the arid, high-altitude northeastern Qinghai–Tibet Plateau in China, is one of the most sensitive regions to global climate change. Several studies have been carried out to reconstruct the paleoenvironment of this area using the stable oxygen isotopic composition of lacustrine carbonate (Zhang et al., 1989, Lister et al., 1991, Zhang et al., 1994, Henderson et al., 2003, Xu et al., 2006, Liu et al., 2007). In one study, the oxygen isotopic composition of ostracods from a 12 ka core was used to indicate relative changes in paleotemperature (Zhang et al., 1989). Based on the same core, Lister et al. (1991) suggested that effective humidity (evaporation/input water ratio) controls the δ¹⁸O values of lake water and its corresponding ostracod. One study of a short core collected from Lake Qinghai suggested that the oxygen isotopic composition of lake water might reflect the oxygen isotopic composition of input water and evaporative enrichment (Henderson et al., 2003). The oxygen isotopic composition of ostracod and fine-grained carbonate from this short core was used to evaluate the change in water level and effective precipitation of Lake Qinghai (Henderson et al., 2003). Recently, new data from lake sediments deposited during the past 600 years suggest that the oxygen isotopic composition of bulk carbonate can be used as a proxy for paleotemperature (Xu et al., 2006). However, previous results have all been based on the oxygen isotopic composition of carbonate from lake sediment cores, and little information is available regarding the oxygen-isotope distribution of carbonate and its environmental significance in modern lakes.

In this paper, we systematically investigate the oxygen isotopic distribution patterns of carbonate, including ostracod, bulk carbonate, and fine-grained carbonate, all of which were collected from modern lakes, together with water from Lake Qinghai and the small surrounding lakes and ponds. The goals of this study were: (1) to examine the oxygen isotopic distribution of carbonate in a lake and its response to environmental conditions and (2) to investigate the isotopic differences between ostracods, fine-grained carbonate, and bulk carbonate as proxies for paleoenvironmental reconstruction.