Distribution of the C 37 tetra-unsaturated alkenone in Lake Qinghai, China: A potential lake salinity indicator

Abstract

The alkenone unsaturation index U K′ 37 has been applied to reconstruct past temperature changes in both marine and lacustrine systems. However, few studies have addressed whether the relative abundance of the C 37:4 alkenone to the total C 37 production (%C 37:4) can reflect surface salinity changes in lacustrine systems. Here we present long-chain C 37 alkenone distribution patterns in surface sediments from Lake Qinghai, China. Surface sediments were sampled over a large range of surface salinity changes (1.7–25 g/l) within Lake Qinghai and its surrounding lakes, while temperature differences at these sampling locations should be relatively small. We have found that %C 37:4 varies from 15% to 49% as surface salinity decreases. We tentatively describe this %C 37:4–salinity link with a general linear regression: %C 37:4 = 53.4 (±7.8) − 1.73 (±0.45) × S ( n = 28, r 2 = 0.62), although step-wise %C 37:4 changes in response to salinity variation may exist. U K′ 37 values vary between 0.10 and 0.16 at these sites and the inferred range of lake water temperature changes is ∼2–3 °C, suggesting that U K′ 37 largely reflects temperature signal across a large salinity range, consistent with previous findings that U K′ 37 can indicate temperature changes over a large diversity of environmental settings. We have also found that U K′ 37 values are correlated with salinity changes ( r 2 = 0.4), and thus cannot exclude potential temperature effect on %C 37:4 and salinity effect on U K′ 37 in this study. However, even extreme estimates of temperature differences within the lake are still unable to explain the observed %C 37:4 changes. We therefore suggest that %C 37:4 could be used to infer past lake salinity changes at a regional scale.