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

The alkenone unsaturation index U super(K') sub(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 sub(37:4) alkenone to the total C sub(37) production (%C sub(37:4)) can reflect surface salinity changes in lacustrine systems. Here we present long-chain C sub(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 sub(37:4) varies from 15% to 49% as surface salinity decreases. We tentatively describe this %C sub(37:4)-salinity link with a general linear regression: %C sub(37:4) = 53.4 (+/-7.8) - 1.73 (+/- 0.45) x S (n = 28, r super(2) = 0.62), although step-wise %C sub(37:4) changes in response to salinity variation may exist. U super(K') sub(37) values vary between 0.10 and 0.16 at these sites and the inferred range of lake water temperature changes is [not, vert, similar]2-3 C, suggesting that U super(K') sub(37) largely reflects temperature signal across a large salinity range, consistent with previous findings that U super(K') sub(37) can indicate temperature changes over a large diversity of environmental settings. We have also found that U super(K') sub(37) values are correlated with salinity changes (r super(2) = 0.4), and thus cannot exclude potential temperature effect on %C sub(37:4) and salinity effect on U super(K') sub(37) in this study. However, even extreme estimates of temperature differences within the lake are still unable to explain the observed %C sub(37:4) changes. We therefore suggest that %C sub(37:4) could be used to infer past lake salinity changes at a regional scale.