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Abstract

Abstract

During the last decade, the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in lacustrine sediments has been widely used to reconstruct past variations in lake temperature. A prerequisite for the application of brGDGTs to lacustrine paleoclimate reconstructions is to understand the sources of brGDGTs in lake systems and the processes that influence their distribution. In this study, we investigated the distribution of brGDGTs in core-top sediments from 35 lakes across China, with a broad mean annual air temperature (MAAT) range, but a constrained pH range, to explore the effect of temperature. The results reveal a contrasting response of MBT' and MBT' to temperature in lake environments compared to that in soils. The sedimentary distributions of 5- and 6-methyl brGDGTs exhibit different relationships with temperature, with most of the latter being correlated to MAAT while the former responding to temperature by only hexamethylated (series III) compounds ( Table 1 ). In both global and Chinese soils, most 6-methyl brGDGTs have no relationship with MAAT but the distribution of 5-methyl brGDGTs is correlated with MAAT (Table 1). Therefore, the MBT'5ME index, which is mainly governed by variations in the proportion of series I brGDGTs (I%), is sensitive to temperature in soils but is not influenced by MAAT in lacustrine environments. The different behaviors suggest that 5- or 6-methyl brGDGTs-producing communities might be different in lakes and soils. In addition, in lakes from cold regions (MAAT > 5 °C), the brGDGT distributions correlates only with warm season temperatures (April to October) but exhibit no correlation with cold seasons, suggesting a seasonal bias in brGDGT production in these lakes. This bias towards the warm season is not found in lakes from warmer regions (MAAT > 5 °C). Based on these results we propose new temperature calibrations for paleotemperature reconstructions in Chinese alkaline lakes.

Table 1

The correlation coefficients between different indices and temperature, showing different ways of methylation of brGDGTs responding to temperature. The 5-methyl brGDGTs use solely III to respond to temperature in Chinese lakes, but adapt to MAAT by regulating (III + II)/I in soils. The 6-methyl bGDGTs adapt to temperature with no selectivity of compound series (using all series) in Chinese lakes, but do not respond to temperature in soils. Soil data without MAAT or ratio values are excluded.

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/content/papers/10.3997/2214-4609.201902988
2019-09-01
2024-03-28
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References

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