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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'5ME and MBT'6ME 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 compounds. 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. The different behaviors suggest that both 5-and 6methyl brGDGTs-producing communities might be different in lakes and soils. In addition, in lakes from cold regions (MAAT < 5 °C), the brGDGT distribution correlates only with warm season temperatures (April to October) but exhibits 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. Highlights 5-and 6-methyl brGDGTs measured in 35 Chinese lakes Seasonal bias towards warm months in cold region lakes Different responses to temperature between lakes and soils Highlights (for review)

The distribution of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) is 14 influenced by growth temperature and pH. This results in the widespread application of the 15 brGDGT-based MBT(')/CBT proxy (MBT-methylation of branched tetraethers, CBT-cyclization 16 of branched tetraethers) in terrestrial paleo-environmental reconstructions. Recently, it was shown 17 that the amount of precipitation could also have an impact on CBT, as well as the abundance of 18 brGDGTs relative to that of archaeal isoprenoidal (iso)GDGTs (Ri/b) and the absolute abundance 19 of brGDGTs, potentially complicating the use of MBT/CBT as paleothermometer. However, the 20 full influence of hydrology, and in particular soil water content (SWC), on GDGT distributions 21 remains unclear. Here we investigated variations in the GDGT distribution across a SWC gradient 22 (0-61%) around Qinghai Lake in the Tibetan Plateau, an arid to semiarid region in China. Our 23 results demonstrate that SWC affects the brGDGT distribution. In particular, we show that SWC 24 has a clear impact on the degree of methylation of C6-methylated brGDGTs, whereas 25 C5-methylated brGDGTs are more impacted by temperature. This results in a combined SWC and 26 temperature control on MBT'. In this context we propose a diagnostic parameter, the IR6ME 27 (relative abundance of C6-methylated GDGTs) index, to evaluate the applicability of 28 brGDGT-based paleotemperature reconstructions. Using the global dataset, expanded with our 29 own data, MBT' has a significant correlation with mean annual air temperature when IR6ME < 0.5, 30 2 allowing for the use of MBT'/CBT as temperature proxy. However, MBT' has a significant 31 correlation with mean annual precipitation (i.e., a substantial reflection of SWC impact) when 32 IR6ME > 0.5, implying that MBT' may respond to hydrological change in these regions and can be 33 used as a proxy for MAP. 34 35

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'5ME and MBT'6ME 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 compounds. 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. The different behaviors suggest that both 5-and 6methyl brGDGTs-producing communities might be different in lakes and soils. In addition, in lakes from cold regions (MAAT < 5 °C), the brGDGT distribution correlates only with warm season temperatures (April to October) but exhibits 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.Highlights 5-and 6-methyl brGDGTs measured in 35 Chinese lakes Seasonal bias towards warm months in cold region lakes Different responses to temperature between lakes and soils Highlights (for review)