Influence of water conditions on peat brGDGTs: A modern investigation and its paleoclimatic implications

Abstract

Peat brGDGTs (branched glycerol dialkyl glycerol tetraether lipids) have been proposed as an indicator of past temperature changes. However, recent Chinese peat brGDGTs-based Holocene temperature ranges, of >10 °C, are implausible as they are greater than the corresponding ranges obtained using other proxies. This indicates the possible role of additional factor(s) affecting peat brGDGTs, other than temperature. Here, we report the results of a comparative study of surface peat samples, from dry sites (without water cover when sampling) and water sites (with water cover when sampling), from three peatlands with contrasting environmental backgrounds in China. The results clearly demonstrate the influence of water conditions on the molecular distribution of peat brGDGTs. Specifically, the relevant methylation index (MBT′/MBT′5ME, which is proposed as a temperature indicator) of brGDGTs from the water sites is generally lower than that from the dry sites, which may be caused by the decoupling of water/air temperatures and/or different brGDGTs-producing bacterial communities controlled by the water conditions. Considering the long-term process of hydroseral succession from a water body (an analogue of a water site) to a peat bog (an analogue of a dry site), we estimate that around half of the range of the peat brGDGTs-based Holocene temperature record could result from long-term changes in hydrological conditions. If this effect is removed from the peat core MBT′5ME record, the residual range of the peat brGDGTs-based Holocene temperature record from the Narenxia peatland in the Altai Mountains, in arid inland China, is consistent with that of the quantitative pollen-based Holocene temperature record from the same peatland, highlighting the paleoclimatic significance of our findings.