Temperature changes reconstructed from branched GDGTs on the central Loess Plateau during the past 130–5 ka

Abstract

This study investigated the glycerol dialkyl glycerol tetraethers (GDGTs) in a loess-paleosol sequence (LPS) to quantitatively reconstruct the mean annual air temperature (MAT) on the central Chinese Loess Plateau (CLP) during the past 130–5 ka. The results show that: (1) MAT variations reconstructed by global MBT (methylation index of branched tetraethers) and CBT (cyclisation ratio of branched tetraethers) transfer functions at Xifeng range from 7.3 °C to 28.4 °C. The MAT variation on the central CLP (Xifeng site) is between the previously reported MAT variations on the western (Yuanbao site) and eastern CLP (Mangshan site), in agreement with the pattern that temperature increases from the northwestern CLP to the southeastern CLP. (2) MAT variations estimated by transfer functions of global MBT-CBT, global MBT′-CBT and Chinese MBT-CBT for LPS are nearly all higher than the modern MAT at Xifeng, indicating that the reconstructed MAT values are probably overestimated. (3) The maximum value of magnetic susceptibility at Xifeng lagged that of the MAT derived by MBT-CBT proxy during the last interglacial (∼9 ka lag) and since the last deglacial (∼6 ka lag).

Introduction

Loess-paleosol sequences (LPS) on the Chinese Loess Plateau (CLP) are important terrestrial archives of the evolution of past climate changes since the Neogene (Heller and Liu, 1982; Porter and An, 1995; Guo et al., 1998, 2002; Liu and Ding, 1998; Hao et al., 2012). Quantitative reconstruction of the temperature variations recorded in LPS on the CLP is crucial for understanding the mechanism of past climate change in Northwest China. During the last two decades, magnetic susceptibility (Lu et al., 1994; Han et al., 1996) and phytoliths (Lu et al., 2007) have been used as proxies of paleotemperature to quantitatively reconstruct the air temperature history recorded in LPS on the CLP. However, different proxies often produce distinct temperature variations, perhaps due to their complicated sensitivity to air temperature change.

Archaeal and bacterial tetraether membrane lipids, i.e. GDGTs (glycerol dialkyl glycerol tetraethers), are recognized to being sensitive to ambient temperature change (Wuchter et al., 2004; Weijers et al., 2007; Yang et al., 2014; Ding et al., 2015). Therefore, GDGT distributions can be applied to reconstruct past temperature variations on the CLP (Peterse et al., 2011, 2014; Gao et al., 2012; Jia et al., 2013; Lu et al., 2016; Tang et al., 2017).

The branched GDGTs (brGDGTs) occur widely in soils and are considered to be of bacterial origin (Weijers et al., 2007). The MBT (methylation index of branched tetraethers) and CBT (cyclisation ratio of branched tetraethers) proxies were defined based on brGDGTs in surface soils (Weijers et al., 2007). The MBT proxy in brGDGTs has been proven to have a strong linear correlation with mean annual air temperature (MAT) and soil pH, while the CBT proxy is primarily related to soil pH (Weijers et al., 2007). Thus, the MBT-CBT proxies have been applied to reconstruct MAT variation in terrestrial settings (Peterse et al., 2011).

Global transfer functions (Weijers et al., 2007; Peterse et al., 2012) and a Chinese transfer function (Yang et al., 2014) have been established to reconstruct ancient MAT by MBT-CBT proxies. Up to now, GDGT compositions in the LPS at Mangshan (Peterse et al., 2011, 2014) on the eastern CLP, at Lantian (Gao et al., 2012) and Weinan (Yang et al., 2014; Tang et al., 2017) on the southern CLP, and at Yuanbao (Jia et al., 2013) on the western CLP have been studied. Although the temperature histories on the eastern, southern and western CLP are reconstructed by the application of brGDGT indices, these studies are inadequate to provide comprehensive information of the spatial-temporal temperature variations with regard to the whole CLP because of the lack of the GDGT composition of LPS on the central CLP. In addition, the previously reported MAT variations estimated by brGDGT indices are higher than the current MAT value at Mangshan and Lantian sections (Peterse et al., 2011; Gao et al., 2012) during the Last Glacial Maximum (LGM), while the MAT variations calculated by global MBT-CBT indices (Weijers et al., 2007) are lower than the current MAT during the LGM at Yuanbao on the western CLP (Jia et al., 2013).

Investigation of the GDGT composition of the LPS on the central CLP is significant for quantitative reconstruction of the MAT variations on the whole CLP. This study determined the GDGT composition of the LPS in Xifeng section on the central CLP. The aims of this study are to (1) quantitatively reconstruct the past air temperature changes at Xifeng during the past 130–5 ka, and (2) compare the temperature variations at Xifeng on the central CLP with those at other sites on the eastern and western CLP.