Abstract
The ubiquitous occurrence of glycerol dialkyl glycerol tetraethers (GDGTs) in soils and their ability to record temperature and environmental changes offer the prospect of independently reconstructing continental paleotemperature and paleoenvironment from the loess-paleosol sequences (LPS) from the Chinese Loess Plateau (CLP). In this study we present records of GDGT-derived proxies for the last 70 kyr from the Yuanbao LPS, western CLP. Temperature record reconstructed from the cyclization and methylation index of branched tetraethers (MBT-CBT) displays that the onset of deglacial warming at ~20 kyr before present (BP) precedes the strengthening of summer monsoon at ~15 kyr BP, which is in agreement in timing with previous MBT-CBT temperature records from the southeastern CLP. The maximal deglacial warming of ~10°C is slightly higher than those in the southeastern CLP, perhaps due to the higher latitude and farther inland of the study site. The Branched and Isoprenoid Tetraether (BIT) index shows higher values (0.87–0.96 range, 0.93 average) in the glacial loess and lower values (0.76–0.91 range, 0.83 average) in the Holocene paleosols, with a steady decreasing trend since the early Holocene. The decreasing trend could suggest enhanced Thaumarchaeota relative to GDGT producing bacteria activity since the early Holocene, but other possibilities, such as preferential degradation of isoprenoid GDGTs or upward increase in living archaea relative to bacteria in the paleosol profile, cannot be fully excluded. Our results thus demonstrate the need of future study on microbial community structure in soil column and differential degradation of GDGT molecules.
Highlights
The microbial membrane lipids glycerol dialkyl glycerol tetraethers (GDGTs) are widespread, but different in structure, in archaea and bacteria (Figure 1)
Based on the age model, magnetic susceptibility (MS) and Corg content in the Yuanbao sequence changed in parallel, with lower values during the glacial time, the deglacial onset of increases at ∼15 kyr before present (BP), and higher values during the Holocene (Figures 4B,C)
The two records are similar to the changes of speleothem δ18O (Figure 4A), as is expected because all of these proxies are associated with the development of the East Asian summer monsoon (EASM) precipitation (Zhou et al, 1990; Wang et al, 2001; Dykoski et al, 2005; Peterse et al, 2011)
Summary
The microbial membrane lipids glycerol dialkyl glycerol tetraethers (GDGTs) are widespread, but different in structure, in archaea and bacteria (Figure 1). Whereas some specific bacteria have branched alkyl chains with 4–6 methyl groups, containing up to two cyclopentyl moieties, at sn 1,2 positions between the glycerol backbones (Weijers et al, 2006a). Both the archaea and bacteria produced GDGTs have been used to develop temperature proxies that are promising and valuable in paleoclimatic studies. Sea surface temperatures were found being recorded by the distribution of isoprenoidal GDGTs (i-GDGTs) of marine Thaumarchaeota with a varying number of clyclopentane moieties, and a proxy called TEX86 (TetraEther Index of tetraethers having 86 carbons) has been developed (Schouten et al, 2002). A proxy for continental air temperatures and soil pH was proposed based on the distribution of soil bacterial branched
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