Ubiquitous production of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in global marine environments: a new source indicator for brGDGTs

Wenjie Xiao,Yinghui Wang,Huan Yang,Yunping Xu,Shangzhe Zhou,Limin Hu

doi:10.5194/bg-13-5883-2016

Abstract

Abstract. Presumed source specificity of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from bacteria thriving in soil/peat and isoprenoid GDGTs (iGDGTs) from aquatic organisms led to the development of several biomarker proxies for biogeochemical cycle and paleoenvironmental reconstructions. However, recent studies reveal that brGDGTs are also produced in aquatic environments besides soils and peat. Here we examined three cores from the Bohai Sea, and found distinct difference in brGDGT compositions varying with the distance from the Yellow River mouth. We thus propose an abundance ratio of hexamethylated to pentamethylated brGDGT (IIIa ∕ IIa) to evaluate brGDGT sources. The compilation of globally distributed 1354 marine sediments and 589 soils shows that the IIIa ∕ IIa ratio is generally < 0.59 in soils and 0.59–0.92 and > 0.92 in marine sediments with and without significant terrestrial inputs, respectively. Such disparity confirms the existence of two sources for brGDGTs, a terrestrial origin with lower IIIa ∕ IIa and a marine origin with higher IIIa ∕ IIa, which is likely attributed to a generally higher pH and the production of brGDGTs in cold deep water in marine waters. The application of the IIIa ∕ IIa ratio to the East Siberian Arctic Shelf proves it to be a sensitive source indicator for brGDGTs, which is helpful for accurate estimation of organic carbon source and paleoclimates in marine settings.

Highlights

Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids of archaea and certain bacteria, are widely distributed in marine and terrestrial environments (Schouten et al, 2013)
A series of isoprenoid GDGTs (iGDGTs) including crenarchaeol and branched GDGTs (brGDGTs) including 5-methyl and 6-methyl isomers were detected in Bohai Sea sediments
The branched and isoprenoid tetraether (BIT) index itself has no ability to determine the source of brGDGTs because brGDGTs and crenarchaeol used in this index are thought to be specific to soil organic carbon and marine organic carbon, respectively (Hopmans et al, 2004), crenarchaeol is present in soils at low abundance (Weijers et al, 2006)

Summary

Introduction

Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids of archaea and certain bacteria, are widely distributed in marine and terrestrial environments (Schouten et al, 2013) These lipids have been a focus of attention of organic geochemists for more than 10 years because they can be used to estimate environmental variables in the past such as temperature, soil pH, organic carbon source and microbial community structure (e.g., Schouten et al, 2002; Hopmans et al, 2004; Weijers et al, 2006; Lipp et al, 2008; Kim et al, 2010; Peterse et al, 2012; Zhu et al, 2016). Only one brGDGT with two 13,16-dimethyl octacosanyl moieties was unambiguously detected in two species of Acidobacteria (Sinninghe Damsté et al, 2011), which hardly explains high

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