Hydroxylated isoprenoid GDGTs in Chinese coastal seas and their potential as a paleotemperature proxy for mid-to-low latitude marginal seas

Highlights

• The global distribution of OH-GDGTs is significantly governed by SST.

• RI-OH is an alternative SST proxy for coastal settings with high terrestrial input.

• RI-OH is a more robust tracer of warmer SSTs (>15 °C), while RI-OH′ is more applicable to cool, polar environments (<15 °C).

Abstract

Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-GDGTs), with one or two hydroxyl groups in one of the biphytanyl moieties, are major compounds in planktonic Thaumarchaeota, and occur widely in marine and lacustrine sediments. In order to examine their potential as an indicator of local variation in sea surface temperature (SST), we collected 70 surface sediments from the Chinese coastal seas (CCSs), including 16 from the Yangtze Estuary, to determine their tetraether lipid composition. The proportion of OH-GDGTs relative to total isoprenoid GDGTs increased with latitude from 1.8% to 12.8% and correlated strongly with SST, as indicated from redundancy analysis. The variation in OH-GDGT composition was best captured by the weighted average number of cyclopentane moieties in OH-GDGT-1 and -2 (RI-OH), which correlated significantly with both summer and autumn SST and was associated with minimum residual SST relative to remote sensing data. The large input of terrigenous organic matter to the Yangtze Estuary appears to have no significant influence on the OH-GDGT based proxy. Our study suggests that RI-OH could serve as a useful proxy for recording warm seasonal SST in the CCSs. A global surface sediment compilation further indicates that RI-OH and its derivative, RI-OH′, which additionally contains OH-GDGT-0, may be used to estimate SST in warm (ca. > 15 °C annual mean SST) and cold (ca. < 15 °C) environments, respectively.

Introduction

Archaea of the phylum Thaumarchaeota (Brochier-Armanet et al., 2008, Spang et al., 2010) are ubiquitous in marine environments and account for up to 20–40% of marine picoplankton (DeLong, 1992, Fuhrman et al., 1992, Karner et al., 2001, DeLong, 1998, Schattenhofer et al., 2009). The predominant membrane lipids of Thaumarchaeota are isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs; e.g., Schouten et al., 2008, Pitcher et al., 2011). It is assumed that the mesophilic planktonic Thaumarchaeota adjust the number of cyclopentyl rings in their GDGT membrane lipids in response to change in temperature, as demonstrated for several mesophilic and thermophilic archaeal cultures of the phyla Thaumarchaeota, Euryarchaeota and Crenarchaeota (de Rosa et al., 1980, Gliozzi et al., 1983, Uda et al., 2001, Uda et al., 2004, Lai et al., 2007, Boyd et al., 2011, Elling et al., 2015). The observation that the degree of cyclization of GDGTs found in surface sediments correlates with sea surface temperature (SST) led to the proposal for a novel proxy, TEX₈₆ (Schouten et al., 2002, Wuchter et al., 2004), which showed potential in SST reconstruction (cf. Pearson and Ingalls, 2013, Schouten et al., 2013 and references therein). However, in coastal and lacustrine environments, the TEX₈₆ signal may be biased by the presence of terrigenous organic matter (OM) containing GDGTs derived from soil Thaumarchaeota and other archaeal groups (e.g. Weijers et al., 2006, Sinninghe Damsté et al., 2010). The branched and isoprenoid tetraether (BIT) index (Hopmans et al., 2004) has been used as a tool for assessing the input of terrigenous OM and thus a potential bias in the TEX₈₆ signal (e.g. Weijers et al., 2006, Zhu et al., 2011). However, in situ production of marine/lacustrine branched GDGTs, as well as very low BIT values in soils from semi-arid and arid regions (e.g. Peterse et al., 2009, Sinninghe Damsté et al., 2009, Zhu et al., 2011; Yang et al., 2014) may complicate application of the BIT index to constrain terrigenous input, hindering TEX₈₆-based SST reconstruction severely in some environments (e.g. Fietz et al., 2012, Smith et al., 2012, Ge et al., 2014).

The hydroxylated GDGTs (OH-GDGTs), initially detected in subsurface sediments by Lipp and Hinrichs (2009) and subsequently identified by Liu et al. (2012), are a suite of GDGTs with up to two OH groups in one of their alkyl chains (structures in Fig. 1) and are widespread in marine sediments (Liu et al., 2012, Huguet et al., 2013, Fietz et al., 2013). Those in marine sediments likely originate from planktonic Thaumarchaeota, though their presence in a thermophilic euryarchaeon hints at a wider taxonomic distribution and possibly points to multiple sources of OH-GDGTs in environmental samples (cf. Liu et al., 2012). Within the Thaumarchaeota, OH-GDGT biosynthesis has only been observed in cultivated representatives of Group 1.1a (Pitcher et al., 2011, Liu et al., 2012, Elling et al., 2014, Elling et al., 2015) but not in strains affiliated to Group 1.1b (Sinninghe Damsté et al., 2012), which represent the dominant thaumarchaeotal group in most soils (Bates et al., 2011, Bartossek et al., 2012). Therefore, OH-GDGTs are thought to be primarily of planktonic origin and have potential for use as biomarkers for thaumarchaeotal taxonomy (Liu et al., 2012, Sinninghe Damsté et al., 2012). In addition, the presence of OH-GDGTs in ancient downcore sediments (Liu et al., 2012) suggests potential for these compounds for paleoenvironmental applications. The relative abundance of OH-GDGTs vs. iGDGTs in marine surface sediments was found to increase with increasing latitude and showed a significant correlation with sea surface temperature (SST; Huguet et al., 2013). In addition, Fietz et al. (2013) observed that the relative number of cyclopentane rings in OH-GDGTs increased with increasing SST in subpolar and polar regions, and suggested that the number of rings could be used as a proxy for tracing SST in polar seas. The CCSs, including the Yellow Sea (YS), East China Sea (ECS) and the northern part of the South China Sea (SCS) are marginal seas in the western Pacific Ocean (Fig. 2). The sediments in the CCSs are contributed to mainly by terrigenous input from the Yangtze River, Yellow River and Pearl River (Niino and Emery, 1961, Chen and Zheng, 1985, DeMaster et al., 1985, Milliman et al., 1985a, Milliman et al., 1985b, Zhao and Yan, 1994, Yang and Youn, 2007). The OM in CCS sediments originates generally from both marine authigenic and terrigenous organic input (Lü and Zhai, 2006). If soils contribute GDGTs to marine sediments in the CCSs (cf. Dang et al., 2008, Hu et al., 2013), TEX₈₆ records would likely be biased towards higher values due to a relatively greater contribution from GDGT-1, GDGT-2, GDGT-3 and the crenarchaeol regio isomer from soil Thaumarchaeota (Sinninghe Damsté et al., 2010), especially in areas with a high sedimentation rate (Lü et al., 2014), given the prevalence of a high degree of GDGT cyclization in soil Thaumarchaeota (Sinninghe Damsté et al., 2012). Therefore, it is necessary to develop novel paleotemperature proxies for tracing SST in marginal seas with a high terrigenous input. Here, we have examined the SST proxy potential of OH-GDGTs in surface sediments from the CCSs and propose the ring index as suitable proxy for SST reconstruction in these seas.