Origins of archaeal tetraether lipids in sediments: Insights from radiocarbon analysis

Sunita R Shah,Gesine Mollenhauer,Naohiko Ohkouchi,Timothy I Eglinton,Ann Pearson

doi:10.1016/j.gca.2008.06.021

Abstract

Understanding the supply and preservation of glycerol dibiphytanyl glycerol tetraethers (GDGTs) in marine sediments helps inform their use in paleoceanography. Compound-specific radiocarbon measurements of sedimentary alkenones from multiple environments have been used to gain insight into processes that affect U 37 K ′ paleotemperature reconstructions. Similar analyses are warranted to investigate how analogous processes affecting GDGTs impact TEX 86 paleotemperatures. Here we present radiocarbon measurements on individual GDGTs from Bermuda Rise and Santa Monica Basin sediments and discuss the results in the context of previous studies of co-depositional alkenones and foraminifera. The 14C contents of GDGTs and planktonic foraminifera in Bermuda Rise are very similar, suggesting a local source; and TEX 86-derived temperatures agree more closely with foraminiferal temperatures than do U 37 K ′ temperatures. In contrast, GDGTs in Santa Monica Basin are depleted in 14C relative to both alkenones and foraminifera, and TEX 86 temperatures agree poorly with known surface water values. We propose three possible factors that could explain these results: (i) GDGTs may be labile relative to alkenones during advective transport through oxic waters; (ii) archaeal production deep in the water column may contribute 14C-depleted GDGTs to sediments; and (iii) some GDGTs also may derive from sedimentary archaeal communities. Each of these three processes is likely to occur with varying relative importance depending on geographic location. The latter two may help to explain why TEX 86 temperature reconstructions from Santa Monica Basin do not appear to reflect actual sea surface temperatures. Terrigenous GDGTs are unlikely to be major contributors to Bermuda Rise or Santa Monica Basin sediments, based on values of the BIT index. The results also indicate that the crenarchaeol regioisomer is governed by processes different from other GDGTs. Individual measurements of the crenarchaeol regioisomer are significantly depleted in 14C relative to co-occurring GDGTs, indicating an alternative origin for this compound that presently remains unknown. Re-examination of the contribution of crenarchaeol regioisomer to the TEX 86 index shows that it is a significant influence on the sensitivity of temperature reconstructions.

Highlights

Archaea are found ubiquitously in the water column (e.g., Fuhrman et al, 1992; DeLong et al, 1992; Karner et al, 2001; Herndl et al, 2005) and in the sediments (e.g., Vetriani et al., 1998; Vetriani et al, 1999; Teske et al, 2002) of the modern ocean
This suggests that sedimentary alkenones are more adversely susceptible to artifacts when constructing paleo-sea surface temperature (SST) records; GDGTs are more likely to be of local origin
There may be export of GDGTs from archaea that live deeply in the water column. It appears that deep export may not necessarily explain 13 all of the 14C-aged GDGTs found in Santa Monica Basin

Summary

Introduction

Archaea are found ubiquitously in the water column (e.g., Fuhrman et al, 1992; DeLong et al, 1992; Karner et al, 2001; Herndl et al, 2005) and in the sediments (e.g., Vetriani et al., 1998; Vetriani et al, 1999; Teske et al, 2002) of the modern ocean. The record of archaea through geologic time is preserved by their characteristic membrane lipids, which are abundant in marine sediments since the Cretaceous (Kuypers et al, 2002; Schouten et al, 2003). A significant correlation has been observed between the composition of particular archaeal lipids in modern surface sediments, the glycerol dibiphytanyl glycerol tetraethers (GDGTs, molecular structures in Figure 1) and overlying sea surface temperature. This correlation has inspired a paleotemperature proxy, the TEX86 index (Schouten et al, 2002). The correlation between sedimentary GDGTs and sea surface temperatures suggests that GDGTs found in sediments originate predominantly from surface waters. Marine archaea are found at all depths in the water column, primary export of GDGTs to the sediments is thought to occur by processes that dominantly exist in the upper water column (e.g., consumption by zooplankton and packaging into fecal pellets (Schouten et al, 2002; Wakeham et al, 2003; Wuchter et al, 2006))

Results

Discussion

Conclusion