Constraining sources of archaeal tetra-ether lipids in marine sediments using compound-specific isotope analysis

Abstract

Archaeal isoprenoid glycerol dialkyl glycerol tetraether lipids (iGDGTs) are commonly used tools in paleooceanography for reconstructing past surface temperatures and carbon cycle dynamics. However, uncertainties persist regarding the origins of iGDGTs in marine sediments, requiring a deeper understanding of their diverse sources. The TEX86 sea-surface temperature proxy, based on the ratio of cyclized iGDGTs, assumes that majority of sedimentary iGDGTs originate from a single, planktonic archaeal representative. Yet, the influence of allochthonous input from benthic in-situ and terrestrial sources is unresolved, potentially calling into question the fidelity of iGDGT-based proxies. This thesis aims to address these uncertainties by constraining sources of sedimentary iGDGTs through compound-specific carbon and hydrogen isotope analysis. We develop a dual-isotopic framework to determine iGDGT source apportionment in methane-impacted systems, characterize the d13C and d2H isotopic signatures and lipid profiles of potential archaeal sources, and evaluate the impact of heterogenous inputs on the reliability and application of iGDGT-based proxies. In Chapter 2, we conducted analytical tests to evaluate the effects of high-performance liquid chromatography (HPLC) on the carbon isotopic (d13C) analysis of iGDGTs. Our findings show that normal phase HPLC can cause significant isotopic fractionation, exceeding analytical thresholds, emphasizing the need for precise and complete peak collection during normal phase HPLC for accurate isotope ratio quantification. In Chapter 3, we measured the d13C values and relative lipid abundances of iGDGTs from three cold-seep locations along Cascadia Margin, using a Bayesian mixing model to robustly characterize the d13C signatures, lipid profiles and relative contributions to the marine sediment of three potential archaeal endmembers. Our results indicate that the cold seep systems in this region can be described as a binary mixing system between planktonic and benthic-methane cycling archaeal groups, with minimal contribution from benthic non-methane-cycling archaea. Finally, in Chapter 4, we introduce a novel dual-isotope approach, combining d2H and d13C analyses of biphytanes derivatives and parent iGDGTs, respectively, to quantitatively distinguish lipid inputs to marine systems. Our findings indicate that planktonic and benthic methane-cycling archaea both yield highly 2H-fractionated lipids, providing an interpretative basis for applying dual isotopic analysis to disentangle iGDGT sources in marine systems.