Abstract

Alkenone signatures preserved in marine sedimentary records are considered one of the most robust paleothermometers available, and are often used as a proxy for paleoproductivity. However, important gaps remain on the provenance and fate of alkenones, and their impact on derived environmental signals in marine sediments. Here, we analyze the abundance, distribution, and radiocarbon (14C) age of alkenones in bulk sediments and corresponding grain-size fractions in surficial sediments from seven continental margin settings in the Pacific and Atlantic Oceans in order to evaluate the impact of organo-mineral associations and hydrodynamic sorting on sedimentary alkenone signals. We find that alkenones preferentially reside within fine-grained mineral fractions of continental margin sediments, with the preponderance of alkenones residing within the fine silt fraction (2–10 µm), and most strongly influencing alkenone 14C age, and SST signals from bulk sediments as a consequence of their proportional abundance and higher degree of OM protection relative to other fractions. Our results demonstrate that selective association of alkenones with mineral surfaces and associated hydrodynamic mineral sorting processes can alter alkenone signals encoded in marine sediments (14C age, content, and distribution) and confound corresponding proxy records (productivity and SST) in the spatial and temporal domain.

Highlights

  • Since the initial discovery of alkenones (Boon et al, 1978; Volkman et al, 1980), these molecular biomarkers have become one of the most applied and well-established paleoclimate proxies, allowing estimation of sea surface temperature (SST) and primary productivity in most oceanographic settings (Raja and Rosell-Melé, 2021; Sachs et al, 2000)

  • Except for NW African margin (NAF) and Bermuda Rise (BER) sediments, where the clay fraction hosts the largest proportion of alkenones followed by fine silt, alkenone concentrations are highest within the fine-silt fraction at all sites

  • Our results suggest lateral supply of pre-aged alkenones influenced by hydrodynamic particle sorting and potentially originating from different locations on the margin, and they are consistent with prior models of sediment transport by bottom and intermediate nepheloid layers leading to the formation of an upper slope organic carbon (OC) depocenter (Inthorn et al, 2006a, b)

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Summary

Introduction

Since the initial discovery of alkenones (Boon et al, 1978; Volkman et al, 1980), these molecular biomarkers have become one of the most applied and well-established paleoclimate proxies, allowing estimation of sea surface temperature (SST) and primary productivity in most oceanographic settings (Raja and Rosell-Melé, 2021; Sachs et al, 2000). The total abundance of C37 alkenones (C37:2 + C37:3) in marine sediments is widely used as a qualitative proxy for primary productivity on the basis that alkenones are a large component of the total carbon of Emiliania huxleyi (Prahl et al, 1988) and that alkenone degradation is not observed upon zooplankton digestion (Grice et al, 1998; Grimalt et al, 2000; Volkman et al, 1980). This signal can be altered in marine sediments by the significant loss of alkenones that occurs during their export to and deposition on the seafloor. The influence of lateral transport on the radio-

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