An integrated carbon and oxygen isotope approach to reconstructing past environmental variability in the northeast Atlantic Ocean

David J Reynolds,Ian R Hall,Sophie M Slater

doi:10.1016/j.palaeo.2019.03.005

David J Reynolds, Ian R Hall + Show 1 more

Open Access

https://doi.org/10.1016/j.palaeo.2019.03.005

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Abstract

The combined influence of temperature and the isotopic composition of the seawater (δ18Ow) often precludes the use of oxygen isotope (δ18O) records, derived from marine carbonates, to reconstruct absolute seawater temperatures, without the application of an independent δ18Ow proxy. Here we investigate the application of carbon isotope records (δ13Cshell), derived from the long-lived marine bivalve Glycymeris glycymeris, as a proxy for δ18Ow variability. Our analyses indicate G. glycymeris δ13Cshell data derived from growth increments >20 years of age contain strong ontogenetic trends (−0.013‰ yr−1, R = 0.98, P < 0.001, N = 51). These analyses demonstrate that, coupled with the ontogenetic trends, 54% of the variability in G. glycymeris δ13Cshell records can be explained by a combination of the marine Suess effect and physical (salinity and riverine input) and biological processes (primary production). The application of these δ13Cshell data in conjunction with co-registered δ18Oshell and growth increment width series, each of which have been shown to be sensitive to seawater temperature and primary productivity respectively, can therefore provide new insights into past environmental variability and help constrain uncertainties on reconstructions of past seawater temperature variability.

Highlights

The widespread application of oxygen stable isotope (δ18O) records for reconstructing past oceanographic variability is largely due to the well characterised mechanisms that drive δ18O variability in marine carbonates (δ18Ocarb; temperature and the δ18O composition of seawater [δ18Ow], which is closely related to salinity), and the broad spectrum of marine archives that provide long-term records of δ18O variability
The dates of the samples derived from the modern shells were based on the Tiree Passage G. glycymeris master growth increment width sclerochronology (Reynolds et al, 2013)
The mean population δ13Cpre-industrial curve was constructed from δ13Cshell data derived from 1269 unique growth increments sampled at annual resolution from a total of 18 individual shells

Summary

Introduction

The widespread application of oxygen stable isotope (δ18O) records for reconstructing past oceanographic variability is largely due to the well characterised mechanisms that drive δ18O variability in marine carbonates (δ18Ocarb; temperature and the δ18O composition of seawater [δ18Ow], which is closely related to salinity), and the broad spectrum of marine archives that provide long-term records of δ18O variability (e.g. planktonic and benthic foraminifera, corals, otoliths and molluscs; Urey, 1948, Shackleton et al, 1984, Chappell and Shackleton, 1986, Fairbanks and Matthews, 1978, Chappell et al, 1996, Schöne et al, 2005, Wanamaker Jr. et al, 2011, Reynolds et al, 2016). Whilst δ18Ocarb records have proved invaluable in developing our understanding of past marine variability, the combined influence of both temperature and δ18Ow on the δ18Ocarb variability typically precludes the reconstruction of absolute water temperature or δ18Ow without using an independent proxy for δ18Ow or seawater temperature respectively. To mitigate this issue δ18Ocarb records derived from foraminifera can be integrated with coupled (co-registered) magnesium/ calcium (Mg/Ca) ratios Given the lack of a stable coherence between molluscan Mg/Ca ratios and temperature alternative approaches have to be employed for detangling the influence of seawater temperature and δ18Ow on δ18Oshell records

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