The preservation of [formula omitted] and [formula omitted] in carbonate-associated sulfate during marine diagenesis: A 25 Myr test case using marine sediments

Alexandra V Turchyn,Victoria C.F Rennie

doi:10.1016/j.epsl.2014.03.025

Alexandra V Turchyn, Victoria C.F Rennie

Open Access

https://doi.org/10.1016/j.epsl.2014.03.025

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Journal: Earth and Planetary Science Letters	Publication Date: Apr 3, 2014
Citations: 101	License type: cc-by

Affiliation: University of Cambridge

Abstract

A 25 million year record of the sulfur and oxygen isotope composition of marine carbonate-associated sulfate (CAS) was constructed from 55 nannofossil ooze samples at three different locations. We tested the impact of early marine diagenesis on CAS by comparing the CAS extractions to the sulfur and oxygen isotope composition of the associated pore fluid sulfate, to determine the degree of pore fluid incorporation during carbonate recrystallization. Neither the sulfur nor the oxygen isotope composition of CAS are completely overprinted by incorporation of pore fluid sulfate. We compare our record to the sulfur and oxygen isotope records of coeval barite. The sulfur isotope record is in agreement with the barite record within ±2‰, except where very young (<2 Ma) sediments are in the presence of highly evolved pore fluid sulfate (δS34=70‰). A simple recrystallization model is used to illustrate the sensitivity of CAS δSSO434 to sedimentation rate, and to emphasize that careful sample selection, along with an analysis of early diagenetic environmental conditions is crucial when interpreting CAS sulfur isotopes. Oxygen isotopes in CAS are more complex and do not reproduce similar values to those of coeval barite. We conclude that oxygen isotopes in sulfate may remain a useful proxy but merit closer attention in the future.

Highlights

The marine biogeochemical sulfur cycle has played a critical role in the evolution of Earth’s surface environment (Holland, 1984)
We present data from three carbonate-rich sediments collected during the Ocean Drilling Program, chosen for their range of bacterial sulfate reduction and sedimentation rates
In all cores there is no correlation between the sulfur and oxygen isotope composition of carbonate-associated sulfate (CAS) and the sulfur or oxygen isotopic composition of the pore fluid sulfate

Summary

Introduction

The marine biogeochemical sulfur cycle has played a critical role in the evolution of Earth’s surface environment (Holland, 1984). Reconstructing the relative fluxes of weathering versus burial within the marine sulfur cycle may yield critical insight into the evolution of both the carbon cycle and the oxidation state of Earth’s surface (Berner, 1987; Canfield, 2005). The sulfur and, more recently, oxygen isotope composition of sulfur-bearing minerals precipitated in the ocean (δ34SSO4 and δ18OSO4 respectively) are the primary tools used for reconstructing the various fluxes within the marine sulfur cycle over geological time (Claypool et al, 1980; Canfield, 1998; Paytan et al, 1998, 2004; Newton et al, 2004). Whilst questions remain as to the mechanism of sulfate incorporation into carbonate minerals, CAS has been shown to record the isotopic composition of sulfate of the coeval seawater with no apparent fractionation for either δ18OSO4 (Cortecci and Longinelli, 1971; Newton et al, 2004) or δ34SSO4 (Burdett et al, 1989; Kampschulte et al, 2001; Kampschulte and Strauss, 2004)

Results

Discussion

Conclusion