The preservation of δSSO434 and δOSO418 in carbonate-associated sulfate during marine diagenesis: A 25 Myr test case using marine sediments

- We measured δO18 and δS34 in carbonate-associated sulfate and pore fluid sulfate.
- We compared CAS δOSO418 and δSSO434 with previously published coeval barite data.
- CAS shows limited diagenetic overprinting from pore fluid sulfate incorporation.
- CAS δS34 is generally ±2‰ of barite δS34, however CAS δO18 and barite δO18 differ.
- We use a model to outline conditions that render CAS susceptible to overprinting.

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.