Matrix effect during sulfur isotope determination of apatite using SIMS and the potential of using bioapatite to reconstruct the sulfur isotopic composition of seawater

Recent studies have shown that the sulfur isotopic composition of apatite can be determined using secondary ion mass spectrometry (SIMS). However, the matrix effect during SIMS analysis of sulfur isotopic composition of apatite has not been well constrained. We investigate this topic by analyzing a set of reported apatite standards (i.e., Big1, SAP1, Durango-B, Hormuz and Mdg-1) with various sulfur concentrations (160–3003 ppm). Our results show that the instrumental mass fractionation (IMF) is negatively correlated with sulfur concentration, indicating that there is a significant matrix effect during sulfur isotope determination of apatite using SIMS. But fortunately, the matrix effect can be corrected using the linear correlation between IMF and sulfur concentration. Using the method reported in this study, we investigated the potential of using the sulfur isotopic composition of bioapatite to reconstruct the sulfur isotopic composition of ancient seawater. Two bioapatite samples collected from modern marine sediments were analyzed. Both samples show significant variation in their δ³⁴S values, and both samples have average δ³⁴S values lower than that of modern seawater. The data can be explained if sulfur sourced from reoxidation of the sulfide produced by bacterial sulfate reduction (BSR) and evolved seawater sulfate in porewater was incorporated into bioapatite during early diagenesis. This means that the primary sulfur isotope signal in bioapatite was possibly altered during early diagenesis, and the sulfur isotopic composition of bioapatite cannot directly be used to reconstruct the sulfur isotopic composition of ancient seawater. However, we note that one bioapatite has δ³⁴S values very close to that of modern seawater, indicating that the primary sulfur isotope signal may be retained in well preserved bioapatite samples. Future research is required to identify the exact location where the sulfur isotopic composition remains unaltered, prior to utilizing the sulfur isotopic composition of bioapatite to reconstruct the sulfur isotopic composition of ancient seawater.