60th Annual Report on Research 2015

During the last year we have made considerable progress on our goal of better understanding the record of sulfur isotopes in the ocean through time. Graduate student Ted Present has published his first paper, been to the West Texas field site, compiled all of the published marine δ³⁴S data, and started developing a new triple quadrupole ICP-MS method for the measurement of S/Ca (and other Me/Ca) ratios in carbonate rocks.

Ted has been using a collection of end Ordovician rocks, collected from Anticosti Island by a Caltech led team in Prof. Fischer's group, to assess the sources of variability in the δ³⁴S of carbonate associated sulfate (CAS). Our new MC-ICP-MS method for measuring sulfur isotope ratios allows us to use very small samples, typically ~20 nmoles of S. Ted combined the unique, fossil rich samples with the new analytical ability to measure texture specific δ³⁴S values from within several hand samples. The results show a ~25‰ in isotope ratio within a single sample. To put this in some context, this range is almost the full range typically shown for the entire Phanerozoic. That is 550 million years of ocean variance in a single hand sample! Isotope ratios are tightly correlated with rock texture. Brachiopod fossils show the least variance and are tightly grouped around the supposed past ocean value. Micrites are highly variable, but always lower than the brachiopods. Rugose corals, trilobites, and other fossils tend to be enriched relative to the brachiopods. The light micrites probably formed in pore water solutions where active sulfur oxidation was occurring. The low δ³⁴S signal of dissolved sulfides, and perhaps pyrites, is retained during oxidation. Heavier fossils, on the other hand, show the effect of carbonate recrystallization in the zone of sulfate reduction where pore water SO4 isotopes are driven to higher values by distilling off isotopically depleted sulfides. This study is already creating a stir in the field as it offers a way to more precisely constrain the history of seawater δ³⁴S and potentially constrain the diagenetic environment of rock deposition. It was published in EPSL this year.

Ted continued fieldwork in the Guadalupe Mountains of West Texas. This area is well characterized for its diagenetic alteration of a complete end Permian reef complex. We are trying to understand how δ³⁴S behaves in a system where the diagenetic constraints are relatively well understood already. Samples have been collected, slabbed and polished. We are starting to measure their δ³⁴S, CAS concentration, and trace metal content. This geochemical work is complemented by on going thin section analysis. We anticipate at least one more field trip to fully constrain the stratigraphy and collect more samples.

As part of our overall effort to better understand the history of the marine sulfur cycle, Ted has complied all of the published marine δ³⁴S data (including those hidden in thesis chapters). The resulting plot is fascinating and is letting us choose candidate regions and times in earth history where there is either a data gap, or large disagreement in the δ³⁴S data. Our new 1000x smaller sample size requirement should let us expand our end Ordovician study discussed above to other parts of the record.

Finally, we have been developing a new triple quadrupole ICP-MS method for the analysis of S/Ca and trace metal values in carbonate rocks. This technique should greatly increase throughput of samples in the lab. It should also give us more precise [SO4] data compared to our current ion chromatography technique. The minor and trace metal data will help provide context for all of our carbonate samples.