61st Annual Report on Research 2016

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 been to his field site in the Guadelupe Mountains several times, accompanied by PI Adkins once. He has characterized many of his samples for elemental abundance by x-ray adsorption spectroscopy at SSRL, and done thin section analysis on this same sample set. Ted has collected over one hundred new δ³⁴S data points on these samples and is starting the synthesis of all his geochemical and petrographic data into a coherent story of primary carbonate associated sulfate (CAS) deposition and it subsequent diagenetic history.

We now have a complete stratigraphic section of several different sea level cycles in McKitrick Canyon, Gudalupe National Park. This region has been well studied for its diagenetic overprints on geochemical tracers, especially δ18O and δ13C, but never for sulfur. Ted has mapped his sections and done the detailed work of siting them within the previous literature. There are strong trends in the geochemical cross plots that we can related to early diagenesis when the rocks were still unconsolidated sediments and to later post-depositional alteration. Being a nearly intact paleo reef lets us conduct several experiments as grains formed in one environment have fallen into deeper waters with lower oxygen and more intense sulfate reduction in the sediments. With several cycles of sea level change we can assess the reproducibility of process that alter the geochemical signatures.

Using beam lines 2-3 and 14-2 at SSRL, Ted has created elemental maps of sulfer and associated elements at the micron scale. Given our new 1000x small detection limit for δ³⁴S we have been able to relate this texture specific information to unique isotope signatures. Multiple generations of alteration can be distinquished and quantified.

Given the larger goals of the project to understand the general rules for sulfur preservation and alteration in the rock record, Ted has continued to build a 1-D diffusion-reation model of the modern sedimentary column. He has samples from long cores in the North Atlantic that we are analyzing for δ³⁴S of SO4 and H2S this summer. The combination of the new model and the new data should help to better understand the Permian data from Texas. With all of these projects moving along, Ted is going to defend somewhere in the next year.