61st Annual Report on Research 2016

ACS PRF 54354-ND2 Reporting for October 2016 (late) Introduction: Recent sulfur isotope analyses (cf. Thompson and Kah, 2013) suggest dramatic changes in the redox structure of the ocean in the Middle Ordovician, wherein progressive ventilation of stratified, euxinic deep-oceans enhanced nutrient availability, potentially driving biological diversification. Intriguingly, evidence for progressive ventilation of a deep-ocean water body is coincident with a dramatic, globally recognized change in the 87Sr/86Sr composition of the oceans, from radiogenic values near 0.7092 to values near 0.7079. The rapidity of this Sr-isotope excursion, which occurs on time-scales (3-5 m.y.) comparable to the residence time of Sr, is challenging to intepret via conventional flux models. Within this context, our project is based on the hypothesis that extraordinarily rapid changes in marine Sr-isotope composition may be related to paleoceanographic changes in ocean circulation arising from enhanced thermo-haline circulation and disruption of stratified ocean bodies. ACS-PRF funding was received to produce a high-resolution stratigraphic and geochemical investigation across the Middle to Late Ordovician (Dariwillian-Sandbian) boundary in strata of the Argentinian Precordillera to evaluate onshore-offshore pattern of geochemical change, its to changing ocean circulation conditions, and the potential effects on the Sr-isotope composition of marine systems. Completed Tasks: The current ACS-PRF grant is in a no-cost extension year (to end August 31, 2017). Field work was completed prior to the original end-date of the granting period (August 31, 2016). During the original granting period, we completed all preliminary petrographic analyses, and the necessary carbon isotope analyses to correlate our onshore and offshore sections. During this extension year, we have completed all sulfur isotope analyses, as well as iron speciation geochemistry necessary to constrain redox conditions in both onshore and offshore sections. Results and Ongoing Analyses: Results of our initial geochemical analyses suggest that we have captured a marine redox boundary in the sections that we sampled. This redox boundary appears to be relatively shallow, as it was captured in the carbonate-dominated successions. We are now integrating our geochemical results from a coeval, deeper-water shale succession. Correlation of these successions has been aided by ongoing collaborative work with Argentinian colleagues, who have been working on the detailed conodont biostratigraphy of these measured sections. They will then provide conodonts (expected in early 2017) for Sr-isotope analysis, which is the last stage of the proposed project. Publications (Bold = PI; * = Graduate Student; ** = Undergraduate Student): (1) Serra, F., Feltes, N.A., *Henderson, M.A., Albanesi, G.L., 2017. Darriwilian (Middle Ordovician) conodont biofacies from the Central Precordillera of Argentina. Marine Micropaleontology, v. 130, p. 15-28. (2) Kah, L.C., Thompson, C.K., *Henderson, M.A., Zhan, R., 2016. The behavior of marine sulfur in the Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 458, p. 133-153. (3) **Bales, R.D., *Henderson, M.A., Kah, L.C., 2016. Constraining diagenetic alteration of middle Ordovician (Darriwilian) limestone from the Argentine Precordillera: Implicaitons for primary seawater signatures. Geological Society of America Abstracts with Programs, v. 48, no. 3, doi: 10.1130/abs/2016SE-273244. (4) *Henderson, M.A., Kah, L.C., 2016. Sulfur cycling in the Sarriwilian (Middle Ordovician) of Argentina: Implications for ocean circulation and the evolution of the Ordovician Ocean. Geological Society of America, v. 48, no. 7, doi: 10.1130/abs/2016AM-284859. (5) *Henderson, M.A., Kah, L.C., 2015. Paired isotope records of carbonate and organic matter, middle Ordovician (Darriwilian) of Argentina: implications for global climate change. Geological Society of America, v. 47, p. 641.