60th Annual Report on Research 2015

Introduction: Analysis of stratigraphic change in the stable isotopic composition of marine systems has proven to be powerful tool to enhance our understanding of the both biogeochemical cycles and their relationship to the evolution of life. Perhaps most profound change in Earth surface environments in the Ordovician was the expansion of marine life during the Great Ordovician Biodiversification Event (GOBE), yet there currently remains no single explanation for this global event. Potential mechanisms under investigation include increased biotic interaction within marine ecosystems, effects of global climate change, changes in nutrient flux, and even extraterrestrially driven ecosystem disturbance. Recently, chemostratigraphic analysis of stable carbon and sulfur isotopes has been used to explore hypotheses regarding ocean oxygenation and effects on nutrient availability. Specifically, increased carbon isotope volatility in the late Middle Ordovician (Dariwillian-Sandbian has been interpreted to reflect increasingly vigorous oceanic circulation and deep-ocean ventilation, which could have driven expansion of phytoplanton productivity. The marine S-isotope record supports dramatic changes in the redox structure of the ocean (Thompson and Kah, 2012; Kah et al., 2015; funded, in part, by ACS-PRF).

Currently, we are focused on exploring additional geochemical tracers, such as elemental ratios, speciation and concentration of redox sensitive metals, and behavior of radiogenic isotopes to evaluate the range of physiochemical change in the ocean and its potential linkage, via tectonic- or climate-driven paleoceanographic events, on the extraordinary biogeochemical events that occurred during this time.

Completed Tasks: At the present time, we have completed a large number of our original goals (see below). A combination of factors (difficulty in scheduling time on the necessary laboratory equipment, the birth of a new baby into the lab, and unavoidable delays on the part of our Argentinian colleagues), however, has placed us behind schedule on the final radiogenic analyses and we expect be requesting a no-cost extension at the expiration of this grant in Fall 2016. A no-cost extension will permit completion of the remaining analytical and modeling activities.

(1) We have completed all elements of fieldwork in Argentina. This has included detailed measurement and sampling of late Middle Ordovician sedimentary strata (sampled at 1-3 meter resolution) across localities that represent a continuum from onshore and offshore depositional environments.

(2) We have completed petrographic analysis, cathodoluminescence analysis, microsampling, and analysis of C-, O-, and S-isotopes and major and trace elements for all samples. These steps of analysis have beencritical for determining the diagenetic history of the carbonate rocks that might compromise radiogenic isotope values. Additionally, this data was used to establish a set of analytical priorities. During this analysis we have been able to demonstrate the effect of postdepositional fluids on some of the tectonically compromised sections, and we have located a critical unconformity that had not previously been recognized. S-isotope analyses (of both carbonate associated sulfate and pyrite) have allowed us to identify the interval of interest and trace this interval accurately from onshore to offshore localities. In two of our sections, the interval of interest has been removed by unconformities, so these localities have been downgraded in importance.

(3) We have nearly completed analysis of redox-sensitive metals in the shales, to provide an additional line of evidence regarding potential ocean oxygenation.

(4) We have completed the mathematical formulation for our 2-box model to explore sulfur cycling in an ocean with two reactive reservoirs. We are continuing analyses and sensitivitiy testing, and will utilize results of this analysis in later modeling of Sr- and Nd-isotopic change.

Results: This project has produced high-resolution isotopic records of both oxidized and reduced forms of carbon (carbonate and organic matter) and sulfur (carbonate-associated sulfate and pyrite) from late Middle Ordovician rocks of the Argentine Precordillera. C-isotope compositions are consistent with the global C-isotope curve for this time period (Saltzman 2005) and show minor isotopic enrichment associated with the MDICE excursion. S-isotope analyses record an interval of potential ocean ventilation across shallow to deeper-water environments. Two measured sections, however, lack characteristic chemistry and show both sedimentological and geochemical indications of previously unrecognized unconformities. Our ability to track this interval in the sedimentary geochemistry is critical to correlate sections across different environments and to determine, in collaboration with detailed biostratigraphic analysis by our Argentinian colleagues, whether or not the observed biogeochemical event is time-syncronous.

Continuing Work: The remainder of this project will consist of completing geochemical analyses (focused on the isotopic analysis of strontium and neodymium in conodonts collected by our Argentinian colleagues), and the analytica modeling of geochemical results. We are currently awaiting final extraction of conodonts from our Argentinian colleagues, and we expect to be able to complete the remaining analyses before February of 2017.

Publications: We currently have one manuscript published (Kah et al., 2016) that explores the geochemical evidence for ocean ventilation in the late Middle Ordovician, a second manuscript in preparation for submission (submission by July 2016) that notes the occurrence of previously unrecognized unconformities in the section, and expect at least 3 more peer-reviewed publications to arise from this project, to be submitted in October 2016, December 2016, and May 2017. Additionally, two undergraduate students have completed directed research on aspects of this project (mentored by both myself and PhD student Miles Henderson).

Post-script (added in April 2016): One of the undergraduates working on this project, Robert Bales, presented results of his research at the SE GSA conference in March 2016, and was also awarded one of two departmental awards for outstanding undergraduate research in April 2016. Miles Henderson and I are extraordinarily proud of Robert's work. Our department has a focus on undergraduate research and typically 8-12 students per year present their research at local, regional, and national scientific forums.