62nd Annual Report on Research 2017

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:

Field work (for collection of samples), petrographic analysis (to determine the best samples for geochemical analysis), and preliminary isotopic and elemental analyses (to determine the paleoenvironmental context of sections) were August 31, 2016. During the 1-year no-cost extension of the project (to August 31, 2017), we completed sulfur isotope analyses (to explore hypotheses of ventilation), iron speciation analyses (to examine the redox conditions of environments), and Sr- and Nd-isotope analyses (to model potential changes in ocean circulation).

Ongoing tasks:

We are now in the stage of completing analysis of all data, and putting together manuscripts for publication. The primary researcher on this project, Mr. Miles Henderson, is expecting to complete and defend his PhD dissertation within the next 6 months. Mr. Henderson will be the primary author on all manuscripts from this research.

Results:

• Carbon isotope analysis. Our carbon isotope analyses, in conjunction with biostratigraphic analysis performed by our Argentinian colleagues, confirm that our sampling captured the time interval of interest. This interval is marked by a globally-recognized, positive excursion in carbon isotopes (the MDICE excursion). When we compare our onshore (Las Chacritas) and offshore (Las Aguaditas) sections, we see parallel carbon isotope signals until the start of the MDICE excursion. At the start of the excursion, the Las Chacritas section records an expected transition to more positive carbon isotope values. By contrast, the Las Aguaditas section is marked by an abrupt jump to more negative carbon isotope values. We have interpreted this disparity to reflect deposition of the Las Aguaditas section beneath a local chemocline.
• Sulfur isotope analysis. High-resolution sulfur isotope analyses through all sections that contain the MDICE interval are consistent with earlier data (cf. Thompson and Kah, 2012; Kah et al., 2016) that suggest deposition under sulfate-limited conditions, and the onset of regional oxidation of a marine H₂S reservoir. Unlike earlier data, which showed culmination of this oxidation event with an interval of “superheavy” pyrite, data from the current sections show initiation of this trend, prior to truncation by a regional
• Iron speciation analysis. Despite evidence of a relatively shallow chemocline (i.e. captured within carbonate-dominated shelf strata), analysis of iron speciation within shale of the more basinal Gualcamayo section is inconclusive. Fe-speciation analysis yields Fe_Py/Fe_HRvalues typically less than 0.7 and FeHR/FeT values ranging from 0 to 0.9. Such values are consistent with a combination of oxic and ferruginous depositional conditions, or potentially oxidation of reduced Fe during exposure in the high desert environments of the Precordillera. Preliminary XRF analysis suggests elevated Mn concentration in some samples, which is consistent with recent reports of seasonally euxinic conditions in anoxic basins. We are currently investigating additional compositional features of these shales (Mo, V, and U) to diagnose the nature of the geochemical signatures of the black shales in the Gualcamayo Formation.
• Strontium and Neodymium isotope analysis. Dramatic changes in carbon and sulfur isotopic compositions (above) have been attributed to potential changes in marine circulation, associated with deep-water ventilation and shifting nutrient fluxes. Because these biogeochemical changes appear coincident with a globally recognized shift in the ⁸⁷Sr/⁸⁶Sr value of seawater. We used a combination of Sr-Nd isotope analyses to evaluate the potential for ocean circulation to be driving marine isotope compositions.

Our ⁸⁷Sr/⁸⁶Sr data from the Las Chacritas and Las Aguaditas formations fit within the global framework for Sr-isotope change with values near 0.709050 falling rapidly to 0.708637 prior to the unconformity that caps the sampled section. Neodymium, which has a much shorter residence time in marine systems is then used to distinguish the potential for discrete oceanic water masses. Our initial εNd values showed an increase from approximately -8 to -4 through the shallow-water sections, consistent with marine strata recording the upwelling of deep marine water into shallow water environments during this time. However, εNd values from the deeper water do not vary through the interval, suggesting that changes in the shallow water environments have a different origin.

New Collaborations resulting from research:

Presentation of our results at numerous conferences has resulted in the initiation of an additional collaboration which has potential to further broaden our understanding of the Middle Ordovician oceans. We, along with our Argentinian colleagues, have started a collaboration with Dr. Ken Macleod (University of Missouri) to explore the oxygen isotope composition of conodont apatite to test whether the MDICE interval may be related to the transition between Ordovician greenhouse and icehouse climates.