[Eoas-seminar] Reminder: Geology Thesis Defense - Nevin Kozik - June 29, 10:00am - 104 CAR

eoas-seminar at lists.fsu.edu eoas-seminar at lists.fsu.edu
Wed Jun 28 11:56:30 EDT 2017


Title: Sea Level and Marine Redox Dynamics Within the Appalachian Basin During the Middle-Late Ordovician: Implication for the Great Ordovician Biodiversification Event (GOBE)
Major Professor: Seth Young


Abstract:

Two Middle-Late Ordovician sections from the Appalachian Basin of eastern North America were analyzed for chemostratigraphic and sequence stratigraphic correlations to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) and shale redox proxies were used in both the carbonate-dominated and shale-dominated localities from the Appalachian Basin to reconstruct the marine redox states during integral periods of biodiversification, while sequence stratigraphic analyses were utilized to reconstruct fluctuations both local and eustatic sea level. Geochemical investigations of the shale-dominated sequence will bring potential insights into the redox environment of the deeper portions of the Appalachian Basin. The carbonate-dominated sequence is one of the most expanded Sandbian Stage (Late Ordovician) sequences known from North America, and brings new high-resolution reconstructions of long-term global carbon and sulfur cycles fluctuations and eustatic sea level changes. Integrating paleoredox reconstructions from relatively shallow and deep water sequences within the same basin can help to understand any contraction or expansion of oxidative or reducing conditions during the Middle-Late Ordovician within this portion of the Appalachian Basin that may be linked to global paleoredox dynamics. We find that although this part of the Appalachian Basin may have been slightly restricted from open marine settings during the latest Darriwilian, trends in stable isotopes and reconstructed sea level (sequence stratigraphy) can be correlated to global trends utilizing previous conodont and graptolite biostratigraphy along with conodont 87Sr/86Sr values. Here we identify several intervals of decoupled δ13Ccarb and δ34SCAS trends: as δ13C increases or remains constant, δ34S trends drop, signaling a discontinuity between carbon and sulfur cycles. However, these decoupled stratigraphic intervals are followed by coupled trends δ13Ccarb and δ34SCAS (parallel positive shifts in both). Causal mechanisms for the observed δ13C and δ34S trends may be linked to a reduction of pyrite burial due to increased ventilation in Middle-Late Ordovician marine environments, while coupled trends likely represent increased organic carbon and pyrite burial associated with expansion of reducing oceanic conditions. These alternations from more oxic to anoxic/euxinic conditions throughout the Middle-Late Ordovician may in part explain the pulsed nature of marine faunal diversity trends previously observed throughout the GOBE. The results presented here demonstrate the unfolding complex evolution of the long-term sulfur and carbon cycles, marine paleoredox conditions, continental weathering, and climate during the Early Paleozoic world.


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