[Eoas-seminar] Geology dissertation defense - Chelsie Bowman - Nov 4, 10am - Zoom

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Wed Oct 21 11:03:08 EDT 2020

Geology dissertation defense - Chelsie Bowman - Nov 4, 10am

Title: The coupled evolution of the biosphere and global marine redox conditions in the late Silurian
Major Professor: Seth Young

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Meeting ID: 922 8632 8718

The Silurian was a dynamic time of climatic transition punctuated by multiple biotic crises and global carbon cycle perturbations. The most severe of these biotic crises was the late Silurian (Ludfordian) Lau/Kozlowskii extinction event (LKE; 425 Ma), with a loss of ~23% of marine genera and a resurgence of abiotic and microbially-mediated carbonate facies. The LKE has been recognized as asynchronous for more than a decade, with benthic and nektonic fauna experiencing earlier extinctions than planktic fauna. The LKE preceded and partially coincided with an associated positive carbon isotope excursion, the Lau CIE, which is the highest-magnitude CIE in the Phanerozoic with average peak excursion values between +5 and +8‰. It has been suggested, but not directly tested, that both the Lau CIE and the LKE were the result of an expansion of reducing conditions in the Ludfordian global oceans. The mechanisms behind the temporal offset between the extinction and the CIE and the asynchronous nature of the extinction have also not been previously constrained.
New records of the extinction and local and global marine paleo-redox change are presented herein from Ludfordian carbonate- and siliciclastic-dominated successions of multiple paleocontinents. For the first time biotic records of well-studied taxonomic groups affected by the LKE are compiled, highlighting the stepwise nature of the extinction. Global paleo-redox dynamics are first examined using a novel approach combining analyses of the stable isotopes of carbon, sulfur, and thallium. A positive excursion in thallium isotopes in the deep-water shales of the Baltic Basin (modern Scandinavia) records the earliest onset of global marine deoxygenation in the Ludfordian, which was coincident with the earliest extinction stages of the LKE and prior to the enhancement of organic carbon burial that fueled the Lau CIE. A large positive excursion in sulfur isotopes in the shallow water carbonates of the Baltic Basin was recorded parallel to the CIE as pyrite burial increased globally. This suggests an expansion of euxinia (anoxic and sulfidic water column) in the global oceans following the initial deoxygenation and concurrent with the later extinction stages of the LKE. Support for the expansion of reducing conditions in the late Silurian global oceans comes from the study of carbonate successions along the southern and western margins of the paleocontinent Laurentia (modern North America) where paired positive excursions in carbon and sulfur isotopes lend further evidence for the global enhancement of organic carbon and pyrite burial.
Variability in local and regional paleo-redox dynamics in the Ludfordian have been explored along the southern Laurentian margin and in the Baltic and Prague basins. Low I/Ca ratios from the carbonates of southern Laurentia and the shallow shelf of the Baltic Basin indicate proximity to local/regional oxygen minimum zones (OMZ) concurrent with the global expansion of reducing conditions. High pyrite content in the carbonate strata of southern Laurentia and the Prague Basin (peri-Gondwanan terrane; modern Czech Republic) suggest that sulfidic sediment porewaters were common. Comparison of iron geochemistry and trace metal enrichments in the deep shelf of the Baltic Basin to that of modern OMZs imply that the basin may have been dominated by a nitrogenous OMZ throughout most of the Ludfordian. There is also a possible overprint of a global trace metal drawdown in the stratigraphic trends of trace metal enrichments from this locality that would have been coeval with the enhancement of pyrite burial and the expansion of euxinic conditions in the global oceans.
Dynamics of the LKE event have been further explored through microfacies analyses and accounts of previously unreported microbially-mediated facies in Ludfordian carbonate successions. The LKE event is recorded in southern Laurentia as notable biotic reorganizations during the interval of the mid-Ludfordian when reducing conditions would have been the most prevalent in the global ocean. Flat-pebble conglomerate facies and possible microbial mats are reported from the carbonate strata of western Laurentia and the Prague Basin, ooids and micro-oncoids from southern Laurentia. The variety and pervasiveness of microbially-mediated carbonate facies associated with the LKE hint at the severity of the extinction and the restructuring of faunal communities that took place within carbonate platforms during the Ludfordian. Altogether, the geochemical, biotic, and lithologic data presented here represent the first proxy evidence linking the LKE event to the Lau CIE through the progressive expansion of anoxia, and later euxinia, across portions of the late Silurian oceans. This multi-proxy, multi-lithology approach provides a previously unmatched, holistic view of the dramatic changes in the late Silurian marine biosphere and global marine redox conditions.

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