GSA Connects 2021 in Portland, Oregon

Paper No. 61-6
Presentation Time: 2:30 PM-6:30 PM


BOYLE, Alexandra E.1, GILLEAUDEAU, Geoffrey J.1, CHERRY, Lucas1, EVANS, Michael N.2 and KAUFMAN, Alan3, (1)Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Drive, Fairfax, VA 22030, (2)Department of Geology, University of Maryland, College Park, MD 20742, (3)Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742

Given that the dissolved inorganic carbon reservoir of the oceans is well-mixed and its isotopic composition is faithfully recorded in well-preserved marine carbonates, stratigraphic variations in δ13C compositions of discrete phases often provide critical information about biogeochemical cycling and stratigraphic correlation through Earth history. For example, the Silurian and Devonian periods are notable for repeated oscillations in carbon isotope abundances that are believed to reflect changes in organic carbon burial rates associated with the emergence of vascular land plants and their effect on continental weathering and nutrient delivery to seawater. The temporal δ13C record of the Silurian and Early Devonian periods shows a series of major positive excursions before temporary stabilization beginning in the Middle Devonian. One such positive excursion, the Klonk Event, characterizes the Silurian-Devonian boundary interval (419.2 Ma), featuring a rise in δ13C values to a maximum of +6‰ in many localities, making it one of the largest positive carbon isotope excursions of the Phanerozoic Eon. The Klonk Event has been identified in Europe, the western US, and at several localities in the northern Appalachian Basin, but has only been identified at one locality in the central Appalachian Basin— Smoke Hole, West Virginia. Here we present a new high-resolution δ13C dataset from a fresh exposure of the Helderberg Group along Corridor H near Moorefield, West Virginia. We identify 12 carbonate facies ranging from shaly carbonate mudstones to fossiliferous grainstones, and micro-drilled carbonate from least-altered phases clearly captures a two-step positive δ13C excursion that begins at −1.5‰ and peaks at +5.2‰. Time-series δ18O values also match well with global trends across the Silurian-Devonian boundary, supporting a progressive cooling trend through the Klonk Event. We also investigated time-series trends in δ13Corg and δ34Spy from insoluble residues of the carbonates in order to test whether these variables respond to the same processes and environments that drove changes in δ13Ccarb across the Klonk Event. Overall, the new Moorefield section identified along Corridor H (West Virginia) has great potential for unraveling global biogeochemistry across the Silurian-Devonian transition.