A NEW RECORD OF THE KLONK POSITIVE CARBON ISOTOPE EXCURSION ACROSS THE SILURIAN-DEVONIAN BOUNDARY, WEST VIRGINIA, USA

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 δ¹³C_org and δ³⁴S_py from insoluble residues of the carbonates in order to test whether these variables respond to the same processes and environments that drove changes in δ¹³C_carb 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.