Chemostratigraphic and U-Pb geochronologic constraints on carbon cycling across the Silurian-Devonian boundary

The Devonian Period hosts extraordinary changes to Earth's biosphere. Land plants began their rise to prominence, with early vascular vegetation beginning its colonization of near-shore environments in the latest Silurian. Across the Silurian-Devonian (Pridoli-Lochkovian) transition, carbon isotope analyses of bulk marine carbonates (δCcarb13) from Laurentian and Baltic successions reveal a positive δCcarb13 shift. Known as the Klonk Event, values reach +5.8‰, making it one of the largest carbon isotope excursions in the Phanerozoic. Assigning rates and durations to these significant events requires a robust, precise Devonian time scale. Here we present 675 micritic matrix and 357 fossil-specific δCcarb13 analyses from the lower Devonian Helderberg Group of New York and West Virginia that exhibit the very positive δCcarb13 values observed in other Silurian-Devonian basins. This chemostratigraphic dataset is coupled with 66 ID-TIMS U-Pb dates on single zircons from six ash falls intercalated within Helderberg sediments, including dates on the stratigraphically lowest Devonian ashes yet developed. In this work, we (a) demonstrate that matrix and fossil-specific δCcarb13 values track one another closely in the Helderberg Group, (b) estimate the Silurian-Devonian boundary age in New York to be 421.3 ± 1.2 Ma (2 σ; including decay constant uncertainties), and (c) calculate the time required to evolve from baseline to peak δCcarb13 values at the onset of the Klonk event to be 1.00 ± 0.25 Myr. Under these constraints, a steady-state perturbation to the global carbon cycle can explain the observed excursion with modern fluxes, as long as DIC concentration in the Devonian ocean remained below ~2× the modern value. Therefore, potential drivers, such as enhanced burial of organic carbon, need not rely on anomalously high total fluxes of carbon to explain the Klonk Event.