Carbon cycling across the Silurian-Devonian Boundary
The Devonian Period hosts extraordinary changes to Earth's biosphere, marking the first appearance of ammonoids, amphibians, insects and the radiation of fish. 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 δ13C from Laurentian and Baltic successions reveal a positive δ13C shift. Known as the Klonk Event, values reach +6‰, 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. Working with Adam Maloof, Blair Schoene and Princeton undergraduate ('14) Sarah Bluher, we have developed a comprehensive chemostratigraphic dataset on carbonates of the lower Devonian Helderberg Group of New York and West Virginia (figure right, displaying data from the famous outcrop of the Taconic Unconformity in Catskill, NY; panorama courtesy of Adam Maloof). Carbon isotopic data, both from ground-mass micrite and fossil-specific carbonate, exhibit the very positive δ13C 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. With these data, we (a) demonstrate that matrix and fossil-specific δ13C 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 (including decay constant uncertainties; the 2012 Geological Time Scale estimates the boundary to be 419.2±3.2 Ma), and (c) calculate the duration of the Silurian-Devonian Boundary δ13C excursion 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 ~2x the modern value.
U-Pb geochronology via CA-ID-TIMS
Helderberg Group ash falls were dated via U-Pb geochronology on zircon using isotope-dilution thermal ionization mass spectrometry (ID-TIMS). Analyses were performed at the Princeton Radiogenic Isotopes Lab, where low Pb-blank levels (< 1 pg per analysis) allows for the dating of single crystals at high precision (0.05-0.075% uncertainty (2σ) on the measured age). Zircons were separated from field samples collected using standard magnetic and heavy liquid separation techniques, and the grains crystals were annealed in quartz crucibles at 900°C for 60 hours. Annealed grains were mounted in epoxy resin and imaged by cathodoluminescence (CL) to help screen for potential inherited populations, with crystals selected for dating plucked from these mounts. Selected grains were then prepared for dating using a modified version of the chemical abrasion technique of Mattinson (2005). After chemical abrasion and washing, grains were spiked with EARTHTIME 233U-235U-205Pb(-202Pb) tracer solution (Condon et al 2015; McLean et al 2015) and dissolved in 60-70 μl of HF + trace HNO3 for 48 hours at 210°C (figure above, left panel). Dissolved samples were dried to completion, redissolved in 6N HCl and left overnight in Parr bombs at 185°C. Samples were dried down to completion again, redissolved in 3N HCl, and put through modified, single-column (50 μl) HCl-based anion exchange chemistry (Krogh 1973) to separate the U and Pb ions from all other cations (left panel). U and Pb were collected in single, 7 ml Savillex Teflon beakers, dried down with a drop of Pb-blank checked 0.02 N H3PO4, and analyzed on a single, outgassed Re filament in a Si-gel emitter (middle panel; Gerstenberger and Hasse, 1997). Measurements were performed on an IsotopX Phoenix62 thermal ionization mass spectrometer at Princeton University (right panel). Column chemistry cartoon courtesy of Blair Schoene. Mass spectrometer photographs courtesy of Brenhin Keller.