AN 85-MILLION-YEAR RECORD OF CARBON ISOTOPIC VARIATION IN THE PERMO-CARBONIFEROUS: IMPLICATIONS FOR CARBON CYLING AND PALEOCLIMATE

We present a new compilation of the isotopic record for the Permo-Carboniferous, with much new data for the Lower Permian of North America. Based on 1500 specimens, this compilation is anchored with data for thick-shelled, nonluminescent brachiopod shells from North America, the Russian Platform, and Australia. Taking a selective approach in evaluating literature data, the compilation avoids samples from localities that yield ¹⁸O-depleted shells (<-5‰). We infer that these samples were compromised by diagenesis. The carbon isotopic record for the Carboniferous can be divided into three isotopic stages (C1 - C3), each with relatively constant values. The Carboniferous begins with a 2.5‰ increase to 3.6‰ (C1: Mid-Tournaisian to Mid-Visean). Carbon isotopic compositions decrease to 2.5‰ in the Mid-Visean to Serpukhovian (C2), then increase dramatically near the Mid-Carboniferous boundary to a Late Carboniferous (C3) average of 4.0‰ in the U.S mid-continent, and 5.5‰ in northern Canada and Europe. This difference is attributed to upwelling in North America, though the magnitude of the effect is large by modern standards. The d¹³C difference between the epicontinental seas of the U.S. mid-continent and other regions (including northern Canada, Europe, and Australia) continues through the early Permian, with no significant change in average values (4.0‰ and 5.4‰, respectively). The Permo-Carboniferous ¹³C record is punctuated by at least two positive anomalies, in the Tournaisian and in the Late Permian (Kazanian?). These carbon isotopic variations argue for substantial changes in the relative burial rates of organic and inorganic carbon due in part to the spread of land plants. The major carbon-isotope shifts occurring in the Tournaisian, Visean, and Serpukhovian are accompanied by parallel shifts in oxygen isotopic composition, evidence for a linkage between paleoclimate and the carbon cycle through variations in atmospheric CO₂ levels.