PALEOOCEANOGRAGHIC ASPECTS OF THE EARLY CHATFIELDIAN (UPPER MIDDLE ORDOVICIAN) POSITIVE d¹³C EXCURSION (GICE)

The Guttenberg d¹³C excursion (GICE) has now been identified in sections worldwide including the Appalachian Basin (central Pennsylvania, New York, southwestern Virginia), the U.S. Midcontinent (Iowa, Wisconsin, Missouri), the Cincinnati Arch and Nashville Dome regions (north central Kentucky, Tennessee), the Arkoma Basin (Oklahoma), the Great Basin (Nevada), and Baltoscandia (Sweden, Estonia). There is a 2‰ offset in the peak GICE values from the Upper Mississippi Valley to the Appalachian Foreland Basin that reflects different temperature-salinity- defined water masses, or aquafacies, of Holmden and others (1998). The GICE in the Midcontinent aquafacies has beginning values ~–2‰ and peak values at +1.5‰, in the Southern and Taconic aquafacies the beginning values are ~0‰ and peak values are ~³+3‰, and Baltioscandia values beginning values near 0‰ and peak values near ~+2‰, These isotopic differences reflect local environmental factors: riverine discharge ( U.S. Midcontinent), upwelling (Oklahoma), and biological pumping of ¹²C-enriched organic carbon (Virginia, Tennessee, Kentucky, Pensylvania).The causes of the GICE (e.g. enhanced organic matter burial in the Appalachian Basin) are still being assessed, but are related to the environmental changes also occurring during the late Middle Ordovician. These changes were previously interpreted as a response to the combined effects of the Taconic Orogeny and a eustatic sea-level rise (Holland and Patzkowsky, 1996), global cooling associated with the onset of glaciation (Pope and Read, 1997), and upwelling (Pope and Steffan, 2003).

The positive (d¹³C) isotope excursion begins in the uppermost part of the Midcontinent P. undatus Conodont Zone, reaches its peak values in the P.tenuis Conodont Zone, and returns to pre-excursion values at and/or just before the P.tenuis/B. confluens Conodont Zonal boundary. The GICE also lies within the North American C. americanus and/or O. ruedemanni Graptolite Zones. The GICE is geographically one of the most widely documented carbon isotope (d¹³C) excursions in the Paleozoic, and apart from the end Ordovician positive excursion it is the most significant excursion documented in the Ordovician.