Late-Middle Ordovician Environmental Change: Molecular and Isotopic Evidence from North America

Abstract

We recently reported inorganic and organic carbonisotope records for Caradocian units that crop out in central Pennsylvania (Patzkowsky et al., 1997). These data reveal a positive 3 %o excursion in the 13C content of micrites (~13CcARB) sampled across the P. undatus-P, tenuis boundary in Pennsylvania, and show the same magnitude of change observed in micrites by Ludvigson et al. (1996) in coeval core samples from Iowa. These findings confirm that the isotopic excursion extended across the North America, in both the Appalachian Basin and the midcontinental region. Both regions also record a positive excursion in the carbon-isotope composition of total organic matter (*13Cxoc). Data from the Iowa section show a +6 %0 excursion during this time (Hatch et al., 1987; Pancost, 1998), whereas ~13Cxo c data from our Pennsylvania samples show only +3.5 %0 change (Patzkowsky et al., 1997). We suggested these results indicate major changes in the carbon cycle accompanied by the lithologic changes noted above. In particular, the similarity in the timing and magnitude of the inorganic isotope excursions observed at both locations indicate an increase in organic carbon burial, possibly due to the widespread deposition of black shales in the foreland basin of the Taconic orogeny and elsewhere. The increased burial of organic matter during this time could have been accompanied by a marked decrease in the concentration of carbon dioxide in the ancient atmosphere. If so, it would have led to a decrease in the isotopic fractionation by marine algae, recorded by a decrease in the isotopic difference between inorganic and total organic carbon (A)13C = ~13CcARB-~13CToc). In both sections, values for A13C decrease beginning at the base of the P. tenuis zone (Patzkowsky et al., 1997), but the extent of change differs between the two locations. Specficially, the Iowa section records a slightly larger change (c. 4%o than in Pennsylvania (3%o). Thus A13C data suggest isotopic fractionation decreased at both sections, consistent with a drop in global atmospheric CO2 levels. However, the regional variation in the magnitude of change, and the coincident lithologic changes suggest the shift in A13C records that fractionation was driven, at least in part, by increased availability of nutrients (Laws et aL, 1995) and/or possible changes in the dominant phytoplankton taxa due to the pronounced change in oceanographic conditions. In this work, we examined biomarker distributions in the Iowa section, as well as an addition set of core samples from Ontario. Our specific goals were to determine if the inorganic and organic excursions are also recorded in the Canadian site to evaluate better the geographic extent of these signals. We hoped to use molecular evidence to evaluate if changes in either productivity or in the dominant phytoplankters can account for the regional differences in A13C. Further, we expected that biomarker distributions could reveal changes in mechanisms underlying the preservation and/or production of organic matter and possibly enhance our understanding of North American Caradocian palaeoceanography.