POSITIVE CARBON ISOTOPE SHIFT IN THE MIDDLE TO LATE ORDOVICIAN: NO LINK WITH SEA LEVEL?

Positive δ¹³C trends in ancient carbonate rocks are commonly interpreted to reflect increases in the burial of organic carbon in deep ocean sediments. An alternative model to explain positive δ¹³C trends is related to an increase in the water depth during the formation of marine carbonate rocks. Rock samples from four age-correlated, stratigraphic sections from Tumbling Run, Virginia; Arbuckle Mountains, Oklahoma; Clear Springs, Maryland; and Antelope Range, Nevada, of Middle to Upper Ordovician age were measured and sampled for δ¹³C and δ¹⁸O. The δ¹³C curves reveal 3-4 per mil positive shifts in the upper Darriwilian to lower Sandbian stages (C. sweeti to gerdae conodont zones). Lithological analysis indicates transgressive events (shallow to deeper marine) associated with the positive δ¹³C shifts in the Appalachian foreland basin but indicates a transgressive-regressive sequence in the upper portions of the Arbuckle Mountains and the Antelope Range. Plots of δ¹³C versus δ¹⁸O reveal minimal covariance, potentially invalidating diagenetic explanations for the trend. Because δ¹³C does not show an obvious correlation with water depth or diagenesis, these results suggest that the positive δ¹³C positive trends of each section may be closely related to changes in organic carbon burial.