143Nd/ 144Nd and Sm/Nd stratigraphy of Upper Ordovician epeiric sea carbonates

Abstract

Stratigraphic changes in the ε Nd of epeiric sea carbonates from central North America track the submergence history of the interior craton during the Late Ordovician. Fluctuations in sea level changed the Nd isotope balance of the epeiric sea by modifying the flux of Nd weathered from the highlands of the Taconic Orogen (ε Nd = −6 to −9) and from the low relief Precambrian basement (ε Nd = −22 to −15) of the Transcontinental Arch and Canadian Shield. Transgressions over the Arch and Shield, which diminished the weathering flux of Nd from the Precambrian basement, are recorded as positive shifts in the ε Nd profiles of carbonates. Negative ε Nd shifts reflect regression and reexposure of the Precambrian basement to erosion. Correlation of Upper Ordovician carbonates by use of the ε Nd profiles demonstrates the potential for Nd isotope stratigraphy. Comparison of stratigraphic variations in carbonate Sm/Nd ratios with sea level curves, conodont paleoecology, and the ε Nd profiles suggest that our observed variations in Sm/Nd ratios are related to changes in depth. Increasing Sm/Nd ratios correlate with increasing depth, whereas decreasing Sm/Nd ratios correlate with decreasing depth. This relationship between Sm/Nd ratios and depth suggests Sm/Nd profiles have potentially wide applications in understanding the paleoceanography of ancient epeiric seas.