Author: Amanda Howard

Publisher:

Published: 2011

Total Pages: 24

ISBN-13:

Abstract: Regional tectonic events may alter the lithology and age of material weathered from the continents, leaving an imprint on the 143Nd/144Nd and 87Sr/86Sr compositions of seawater. Because of the long residence time of Sr compared to Nd, seawater 87Sr/86Sr changes should be globally synchronous and slow whereas 143Nd/144Nd changes may be rapid and regional. In the Ordovician, major stratigraphic shifts in both Sr (~ 0.001) and Nd (~ 10 epsilon units) have been documented. In the case of Sr, the global shift has been broadly attributed to changes in plate tectonics. For Nd, the shift has specifically been attributed to the uplift of the Appalachian Mountains (Taconic orogeny). However, no studies have been conducted that specifically address whether these changes in Sr and Nd are linked. Because the oceanic inventory of Nd is not balanced by seafloor hydrothermal input, the shift could be due to changes in continental weathering or ocean circulation patterns. A large seawater Nd shift that is due to enhanced continental weathering of young crustal rocks in the uplifted Appalachian Mountains may also be predicted to increase the flux of nonradiogenic Sr into the oceans. We have begun to test this hypothesis by producing the first integrated stratigraphic records of changes in seawater 87Sr/86Sr and 143Nd/144Nd. Our initial high-resolution curves were generated using bulk carbonate dissolved in acid and analyzed on a thermal ionization mass spectrometer, although we have also begun analyses with conodont apatite. These results from outcrops at Rocky Gap, Virginia and Roaring Spring and Union Furnace, Pennsylvania indicate a broad correlation of the initiation of Sr and Nd shifts in the upper Darriwilian to lower Sandbian stages (Middle-Late Ordovician transition). Other sections further west (e.g., Nevada) will be analyzed to compare the timing of Sr and Nd shifts in different epeiric sea water masses.