Paper No. 1
Presentation Time: 8:00 AM-6:00 PM
STRONTIUM AND NEODYMIUM ISOTOPE STRATIGRAPHY OF THE MIDDLE ORDOVICIAN AND WEATHERING OF THE APPALACHIAN MOUNTAINS
HOWARD, Amanda1, SALTZMAN, Matthew R.
2, SEDLACEK, Alexa R.C.
2, SEDLAK, Christopher
2, FOLAND, Kenneth
2, LINDER, Jeff S.
2, LESLIE, Stephen A.
3 and YOUNG, Seth A.
4, (1)School of Earth Sciences, The Ohio State University, Columbus, OH 43210, (2)School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, (3)Department of Geology and Environmental Sciences, James Madison University, MSC 6903, Harrisonburg, VA 22807, (4)Department of Geological Sciences, Indiana University, Bloomington, IN 47405, howard.616@osu.edu
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, possibly 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, although we are also working 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 uppper Darriwilian to lower Sandbian stages (Middle-Late Ordovician transition). However, the Nd shift is more gradual than expected given the residence time of Nd, which suggests a gradual change in the Nd source. 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.
