T198 Epi-continental Seas in the Geological Record: The Limitations of the Uniformitarian Paradigm I

Wednesday, 25 October 2006
8:00 AM - 12:00 PM

A LARGE, RAPID 87/86SR DROP ACROSS THE MIDDLE TO LATE ORDOVICIAN BOUNDARY: IMPLICATIONS FOR PALEOCEANOGRAPHY & WEATHERING OF THE TACONIC HIGHLANDS

Paper number: T198-11
Program: Technical Sessions
Time: 10:30 AM – 10:45 AM

Authors

Abstract

Here we present new Middle and Upper Ordovician 87Sr/86Sr values from central Nevada, integrated with δ13C, sequence and event stratigraphy, which documents the large Ordovician seawater 87Sr/86Sr drop in a single, well biostratigraphically studied sequence for the first time. The duration of this 87Sr/86Sr drop is now confined to ~7-8 m.y., beginning in the Middle Ordovician (late Darriwillian Stage) and ending in the Upper Ordovician (Turinian/ Chatfieldian Stages). This Ordovician 87Sr/86Sr drop is the largest (~0.0009) and most rapid in the Phanerozoic (Viezer et al., 1999), comparable in magnitude to the well-known late Cenozoic slow (~40 m.y.) 87Sr/86Sr rise (Burke et al., 1982). δ13C values in Whiterockian through lower Mohawkian carbonates are steady, however a significant positive δ13C excursion occurs in the upper Copenhagen Formation (Chatfieldian Stage). This positive shift has been documented globally, and was closely followed by a relative sea level fall represented by the prominent Eureka Quartzite. In the Cincinnatian δ13C values remain steady until the large Hirnantian δ13C excursion in the upper Hanson Creek Formation (Kump et al., 1999).

Our integrated data show that seawater 87Sr/86Sr values drop prior to and during a major sea-level fall, Sauk-Tippecanoe sequence boundary, and overlaps with the beginning stages of the Taconic Orogeny. Weathering fluxes from uplifted young basaltic rocks overwhelmed the radiogenic Sr inputs from low-lying cratonal rocks causing a lowering seawater 87Sr/86Sr values. In addition to lowering the oceanic 87Sr/86Sr values, enhanced weathering of these juvenile silicate rocks likely began to lower atmospheric CO2 levels. The Chatfieldian δ13C excursion that closely follows stabilized 87Sr/86Sr values signals a period of enhanced organic carbon burial, which likely enhanced pCO2 drawdown to near the threshold for ice buildup in an Ordovician greenhouse. 87Sr/86Sr likely stabilized due to the evolution from basaltic to more felsic Taconic Arc magmas (i.e. peak K-bentonite ash falls in the Mid-Mohawkian), which have more radiogenic 87Sr/86Sr values. 87Sr/86Sr remained buffered at relatively non-radiogenic values through the rest of the Ordovician possibly due to a combination of carbonate weathering and continued high input from young volcanics.