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

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.