Did Taconic orogenesis drive the Ordovician greenhouse-icehouse transition?

Previous geochemical, tectonic, paleomagnetic, and stratigraphic studies have suggested that island arc accretion during the Ordovician Taconic orogeny enhanced regional mafic weathering rates and ultimately led to the Ordovician greenhouse-icehouse transition. However, the precise timing of island arc exhumation and weathering, and therefore its relation to global cooling, is uncertain. Also, it remains unclear if a regional event like the Taconic orogeny alone could have affected global climate. This work pairs seawater strontium (⁸⁷Sr/⁸⁶Sr) and neodymium (ɛ_Nd(t)) signals preserved in Middle-Upper Ordovician (~470-450 Ma) marine carbonate strata and conodont bioapatite from (1) the Antelope Range in central Nevada and (2) central and southern Sweden to test the hypothesis that the chemical weathering of mafic Taconic island arcs was a primary driver of Ordovician cooling. Data from Nevada, a location distal to the Taconic orogen, will be compared to more proximal published data from the Appalachian region (Swanson-Hysell and Macdonald, 2017, Geology). A change in seawater ɛ_Nd(t) associated with the onset of weathering of Taconic arcs should appear later in distally deposited sediments due to ɛ_Nd(t) being a local signal.

­­Bulk rock from Nevada decreases from ɛ_Nd(t) = -18.5 at ~470 Ma to ɛ_Nd(t) = -20 at ~465 Ma and then increases to ɛ_Nd(t) = -13 at ~453 Ma. Nevada bulk rock and conodont ⁸⁷Sr/⁸⁶Sr = ~0.7087 at ~464 Ma and decreases steadily to ~0.7080 at ~453 Ma. The trend in Nevada samples of ɛ_Nd(t) to higher (more radiogenic) and ⁸⁷Sr/⁸⁶Sr to lower (less radiogenic) values is consistent with an increase in mafic weathering input beginning at ~465 Ma. Ongoing data collection will reveal if: 1) the change in the ⁸⁷Sr/⁸⁶Sr signal lags that of the 2) the ɛ_Nd(t) shift in Nevada lags the change in the more proximal Appalachian region. ⁸⁷Sr/⁸⁶Sr and ɛ_Nd(t) data from Sweden will reveal if higher-latitude (i.e., slower chemical weathering rate; ~25-30° S) island arc accretion in the Jämtlandian orogeny (~458 Ma; coincident with Taconic island arc accretion) contributed to atmospheric CO₂ removal during the Ordovician. This work is the first to pair these seawater signals from the same rock samples to evaluate the precise timing and nature of changes in continental weathering during the Ordovician.