DID TACONIC OROGENESIS ENHANCE MAFIC WEATHERING RATES IN THE MIDDLE–LATE ORDOVICIAN? A TEST USING SEAWATER STRONTIUM AND NEODYMIUM ISOTOPES

While Ca, Mg (mafic) silicate weathering is thought to be a dominant driver of long-term climate, its role in the dramatic global cooling and accumulation of ice sheets in the Ordovician is not clear. Previous geochemical, tectonic, and stratigraphic studies have suggested that island arc accretion to the southern Laurentian margin during the Taconic orogeny lead to enhanced global rates of mafic weathering (i.e., global weatherability), but the precise timing of island arc exhumation and weathering, and therefore its relation to global cooling, is ambiguous. This work pairs seawater strontium (⁸⁷Sr/⁸⁶Sr) and neodymium (ε_Nd(t)) signals preserved in Middle–Upper Ordovician (~470–450 Ma) marine carbonate strata and conodont bioapatite from the Antelope Range in central Nevada to test the hypothesis that accretion of Taconic island arcs enhanced mafic weathering rates. These data from 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). From bulk rock ε_Nd(t) = –18.5 at ~470 Ma, values fall slightly to ε_Nd(t) = –20 at ~465 Ma and then increase to ε_Nd(t) = –13 at ~453 Ma. Bulk rock and conodont ⁸⁷Sr/⁸⁶Sr = ~0.7087 at ~464 Ma and decreases steadily to ~0.7080 at ~453 Ma. The trend of ε_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 ε_Nd(t) by ~2–3 myr due to the longer oceanic residence time of Sr; 2) the ε_Nd(t) shift in Nevada lags the change in the more proximal Appalachian region. This work is the first to pair these seawater signals from the same rock samples to evaluate the precise timing and nature of changes, allowing for more detailed study of continental weathering during the Taconic orogeny and its relation to global Ordovician cooling.