T169 Chemostratigraphy: Physical and Temporal Applications

Tuesday, 24 September 2019
8:00 AM - 12:00 PM
Room 224B, North Building

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

Paper number: T169-3
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

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 (εNd(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 εNd(t) = –18.5 at ~470 Ma, values fall slightly to εNd(t) = –20 at ~465 Ma and then increase to εNd(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 εNd(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 εNd(t) by ~2–3 myr due to the longer oceanic residence time of Sr; 2) the εNd(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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340995

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