CONSTRAINING CHANGES IN SILICATE WEATHERING DURING THE EARLY ORDOVICIAN USING LITHIUM ISOTOPE CHEMOSTRATIGRAPHY

Ordovician strata represent a transition from a greenhouse to an icehouse climate. While recent studies on paleotemperature reconstructions have bolstered this notion and elucidated the overall cooling trend during the Middle Ordovician, the cause of the larger cooling trend characterizing the Early Ordovician climate remains ambiguous. Several hypotheses have been put forward to explain the cause of the Early Ordovician cooling which include a decline in volcanic outgassing and increasing silicate weathering rates. However, there are some questions that remain regarding the relative importance of these processes and possible feedback mechanisms that took place during this period. In addition, the Early Ordovician cooling trend is coeval with a decline in seawater ⁸⁷Sr/⁸⁶Sr which suggests a possible change in the proportion of basaltic versus granitic weathering. While changes in the weathering lithology could potentially change the efficiency of CO₂ consumption via silicate weathering (i.e., weatherability), relying on traditional weathering proxies such as strontium isotope data does not provide a complete picture of weathering dynamics in deep time as it is difficult to isolate signals of changes in silicate weathering from changes in provenance.

Here we use chemostratigraphic variations of lithium isotopic ratios (δ⁷Li) from 7 carbonate sections across the Laurentia paleo continent that span the Lower to Middle Ordovician to constrain changes in global silicate weathering during this period. Our data suggest that Early Ordovician seawater is characterized by a relatively light δ⁷Li isotopic composition (~14.5‰) compared to modern seawater (~31‰). The initial results from our stochastic modeling suggest that these lighter baseline values in seawater δ⁷Li are likely due to a relatively lighter riverine lithium isotopic composition. This implies that the Early to Middle Ordovician had a relatively low weathering intensity (i.e., relatively high erosion compared to the total denudation rate). Furthermore, previous studies have suggested that low weathering intensity is linked to high cation delivery efficiency during weathering.