GLOBAL COOLING LINKED TO BASALTIC WEATHERING DURING THE ORDOVICIAN GREENHOUSE–ICEHOUSE TRANSITION

During the Ordovician period, global climate cooled from an exceedingly warm, high-CO₂ “greenhouse” state to an “icehouse” state with extensive polar ice sheets. This greenhouse–icehouse transition has been explained by two competing hypotheses for atmospheric CO₂ drawdown: an increase in the Ca-silicate weathering CO₂ sink flux due to uplift of basaltic ocean crust or a decrease in the continental arc volcanic CO₂ source flux. Therefore, it remains unclear whether changes in the sources or sinks of atmospheric CO₂ are most responsible for long-term climate regulation during the Ordovician.

Here, we show paired weathering proxy data (⁸⁷Sr/⁸⁶Sr and ε_Nd(t)) from Middle–Late Ordovician (470–455 Ma) carbonate strata in central Nevada that indicate an increase in juvenile weathering input, similar to weathering proxy reconstructions in the Appalachian and surrounding basins. Seawater ⁸⁷Sr/⁸⁶Sr falls by 0.0008 (0.7088 at 470 Ma to 0.7080 at 455 Ma) at a rate among the fastest in the Phanerozoic, while seawater ε_Nd(t) rises by ~7 ε units (–20 at 470 Ma to –13 at 455 Ma). These data are paired with δ¹⁸O measurements from conodont apatite collected from the same section that indicate a ~3‰ increase (cooling; 17.5‰ at 464 Ma to 20.2‰ at 458 Ma) during the Middle–Late Ordovician. The δ¹⁸O cooling trend contrasts with previous globally-compiled conodont δ¹⁸O datasets that show no significant cooling in the Middle–Late Ordovician prior to the Hirnantian glaciation.

We constructed a simple box model to estimate the volume of basaltic weathering required to balance the ⁸⁷Sr/⁸⁶Sr and ε_Nd(t) isotopic budgets in the Middle–Late Ordovician. Assuming steady state of Sr and Nd in the Laurentian continental ocean, we estimate the dissolution of 1.5x10¹¹ kg/yr of basalt, delivering 1.3x10¹¹ mols Ca²⁺/yr to the ocean. With a volcanic degassing CO₂ source flux set to 70% of the silicate weathering sink flux, we estimate that an atmosphere beginning with 4000 ppm CO₂ at 470 Ma is reduced to 1880 ppm CO₂ at 450 Ma, reducing modeled equatorial sea surface paleotemperature from 30 °C at 470 Ma to 23.5 °C at 450 Ma. These model results are in broad agreement with our reported δ¹⁸O measurements, demonstrating the plausibility of basaltic weathering during the Taconic orogeny as a driver for the Ordovician greenhouse–icehouse transition.