T179 The Ordovician Earth: New Insights to Environmental and Biotic Responses in the Fossil and Rock Record I

Thursday, 29 October 2020
10:00 AM - 12:00 PM
Meeting Rooms

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

Paper number: T179-8
Program: Technical Sessions
Time: 11:30 AM – 11:45 AM

Authors

Abstract

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 εNd(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 εNd(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 εNd(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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-357955

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