252-2 Paleozoic temperatures: The oxygen isotope record

Session: Climate Transitions in the Paleozoic

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Abstract:

The Paleozoic’s 287 million years of Earth history represent the first and only exposure (to-date) of animal life to a greenhouse-icehouse-greenhouse climate cycle.  Fundamental to characterizing the interplay between temperature and early metabolism is accurate quantitation of temperature. The most common proxy for deep-time paleothermometry is the oxygen isotope compositions of carbonate and phosphate fossils and microfossils. Such data are compiled in the online database StabisoDB (StabisoDB.org), with roughly 9,000 oxygen isotope analyses of Paleozoic brachiopods, mollusks, and conodonts. These phosphate and carbonate δ¹⁸O data show the lowest averages by stage for the Ordovician and Silurian (17.1‰ VSMOW and -5.6‰ VPDB respectively) and the highest averages for the Carboniferous and Permian (21.5‰ VSMOW and -0.9‰ VPDB respectively). Data interpretation, however, is hampered by uncertainty in seawater δ¹⁸O and concerns about signal preservation. Do low δ¹⁸O values in Early Paleozoic fossils reflect lower seawater δ¹⁸O, warmer oceans, or progressive diagenesis with age? For carefully screened samples, carbonate and phosphate δ¹⁸O data show excellent correlation at the stage level, despite the greater resistance of phosphate oxygen to diagenesis, arguing for preservation of both δ¹⁸O records.

Researchers have argued for decreasing seawater δ¹⁸O with age to explain the low δ¹⁸O values of Ordovician, Silurian, and Devonian brachiopods, tacitly assuming a constant low-latitude temperature of roughly 24 °C. Others have argued that global seawater δ¹⁸O is crustally buffered at roughly -1‰. Both arguments are supported by modeling efforts. However, a crustally-buffered hydrosphere is suggested by clumped isotope data.  Assuming a baseline global seawater δ¹⁸O of -1.1‰ and correcting for ice volume and paleolatitude changes, low-latitude δ¹⁸O data yield average temperatures of ~42 °C during the Early-to-Middle Ordovician, ~36 °C during the Late Ordovician through the Devonian, ~25 °C during the Mississippian through Permian. These results suggest that early marine animals had a greater thermal tolerance than modern species. They also imply that Earth’s global thermostat can have a higher setting than previously thought. However, an important caveat is that oxygen isotope temperatures can be biased by local variations in seawater δ¹⁸O in restricted epicontinental seas subject to freshening and excess evaporation. This uncertainty highlights the need for high-quality, high-resolution clumped isotope temperatures from a variety of regions to complement and confirm these determinations.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9647

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