218-7 Middle Devonian Paleotemperatures Derived from Triple Oxygen Isotope Values of Appalachian Basin Brachiopod Fossils

Abstract:

The Middle Devonian was an important interval in Earth’s history, seeing the emergence of the first tetrapods and deep-rooted terrestrial forests. It was also an interval of faunal turnover, with multiple instances of severe ocean anoxia causing both local and global extinctions, such as the Kačák Event and the Taghanic Event. Determining global surface temperatures over this time interval is essential for contextualizing the observed biological and geological changes. Biogenic calcite and apatite δ¹⁸O values are commonly used geochemical proxies to reconstruct paleotemperatures across the Devonian (and the Paleozoic more broadly). However, there are some intervals where the temperatures derived from these geochemical proxies disagree. For the Middle Devonian, brachiopod calcite generally yields unrealistically high paleotemperatures relative to those derived from conodont δ¹⁸O values, even when screened for diagenesis. Here, we measured the triple oxygen isotope values (δ¹⁸O & δ¹⁷O) of brachiopod shell and sparry infill calcite from 23 specimens spanning a six-million-year interval in the Middle Devonian Appalachian Basin to re-evaluate the discrepancy between conodont- and brachiopod-derived paleotemperatures. We show minimally altered samples have triple oxygen isotope values that yield relatively cool paleotemperatures ranging from 15 °C to 27 °C. We found that brachiopod calcite from the Tully Formation (the youngest formation in our dataset) had suffered pervasive alteration, however, we were able to reconstruct the initial formation temperature in these highly altered fossil shells using a fluid-rock mixing model. Their calculated formation temperatures (12 °C to 28 °C) overlap with the range of temperatures derived from our minimally altered fossils. Overall, the temperatures derived from our brachiopod triple oxygen isotope dataset agree with contemporaneous conodont-derived paleotemperatures. This work highlights the utility of triple oxygen isotope measurements for brachiopod-based paleotemperature reconstruction from fossils with a wide range of alteration conditions.

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