Reassessment of ocean paleotemperatures during the Late Ordovician

Abstract

The Late Ordovician is a paradox, with a greenhouse climate shifting to an icehouse climate during a time of presumably high CO2 levels. We used clumped isotope (Δ47) microanalysis of fossil brachiopod shells to determine paleotemperatures of North America (Cincinnati Arch) during the Katian (453–443 Ma). Microanalyses of fossil brachiopods yielded a right-skewed distribution of clumped isotope temperatures [T(Δ47)] ranging from 25 °C to 55 °C with a mode of 32 °C and a mean of 36 °C. To test the impact of Δ47 reordering (reequilibration of 13C-18O bonding) at burial temperatures on T(Δ47), we applied a Monte Carlo simulation to a Δ47 reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(Δ47) and correct the reordering effect. Correcting for reordering decreased the mean T(Δ47) by only ~1 °C, to 35 °C, and did not change the mode (32 °C). These temperatures are lower than those of previous studies, but still suggest that the Late Ordovician climate of the Cincinnati Arch was warmer than the modern subtropical seas. However, carbonate facies in these strata argue for upwelling of cool water; thus, typical subtropical surface waters may have been as warm as 35 °C approaching the Hirnantian glaciation. Seawater δ18O values derived from back-calculated T(Δ47) and brachiopod δ18O values average –0.3‰ ± 0.6‰ relative to Vienna standard mean ocean water (VSMOW), consistent with the value expected for subtropical surface waters in a greenhouse Earth, assuming a constant seawater-δ18O model.

Ocean temperatures through the Phanerozoic reassessed

“…These temperatures and associated δ 18 O values suggest δ 18 O sw values of −0.5 to 3.5‰. Reexamination of Katian brachiopods using microanalytical techniques yields clumped isotope temperatures with a mode of ~ 33 °C (mean = 35 ± 2.8 °C) for a subtropical upwelling setting 24 . These temperatures and carbonate δ 18 O values equate to a mean seawater δ 18 O of −0.3 ± 0.6‰ VSMOW.…”

“…Locfit regression (α = 0.05) with 95% Cl calculated for combined carbonate and phosphate δ 18 O paleotemperatures. Paleozoic clumped isotope paleotemperatures (clumped T) from Henkes et al 23 and Barney and Grossman 24 . Also shown are TEX 86 paleotemperatures using the TEX 86 H -SST calibration 16 , 17 .…”

Environmental conditions in the preglacial late Ordovician subtropical Iapetus Ocean: geochemistry of the type section of the Lower Hartfell Shales (Katian) at Hartfell, Dumfries & Galloway, Scotland.

“…both today and in the past (Geen et al, 1992;Morford and Emerson, 1999;Henderson, 2002;Wilde et al, 2004;Tribovillard et al, 2006;Calvert and Pederson, 2007;Gallego-Torres et al, 2010). (Barney and Grossman, 2022). If the apparent increase in temperature from Ga/Rb ratios from sample 1 to 5 (bicornis to caudatus zones) is true (Fig.…”

Re‐Assessing the Need for Apatite‐ and Dolomite‐Specific Calibrations of the Carbonate Clumped Isotope Thermometer

“…Carbonate clumped isotope thermometry has the potential to constrain the formation temperature (TΔ 47 ) and oxygen isotope composition of formation water (water δ 18 O) of carbonate minerals by exploiting the thermodynamic preference for 13 C– 18 O bonds in carbonate molecules with decreasing temperature (Eiler, 2007; Wang et al., 2004). TΔ 47 and water δ 18 O have important applications, including the temperature history of Earth's oceans (Barney & Grossman, 2022; Bergmann et al., 2018; Henkes et al., 2018; Meckler et al., 2022; Modestou et al., 2020), paleoaltimetry (Kelson et al., 2022), the diagenetic history of carbonates (Goldberg et al., 2021; Mackey et al., 2020; Shenton et al., 2015), and basin thermochronology and tectonics (Brigaud et al., 2020; Gasparrini et al., 2023; Mangenot et al., 2018). Most clumped isotope studies have focused on CaCO 3 polymorphs (calcite, aragonite) due to their ubiquity in the geologic record, pervasiveness in shells of biomineralizing organisms, relative ease of laboratory precipitation, rapid dissolution in phosphoric acid, and generally better understood precipitation pathways.…”