Application of Carbonate ‘Clumped Isotope' Thermometry to Marine Brachiopods from Icehouse and Greenhouse Periods in the Paleozoic Era — Preliminary Results

The carbonate ‘clumped isotope' paleothermometer provides constraints on the temperatures of growth or diffusional re-equilibration that depend only on the isotopic composition of carbonate, without requiring assumptions regarding seawater δ¹⁸O (Ghosh et al., 2006). Where primary depositional temperatures are preserved, this method permits the calculation of seawater δ¹⁸O based on ‘clumped isotope' temperatures and the δ¹⁸O of carbonate.

A previous ‘clumped isotope' reconstruction of Paleozoic temperatures suggests that the mid Silurian (Telychian; greenhouse conditions) was characterized by high equatorial shallow-marine temperatures of ~35°C, consistent with the expectations of models in which the elevated atmospheric CO₂ during the Silurian drove or amplified increased temperatures (Came et al., 2007). Here, we investigate Paleozoic temperature changes by applying the ‘clumped isotope' paleothermometer to brachiopods from the Ordovician (Early Ashgill; greenhouse conditions) and Silurian (Early Wenlock; icehouse conditions). Our results show that both sample suites are compromised due to burial alteration (i.e., their temperatures are variably reset by post-depositional heating and/or water-rock reaction). Further work will be required to determine whether these or contemporaneous sections contain samples preserving primary temperatures. However, for the present we use the trends in ‘clumped isotope' space defined by these altered samples to place bounds on possible primary temperatures and seawater δ¹⁸O. Assuming that the lowest temperature in our early Wenlock suite represents the maximum possible depositional temperature, the data suggest that Silurian icehouse temperatures were ≤29°C, or ≥6° cooler than during Silurian greenhouse conditions. In this case, seawater δ¹⁸O was ≥1.5 permil higher during icehouse conditions, consistent with the presence of ice sheets comparable in size to those extant today. No plausibly primary temperatures are preserved in the Early Ashgill suite. However, assuming the previously reported Paleozoic ‘greenhouse' seawater δ¹⁸O of -1.2 permil (Came et al., 2007), our data imply depositional temperatures of ~30°C during Ordovician greenhouse conditions.