2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 12
Presentation Time: 4:35 PM

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

CAME, Rosemarie E., Department of Earth Sciences, The University of New Hampshire, Durham, NH 03824-3589, BRAND, Uwe, Earth Sciences, Brock University, 500 Glenridge Ave, St. Catharines, ON L2S 3A1, Canada, GUO, Weifu, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, VEIZER, Jan, Ottawa-Carleton Geoscience Center, Univ of Ottawa, Ottawa, K1N 6N5, Canada, AZMY, Karem, Earth Sciences, Memorial University, St. Johns, NF A1B 3X5, Canada and EILER, John, Geology and Planetary Sciences, California Institute of Technology, MC100-23, Pasadena, 91125, rose.came@gmail.com

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.