OXYGEN ISOTOPIC COMPOSITION OF BIOGENIC PHOSPHATE AND THE TEMPERATURE OF EARLY ORDOVICIAN SEAWATER

We measured oxygen isotope values of early Ordovician biogenic phosphate (d¹⁸O_phos) to better constrain the relative importance of temperature, oxygen isotopic composition of seawater, and diagenesis in controlling measured oxygen isotope values of carbonates (d¹⁸O_carb) in early Paleozoic rocks.  Micrite and well-preserved conodont samples were collected from the top of the Cordylodus angulatus Zone through the Rossodus manitouensis Zone in the Lower Ordovician Tanyard Formation in central Texas.  The d¹⁸O_carb of the micrite samples ranged from –7.7 to –5.1 ‰ PDB (22.9 to 25.6 VSMOW).  These values are consistent with results from other studies but are significantly lower than expected for carbonate deposited in equilibrium with tropical seawater.  According to standard paleotemperature equations these values suggest temperatures of 36 to 50 ^oC.  Common explanations for low early Paleozoic d¹⁸O_carb include: high temperatures relative to modern, lower seawater d¹⁸O than can be explained only by melting glacial ice, or pervasive diagenetic bias toward lower values of d¹⁸O_carb.

To test these alternatives we analyzed biogenic phosphate, which has lower diagenetic alteration potential than carbonate.  Well-preserved conodonts (Color Alteration Index 1-1.5) were targeted to try to minimize diagenetic effects.  Initial d¹⁸O_phos data (14.4 to 16.5‰ VSMOW) suggest paleotemperatures slightly less than d¹⁸O_carb suggest, but still warm relative to the modern.  Additional analyses will test the reproducibility of the results.