T234 Phosphates: Geochemical, Geobiological, and Materials Importance II

Wednesday, 30 October 2002
1:30 PM - 5:30 PM

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

Paper number: T234-12
Program: Technical Sessions
Time: 4:10 PM – 4:25 PM

Authors

Abstract

We measured oxygen isotope values of early Ordovician biogenic phosphate (d18Ophos) to better constrain the relative importance of temperature, oxygen isotopic composition of seawater, and diagenesis in controlling measured oxygen isotope values of carbonates (d18Ocarb) 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 d18Ocarb 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 d18Ocarb 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 d18Ocarb.

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 d18Ophos data (14.4 to 16.5‰ VSMOW) suggest paleotemperatures slightly less than d18Ocarb suggest, but still warm relative to the modern.  Additional analyses will test the reproducibility of the results.