Oxygen isotopic composition of biogenic phosphate and the temperature of Early Ordovician seawater

Stable isotopic values were measured on micrite, sparry calcite, dolomite, inarticulated brachiopods, and conodonts from the Lange Ranch section (central Texas) of the Lower Ordovician Tanyard Formation. The section spans the upper Cordylodus angulatus Zone through the lower Rossodus manitouensis Zone. An similar to 2 parts per thousand negative delta C-13 shift from > 0 parts per thousand to <-1.5 parts per thousand(VPDB) through the section suggests the lower third of the Rossodus manitouensis Zone was sampled. Consistent with previous studies, the delta O-18 values of carbonates are low, ranging from -3.3 parts per thousand to -8.1 parts per thousand(VPDB). Phosphate delta O-18 values range from 15.4 parts per thousand to 17.1 parts per thousand(VSMOW). Paleotemperature estimates calculated from micrite delta O-18 values assuming an ice-free seawater delta O-18 value of -1 parts per thousand(VSMOW) indicate Early Ordovician tropical seawater temperatures averaged 42 degrees C, whereas delta O-18 values of co-occurring biogenic phosphate assuming the same seawater value yield paleotemperature estimates averaging 37 degrees C. The phosphate values are interpreted as less affected by diagenesis than carbonate values and suggest Early Ordovician tropical paleotemperatures were not more than 10 degrees C warmer or the oxygen isotopic composition of Early Ordovician hydrosphere was not more than 2 parts per thousand lower than present.