PALEOTEMPERATURE ESTIMATE FOR A SHALLOW EPICONTINETAL SEA (LATE ORDOVICIAN; LAURENTIA) BASED ON OXYGEN ISOTOPIC COMPOSITION OF CONODONT APATITE

During the Mohawkian, a shallow, subequatorial epicontinental sea covered most of Laurentia. Despite this paleogeographic position, lithologic and faunal evidence suggest a shift from warm-water to cool-water conditions, accompanied by a major biotic turnover event. So far, though, this proposed drop in temperature has not been confirmed let alone calibrated using a geochemical paleothermometer. The oxygen isotopic composition of conodont apatite could provide such constraints because of its high potential to preserve a primary signature. Preliminary data confirm warm conditions before the shift and indicate differences in values among conodont taxa.

The δ¹⁸O_phos values of four conodont species (P. gracilis, B. confluens, P. ramosus, D. suberectus) from two locations in Minnesota (deposited in the Hollandale Embayment near the Transcontinental Arch) and one bulk sample from Missouri (located in an open platform setting) range from 18.1‰ to 19.1‰ (V-SMOW). The samples were all collected from limestone beds immediately below the widespread Deicke K-bentonite (457.1±1.0 My). Thus, the samples represent a transect through part of the epicontinental sea shortly before the cooling event.

The δ¹⁸O_phos results are very close to, or within, the range of Modern and Mesozoic tropical biogenic apatites. Assuming δ¹⁸O_water of –1‰, a reasonable value for a time period for which no ice-sheet related sediments are known, these δ¹⁸O_phos suggest a temperature range from 29.4°C to 25.2°C for the Minnesota samples. Measured P. gracilis values indicate an average temperature of 29.1°C. The δ¹⁸O_phos values of P. ramosus, B. confluens and D. suberectus, measured from only one sample in Minnesota, yield estimates of 28.4°C, 26.7°C and 25.2°C, respectively. Taxon specific differences could be related to different habitats, and thus might give insight into ocean stratification. The δ¹⁸O_phos value of the bulk sample from Missouri corresponds to a paleotemperature of 28.2°C. Ongoing studies will increase the geographic coverage of data from below the Deicke K-bentonite and test apparent temperatures in samples from above this horizon.