“Warm” palaeotemperatures from high Late Jurassic palaeolatitudes (Falkland Plateau): Ecological, environmental or diagenetic controls?

Oxygen and carbon isotopes have been determined from Late Jurassic (Oxfordian-Tithonian) belemnites and inoceramid bivalves from two Deep Sea Drilling Project (DSDP) sites located on the Falkland Plateau. Mean belemnite ∂18O values, derived from well preserved skeletal material, were −1.29‰ from DSDP site 330 and −1.45‰ from DSDP site 511. Assuming a seawater SMOW value of −1.0‰, mean palaeotemperatures calculated from the oxygen isotopic composition are 17.2°C and 17.9°C, respectively. The inoceramid bivalves yielded much lighter ∂18O values (mean −3.58‰). Petrographic and geochemical evidence points to the inoceramid bivalves being altered by diagenesis which accordingly accounts for the observed differences in isotopic values. “Vital effects” or the importation of belemnites or inoceramids from another area, are considered not to account for the observed isotopic trends. The palaeotemperatures interpreted from the belemnites are significantly warmer than other recent estimates of Late Jurassic temperature (from oxygen isotope studies and climate model predictions) from similar southern palaeolatitudes. We suspect our apparent warmer temperatures are because of a combination of increased freshwater runoff depleting surface waters with respect to ∂18O and related to the semi-enclosed nature of the depositional basin retaining warmth, relative to the open ocean of similar latitudes.