USING STABLE ISOTOPE ANALYSIS AS A PROXY FOR PALEOTEMPERATURES ACROSS THE EOCENE-OLIGOCENE BOUNDARY ON THE PACIFIC MARGIN

The Eocene-Oligocene boundary marks the last major transition from global hothouse to icehouse. Oxygen isotope signatures obtained from deep-sea core microfossils across the boundary indicate temperature changes that ranged from 4° to 16°C. However, there is very little isotope data from the North Pacific Ocean, and none from coastal environments. Taxonomic changes across the Eocene-Oligocene boundary in western Oregon and Washington have already been detailed for mollusks, showing rapid species turnover at both the middle-late Eocene and the Eocene-Oligocene boundaries. This study included two different stable isotopic techniques applied to macro and microfossils from the marine shallow shelf to upper bathyal depositional settings of the Lincoln Creek Formation, western Washington. These data now provide a link between the deep-sea temperature data, and climate estimate from angiosperm leaf morphology analysis obtained from coastal floras of central western Oregon (Kester, 2001; Myers, 2003).

Tests of four selected benthic foraminifera species and one bivalve species were collected at close stratigraphic intervals from several measured sections of the Lincoln Creek Formation, southwestern Washington. Paleodepth was constrained using grain-size analysis, and the exact age of the sections was determined from prior studies of magnetostratigraphy and biozonations. Shells were processed for both d¹⁸O and Mg/Ca ratio isotopic records. Unlike oxygen isotope signatures, Mg/Ca measurements are an ice-volume independent measurement, and comparison of the two allows for an elucidation of Antarctic ice volume and timing of its growth in the mid-Cenozoic. Whereas the record we created has insufficient resolution to provide specific data on climate change in the region, comparison of the different methods used provides new insight into the type of studies that can be done in shallow marine paleoenvironments. In addition, this study successfully tested the application of the Mg/Ca isotopic system in a cool water environment. These results do suggest that the effect of global ice formation on the isotope record of the northern Pacific margin may be smaller than currently understood. This corroborates the lower magnitude of cooling derived from the late Eocene through early Oligocene floras of western Oregon.