INITIATION OF MODERN PACIFIC ATMOSPHERIC CIRCULATION AT THE PLIO-PLEISTOCENE BOUNDARY RECORDED IN HORSE TEETH ENAMEL GEOCHEMISTRY

A major challenge for paleontologists and paleoceanographers is the development of a consistent record of environmental change that interlinks geochemical proxies derived from organisms in the terrestrial and marine environments. The Pliocene-Pleistocene boundary at 2 Ma represents a period of time when the Earth’s climate underwent a significant shift from warm to cool mode. Recent geochemical data from ODP sites along the tropical and eastern north Pacific suggest that this boundary coincides with the development of east-west temperature gradients in the tropics, increased upwelling along the California margin and the initiation of Walker Circulation in the tropical Pacific (Ravelo et al 2001; Fall AGU abstract PP12A-0477).

Here we present stable isotope data obtained from fossil Equus sp. teeth enamel from Anza Borrego Desert State Park^TM (ABDSP), California, that show abrupt geochemical changes in oxygen and carbon isotope values at 2 Ma. We find that teeth enamel d¹³C values averaged –4.6‰ from 3 to 2 Ma (n=5 specimens) and abruptly decreased to –7.1‰ from 2 to 0.8 Ma (n=8 specimens). At the same time, d¹⁸O values increased from Pliocene values of 23‰ (relative to V-SMOW) to Pleistocene values of 25‰. We interpret these data to reflect a shift from diets rich in C4 grasses during the dry Pliocene climate of the ABDSP region to a mixed diet of C3 and C4 vegetation during the Pleistocene. Because C3 grasses are found in semi-arid environments when winter precipitation is common, this vegetation shift may reflect the impact of tropical Pacific Walker Circulation initiation on California borderland climate. The shift in teeth oxygen isotopes at 2 Ma could reflect a change in the source of atmospheric water vapor from the N. Pacific Aleutian low to more subtropical sources common to today’s El Niño weather pattern. Alternatively, enriched d¹⁸O values could signal the start of generally drier conditions at ABDSP with episodic winter rainfall providing the bulk of the annual moisture for the region.