ENSO DYNAMICS AND BAJA CALIFORNIA OXYGEN MINIMUM ZONE STRENGTH SINCE THE LAST GLACIATION

Shifts in the mean state or spectral character of the El Niño-Southern Oscillation (ENSO) system during past climate intervals may have had major impacts on global climate. We are testing hypotheses regarding ENSO system behavior on both orbital and millennial time scales using a remarkable pair of sediment cores from off the southern tip of Baja California. This region is today situated with the eastern tropical Pacific oxygen minimum zone (OMZ), a widespread feature caused by a combination of high upwelling-driven productivity and poor ventilation of intermediate waters. During modern El Niño events, upwelled nutrients are sharply reduced due to a regionally deeper nutricline, suggesting that the sedimentary record may preserve a history of ENSO conditions.

One of our cores, recovered near the core of the OMZ (705 m), exhibits fluctuations in organic matter content and sediment color that bear a striking resemblance to Greenland oxygen isotope records. Specifically, organic matter was lower (suggesting a weaker OMZ) during Dansgaard-Oeschger stadial events. As noted previously at Santa Barbara Basin, OMZ weakening could have been due to reduced productivity or to increased ventilation from the North Pacific. Several lines of evidence point to the dominance of productivity over ventilation. First, benthic foraminiferal Mg/Ca and δ18O are inconsistent with cooler (North Pacific derived) intermediate waters during stadials. Second, benthic radiocarbon ventilation ages suggest significant changes in intermediate depth circulation that appear to be unrelated to the OMZ fluctuations. Finally, benthic foraminiferal fluxes decrease dramatically during stadials, indicative of lower productivity. The productivity record is consistent with El Niño-like conditions during stadials, though other mechanisms cannot be ruled out.

Our second core is from nearby Soledad Basin, which has an effective sill depth of 290 m. In contrast to the deeper core, planktonic foraminiferal preservation is excellent in Soledad Basin. We are currently analyzing planktonic Mg/Ca, Cd/Ca, and Zn/Ca in an attempt to reconstruct the upwelling history of this site since the Bølling/Allerød. We will test model results that suggest ENSO has varied with orbital forcing, and search for millennial-scale variability within the Holocene.