VENTILATION INFLUENCE ON BAJA CALIFORNIA OXYGEN MINIMUM ZONE STRENGTH SINCE THE LGM: CONSTRAINTS FROM BENTHIC FORAMINIFERAL MG/CA

Organic matter concentrations in a sediment core from the oxygen minimum zone (OMZ) off southern Baja California (705 m water depth) vary in concert with Dansgaard/Oeschger climate oscillations. A measure of sediment color (the third principal component of the diffuse spectral reflectance) displays a particularly striking similarity to Greenland ice core temperature records over the past 50 kyr. High organic matter concentrations during interstadials and during the Holocene may have been caused by increased productivity and/or increased preservation due to reduced OMZ ventilation. Changes in ventilation could have been caused by competition between warm, salty, low-oxygen (tropical) intermediate waters and cool, fresh, high-oxygen (North Pacific) intermediate waters. Such fluctuations, if significant, should be recorded by benthic foraminiferal Mg/Ca (temperature) and d¹⁸O (temperature plus salinity).

We present high resolution records of Uvigerina Mg/Ca and d¹⁸O spanning the past 16 kyr. d¹⁸O displays two apparently abrupt decreases superimposed on the deglacial ice volume trend: one at the time of the Bolling warming, and another at the end of the Younger Dryas. Mg/Ca remains flat across the start of the Bolling, and actually suggests a slight cooling at the end of the Younger Dryas. These results suggest that the d¹⁸O is dominated by salinity changes, with freshening occurring at the starts of the Bolling and the Holocene. For the latter, the coincidence of cooling and freshening implies a shift toward more North Pacific-like intermediate waters. Since neither transition is consistent with reduced ventilation as a driver of organic matter increases, productivity was likely the dominant influence. Additionally, there is an abrupt shift toward cooler, fresher waters after ~5 ka that is not obviously reflected in organic carbon content. We also consider the possibility that Uvigerina Mg/Ca is complicated by environmental factors other than temperature.