Late Pleistocene Variations in Oxygen Minimum Zone: Southern California Margin

Oceanic carbon cycling is strongly influenced by the Oxygen Minimum Zone (OMZ). Studies of sediments from ODP Sites 893 (~595m) and 1017 (~955m) demonstrate that the strength of the OMZ off the Southern California margin (between ~500 and ~1000m) has fluctuated on orbital and millennial time scales during the late Pleistocene, affecting the preservation of organic carbon in marine sediments. Existing records do not adequately describe the history over the depth range of the OMZ. To augment the existing depth transect, we generated oxygen isotope and benthic foraminiferal assemblage records from core NBP0206 JPC1 (755m) located near the center of the OMZ. The chronology for this core was determined using radiocarbon analyses and oxygen isotope stratigraphy. The planktonic d18O record correlates well with the orbital and millennial- scale fluctuations recorded in nearby ODP Holes 893 and 1017. Benthic d18O data reflect orbital variations in upper intermediate waters, including a decrease of 1.6 ‰ across the last glacial termination, implying a temperature increase of 1-2 deg C (after ice volume correction). The millennial scale isotopic oscillations recorded in Hole 893 are absent here. Benthic foraminiferal assemblages are sensitive recorders of bottom water oxygenation. On orbital time scales, the fauna record an oxic Marine Isotope Stage 3 and Last Glacial Interval (LGI), and a dysoxic Holocene, supporting previous interpretations of orbital scale ventilation changes in intermediate water depths. The benthic assemblages record as low oxygen intervals 9 of the 13 interstadials of the last 50ka. An absence of associated changes in the benthic d18O on millennial scales implies that the low oxygen interstadials largely reflect changes in surface productivity, similar to those seen at Hole 1017. Millennial scale intermediate water ventilation changes, as interpreted in Hole 893, occurred above the depth of this core. During the LGI and the stadials of MIS 3, the OMZ appears to have entirely disappeared on the Southern California margin. This has major implications for the cycling of oceanic carbon.