The negligible role of intermediate water circulation in stadial–interstadial oxygenation variations along the southern California margin: Evidence from Nd isotopes

Abstract

Changes in the source of intermediate waters to the southern California margin may have caused variations in seafloor oxygen levels on stadial–interstadial time scales. We test this hypothesis using the Nd isotopic composition of benthic foraminifera and fossil fish debris from ODP Sites 893 and 1017 to track the composition of intermediate waters across interstadials 8–14 (∼37–52 ka) during Marine Isotope Stage 3. The ɛ Nd values of waters bathing the seafloor at Site 893 were typically ∼−9 and those bathing Site 1017 were ∼−7, both of which are significantly less radiogenic than waters that had originated in either the North Pacific or Southern Ocean (by the time such waters reached the southern California margin). Detrital silicate ɛ Nd values of nearly −12 suggest that this offset toward lower ɛ Nd values was likely caused by boundary scavenging that partially overprinted the water mass composition with local/regional fluvial Nd inputs. In spite of the evidence for boundary scavenging, the lack of systematic seawater Nd isotope changes on a stadial–interstadial basis suggests that the provenance of the intermediate waters did not change, and that the waters were derived from the Southern Ocean. Instead, changes in local/regional sea surface productivity may have caused the recorded changes in seafloor oxygenation.