Exploring changes to ocean oxygenation and seafloor ecosystems in Southern California Borderlands through the Holocene

Abstract:

Quantifying past changes in marine oxygenation and ventilation is critical for understanding future ocean oxygen change. In particular, identifying the drivers and impacts of changes in oxygenation and oxygen deficient zones through the Holocene informs our understanding of modern climate change. Benthic foraminifera and microfossil metazoan assemblages can be utilized both as proxies for past change, using known environmental associations, as well as examined ecologically in conjunction with geochemical environmental reconstructions. Here, we combine geochemical and faunal records to understand oxygenation history of the Southern California Borderlands and to identify relationships between environmental and seafloor ecosystem change. We investigate three sediment cores from the Southern California Borderlands, EW9504-05PC (San Clemente Basin, 1818 m water depth), EW9504-08PC (San Nicolas Basin, 1442 m water depth), and EW9504-09PC (Tanner Basin, 1194 m water depth). Radiocarbon based age models are developed for all three cores, with at least two age dates from each core. We generated a stable carbon and oxygen isotope record (centennial resolution) from Tanner Basin using planktonic foraminifera Globigerina bulloides and benthic foraminifera Cibicides mckannai. In Tanner Basin, we document changes in benthic foraminiferal assemblages (using multivariate statistical analyses NMDS and RDA) from 6000 – 2000 ybp, without concurrent changes in benthic ẟ¹³C and ẟ¹⁸O. This faunal change is driven by an increase in Bolivina spissa and decrease in Hansenisca sp. and may be attributed to a decrease in oxygenation across this interval. These findings will be compared with similar analyses from adjacent basins (San Nicolas and San Clemente) and the well-documented Santa Barbara and Santa Monica Basins to investigate whether these ecosystem and environmental changes are driven by changes in intermediate water or export from the surface ocean.