Recent paleoceanographic history of the San Diego margin, California: A benthic foraminiferal perspective

Foraminiferal assemblages respond to changes in marine biogeochemistry and are widely used as biological proxies for oceanographic parameters. In this study, modern marine conditions are compared against recent environmental records using foraminiferal tests, in order to detail foraminiferal community response to oceanographic conditions. A set of sediment cores along a depth transect on the San Diego margin (7.5 km across, 86- 1175 m below sea level, 32°48\u2736.00 N, 117°24\u2759.76 W) is utilized to explore how benthic foraminiferal assemblages respond to environmental variation across the modern continental margin and through a down-core record of the past 1800 years. Two types of benthic foraminifera, Bolivina spp. and Uvigerina peregrina, reveal a complex response to a suite of hydrographic parameters, including temperature, dissolved oxygen, carbonate chemistry, and organic matter flux. Core-top samples showed that the proportional abundance of both types increases into the oxygen minimum zone (OMZ) upper margin, below which abundance decrease substantially. Foraminiferal assemblages from two short cores, MV1217-3-3 (700 m water depth, 20 cm length) and MV1217-2-3 (528 m water depth, 26 cm length), are used to investigate recent temporal changes along this margin. Core MV1217-3-3, located within the modern OMZ, exhibits limited variation in foraminiferal abundances and supports a stable OMZ over the past 1800 years. Core MV1217-2-3, at the upper margin of the OMZ, exhibits variation in abundances indicating vertical range fluctuations of low oxygen waters over the same time period. Carbon, oxygen, and nitrogen isotopes suggest OMZ shifts are caused by climate changes that alter upwelling, ventilation, and productivity. These results provide an example for how evaluating oceanographic conditions over recent timescales via foraminiferal assemblages and geochemical data provides a mechanism for predicting and interpreting the future alteration of significant environmental parameters like oxygenation of coastal waters