Southern California margin benthic foraminiferal assemblages across a modern environmental gradient record recent centennial-scale changes in oxygen minimum zone

Abstract. Microfossil assemblages provide valuable records to investigate variability in continental margin biogeochemical cycles, including dynamics of the oxygen minimum zone (OMZ). Analyses of modern assemblages across environmental gradients are necessary to understand relationships between assemblage characteristics and environmental factors. Here, we analyzed five cores from the San Diego Margin (32°42'00'' N, 117°30'00'' W, 300–1175 m water depth) for core top benthic foraminiferal assemblages to understand relationships between community assemblages and spatial hydrographic gradients and down core to identify changes in the oxygen minimum zone through time. Comparisons of benthic foraminiferal assemblages from two size fractions (63–150 and > 150 μm) exhibit similar trends across the spatial/environmental gradient, or in some cases exhibit more pronounced spatial trends in the > 150 μm fraction. We identify two hypoxic associated species (B. spissa and U. peregrina), one oxic associated species (G. subglobosa) and one OMZ edge-associated species (B. argentea). A range of species diversity exists within the modern OMZ (1.5–2.6 H, Shannon Index), suggesting that diversity is not driven by oxygenation alone. Down core analysis of indicator species reveal variability in upper margin of the OMZ (528 m water depth) while the core of the OMZ (800 m) and below the OMZ (1175 m) remained stable in the last 1.5 ka. We document expansion of the upper margin of the OMZ beginning 400 ybp on the San Diego Margin that is synchronous with other regional records of oxygenation.