Abstract
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. Five cores were analyzed 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 as well as for down-core benthic foraminiferal assemblages to identify changes in the OMZ through time. Comparisons of benthic foraminiferal assemblages from two size fractions (63–150 and >150 µm) exhibit similar trends across the spatial and environmental gradient or in some cases exhibit more pronounced spatial trends in the >150 µm fraction. A range of species diversity exists within the modern OMZ (1.910–2.586 H, Shannon index), suggesting that diversity is not driven by oxygenation alone. 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). Down-core analysis of indicator species reveals variability in the 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 kyr. We document expansion of the upper margin of the OMZ beginning 400 BP on the San Diego margin that is synchronous with other regional records of oxygenation.
Highlights
Ocean oxygenation is declining globally; rising ocean temperatures decrease oxygen solubility at the sea surface and increased stratification inhibits ventilation, leading to decreased oxygen at depth (Breitburg et al, 2018; Levin et al, 2009; Stramma et al, 2010)
We interpret the transition in assemblage as a decrease in oxygenation at this depth and a shoaling of the upper margin of the oxygen minimum zone (OMZ) beginning at 400 BP and continuing to the present
This spatial and temporal analysis of benthic foraminifera assemblages across a modern oxygen gradient on the San Diego margin improves our understanding of the relationship between assemblages and their environment
Summary
Ocean oxygenation is declining globally; rising ocean temperatures decrease oxygen solubility at the sea surface and increased stratification inhibits ventilation, leading to decreased oxygen at depth (Breitburg et al, 2018; Levin et al, 2009; Stramma et al, 2010). Expansions of oxygen minimum zones (OMZs) have already been documented, and further expansions are predicted (Bograd et al, 2008; Schmidtko et al, 2017; Stramma et al, 2010). The intensity and geographic extent of the CA margin OMZ has oscillated in response to past changes in climate and ocean temperatures on millennial timescales – weakening during cool periods and strengthening during warm periods (Cannariato and Kennett, 1999; Jaccard et al, 2014; Moffitt et al, 2014, 2015a; Ohkushi et al, 2013). Determination of timing and drivers of past expansions and contractions of OMZs is critical to developing accurate predictions of future change (Jaccard et al, 2014)
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call