Southern California margin benthic foraminiferal assemblages record recent centennial-scale changes in oxygen minimum zone
2020
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 µ m) exhibit similar trends across the spatial and environmental gradient or in some cases exhibit more pronounced
spatial trends in the >150 µ m 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.
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