Constraining planktic foraminiferal depth habitats for improved polar and subpolar paleoreconstructions

Abstract. Planktic foraminifera serve as critical proxy substrates in paleoceanographic reconstructions. However, the vertical structure of foraminiferal assemblages in the water column remains poorly constrained, particularly in the Pacific Ocean, thus impacting paleoceanographic interpretations from this basin. Here we use depth-stratified plankton tows (0–250 m) collected from the Northern California Current (NCC) to investigate spatiotemporal variability in community composition and depth habitat, with particular emphasis on taxa commonly used in polar and subpolar paleoreconstructions, specifically Neogloboquadrina pachyderma, N. incompta, and Turborotalita quinqueloba. Community composition varied significantly across years but not across latitudes, and comparison to previous NCC studies revealed distinct assemblages across end-member states of the Pacific Decadal Oscillation. Spinose species were most abundant in the upper 25 m of the water column and exhibited consistently shallower average living depths (ALD; 24–31 m) and narrower vertical distributions (VD; ± 13–19 m) compared to non-spinose taxa (38–94 m ± 23–49 m). Turborotalita quinqueloba and N. pachyderma displayed greater depth habitat variability across the study interval, whereas N. incompta exhibited a more stable, constrained depth habitat. Depth habitats in the NCC were shallower when compared to previous studies from other ocean basins, likely due to distinct genotypes inhabiting the study region compared to previous studies concentrated in the North Atlantic and Arctic Oceans. Results herein highlight the need to develop a nuanced understanding of the ecological and environmental processes that govern foraminiferal distribution in the water column to better refine proxy interpretations, particularly in climatically sensitive polar and subpolar regions.