Abstract
Abstract. The close association between planktic foraminiferal assemblages and local hydrography make foraminifera invaluable proxies for environmental conditions. Modern foraminiferal seasonality is important for interpreting fossil distributions and shell geochemistry as paleoclimate proxies. Understanding this seasonality in an active upwelling area is also critical for anticipating which species may be vulnerable to future changes in upwelling intensity and ocean acidification. Two years (2012–2014) of plankton tows, along with conductivity–temperature–depth profiles and carbonate chemistry measurements taken along the north-central California shelf, offer new insights into the seasonal dynamics of planktic foraminifera in a seasonal coastal upwelling regime. This study finds an upwelling affinity for Neogloboquadrina pachyderma as well as a seasonal and upwelling associated alternation between dominance of N. pachyderma and Neogloboquadrina incompta, consistent with previous observations. Globigerina bulloides, however, shows a strong affinity for non-upwelled waters, in contrast to findings in Southern California where the species is often associated with upwelling. We also find an apparent lunar periodicity in the abundances of all species and document the presence of foraminifera even at very low saturation states of calcite.
Highlights
Planktic foraminifera have a long history as paleoceanographic proxies due to their environmental sensitivity, cosmopolitan distribution, and extensive fossil record
We focus on planktic foraminiferal assemblages sampled along a cross-shore transect over the central California shelf extending from 1 km offshore to the shelf break (30– 60 km offshore)
The assemblage was heavily dominated by the planktic species N. pachyderma, N. incompta, T. quinqueloba, and G. bulloides, representing 35.3, 23.1, 13.5, and 11.7 % of all recovered foraminifera, respectively
Summary
Planktic foraminifera have a long history as paleoceanographic proxies due to their environmental sensitivity, cosmopolitan distribution, and extensive fossil record. At sites where overlying water masses change seasonally, the foraminiferal fossil record will represent a combination of individuals that may have grown under vastly different conditions This averaging of short-term variability has the potential to impact the interpretation of any proxy based on foraminifera. Previous studies have explored seasonal assemblage shifts in the North Pacific, including at Station Papa (50◦ N, 145◦ W; Thunell and Reynolds, 1984; Reynolds and Thunell, 1985), in the California Current off of Oregon (> 130 km offshore) (Ortiz et al, 1995), in the Santa Barbara Basin (Kincaid et al, 2000; Darling et al, 2003), off Southern California (Sautter and Thunnell, 1991), and in the western Pacific (Eguchi et al, 2003). The majority of this prior work has focused on openocean assemblages, leaving a gap in understanding the seasonal dynamics in coastal upwelling regions, as well as a significant spatial gap within the California Current system between the Southern California Bight and Oregon
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