Planktonic foraminifera in the California current: Vertical distributions, decadal climate variability, and 20th century warming

Planktonic foraminifera from varved Santa Barbara Basin sediments are ideally suited to reveal decadal-to-centennial changes in hydrographic structure of the California Current. Here I examine the distributions of living foraminifera from vertically stratified plankton samples taken in different hydrographic regions of the California Current to better interpret variations in the fossil record. I then reconstruct variations in abundance and isotopic composition of foraminiferal species from the past 260 years (1700 years) at continuous 2-year (5-year) sampling intervals from sediment cores.; Many tropical and subtropical species increased in abundance in the early 20th century and reflect both the variations and trend in Sea Surface Temperature (SST) within the 20th century. Although species with subpolar affinities generally covaried with many subtropical species in both plankton samples and in most of the sedimentary record, this relationship changed in the late 20th century. The divergence of these assemblages after the mid-1970s indicates a warming throughout the water column that is atypical of decadal fluctuations of the preceding 200 years.; Near-surface temperatures inferred from delta18O of Globigerina bulloides were negatively related to reconstructed northern hemisphere air temperatures for nearly 1700 years prior to the 20th century, which emphasizes that regional climate records may differ in response to changes such as the Medieval Warm Period and Little Ice Age. In contrast, in the 20th century inferred ocean temperatures show a positive relationship with northern hemisphere temperatures, reflecting a more widespread warming that overrides other modes of variability. However, the vertically stratified plankton samples show that G. bulloides lives deeper under increasing stratification, in response to deepening chlorophyll concentrations, which suggests an underestimate of the true magnitude of 20th century warming from delta18O of G. bulloides.; The increase in near-surface temperatures beginning around 1930 is attributed to increased ocean-atmosphere heat exchange and is accompanied by an underlying shoaling of isotherms from greater alongshore winds. In addition, the mid-1970s change is characterized by a more penetrative warming due to the wintertime expansion of the Aleutian low pressure system. The distinctive patterns of the 20th century changes inferred from foraminiferal tests appear to be best explained by anthropogenic activity.