Foraminiferal proxy response to ocean temperature variability and acidification over the last 150 years in the Santa Barbara Basin (California)

Abstract

The last 150 years on the California margin have been marked by high variability in precipitation, temperature, sea surface temperature, and ocean chemistry, in response to both natural and anthropogenic changes. Deciphering climate history from ocean sediments is dependent on our understanding of the various influences on marine proxies. Sediment from Santa Barbara Basin (SBB), California (box core SPR0901-10BC) provides an opportunity to compare multiple marine proxies with instrumental records of climate of the past 150 years and extend them beyond the historical record. Here we use three planktonic foraminiferal proxies, δ18Ocalcite, Mg/Ca, and size-normalized shell weight, to reconstruct the surface temperature and δ18Owater in SBB California at near-annual resolution from AD 1850–2000. Results indicate that Globigerina bulloides Mg/Ca reflects the instrumental temperature trends over the last 150 years, with average Mg/Ca temperatures approximately 2 °C lower than instrumental values, due to the slightly deeper habitat preference of G. bulloides. Similarly, temperature is the dominant control on G. bulloides δ18Ocalcite, and both Mg/Ca and δ18Ocalcite reflect variability consistent with changes in the intensification of ENSO over the past 150 years.Shell weights of the planktic foraminifer G. bulloides are inversely correlated with historical sea surface temperatures over the last 150 years in SBB sediments. Greater shell weight associated with cooler surface ocean temperatures does not support the anticipated relationship between coastal upwelling of cool, low pH water and shell thinning due to CO32− undersaturation. However, in contrast to the previous 130 years, G. bulloides shells deposited on the sea floor since the late 1970s exhibit a distinctive smooth shell structure indicative of partial dissolution. We hypothesize that a mid-1970s shift in intermediate water flow from predominantly equatorward to anomalously poleward led to the influx of relatively corrosive bottom water into SBB with subsequent carbonate dissolution at the sediment–water interface.