Oceanographic variability in the southern Gulf of California over the past 400 years: Evidence from faunal and isotopic records from planktic foraminifera

Oceanographic conditions during the past four centuries–sea surface temperature (SST) centennial and decadal variability, and changes in the water column structure–were reconstructed in high resolution from foraminiferal assemblages and stable isotope analyses completed on laminated sediments from two multicores (MC15, 415 m depth and MC26, 600 m depth) from the southern Gulf of California (eastern subtropical Pacific). Total abundance counts and oxygen isotopic analyses were performed on two planktonic species, Globigerina bulloides (a proxy for winter SST) and Pulleniatina obliquiloculata (a proxy for late fall subsurface water temperature). Multidecadal oscillations in δ18O values are considered to result primarily from extratropical forcing–sea level pressure anomalies in the north Pacific–via northwesterly winds weakening/strengthening in association with changes in the average position and intensity of the Aleutian Low. The downcore evolution of both biological proxies revealed an increase in SST since the 1600s, reflecting a gradual decline in upwelling activity during this period. This temperature trend implies a long-term change that cannot be explained by the same mechanisms associated with the multidecadal oceanographic variability. We proposed that the progressive warming of the surface waters is linked to a general northward shift of the ITZC resulted from increased solar irradiation, which accounts for larger net surface shortwave fluxes, affecting regional heating patterns and consequently the sensitive regional climate.