Cooling trend and enhancement of productivity in the upwelling off Peru since the late 19th century

Reconstructions of past climate and ocean variability and of the response of the marine ecosystems from high-resolution marine archives are critical to our understanding of climate/ocean dynamics and its links with ecosystem change. Laminated sediments preserved in the Peruvian margin within the oxygen minimum zone allow reconstructing past climate/ocean variability and environmental changes from interannual to centennial or longer time-scales (Gutierrez et al., 2009). We investigated proxy records in laminated sediments downstream the main upwelling area of the Peruvian coast (off Pisco, 14° S, 300m water depth) aiming at reconstructing variations of sea-surface temperature and biological productivity during the past century in order to explore potential (global warming related) changes in the intensity of coastal upwelling and ecosystem response. Dating of the sediment core revealed extremely high sedimentation rates of 1.9-2.3 mm/year for the period since the late nineteenth century to the present (Gutierrez et al., 2009). SST values, reconstructed from the alkenone unsaturation index (Uk'37), are within the range observed in the Pisco coastal area during spring/summer, when productivity (inferred from Chl-a) is highest. They show conspicuous positive/negative excursions which are generally in-line with known past El Nino/El Nina events. The record reveals significant multidecadal variability: SSTs smoothly decrease from 1880 to ca.1920, show little variation between 1920 and 1945 followed by a steady rapid cooling of ca. 1.5° C in the latter part of the 20th century (after 1950). This trend is consistent with instrumental inshore SST time-series from central and southern Peru which also exhibit significant cooling since 1950 and with satellite SST data. Yet, for the same period, these data are not in line with ICOADS-SSTs, most likely depicting a strong onshore-offshore gradient. The negative SST changes coincide with increased, upwelling favorable alongshore wind stress recorded off Pisco. They reveal an upwelling intensification through the 20th century with a more pronounced trend during the latter 50 years. Accordingly, proxy records of productivity (TOC fluxes, alkenone fluxes, anchovy scale deposition rates) off Pisco show increased productivity with enhanced alongshore windstress and SST cooling (stronger upwelling) after 1950. The trend to cooler SSTs and inferred upwelling intensification through the 20th century in the upwelling area off Peru is in agreement with studies in the coastal upwelling off NW Africa (McGregor et al., 2007) and supports the mecanism proposed by Bakun (1990) for global warming-related changes in the intensity of coastal upwelling .