Palaeoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidence

Abstract

The Holocene palaeoclimatic history of the Galician continental shelf has been investigated through the analyses of diatom remains, other siliceous compounds, biogenic silica (BSi) and metals content in a multiproxy approach to a gravity core recovered from the Galicia Mud Patch, NW Iberian Peninsula, covering the last 5000 years. Downcore changes in diatom assemblages composition and abundance reflect changes in diatom production related to long-/short-term variations in climate, regional oceanography, upwelling strength and river influx off the coast of NW Spain and Portugal. Palaeoclimatic variability was related to the relative strengths and position of the Azores High and Iceland Low pressure cells. Metals and microflora fluctuations are interpreted as changes in the riverine influence and upwelling intensity paced by oceanographic, atmospheric and climatic changes. Lack of diatoms between 4700—3300 and 1800—1200 cal. yr BP could be linked to early diagenetic processes taking place in the sediment after burial. Biogenic barium (Baexcess), metals and excess diatom assemblages, show a general increase of marine productivity for the last 1200 cal. yr BP. Between 800 and 500 cal. yr BP high production of the microflora is triggered by influx of river-derived nutrients under conditions of SW winds and storms resulting from a NAO negative-like phase. The biosiliceous and geochemical signatures of sediments from the last 500 cal. yr BP indicate conditions of enhanced upwelling and increased phytoplanktonic production associated with the intensification of northerly winds. Upwelling strengthening in this area, attributed to recent global warming, could provoke an increase in phytoplankton biomass with consequent biological, climatological and socioeconomical impacts. The imprint of anthropic activities has been recorded by the increasing Pb/Al ratios for the last 400 cal. yr BP.