Paleoenvironmental evolution of a late Holocen lake in the Azores archipelago

Abstract

[Abstract] A major source of interannual variability in the atmospheric circulation is the North Atlantic Oscillation (NAO), which is associated with changes in the surface westerlies across the North Atlantic onto Europe. Irregularities in the NAO are responsible for storm track paths across the Atlantic Ocean towards Europe and, therefore, for the amount and timing of precipitation on this continent. Despite its importance, few historical and long-term reconstructions have been carried out and few multiproxy reconstructions of NAO evolution have been conducted in the Azores islands located at one end of the dipole. Since the NAO has a strong and direct influence on lacustrine ecosystems in many ways, the study of lake sediments can provide precise information on the long-term evolution of this climate mode. Characterization of sedimentary facies, organic matter geochemistry and remains of diatoms have been used in this study to determine three different climatic phases in a sediment core from Lake Empadadas Norte (São Miguel Island, Azores). The period 1340-1460 AD is defined by a decreasing trend in productivity and moderately shallow waters, as terrestrial inputs dominated, suggesting a humid climate with abundant precipitations indicating a NAOphase. Between 1460 and 1930 AD, and despite gaps in our data, the site was characterized by a more arid climate, related to a NAO+ phase, with lower lake level and productivity, with carbon appearing to be from mixed sources, although a series of catastrophic events are apparent in the form of massive instantaneous alluvial depositions. Finally, the period 1930-1950 AD, corresponds to a humid phase where lake level and productivity is high suggesting a return to low NAO values, although not as high as during 1340-1460 AD, and organic matter has a high percentage of terrestrial content as a result of increased run-off. This indicates NAO variability might be the main factor controlling water level fluctuations. We conclude that in order to accomplish a better understanding of climate change associated to the NAO, further and more ample studies are needed.