Holocene climate and vegetation dynamics on Nightingale Island, South Atlantic—an apparent interglacial bipolar seesaw in action?

Abstract

High resolution analyses of a sediment sequence from an in-filled lake on Nightingale Island at 37°S in the South Atlantic have revealed hydrological variations during the last 10700calyrBP, which are linked to shifts in the Southern Hemisphere westerlies and the Meridional Overturning Circulation (MOC) in the Atlantic. The lowermost part of the sequence, 10700–8600calyrBP, consists of peat. The peat is overlaid by gyttja, which continues up to the final overgrowing of the basin around 500 years ago. The transition from peat to gyttja at 8600calyrBP implies a significant increase in effective humidity, probably caused by a northward shift of the Southern Hemisphere westerlies after the end of the Antarctic thermal optimum. Recurring phases of increased erosion with high deposition of minerogenic particles, terrestrial organic matter (high C/N ratios), and pollen grains from plant taxa growing in more distal parts of the catchment, indicate periods with higher precipitation. These high precipitation periods correspond to periods of southward shifted Southern Hemisphere Westerlies inferred from South America and with periods of weaker MOC in the North Atlantic. Therefore we conclude that these periods, also characterized by decreased influence from marine aerosols, were not caused by the stronger westerlies, but by weaker MOC and less heat transport from the South to the North Atlantic causing warmer sea surface temperatures (SSTs) and increased precipitation in the South Atlantic. This is the expression of the bipolar seesaw mechanism and implies that it may have been operational also during interglacial conditions.