A multi-proxy study of a Holocene peat sequence on Nightingale Island, South Atlantic

Abstract: Nightingale Island is situated at 37°S in the central part of the South Atlantic Ocean, which is a sensitive location for variations within the Atlantic Meridional Overturning Circulation (AMOC) and the Southern Hemisphere Westerlies (SHW) wind belt. A peat sequence from the wetland 2nd Pond has been investigated with emphasis on the composition of the deposits, loss on ignition (LOI), total Carbon (TC), Nitrogen (N), biogenic Silica (BSi), magnetic susceptibility (MS), diatom counting and macrofossil analysis. The proxies reveal a pattern of recurring cycles of short oscillations lasting between 200 and 2000 years. The oscillations are clearly seen by increased MS and decreased TC, N and LOI values. An age-model based on radiocarbon dating gives detailed information about the timing of the oscillations. The study also compares these results with a previous study from the same wetland. Ljung and Björck (2007) did similar investigations and found a pattern of fairly short oscillations, which they interpreted as periods with increased precipitation. The changing pollen assemblages were important for this conclusion. The oscillations are in general well correlated between the two sites within the wetland, but with some differences. These are mainly due to the exact timing of events and can mostly be explained by uncertainties within the age-models. The main forcing factors behind these precipitation-rich oscillations are thought to be changes of the position of SHW or increased SST (sea surface temperature) in the central South Atlantic Ocean. Increased SST may be due to a weaker AMOC and an interglacial bipolar seesaw might be in action although more studies are needed to confirm or reject the hypothesis. A short literature review of South American Holocene climate reconstructions reveals a very diffuse record. Many signs of climate change are found but few of them are consistent and synchronized between different sites, although geographical differences may be part of the explanation.