Holocene climate and environmental dynamics on the Tristan da Cunha island group, South Atlantic

Abstract: It has been shown that the present interglacial has been climatically dynamic with both rapid and severe climatic fluctuations. The spatial and temporal differences of these changes are however not fully resolved. The aim of this thesis is to present a detail high resolution record of past climate changes from the central South Atlantic. Peat and lacustrine sediment sequences were retrieved from the Tristan da Cunha island group in the central South Atlantic. These sequences were analysed with a multiproxy approach including pollen analysis, measurements of total carbon, nitrogen and sulphur, mineral magnetic measurements and detailed radiocarbon dating. In addition tree-ring widths from two trees are presented. The oldest sediment sequence extends back to 10 700 cal. yrs BP and the analyses show that the climate varied considerably during this time. Peat was accumulating from 10 700 cal. yrs BP to around 8600 cal. yrs BP when it was replaced by gyttja deposition. This change implies a substantial increase in effective humidity probably by precipitation. The cause of this change was probably a northward shift of the Southern Hemisphere westerlies, related to a large scale reorganization of the circulation system. This was probably caused by cooling of the Southern Ocean and the Atlantic sector of Antarctica, which caused expansion of sea ice and northward movement of the westerlies. Several periods with increased catchment erosion and further increased precipitation follow this transition. These periods were probably caused by increased air humidity as a response to higher sea surface temperatures. In the North Atlantic conditions were cool with increased deposition of ice rafted debris. Probably, these changes was caused by weaker meridional overturning circulation and less northward transport of warm shallow water in the Atlantic, which caused higher sea surface temperatures in the central South Atlantic and cooler conditions in the North Atlantic. One period of increased precipitation and sea surface temperature is contemporaneous with the 8.2 ka event in the Greenland ice cores and is probably the South Atlantic response to the weaker circulation induced by freshwater forcing in the North Atlantic. Tristan da Cunha was discovered in 1506 AD by Portuguese explorers and their arrival is clearly seen in the pollen diagram. The introduced species Plantago lanceolata appears around 1550 AD. The seal hunting and establishment of the permanent settlement caused introduction of more species and clearing of the lowland forest.