HR: 14:55h
AN: PP23C-06    [Abstracts]
TI: A Holocene Bipolar Seesaw Effect In The South Atlantic?
AU: * Bjorck, S
EM: svante.bjorck@geol.lu.se
AF: Geobiosphere Science Centre, Quaternary Sciences, Lund University, Solvegatan 12, Lund, 223 62, Sweden
AU: Ljung, K
EM: karl.ljung@geol.lu.se
AF: Geobiosphere Science Centre, Quaternary Sciences, Lund University, Solvegatan 12, Lund, 223 62, Sweden
AB: Almost 10 m of sediments were retrieved from an overgrown lake on Nightingale Island, Tristan da Cunha island group, situated in the centre of the South Atlantic. The sequence comprises 1 m of peat overlain by 8 m of lacustrine sediments with peat on top. It shows a high degree of variability in terms of lithology, geochemistry and pollen composition. More than 50 radiocarbon dates, mainly from terrestrial macro-fossils, form the basis of this high resolution study. They show that after 2 kyr of peat formation a hydrologic shift occurred, possibly when today´s westerlies circulation was established at c. 8.7 kyr BP, and an 8.4 kyr long lake phase followed until the lake grew over 0.3 kyr ago. The lake sediments are exceptionally rich in organic matter, interrupted by recurrent phases of increased mineral content. Such phases coincide with elevated C/N ratios and higher percentages of pollen taxa of upland types as well as decreased sulphur content and less local bog/aquatic pollen types. These periods are interpreted as being characterized by higher precipitation, resulting in increased surface run-off to the lake leading to the observed changes in the proxy records. One of the periods coincides with the Northern Hemisphere 8.2 kyr event. Coupled atmosphere-ocean model simulations of this assumed glacially triggered fresh-water pulse in the Labrador Sea have shown an almost immediate response in the Tristan da Cunha region; dwindling thermohaline circulation (THC) in the north increases sea surface temperatures (SST) in the central South Atlantic resulting in increased air humidity and precipitation, possibly exaggerated by the local orographic island effect. The proxy signature of the assumed response of the 8.2 kyr event on Nightingale Island is more or less identical to at least six other phases, which would imply that all the precipitation-rich periods were triggered by increased SSTs. This is, e.g., supported by our low sulphur values (and high C/S ratios) indicating less influx of sea salt sulphates from marine aerosols, which would increase if the elevated precipitation was caused by intensified westerlies. Phases of inferred increased precipitation, but with a different signature, can also be seen in the bottom peat. All these precipitation-rich phases, assumed to have been caused by high SST´s, are fairly well correlated to periods of increased IRD and dwindling THC in the North Atlantic, especially before 4 kyr BP, implying a bipolar seesaw mechanism in action in the Atlantic Ocean during the Holocene.
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1105 Quaternary geochronology
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 9345 Large bodies of water (e.g., lakes and inland seas) (0746)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting