Western Indian Ocean circulation and climate variability on different time scales

Andreas.Mackensen [ at ] awi.de


In order to understand the Earth’s climate evolution it is crucial to evaluate the role of low-latitude oceans in the global climate system, as they are connected to both hemispheres via atmospheric and oceanic circulation and thus hold the potential to disentangle the asynchronicity of short-term Pleistocene climate variability. However, the potential of low latitude oceans to respond to and force large-scale changes of the climate system is still debated. The aim of this thesis is to examine and to understand the causal relationship of both atmospheric and oceanic changes in the tropical western Indian Ocean on centennial-, millennial and glacial-interglacial timescales. For this purpose I investigated stable oxygen and carbon isotope compositions of both planktic and benthic foraminiferal tests, Mg/Ca ratios of planktic foraminiferal tests as well as benthic foraminiferal assemblages and sedimentary geochemical parameters on two sediment cores (GeoB12615-4, 446 m and GeoB12616-4, 1449 m) from the continental slope off Tanzania, East Africa. Time series of tropical Western Indian Ocean Sea Surface Temperatures (SST) based on Mg/Ca paleothermometry, in combination with planktic and benthic oxygen and carbon isotopes from sediment core GeoB12615-4 demonstrate that Southern Ocean Intermediate Waters (SOIW) acted as an interhemispheric transmitter of high southern latitude temperature trends over the past 40 kyr. Furthermore, the SOIW signature shows evidence for the deglacial release of deep-ocean sequestered carbon to the atmosphere, which reveals that the Southern Ocean played a pivotal role in not only modulating tropical climate but also realigning the global carbon system. Stable carbon and oxygen isotope as well as sediment elemental composition time series from sediment core GeoB12616-4 reveal that the Western Boundary Current of the Indian Ocean comprised primarily Southern Ocean sourced Upper Circumpolar Deep Water (UCDW) throughout the past 600 kyr. This uniform Southern Ocean deep water entered via the Amirante Passage or the Mozambique Channel and represents a downstream equivalent of South Atlantic UCDW. Short-term intrusions of Red Sea Water (RSW) along the African continental margin cannot clearly be confirmed at the study site. Benthic foraminiferal assemblages in combination with sediment elemental composition from sediment core GeoB12615-4 indicate dynamic changes of the marine environment as a response to increased East African rainfall and subsequent increased Rufiji River runoff during the East African Humid Period (12-5 kyr), as well as to the arid 8.2 kyr event, both concurrent with continental climate archives. Besides responding dynamically to glacial sea level variability and potential intermediate water rearrangement, benthic foraminiferal assemblages document that the profound postglacial sea level rise favoured the reestablishment of the East African fringe reefs.

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Romahn, S. (2014): Western Indian Ocean circulation and climate variability on different time scales , PhD thesis, Universität Bremen.

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