ALBERT

Description: As a natural sediment trap, the marine sediments of the sheltered central part of the Maldives Inner Sea represent an exceptional archive for paleoenvironmental and climate changes in the equatorial Indian Ocean. To evaluate the complex interplay between high-latitude and monsoonal climate variability, related dust fluxes, and regional oceanographic responses, we focused on Fe ∕ Al, Ti ∕ Al and Si ∕ Ca ratios as proxies for terrigenous sediment delivery and total organic carbon (TOC) and Br XRF counts as proxies for marine productivity. Benthic foraminiferal fauna distributions, grain size and stable δ18O and δ13C data were used for evaluating changes in the benthic ecosystem and changes in the intermediate water circulation, bottom water current velocity and oxygenation. Our multi-proxy data record reveals an enhanced dust supply during the glacial intervals, causing elevated Fe ∕ Al and Si ∕ Ca ratios, an overall coarsening of the sediment and an increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilization during the cold phases resulted in an increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas with certain detritus and suspension feeders was fostered. The difference in the δ13C signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 µmol kg−1 during this time. The precessional fluctuation pattern of oxygen changes resembles that from the deep Arabian Sea, suggesting an expansion of the oxygen minimum zone (OMZ) from the Arabian Sea into the tropical Indian Ocean with a probable regional signal of strengthened winter-monsoon-induced organic matter fluxes and oxygen consumption further controlled by the varying inflow intensity of the Antarctic Intermediate Water (AAIW). In addition, the bottom water oxygenation pattern of the Maldives Inner Sea reveals a long phase of reduced ventilation during the last glacial period. This process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and the restriction of upper bathyal environments in the Inner Sea during sea-level lowstands. Thus, our multi-proxy record reflects a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital timescale.

Description: We present a detailed multi-proxy data record to reveal the late Quaternary changes in marine sedimentation and biogeochemical processes of the upper bathyal Maldives (equatorial Indian Ocean) and how they are related to the benthic ecosystem dynamics. We investigated the sediment core SO-236-052-4 from the central part of the Inner Sea, Maldives, focusing on Fe / Ca and Si / Ca ratios as proxies for terrigenous sediment delivery, as well as Total Organic Carbon (TOC) and Ba / Ca ratios as proxies for marine productivity. Benthic foraminiferal fauna distributions, sortable silt records and stable oxygen and carbon isotope analyses were used for reconstructing the past ecosystem, as well as changes in the intermediate water circulation, bottom water current velocity and oxygenation. This multi-proxy data record shows an enhanced dust supply during the glacial intervals, represented by increased Fe / Ca and Si / Ca ratios, an overall coarsening of the sediment and increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilisation during the cold phases resulted in increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas and the distribution of certain detritus and suspension feeders were fostered. The difference in the stable carbon isotope signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 μmol kg−1. The pattern of oxygen changes resembles that from the deep Arabian Sea suggesting an expansion of the Oxygen Minimum Zone (OMZ) from the Arabian Sea into the tropical Indian Ocean, further controlled by the inflow of the Antarctic Intermediate Water (AAIW). The precessional circulation pattern of the bottom water oxygenation is overprinted by glacial-/interglacial changes resulting in a long phase of reduced ventilation during the last glacial period. The latter process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and restriction of bathyal environments of the Inner Sea of the Maldives during sea-level lowstands. Thus, this multi-proxy record provide a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital time-scale.

Description: 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.

Description: Knowledge of past ocean productivity and changes in Indian Ocean water mass geometry is critical to understand climate variability in the east African tropics. Today, for example, the development of a more or less pronounced Oxygen Minimum Zone (OMZ) in the Somali Basin is triggered by monsoonal strength and weak thermocline ventilation. Up to now, however, Somali Basin paleoceanographic changes during Quaternary times are poorly investigated. Here we present results of micropaleontological, geochemical and sedimentological investigations on two 14CAMS-dated sediment cores from the southern Somali Basin, off Tanzania. Cores were retrieved during Meteor cruise M75 in 2008. Position and water depth of one core from 446 m water depth was chosen such that variations in intensity and extension of the OMZ, as well as changes in the influence of northern sourced Red Sea Water (RSW) and southern sourced Antarctic Intermediate Water (AAIW) were monitored on millenial timescales. A second core site at 1449 m water depth was selected to determine variations in spreading of northern sourced Indian Deep Water (IDW) and southern sourced Upper Circumpolar Deep Water (UCDW) on orbital time scales. In particular, we determined (i) δ18O values and Mg/Ca ratios of planktic foraminifera G. ruber and G. tumida to reconstruct ocean surface salinity and temperature changes and thermocline depth, and (ii) the benthic foraminiferal assemblage composition including δ13C values of epifaunal and deep infaunal species to account for changes in amount and seasonality of ocean surface productivity and deep-water ventilation. In addition, X-ray fluorescence analysis and measurements of magnetic suceptibility were carried out to facilitate easy intercorrelation between cores, and to decipher changes in the amount of terrestrial input.

Description: The processes that control past monsoon variability in the East African Tropics during the Holocene are poorly understood. Especially the role of Sea Surface Temperatures (SST) controlling East African Rainfall on millennial timescales, as it is observed on decadal timescales, is currently intensely debated. In addition, it has been suggested recently that the longitudinal migration of the Congo Air Boundary (CAB) modulates East African precipitation on a regional scale as well [Tierney et al., 2011]. Here, we present a high-resolution marine sediment record for the past 12 kyrs from offshore Tanzania, close to the Rufiji River delta, to contribute to the current debate from a marine point of view. We reconstructed past SST and !18Oseawater, derived from planktic foraminiferal Mg/Ca and !18O, and past Sea Surface Salinity (SSS) variations, derived from planktic foraminiferal Ba/Ca-ratios. In the vicinity of river deltas, Ba/Ca-ratios have potential to record precipitation changes in the rivers’ catchment area. Our records show that East African precipitation, derived from Ba/Ca-ratios, roughly varies in concert with Indian Ocean SST, suggesting higher Indian Ocean SST to be an important prerequisite for stronger precipitation, and hence an intense monsoon episode in East Africa. We calculated the difference ("SST) between our record of Indian Ocean SST and SST of the tropical Atlantic [Weldeab et al., 2005], showing that "SST variability resembles the isotopic pattern of the Kilimanjaro ice core record [Thompson et al., 2002]. We suggest this to be the consequence of a longitudinal movement of the CAB over the African Continent, changing the trajectory of Indian Ocean moisture into the continent and therefore affecting the !18O of the East African rainout.

Description: We reconstructed water column structure and surface productivity in the tropical western Indian Ocean to identify to what extent changes in surface water temperature and paleoproductivity are atmospherically controlled by the East African monsoon system or controlled by the ocean via subsurface water masses of Antarctic origin. For this purpose we analyzed proxy records of a 14C-AMS dated sediment Core GeoB12615-4 off Tanzania (07°08.30’S / 39°50.45’W, 446 m water depth) that spans the last 40 kyr. The hydrography at this site is influenced by a northwestern branch of the South Equatorial Current, the East African Coastal Current, which receives its water mainly via the Indonesian Throughflow and by equatorial upwelling of Subantarctic Mode Water (SAMW) at 5-10°S [Schott and McCreary, 2001]. The East African Monsoon may affect primary productivity and water mass stratification via nutrient and fresh water influx, respectively, from Rufiji River into the Indian Ocean. We used Mg/Ca ratios of the planktic foraminifer Globigerinoides ruber white (s.s.) for past sea surface temperature (SST) reconstructions and stable carbon isotope ratios of G. ruber white (s.s.) and the benthic foraminifer Planulina ariminensis for paleoproductivity calculations. We find strong correlations between paleoproductivity and SST. In addition, high δ13C values of bottom water coincide with low SST on centennial timescales. Most important, however, the SST pattern during the deglaciation resembles temperature records from continental Antarctic ice cores, with an early SST increase at ∼19 kyr BP and a distinct temperature setback, simultaneous to the Antarctic Cold Reversal. We suggest SAMW to be the most likely thermal link between western tropical Indian and Antarctic Oceans during the past 40 kyr and, moreover, to be the main control on marine primary productivity. Our conclusion corroborates earlier studies suggesting the SST in the Western Indian Ocean to be controlled by SAMW and thereby modulated by Antarctic temperature [Kiefer et al., 2006; Naidu and Govil, 2010]. Kiefer, T. McCave, I.N. and Elderfield, H. (2006). Antarctic control on tropical Indian Ocean sea surface temperature and hydrography, Geophys. Res. Lett, 33, L24612. Naidu, P. D., and P. Govil (2010). New evidence on the sequence of deglacial warming in the tropical Indian Ocean, Journal of Quaternary Science, 25(7), 1138-1143. Schott, F. A., and J. P. McCreary (2001). The monsoon circulation of the Indian Ocean, Progress In Oceanography, 51(1), 1-123.

Description: The importance of intermediate water masses in climate change and ocean circulation has been emphasized recently. In particular, Southern Ocean Intermediate Waters (SOIW), such as Antarctic Intermediate Water and Subantarctic Mode Water, are thought to have acted as active interhemispheric transmitter of climate anomalies. Here we reconstruct changes in SOIW signature and spatial and temporal evolution based on a 40 kyr time series of oxygen and carbon isotopes as well as planktic Mg/Ca based thermometry from Site GeoB12615-4 in the western Indian Ocean. Our data suggest that SOIW transmitted Antarctic temperature trends to the equatorial Indian Ocean via the "oceanic tunnel" mechanism. Moreover, our results reveal that deglacial SOIW carried a signature of aged Southern Ocean deep water. We find no evidence of increased formation of intermediate waters during the deglaciation.