Foraminifera of abyssal plains in the E South Atlantic

The abyssal zone (deep sea > 3000 m) of the world`s oceans is the target of an international biodiversity programme called "Census of Marine Life" (CoML). DIVA ("Diversity gradients in the deep sea of the Atlantic") is the German contribution to this international project and focuses on the three deep sea basins of the southern East-Atlantic (Guinea Basin, Angola B., Cape B.). General questions of the DIVA programme are diversity and biogeographic patterns of benthic fauna in the deep sea basins and its correlation with biotic and abiotic variables. The presented project proposal is integrated in the DIVA - programme as the samples which will be examined originate from the DIVA II expedition in February/March 2005 and were collected by the planned project PhD- student. Beside the general questions special interest will be put on the ecology of benthic as well as planktic Foraminifera and sedimentology, oceanography and system evolution. As Foraminifera occupy pelagic and neritic environments as well as epifaunal and infaunal microhabitats this group of protists is predestined to correlate faunal distribution with parameters of the sediment and the water column. Investigations will include the following topics: primary production in the water column, diversity within and between deep-sea basins, gradients of diversity along the N-S transect from the Guinea to the Cape Basin, influence of ecological factors on species diversity and distribution, variability between the different deep sea basins and the definition of biogeographical regions in the deep sea. Due to the expected low accumulation rate of deep sea sediments the sub - fossil record is achieved as the sampled soft-sediment cores reach a depth of at least 35 cm. This allows speculations about the evolution of the ecosystems and the water masses in the three investigated deep sea basins. To validate these results age dating of the sediments is planned by applying C14-dating on foraminiferan tests. As a sound base for all topics listed a detailed taxonomic study with light and electron microscopy will be done. To get ecologically relevant information like water temperature and salinity variations stable isotope studies (C, O) will be carried out. For ecological interpretations a morphogroup approach will be applied to foraminiferan tests and several sediment parameters will be compiled. All these data will be included in statistical analyses. Additional data from other programmes, e.g. DIVA I from the southern Angola Basin, will be available to compare results with the ongoing DIVA II programme.

In the three SE-Atlantic deep-sea basins - Guinea, Angola and Cape Basin - sediments and foraminifers were studied in Holocene sediment cores in similar water depths (c. 5100 - 5600 m). Multicorer samples of Holocene sediments were studied in 5 and 10 mm slices down to 35 mm sediment depth. These sediments represent archives to reconstruct sedimentation, foraminiferal distribution, biodiversity, oceanography and climatic signals in these deep-sea sediments. At large, the three basins show a clearly differing evolution during the Holocene. Live and dead benthic foraminiferal assemblages were investigated to decipher live mode and biodiversity patterns of these protists in abyssal depths. Microhabitat preferences of benthic foraminiferal taxa have been identified and clearly related to specific shell shapes. Both, shell shape and live mode influence the preservation potential of species and assemblages. The dependence of planktic and benthic foraminifers on environmental parameters allowed reconstructing their diversity patterns in the study areas as well as the oceanographic evolution. The most prominent factors influencing diversity trends in the SE Atlantic abyssal plains are nutrient supply and chemical properties of the water masses overlying the sample sites (mainly corrosiveness concerning carbonate). The biogeographical distribution of benthic foraminifers is not strongly influenced by geographical barriers such as the Walvis Ridge and the Guinea Rise. A general increase in diversity with decreasing latitude is evident in the foraminiferal fauna. Sediment analyses (grain size, carbonate content, organic carbon, fragmentation of foraminiferal tests) offered the possibility to reconstruct changes in bioproductivity, surface and deep water circulation and terrigenous sediment flux. Guinea Basin samples are largely unaffected by changes in dissolution and delivered just slightly increasing carbonate contents during the Holocene owing to a predominant influence of North Atlantic Deep Water north of the Guinea Rise. Angola Basin samples delivered a carbonate peak around 8,200 years before present (BP) possibly indicating a connection to the North Atlantic cold event about the same time interconnected with climatic changes on the African Continent. Cape Basin localities show a pattern of enhanced carbonate preservation around 12,000 years BP possibly indicating a delayed Last Glacial Maximum signal. This pattern, which is typical for Pacific records, clearly points to an influence of Antarctic Bottom Water at water depths below 5,000 m in the Northern Cape Basin. Changes in the surface water stratification and thermocline depth during the Late Holocene are evident in the equatorial East Atlantic (Guinea Basin). Increasing stable oxygen isotope trends point to an intensification of equatorial upwelling during the Late Holocene. This trend is interrupted by a sharp decline around 3,200 years BP which can be explained by increasing monsoon activity in the Sahel region and an aligned deepening of the thermocline in the tropical East Atlantic. The following trend of a shallowing thermocline correlates with the final aridification of the Sahara during the Late Holocene corresponding with other terrestrial and shallow marine records and supporting the hypothesis of centennial to millennial scale fluctuations of the West African monsoon system.