Stefanie Schumacher

Microhabitat preferences and stable carbon isotopes of endobenthic Foraminifera: clue to quantitative reconstruction of oceanic new production?

Seventeen surface sediment samples from the North Atlantic Ocean off NE-Greenland between 76° and 81°N, and nine samples from the South Atlantic Ocean close to Bouvet Island between 48° and 55°S were taken with the aid of a Multiple Corer and investigated for their live (Rose Bengal stained) benthic foraminiferal content within the upper 15 cm of sediment. Preferentially endobenthic Melonis barleeanum, M. zaandami, and Bulimina aculeata as well as preferentially epibenthic Lobatula lobatula were counted from 1-cm-thick sediment slices each and analyzed for stable carbon and oxygen isotopic compositions of their calcareous tests. Live and dead specimens were counted and measured separately. The carbon isotopic composition of the foraminifera was compared to that of the dissolved inorganic carbon (DIC) of simultaneously sampled bottom water. During a period of one month one station off NE-Greenland was replicately sampled once every week and samples were processed as above.Live specimens of Lobatula lobatula are confined to the uppermost two centimeters of sediment. Live specimens of Melonis spp. are found down to eight centimeters within the sediment but with a distinct sub-surface maximum between two and five centimeters. The down-core distribution of live Bulimina aculeata shows a distinct surface maximum in the top centimeter and constant but low numbers down to 11 cm subbottom depth.The average stable carbon isotopic composition (d13C per mil VPDB) of live L. lobatula off NE-Greenland is by 0.4 ± 0.1 per mil higher than the d13CDIC of the ambient bottom water at the time of sampling. There is evidence that this species calcify before the ice-free season, when bottom water d13CDIC is supposed to be higher. This would reconfirm the one-to-one relationship between d13C of ambient water DIC and cibicids, widely used by paleoceanographers. Live M. barleeanum show a negative offset from bottom water DIC of -1.7 ± 0.6 per mil in the uppermost sediment and of -2.2 ± 0.5 per mil in three to four centimeters subbottom depth. All d13C values of live Melonis spp. decrease within the upper four centimeters, regardless of the time of sampling and site investigated. The offset of live B. aculeata from bottom water d13CDIC values of eight stations rather constantly amounts -0.6 ± 0.1 per mil, no matter what subbottom depth the specimens are from. At one station however, where is strong indication of elevated organic carbon flux, the negative offset averaged over all sub-bottom depths increases to -1.5 ± 0.2 per mil. Buliminids actively move within the sediment and by this either record an average isotope signal of the pore water or the signal of one specific calcification depth. The recorded signal, however, depends on the organic carbon flux and reflects general but site specific pore water d13CDIC values. If compared with epibenthic d13C values from the same site, not influenced by pore water and related phytodetritus layer effects, Bulimina d13C values bear some potential as a paleoproductivity proxy. Specimens of Melonis spp. seem to prefer a more static way of life and calcify at different but individually fix depths within the sediment. Although live specimens thus record a stratified pore water d13C signal, there is no means yet to correct for bioturbational and early diagenetic effects in fossil faunas.

COMMUNITIES AND MICROHABITATS OF LIVING BENTHIC FORAMINIFERA FROM THE TROPICAL EAST ATLANTIC: IMPACT OF DIFFERENT PRODUCTIVITY REGIMES

Living (Rose Bengal stained) benthic foraminifera were collected with a multicorer from six stations between 2°N and 12°S off West Africa. The foraminiferal communities in the investigated area reflect the direct influence of different productivity regimes, and are characterized by spatially and seasonally varying upwelling activity. At five stations, foraminiferal abundance coincides well with the gradient of surface productivity. However, at one station off the Congo River, the influence of strong fresh water discharge is documented. Although this station lies directly in the center of an upwelling area, foraminiferal standing stocks are surprisingly low. It is suggested that the Congo discharge may induce a fractionation of the organic matter into small and light particles of low nutritional content, by contrast to the relatively fast-sinking aggregates found in the centers of high productivity areas. Quality and quantity of the organic matter seem to influence the distribution of microhabitats as well. The flux of organic carbon to the sea-floor controls the sequence of degradation of organic matter in sediment and the position of different redox fronts. The vertical foraminiferal stratification within sediment closely parallels the distribution of oxygen and nitrate in porewater, and reflects different nutritive strategies and adaptation to different types of organic matter. The epifauna and shallow infauna colonize oxygenated sediments where labile organic matter is available. The intermediate infauna ( M. barleeanum ) is linked to the zone of nitrate reduction in sediments where epifaunal and shallow infaunal species are not competitive anymore, and must feed on bacterial biomass or on metabolizable nutritious particles produced by bacterial degradation of more refractory organic matter. The deep infauna shows its maximum distribution in anoxic sediments, where no easily metabolizable organic matter is available.