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

Abstract

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, Melonis 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 8 cm within the sediment but with a distinct sub-surface maximum between 2 and 5 cm. 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 ( δ 13C versus‰ PDB) of live Lobatula lobatula off NE-Greenland is by 0.4±0.1‰ higher than the δ 13C DIC 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 δ 13C DIC is supposed to be higher. This would reconfirm the one-to-one relationship between δ 13C of ambient water DIC and cibicids, widely used by paleoceanographers. Live Melonis barleeanum show a negative offset from bottom water DIC of −1.7±0.6‰ in the uppermost sediment and of −2.2±0.5‰ in 3–4-cm subbottom depth. All δ 13C values of live Melonis spp. decrease within the upper four centimeters, regardless of the time of sampling and site investigated. The offset of live Bulimina aculeata from bottom water δ 13C DIC values of 8 stations rather constantly amounts to −0.6±0.1‰, 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‰. 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 δ 13C DIC values. If compared with epibenthic δ 13C values from the same site, not influenced by pore water and related phytodetritus layer effects, Bulimina δ 13C 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 δ 13C signal, there is no means yet to correct for bioturbational and early diagenetic effects in fossil faunas.