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-Gre enland between 76 degrees and 81 degreesN, and nine samples from the South Atlantic Ocean close to Bouvet Island between 48 degrees and 55 degreesS we re taken with the aid of a Multiple Corer and investigated for their live ( Rose Bengal stained) benthic foraminiferal content within the upper 15 cm o f sediment. Preferentially endobenthic Melonis barleeanum, Melonis zaandami , and Bulimina aculeata as well as preferentially epibenthic Lobatula lobat ula were counted from 1-cm-thick sediment slices each and analyzed for stab le carbon and oxygen isotopic compositions of their calcareous tests. Live and dead specimens were counted and measured separately. The carbon isotopi c composition of the foraminifera was compared to that of the dissolved ino rganic carbon (DIC) of simultaneously sampled bottom water. During a period of one month, one station off NE-Greenland was replicately sampled once ev ery week and samples were processed as above. Live specimens of Lobatula lobatula are confined to the uppermost two centi meters of sediment. Live specimens of Melonis spp. an found down to 8 cm wi thin the sediment but with a distinct sub-surface maximum between 2 and 5 c m. The down-core distribution of live Bulimina aculeata shows a distinct su rface maximum in the top centimeter and constant but low numbers down to Il -cm subbottom depth. The average stable carbon bun isotopic composition (delta C-13 versus parts per thousand PDB) of live Lobatula lobatula off NE-Greenland is by 0.4 +/- 0.1 parts per thousand higher than the delta C-13(DlC) of the ambient bott om water at the time of sampling. There is evidence that this species calci fy before the ice-free season, when bottom water beta C-13(DIC) is supposed to be higher. This would reconfirm the one-to-one relationship between del ta C-13 of ambient water DIC and cibicids, widely used by paleoceanographer s. Live Melonis barleeanum show a negative offset from bottom water DIC of -1.7 +/- 0.6 parts per thousand in the uppermost sediment and of -2.2 +/- 0 .5 parts per thousand in 3-4-cm subbottom depth. All parts per thousand C-1 3 values of live Melonis spp. decrease within the upper four centimeters, r egardless of the One of sampling and site investigated. The offset of live Bulimina aculeata from bottom water delta C-13(DlC) values of 8 stations ra ther constantly amounts to -0.6 +/- 0.1 parts per thousand, no matter what subbottom depth the specimens are from. At one station however, where is st rong indication of elevated organic carbon flux, the negative offset averag ed over all sub-bottom depths increases to -1.5 +/- 0.2 parts per thousand. Buliminids actively move within the sediment and by this either record an a verage isotope signal of the pore water or the signal of one specific calci fication depth. The recorded signal, however, depends on the organic carbon flux and reflects general but site-specific pore water delta C-13(DlC) val ues. If compared with epibenthic delta C-13 values from the same site, not influenced by pore water and related phytodetritus layer effects, Bulimina delta C-13 values bear some potential as a paleoploductivity proxy. Specime ns 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 sp ecimens thus record a stratified pore water delta C-13 signal, there is no means yet to correct for bioturbational and early diagenetic effects in fos sil faunas. (C) 2000 Elsevier Science B.V. All rights reserved.