Carbon Isotopes of Live Benthic Foraminifera from the South Atlantic: Sensitivity to Bottom Water Carbonate Saturation State and Organic Matter Rain Rates

Abstract

Live (Rose Bengal stained) and dead benthic foraminifera of surface and subsurface sediments from 25 stations in the eastern South Atlantic Ocean and the Atlantic sector of the Southern Ocean were analyzed to decipher a potential influence of seasonally and spatially varying high primary productivity on the stable carbon isotopic composition of foraminiferal tests. Therefore, stations were chosen so that productivity strongly varied, whereas conservative water mass properties changed only little. To define the stable carbon isotopic composition of dissolved inorganic carbon (δ13CDIC) in ambient water masses, we compiled new and previously published δ13CDIC data in a section running from Antarctica through Agulhas, Cape and Angola Basins, via the Guinea Abyssal Plain to the Equator. We found that intraspecific δ13C variability of all species at a single site is constantly low throughout their distribution within the sediments, i.e. species specific and site dependent mean values calculated from all subbottom depths on average only varied by ±0.09 ‰. This is important because it makes the stable carbon isotopic signal of species independent of the particular microhabitat of each single specimen measured and thus more constant and reliable than has been previously assumed. So-called vital and/or microhabitat effects were further quantified: (1) δ13C values of endobenthic Globobulimina ajfinis, Fursenkoina mexicana, and Bulimina mexicana consistently are by between −1.5 and −1.0 ‰ VPDB more depleted than δ13C values of preferentially epibenthic Fontbotia wuellerstorfi, Cibicidoidespachyderma, and Lobatula lobatula. (2) In contrast to the Antarctic Polar Front region, at all stations except one on the African continental slope Fontbotia wuellerstorfi records bottom water δ13CDIC values without significant offset, whereas L. lobatula and C. pachyderma values deviate from bottom water values by about −0.4‰ and −0.6‰, respectively. This adds to the growing amount of data on contrasting cibicid δ13C values which on the one hand support the original 1:1 -calibration of F. wuellerstorfi and bottom water δ13CDIC, and on the other hand document severe depletions of taxonomically close relatives such as L. lobatula and C. pachyderma. At one station close to Bouvet Island at the western rim of Agulhas Basin, we interpret the offset of −1.5 ‰ between bottom water δ13CDIC and δ13C values of infaunal living Bulimina aculeate in contrast to about −0.6 ± 0.1 ‰ measured at eight stations close-by, as a direct reflection of locally increased organic matter fluxes and sedimentation rates. Alternatively, we speculate that methane locally released from gas vents and related to hydrothermal venting at the mid-ocean ridge might have caused this strong depletion of 13C in the benthic foraminiferal carbon isotopic composition. Along the African continental margin, offsets between deep infaunal Globobulimina affinis and epibenthic Fontbotia wuellerstorfi as well as between shallow infaunal Uvigerina peregrine and F wuellerstorfi, δ13C values tend to increase with generally increasing organic matter decomposition rates. Although clearly more data are needed, these offsets between species might be used for quantification of biogeochemical paleogradients within the sediment and thus paleocarbon flux estimates. Furthermore, our data suggest that in high-productivity areas where sedimentary carbonate contents are lower than 15 weight %, epibenthic and endobenthic foraminiferal δ13C values are strongly influenced by 13C enrichment probably due to carbonate-ion undersaturation, whereas above this sedimentary carbonate threshold endobenthic δ13C values reflect depleted pore water δ13CDIC values.