Abstract

A 2800-m-deep station (Station I) from the lower part of Cap-Ferret Canyon (Bay of Biscay) was sampled with a multitube corer in January 1999, June 1999 and April 2000. Four cores (with two replicate cores in April 2000) were investigated to study the stable carbon and oxygen isotopes of live (rose-Bengal-stained) foraminiferal taxa. Eight taxa were analyzed: Hoeglundina elegans, Cibicides wuellerstorfi, Uvigerina peregrina, Bulimina inflata, Melonis barleeanus, Pullenia quinqueloba, Chilostomella oolina and Globobulimina affinis. By using the apparent oxygen utilization of the lower Northeastern Atlantic Deep Water (NEADW) in our study area, we calculated the bottom-water δ13CDIC, which we compared with foraminiferal carbon isotope values. Next, we investigated the relationship between the foraminiferal δ13C and the microhabitat of investigated species. By using the value of δ18O (SMOW) for the lower NEADW, we calculated the equilibrium calcite δ18O of the bottom water, which we compared with the foraminiferal δ18O. The occurrence of a living holothurian in its deep infaunal burrow from one of two replicate cores collected in April 2000 (core B) allowed us to investigate the impact of macrofaunal activity on foraminiferal isotopes. Our results are finally compared with data from shallower open-slope stations close to our study area.

The δ13C signatures of most foraminiferal taxa are not correlated to the bottom-water δ13CDIC but seem to be controlled by a microhabitat effect. Only the δ13C of Cibicides wuellerstorfi is close to the bottom water δ13CDIC. When investigating oxygen isotopes, there is no obvious relationship between the foraminiferal microhabitat and the offset between the foraminiferal δ18O and the equilibrium calcite δ18O. The presence of a living holothurian had no obvious effect on the δ18O and δ13C of foraminifera occurring in the bioturbated interval. However, several individuals of Melonis barleeanus collected in the direct vicinity of the holothurian exhibited lower δ13C values, suggesting a potential influence of macrofaunal activity on the carbon isotopes of some intermediate and deep infaunal taxa calcifying in the deep sediment. The comparison of Δδ13C between Uvigerina peregrina, M. barleeanus and Globobulimina spp. with values recorded at shallower stations suggests that the focusing of organic matter in an intermediate state of decay, at our canyon station, has a weak impact on the biogeochemical processes deeper in the sediment. The δ13C of U. peregrina and the Δδ13C between U. peregrina and Globobulimina affinis appears definitively more sensitive to labile organic matter supplies than to the advection of low-quality, organic matter.

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