Abyssal benthonic foraminifera and the carbonate saturation of sea water and a benthonic foraminferal carbonate saturation history for the Cape Verde Basin for the last 550,000 years
Authors:   ,   Massachusetts Institute of Technology, Bremer Mary Lee ,   Woods Hole Oceanographic Institution
Published: November 2024

The distribution of certain Atlantic Ocean benthonic foraminifera is most consistently correlated with the degree of saturation of seawater with respect to calcium carbonate. The relative abundance of cibicidids, miliolids, Uvigerina and Globocassidulina subglobosa increases in saturate water and the relative abundance of Nuttlaides umbonifera increases in undersaturated water. It is suggested that this is a response of living benthonic foraminifera. Using this relationsship it is possible to recognize changes in the "paleosaturation" of deep and bottom water from fossil deep-sea benthonic foraminifera. The deepest water in the western Atlantic is barred from flowing into the eastern Atlantic by topographic barriers such as the Walvis Ridge and the mid-Atlantic Ridge. The major conduits of the depest and most dense water into the eastern Atlantic are the fracture zones on the ridge such as the Romanche, Chain ema Fracture Zones. The carbonate saturation of the Cape Verde Basin is very snsitive to the hydrography at these sill depths in the western Atlantic. The four cores studied range from 2471-4696m. They are relatively isolated from the influx of terrigenous detritus and have low sedimentation rates which vary between 0.5 and 0.7 cm/1000 years. They do no show the Atlanic Ocean low-carbonate glacial, high-carbonate interglacial cycles seen in other, higher sedimentation rate cores. This supports the contention of Broecker et al. (1958), Needham et al. (1969), Ruddiman (1971), Hays and Perruzza (1972) and Damuth (1977) that the cycles are produced by masking of carbonate by terrigenous detritus. The glacial-interglacial variability (in particular, increases in U. peregrina during glacial times) observed in higher latitude cores (Streeter and Shackleton, 1979; Schnitker, 1979; Croliss, in preparation) was not observed. A carbonate saturation history for the Cape Verde Basin for the last 550,000 years based on benthonic foraminifera shows that undersaturation in the Cape Verde Basin was as great or greater that it is today at 120,000-180,000 yr. B.P., 280,000 yr. B.P., 375-000-425,000 yr. B.P. and 525,000 yr. B..P. The inferred carbonate saturation changes could have been produced by a decrease of less than 1° C in bottom-water temperature at the sill depths of the Romanche, Chain and Vema Fracture Zones. The increases in undersaturation are no correlative with glacial-interglacial climate change. They are, in general, correlative with upward migrations of what is reinterpreted here to be the undersaturation biofacies at the Rio Grande Rise in the southwestern Atlantic (Lohmann, 1978) and in the southeastern Indian Ocean (Corliss, 1979). In both the southeastern Indian Ocean an in the Cape Verde Basin, the greatest increase in undersaturation occurs between 375,000-425,000 yr. B.P. These upward migrations are interpreted to represent shoaling of the deep-water bottom-water transition.