Benthic foraminifera in South Atlantic Ocean surface sediments

Benthic foraminiferal assemblages were analyzed on forty-eight surfacesediment samples which were taken with a multiple corer from the southern South Atlantic Ocean and the northern Angola-Basin. The short cores were split into 1-cm-thick slices down to 15 cm sediment depth. Live (with Rose Bengal stained) and dead faunas were counted separately. The data were used to calculate numerical faunal characteristics like standing stocks, benthic foraminifera accumulation rates, diversity, average living depth, and habitat depth. Additionally the live and dead faunas were grouped into a number of principal faunal endmembers by Q-mode principal component analysis. Faunal composition, numerical faunal characteristics, and isotopic composition of calcareous tests were correlated with available environmental variables. Oxygen content in Pore water and rates of primary production were calculated by faunal composition and benthic foraminifera accumulation rate.
Some principal faunal endmembers of the analysis for the living fauna of the sediment surface, for the living fauna down to 11 cm sediment depth, and for the dead fauna of the sediment surface show almost identical assemblages. The total number of live specimens decreases with decreasing organic carbon flux rates. Living benthic foraminifera are found down to a maximum sediment depth of 11 cm in the southern South Atlantic Ocean and down to 5 cm in the northern Angola-Basin with a maximum number of specimens in the uppermost centimeter. Standing stock, species number, and diversity decrease with increasing sediment depth. Between 30 and 70 % of the total living population settles on the surface and within the uppermost centimeter of the sediment. Epifaunal species are living on the surface and within the first centimeter of the sediment. Most of infaunal species depict a maximum abundance near the surface. In sediments with low oxygen penetration depth specific calcareous species clearly depict a subsurface maximum.
Comparisons of pore water dissolved oxygen contents calculated according to the Benthic Foraminiferal Dissolved-Oxygen Index originally defined for paleobottom water dissolved oxygen contents by Kaiho (1994, doi:10.1130/0091-7613(1994)022<0719:BFDOIA>2.3.CO;2) with measured pore water oxygen data show good agreement only in sediments suffering from low oxygen conditions. Primary production values calculated according to the benthic foraminiferal accumulation rate method of Herguera and Berger (1991, doi:10.1130/0091-7613(1991)019<1173:PFBFAG>2.3.CO;2) can be too high compared to estimates primarily based on satellite observation data ("Dahlem" Map).
The average stable carbon isotopic composition of calcareous tests of live and dead infaunal species with a maximum abundance near the sediment surface (Bulimina aculeata, Trifarina angulosa and Uvigerina peregrina) does not vary significantly with the sediment depth. These species show a negative offset from bottom water d13C(DIC). Infaunal species with a maximum abundance near the sediment surface vertically migrate within the upper sediment. Therefore these species record an average isotope signal of the pore water which depends on the organic carbon flux and reflects general, but site specific average pore water d13C(DIC) values. High organic matter values and low oxygen contents in pore water are documented by low d13C values in the tests.