Down-core distribution of live and dead benthic foraminifera in deep sea sediments from the Weddell Sea and the Californian continental borderland

Five short cores sub-sampled from box cores from three sites in the eastern Weddell Sea off Antarctica and in the eastern Pacific off southern California, covering a range in water depth from 500 to 2000 m, were analysed for the down-core distribution of live (stained with Rose Bengal) and dead benthic foraminifera. In the California continental borderland, Planulina ariminensis, Rosalina columbiensis and Trochammina spp. live attached to agglutinated polychaetes tubes that rise above the sedimentwater interface. Bolivina spissa lives exclusively in or on the uppermost sediment. Stained specimens of Chilostomella ovoidea are found down to 6 cm within the sediment and specimens of Globobulimina pacifica down to a maximum of 8 cm. Delta13C values of live G. pacifica decrease with increasing depth from the sediment surface down to 7 cm core depth, indicating that this infaunal species utilizes13C-depleted carbon from pore waters. In the dead, predominantly calcareous benthic forminiferal assemblage, selective dissolution of small delicate tests in the upper sediment column causes a continuous variation in species proportions. In the eastern Weddell Sea, the calcareous Bulimina aculeata lives in a carbonate corrosive environment exclusively in or on the uppermost sediment. The arenaceous Cribrostomoides subglobosum, Recurvoides contortus and some Reophax species are frequently found within the top 4 cm of the sediment, whereas stained specimens of Haplophragmoides bradyi, Glomospira charoides and Cribrostomoides wiesneri occur in maximum abundance below the uppermost 1.5 cm. Species proportions in the dead, predominantly arenaceous, benthic foraminiferal assemblage change in three distinct steps. The first change is caused by calcite dissolution at the sediment-water interface, the second coincides with the lower boundary of intense bioturbation, and the third results from the geochemical shift from oxidizing to reducing conditions below a compacted ash layer.

Benthic anammox and dentrification rates measured in a slurry incubation experiments at four different stations in the Arabian Sea off Pakistan during the METEOR cruise M74/2 in 2007

Vertical distributions of benthic denitrification and anammox rates within the sediment were estimated from slurry incubation experiments. Rates were used to calculate the contribution of anammox and denitrification to the total N-loss. Briefly, MUC sediment cores were sliced in 2 cm intervals and the sediment was diluted and incubated with degassed bottom water in a gas tight bag. After pre-incubating the bags for 2 h, 15N-labeled substrates were injected into the bags and the slurries were thoroughly mixed. Incubations were performed in the dark at in situ temperatures. The N2 isotope ratio (28N2, 29N2, and 30N2) was determined by gas chromatography-isotopic ratio mass spectrometry (VG Optima, Micromass) and calculated according to Kuypers et al. (2005) and Holtappels et al. (2011), respectively.Furthermore, total organic carbon and nitrogen concentrations were measured of core sediment layers corresponding to those used for rate measurements. Concentrations of organic carbon and nitrogen were determined by combustion/gas chromatography (Carlo Erba NA-1500 CNS analyzer) of dried sediment samples after acidification. The same sediment layer were also used to extract nucleic acids. The concentrations of the DNA in the samples were measured spectrophotometrically with a NanoDrop instrument (Thermo Fisher Scientific Inc.). The biomarker functional gene nirS, encoding the cd1-containing nitrite reductase, for both denitrifiers and marine anammox bacteria were quantified with real-time PCR, using the primers cd3aF/R3cd (5'-GTSAACGTSAAGGARACSGG-3' (Michotey et al., 2000)/5'-GASTTCGGRTGSGTCTTGA-3'; Throback et al., 2004) and Scnir372F/Scnir845R (5'-TGTAGCCAGCATTGTAGCGT-3'/5'-TCAAGCCAGACCCATTTGCT-3'; Lam et al., 2009).