Geochemical analysis of surface sediments from the ocean floor and from sea ice in the northern Atlantic and Arctic Oceans

Barium in marine terrigenous surface sediments of the European Nordic Seas is analysed to evaluate its potential as palaeoproductivity proxy. Biogenic Ba is calculated from Ba and Al data using a conventional approach. For the determination of appropriate detrital Ba/Al ratios a compilation of Ba and Al analyses in rocks and soils of the catchments surrounding the Nordic Seas is presented. The resulting average detrital Ba/Al ratio of 0.0070 is similar to global crustal average values. In the southern Nordic Seas the high input of basaltic material with a low Ba/Al ratio is evident from high values of magnetic susceptibility and low Al/Ti ratios. Most of the Ba in the marine surface sediments is of terrigenous and not of biogenic origin. Variability in the lithogenic composition has been considered by the application of regionally varying Ba/Al ratios. The biogenic Ba values are comparable with those observed in the central Arctic Ocean, they are lower than in other oceanic regions. Biogenic Ba values are correlated with other productivity proxies and with oceanographic data for a validation of the applicability in paleoceanography. In the Iceland Sea and partly in the marginal sea-ice zone of the Greenland Sea elevated values of biogenic Ba indicate seasonal phytoplankton blooms. In both areas paleoproductivities may be reconstructed based on Ba and Al data of sediment cores.

Meiofauna of surface sediments in the Cretan Sea

Quantitative information on metazoan meiofaunal abundance and biomass was obtained from three continental shelf (at 40, 100 and 200 m depth) and four deep-sea stations (at 540, 700, 940 and 1540 m depth) in the Cretan Sea (South Aegean Sea, NE Mediterranean). Samples were collected on a seasonal basis (from August 1994 to September 1995) with the use of a multiple corer. Meiofaunal abundance and biomass on the continental shelf of the Cretan Sea were high, in contrast to the extremely low values reported for the bathyal sediments that showed values comparable to those reported for abyssal and hadal environments. In order to explain the spatial and seasonal changes in metazoan meiofauna these data were compared with: (1) the concentrations of 'food indicators' (such as proteins, lipids, soluble carbohydrates and CPE) (2) the bacterial biomass (3) the flux of labile organic compounds to the sea floor at a fixed station (D7, 1540 m depth). Highly significant relationships between meiofaunal parameters and CPE, protein and lipid concentrations and bacterial biomass were found. Most of the indicators of food quality and quantity (such as CPE, proteins and carbohydrates) showed a clear seasonality with highest values in February and lowest in September. Such changes were more evident on the continental shelf rather than at deeper depths. On the continental shelf, significant seasonal changes in meiofaunal density were related to changes in the input of labile organic carbon whereas meiofaunal assemblages on the deep-sea stations showed time-lagged changes in response to the food input recorded in February 95. At all deep-sea stations meiofaunal density increased with a time lag of 2 months. Indications for a time-lagged meiofaunal response to the food inputs were also provided by the increase in nauplii densities during May 95 and the increase in individual biomass of nematodes, copepods and polychaetes between February and May 1995. The lack of strong seasonal changes in deep sea meiofaunal density suggests that the supply of organic matter below 500 m is not strong enough to support a significant meiofaunal development. Below 700 m depth >92% of the total biomass in the sediment was represented by bacteria. The ratio of bacterial to meiofaunal biomass increased with increasing water depth indicating that bacteria are probably more effective than meiofauna in exploiting refractory organic compounds. These data lead us to hypothesise that the deep-sea sediments of the Cretan Sea are largely dependent upon a benthic microbial loop.

Bulk 15N, and 13Corg ratios, and calculation of unutilized nitrate of surface samples off the southwest coast of Africa (Table 1)

Bulk delta15N values in surface sediment samples off the southwestern coast of Africa were measured to investigate the biogeochemical processes occurring in the water column. Nitrate concentrations and the degree of utilization of the nitrate pool are the predominant controls on sedimentary delta15N in the Benguela Current region. Denitrification does not appear to have had an important effect on the delta15N signal of these sediments and, based on delta15N and delta13C, there is little terrestrial input.