Geochemical analysis of surface sediments along the Portuguese Margin

Twenty-nine surface samples from the Portuguese shelf, recovered offshore from the mouths of the Ave, Douro, Lis and Mira rivers, were analysed using ICP-OES for selected major and trace elements, after total dissolution. Organic carbon, carbonate content and grain size were also determined. Five evaluation tools have been applied in order to compare the three study areas and to evaluate sediment geochemistry and other sediment compositional variability in the acquired samples: (1) empirical methods based on comparison with standard reference criteria, e.g. the NOAA sediment quality guidelines, (2) normalisation ratios using a grain-size proxy element, (3) "Gradient Method", plotting contaminant vs. organic matter or Al, (4) definition of a regional geochemical baseline from a compiled database, and (5) enrichment factors. The evaluation of element and component associations indicates differences related both to the onshore drainage areas and to the environmental shelf setting. Despite the considerable variability in total metal contents indicated by our results, the sediment metal composition is largely of natural origin. Metal enrichments observed in the Mira area are associated with the drainage of mineralised areas rich in Cu, Pb, Zn, Fe and Mn. The near absence of human impact on shelf sediments, despite the vicinity to urban areas with high industrialisation levels, such as the Ave-Douro and Lis areas, is attributed to effective trapping in the estuaries and coastal zones, as well dilution with less contaminated sediments shelf sediments and removal with fine fractions due to grain-size sorting. The character of the contaminated sediments transported to these shelf areas is further influenced by grain-size sorting as well as by dilution with less contaminated marine sediments. The results obtained individually by the different methods complement each other and allow more specific interpretations.

Seasonal variations of contribution of large and fine fractions of phytoplankton to primary production and chlorophyll a in the Sevastopol Bay from April 1985 till March 1986

It has been shown that in the Sevastopol Bay during the year primary production and chlorophyll "a" created by picoplankton (0.45-2.5 µm) consisted on the average 20-44% of total production. It was approximately a half of the level for oligotrophic waters of the ocean. Picoplankton of waters studied is represented by eucaryotes, cell diameter of which is, as a rule, about 2-3 µm. Contribution of the finest fraction of phytoplankton (0.43-0.85 µm) to primary production and con¬tent of chlorophyll "a" was insignificant (0-4%).

(Tables 2) The bPSi:POC ratio of the total diatom community, and abundance of full, empty, and broken diatom cells compared to heavily silicified diatoms during the EIFEX experiment

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms >10 µm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (>20 µm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 µm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

40Ar-39Ar investigation of volcanic clasts in glaciogenic sediments at ODP Sites 1097 and 1103

Three selected diamictite samples recovered within sequence group S3 at Sites 1097 (Sample 178-1097A-27R-1, 35-58 cm) and 1103 (Samples 178-1103A-31R-2, 0-4 cm, and 36R-3, 4-8 cm) of Ocean Drilling Program Leg 178 have been investigated by scanning electron microscope, electron microprobe, and 40Ar-39Ar laser-heating techniques. They contain variable proportions of fragments of volcanic rock groundmass (mostly in the range of 100-150 µm) with textures ranging from microcrystalline to ipocrystalline. Their rounded shapes indicate mechanical reworking. Fresh groundmass glasses, recognized only in grains from samples of Site 1103, show mainly a subalkaline affinity on the basis of total alkali-silica variations. However, they are characterized by relatively high TiO2 and P2O5 contents (1.4-2.8 and 0.1-0.9 wt%, respectively). Because of the small size of homogeneous grains (100-150 µm), they were not suitable for single-grain total fusion 40Ar-39Ar analyses. The incremental laser-heating technique was applied to milligram-sized samples (only for Samples 178-1097A-27R-1, 35-58 cm, and 178-1103A-36R-3, 4-8 cm) and to various small fractions (each consisting of 10 grains for the sample from Site 1097 and 30 grains for samples from Site 1103). The latter approach resulted in more effective resolution of sample heterogeneity. Argon ages from the small fractions show significantly different ranges in the three samples: 75-173 Ma for Sample 178-1097A-27R-1, 35-58 cm, 18-57 Ma for Sample 178-1103A-31R-2, 0-4 cm, and 7.6-50 Ma for Sample 178-1103A-36R-3, 4-8 cm. Ca/K ratios derived from argon isotopes at Site 1103 suggest that the data mainly refer to outgassing of groundmass glass. At Site 1103, we observe an overall apparent age increase with decreasing sample depth. This is compatible with glacial erosion that affected with time deeper levels of a volcanic sequence previously deposited on the continent. The youngest apparent age of 7.6 ± 0.7 Ma detected close to the bottom of Hole 1103A (340 meters below seafloor [mbsf]) is compatible with the age range of the diatom Actinocyclus ingens v. ovalis Zone (6.3-8.0 Ma) determined for the interval 320-355 mbsf and with the maximum ages derived from strontium isotope composition of barnacle fragments obtained at 262-263 mbsf at the same site. Nevertheless, this age cannot be taken as the maximum youngest age of the volcanic sequence sampled by glacial erosion or as the maximum age for the deposition of the Sequence S3 at 340 mbsf unless validated by further research.