Physical oceanography in the Western Bransfield Strait and Gerlache Strait during Austral summer 1995/96

n the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hespérides in the eastern Bellingshausen Sea, western basin of the Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures of the study domain were the Southern Boundary of the ACC and the Bransfield Front. The characteristics and zonation of local water masses are discussed in terms of temperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis of chemical parameters in addition to the common theta/S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses.

Opal concentration of ODP Leg 178 holes

Neogene to Quaternary records of biogenic opal contents and opal accumulation rates are presented for Sites 1095, 1096, and 1101, which were drilled during Ocean Drilling Program Leg 178 in the Bellingshausen Sea, a marginal sea in the eastern Pacific sector of the Southern Ocean. The opal records in the drift sediments on the continental rise west of the Antarctic Peninsula provide signals of paleoproductivity, although they are influenced by dissolution in the water column and the sediment column. Opal contents at Sites 1095, 1096, and 1101 show similar long-term trends through the Neogene and Quaternary, whereas the opal accumulation rates exhibit marked discrepancies, which are caused by local differences in opal preservation linked to local variations of bottom current-induced supply of lithogenic detritus. We used a regression describing the relationship between opal preservation and sedimentation rate to extract the signal of primary opal deposition on the seafloor in the Bellingshausen Sea from the opal accumulation in the drift deposits. On long-term timescales, the reconstructed opal depositional rates show patterns similar to those of the opal contents and a much better coherency between the different locations on the Antarctic Peninsula continental rise. Therefore, the estimated opal depositional rates are suggested to represent a suitable proxy for paleoproductivity in the drift setting of the Bellingshausen Sea. Supposing that the sea-ice coverage within the Antarctic Zone was the main factor controlling biological productivity in the Bellingshausen Sea, and thus the estimated opal depositional rates on the continental rise, we reconstructed paleoceanographic long-term changes during the Neogene and Quaternary considering the climatic control on regional and global scales. Slightly enhanced opal depositional rates during the late Miocene are interpreted to indicate warmer climatic conditions in the vicinity of the Antarctic Peninsula than at present. The contribution of heat from the Northern Component Water (NCW) into the Southern Ocean seems only to have played a subordinate role during that time. High opal depositional rates during the early Pliocene document a strong reduction of sea-ice coverage and relatively warm climatic conditions in the Bellingshausen Sea. The early onset of the Pliocene warmth points to a positive feedback of regional Antarctic climate on the global thermohaline circulation. A decrease of opal deposition between 3.1 and 1.8 Ma likely reflects sea-ice expansion in response to reduced NCW flow, caused by the onset and intensification of Northern Hemisphere glaciation. Throughout the Quaternary, a relatively constant level of opal depositional rates in the Bellingshausen Sea indicates stable climatic conditions in the Antarctic Peninsula area.

Abundance, biomass, production, grazing, and biometric measurements of prokaryotes, nanoflagellates, ciliates, and dinoflagellates in three areas close to the Antarctic Peninsula in spring and summer 1995/96

Two cruises were carried out during the Austral spring-summer (November 1995 - January 1996: FRUELA 95, and January - February 1996: FRUELA 96), sampling in Bellingshausen Sea, western Bransfield Strait and Gerlache Strait. We investigated whether there were any spatial (among locations) or temporal (between cruises) differences in abundance and biomass of microbial heterotrophic and autotrophic assemblages. Changes in the concentration of chlorophyll a, prokaryotes, heterotrophic and phototrophic nanoflagellates abundance and biomass were followed in the above mentioned locations close to the Antarctic Peninsula. Parallel to these measurements we selected seven stations to determine grazing rates on prokaryotes by protists at a depth coincident with the depth of maximum chlorophyll a concentration. Measuring the disappearance of fluorescent minicells over 48 h assessed grazing by the protist community. From prokaryotes grazing rates, we estimated how much prokaryotic carbon was channeled to higher trophic levels (protists), and whether this prokaryotic carbon could maintain protists biomass and growth rates. In general higher values were reported for Gerlache Strait than for the other two areas. Differences between cruises were more evident for the oligotrophic areas in Bellingshausen Sea and Bransfield Strait than in Gerlache Strait (eutrophic area). Higher values for phototrophic (at least for chlorophyll a concentration) and abundance of all heterotrophic microbial populations were recorded in Bellingshausen Sea and Bransfield Strait during late spring - early summer (FRUELA 95) than in mid-summer (FRUELA 96). However, similar results for these variables were observed in Gerlache Strait as in spring-early summer as well as in mid-summer. Also, we found differences in grazing rates on prokaryotes among stations located in the three areas and between cruises. Thus, during late spring-early summer (FRUELA 95), the prokaryotic biomass consumed from the standing stock was higher in Bellingshausen Sea (26%/day) and Gerlache Strait (18-26%/day) than in Bransfield Strait (0.68-14%/day). During mid-summer (FRUELA 96) a different pattern was observed. The station located in Bellingshausen Sea showed higher values of prokaryotic biomass consumed (11%/day) than the one located in Gerlache Strait (2.3%/day). Assuming HNF as the main prokaryotic consumers, we estimated that the prokaryotic carbon consumed by heterotrophic nanoflagellates (HNF) barely covers their carbon requirements for growth. These results suggest that in Antarctic waters, HNF should feed in other carbon sources than prokaryotes.