Age determination of sediment cores close to the Strait of Gibralta

A suit of sediment cores close to and south of the Strait of Gibraltar (12°-36°N, 500-2800 m water depth) were analyzed for stable isotopes in epibenthic foraminifers Cibicidoides wuellerstorfi and Planulina ariminensis. During peak glacial times, the data exhibit higher delta13C values of up to 1.6 per mil at intermediate depths near the Strait of Gibraltar (36°N). The values decrease to the south as evidenced by our data, but also to the north as revealed by data of intermediate depth cores north of 38°N (in Duplessy et al. (1987)). Thus, the distribution pattern of delta13C provides crucial evidence for an increased influence of nutrient depleted Mediterranean Outflow Water (MOW) on the glacial northeast Atlantic hydrography. During oxygen isotope Terminations I and II, the meridional carbon isotope gradient indicates a significantly decreased but still active MOW. As deduced from the delta18O fluctuations, the temperatures of the MOW in the Atlantic were lower during glacial times by as much as 5°C. During glacial times and during Termination I the maximum delta13C values of the MOW correlate with minimum values of the North Atlantic Deep Water (NADW) and vice versa. This inverse response to climatic change of the carbon isotope signals of both water masses indicates, that the supply of saline MOW to the north Atlantic may be less important for the formation of NADW than previously assumed.

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.