ALBERT

Description: Benthic foraminifera from surface Sediments of the Ross Sea were studied to determine modern distributions of important assemblages. Factor analysis of the raw data distinguished nine significant factor assemblages which account for 86% of the raw data. These factor assemblages provide a means of understanding modern oceanographic and ecologic conditions because they show the response of this faunal group to different environments. Environmental conditions are in turn controlled by the modern climatic regime of the region. Four benthic assemblages from the relatively shallow (500 to 700 m) eastern Ross Sea Continental shelf are predominantly arenaceous. This may be because the relatively late seasonal breakup of pack ice inhibits productivity in the surface waters and permits a buildup of CO, thus causing the CCD to occur at shallow depths. On the Western part of the Continental shelf, three assemblages are composed primarily of calcareous species even though water depths are often greater there than they are in the east. One of these calcareous assemblages occurs in samples from water depths as great as 755 m in the southwestern part of the region, below the CCD as defined for the Ross Sea by previous workers. We relate this depressed CCD to early seasonal breakup of pack ice in the Western Ross Sea. Within the eastern arenaceous and Western calcareous regions on the Continental shelf, distributions of benthic assemblages are probably related to water depth and other ecologic variables. Two benthic assemblages from the Continental slope north of the Ross Sea also are calcareous. We attribute their distributions to high rates of productivity in the overlying surface waters, where the interface between eastward-flowing Circumpolar Deep Water and Ross Sea water masses is marked by a narrow zone of intense upwelling.

Description: Plankton records from three ATLANTIS II cruise tracks near the west African coast reveal a deep penetration of northern forms into tropical waters. They are carried there by the clockwise movement of the North Atlantic Gyre and are found mixed with subtropical and tropical forms. In light of these plankton records, and previous ones from other parts of the Gyre, a view of North Atlantic faunal parcelling is given. The traditional three-fold parcelling (northern, subtropical and tropical) is recognizable. However, gyral circulation tends to generate a rotary pattern of distribution with abundant faunal mixing. The subtropical fauna occupies the core of the Gyre and mixed faunal associations occur around the margins. The northern and tropical faunas contribute to these mixed faunal associations along clockwise dispersal routes.

Description: The strontium, magnesium, and calcium contents of the shells of the planktonic foraminifer Globorotalia truncatulinoides from continental rise sediments were determined by electron microprobe. This species spends the early part of its life cycle in the upper water column where it precipitates the lamellar portion of its shell. It then sinks below the permanent thermocline where it is secondarily calcified with a blocky calcite coating. This change of environments through the organism’s life allows a test of the hypothesis that an individual can change the composition of the carbonate material secreted in response to changes in the temperature of of the water mass in which it lives. All individuals except a juvenile showed decreasing magnesium contents in the outermost portion of their shells, corresponding to decreasing temperature in successively deeper water. The juvenile form showing no secondary calcification was uniform in composition. Because the Mg/Ca ratio in seawater is constant, this differentiation strongly supports the contention that an individual organism, as it precipitates shell material, can regulate its composition in response to environmental changes in temperature. No significant differences in composition were found between left- and right-coiling specimens. The difference between lamellar and blocky calcite was found to be significantly less in a sample from 10 cm below the top of the piston core than those from above or below this level. This difference is produced by a decrease in the magnesium content of the lamellar calcite and may correspond to a surface water temperature 2.5°C lower approximately 1,100 years ago.

Description: Plankton records from three ATLANTIS II cruise tracks near the west African coast reveal a deep penetration of northern forms into tropical waters. They are carried there by the clockwise movement of the North Atlantic Gyre and are found mixed with subtropical and tropical forms. In light of these plankton records, and previous ones from other parts of the Gyre, a view of North Atlantic faunal parcelling is given. The traditional three-fold parcelling (northern, subtropical and tropical) is recognizable. However, gyral circulation tends to generate a rotary pattern of distribution with abundant faunal mixing. The subtropical fauna occupies the core of the Gyre and mixed faunal associations occur around the margins. The northern and tropical faunas contribute to these mixed faunal associations along clockwise dispersal routes.