ALBERT

Beschreibung: Analyses of stable isotopes of monospecific planktonic foraminifers (G. quadrilobatus group) and monogeneric benthic foraminifers (Cibicidoides spp.) from late Neogene Atlantic Site 502 and Pacific Site 503 were conducted in order to determine the paleoceanographic changes resulting from the late Neogene uplift of the Panama Isthmus and from climatic cooling. In general, results at each site are similar to those from previous studies for the late Miocene and late Pliocene time interval, documenting the late Miocene (6 Ma) shift in carbon isotopes and the inferred growth of permanent Northern Hemisphere continental ice sheets beginning about 3.2 Ma. Comparison of Atlantic-Pacific planktonic-benthic isotope data for four stratigraphic intervals (~6-8, ~5-6, ~3-5, and ~2-3 Ma) suggests that increasing isolation of Atlantic and Pacific low-latitude waters may be related to the emergence of the Panama Isthmus. The contrast between Atlantic and Pacific benthic foraminiferal d13C increased in two steps from 0.60 per mil to 1 per mil (the modern contrast) at about 6 Ma and 3 Ma. The first increase (0.15 per mil) may represent the end of previously limited deep-water communication between the Atlantic and Pacific at the present location of Panama. The second increase (0.25 per mil) may be due to increased production of North Atlantic Deep Water. This probably reflects the development of modern deep-sea circulation. The d18O of planktonic foraminifers begins to increase in Atlantic Site 502 at 4.2 Ma and may reflect the increasing salinity of the North Atlantic Ocean arising from diminishing surface-water exchange across Panama. This increase is clearly shown by contrasting the d18O of Atlantic and Pacific planktonic foraminifers, as well as the d18O of planktonic and benthic foraminifers at Site 502. This inferred increase in surface-water salinity begins at the time of increasing provinciality of Atlantic and Pacific planktonic foraminifers.

Beschreibung: Scientists from the Woods Hole Oceanographic Institution, the U.S. Navy, the State University of New York at Albany, Wesleyan University, Nine dives in the research submersible ?Alvin? were made into Great Abaco Submarine Canyon to depths ranging from 1850 to 3666 m. Our observations indicate that the walls of this canyon are distinctly terraced, consisting of nearly vertical to overhanging rock cliffs and intervening, less steep sediment-covered slopes. The wall rock consists mostly of massive, shallow-water limestones and dolostones of Cretaceous age, coated on exposed surfaces with manganese oxides. These rocks are heavily jointed/fractured and thus very blocky to angular in appearance, with sponges and other sessile organisms commonly attached. Talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments. Short-term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak, reaching velocities of only 10 cm/sec. This relatively placid setting is further corroborated by the abundance of turtle grass (Thalassia) found along the canyon axis. However, abundant subdued, symmetrical ripple marks and large scour depressions at the base of boulders, indicate that high-energy events sporadically impact the canyon axis. Contemporary erosional activity along the axis of the western (headward) part of the canyon appears to be more significant, as evidenced by asymmetrical ripple marks, sand waves and bioerosion. Great Abaco Canyon has evolved with time via a variety of processes, including: (1) faulting: (2) subsidence; (3) defacement; and (4) erosional down-cutting. The location, orientation and initiation of this canyon appear to be structurally controlled by the Great Abaco Fracture Zone during pre-Santonian time. Regional subsidence during the Mesozoic allowed the walls of Great Abaco Canyon to build vertically by accretion of shallow-water limestones, whereas joint-controlled defacement has widened the canyon while maintaining steep walls. Erosional down-cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically, particularly during the Miocene and Pleistocene.