Geochemistry of Mid-Atlantic Ridge sediments

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.

Silicoflagellates of ODP Leg 119 samples

The occurrence of Quaternary and Oligocene silicoflagellates at two Ocean Drilling Program (ODP) Leg 119 Holes (736A and 744A) on the Kerguelen Plateau in the Southern Ocean was investigated to compare species distributions to Northern Hemisphere floras. This abstract gives the data determined (Tables 1 and 2) for 24 samples and few preliminary remarks. Quaternary assemblages of Hole 736A are noteworthy for the absences of key North Pacific zonal guide species such as Bachmannocena quadrangula, Dictyocha aculeata, Dictyocha subarctios, and Distephanus octangulatus (Bukry and Monechi, 1985). Other species such as Distephanus floridus, Distephanus speculum elongatus, and Mesocena octagona show limited ranges in Hole 736A and may help to subdivide the Quaternary locally. The late Oligocene assemblages of Hole 744A contain widely distributed species of Distephanus and Naviculopsis, which permit correlation to lower latitude assemblages. They also contain the high-latitude acme of Distephanus raupii which was first noted at Deep Sea Drilling Project (DSDP) Hole 278 (56°3.42'S, 160°04.29'E, water depth 3689 m) by Perch-Nielsen (1975) and Bukry (1975). Study of Hole 744A assemblages suggests that D. raupii developed from pentagonal Dictyocha deflandrei deflandrei. A final note on the Hole 744A assemblages is the brief late Oligocene acme (25%) of Dictyocha sp. aff. D. spinosa in Sample 119-744A-13H-4, 65-67 cm, which provides a direct correlation to the acme (16%) in DSDP Sample 29-278-31R-CC (Perch-Nielsen, 1975) in the Southern Ocean. Most of the taxonomy used in the tables is documented in earlier publications of the DSDP Initial Reports (see Bukry in Volumes 16, 35, 37, 40, 44, 49, 54, 67, 68, 69, 81, and 95). Also, see Loeblich et al. (1968) and Perch-Nielsen (1985) for extensive taxonomy and illustrations.

Vertical microalgae fluxes at the mooring station LOMO-2 over the Lomonosov Ridge in the period from September 15, 1995 till August 16, 1996

The first studies of microalgae fluxes over the Lomonosov Ridge in the northern Laptev Sea were carried out with a sediment trap at the year-long mooring station LOMO-2, installed at 150 m depth from September 15, 1995 to August 16, 1996. These studies demonstrated essential seasonal variations of vertical microalgae flux. It was shown that in summer diverse flora (composed mainly of cryophylic diatoms) growed intensively beneath the permanent ice cover. Strongly pronounced seasonal variations of microalgae growth correlate closely with solar radiation. Exactly during the maximum insolation period, from the middle of July until the end of September, the microalgae flux was hundreds of times higher than that in the rest of the year. Summer values of the microalgae flux over the Lomonosov Ridge in the northern Laptev Sea were similar to those in the Weddell Sea (Antarctic) and exceeded summer flux values in the Norwegian and Greenland Seas and in the St. Anna Trough (northwestern Kara Sea).