Calcareous nannofossil biostratigraphy of the central East Pacific Rise

During Leg 92 of the Deep Sea Drilling Project, sediments containing calcareous nannofossils of latest Oligocene to Holocene age were recovered from 14 holes at six sites (597 to 602) along the East Pacific Rise. The combined sections yield a virtually complete record for the region, with a compressed upper Miocene to Pleistocene interval. The nannofossil content of 14 U.S.N.S. Eltanin piston cores from the study area were also examined in order to supplement data generated during Leg 92. Two taxonomically new combinations are presented: Sphenolithus umbellus and Pontosphaera segmenta. Assemblages of calcareous nannofossils juxtaposed in reversed stratigraphic order within the upper Miocene provide strong evidence for downslope transport of sediments along the East Pacific Rise during the Messinian. Narrow bands of dark metalliferous sediment of coccolith Zone CN8b alternate with normal light-colored, in situ, pelagic sequences of Zone CN9b. This may indicate more vigorous bottom current activity between 5.40 and 6.70 Ma.

Physical properties nd chemistry of igneous rocks from the Pigafetta and East Mariana Basins

Physical properties of basalts from Ocean Drilling Program Sites 800 and 801 in the Pigafetta Basin and Site 802 in the East Mariana Basin, including porosity, wet-bulk density, grain density, compressional wave velocity, and thermal conductivity, were measured aboard JOIDES Resolution during Leg 129. The ranges for the properties are large, as typified by the velocity, which varies from 3.46 to 6.59 km/s. Extensively altered basalts immediately above and below a silicified hydrothermal deposit (60-69 m sub-basement depth) at Site 801 display the highest porosity, and lowest bulk density, velocity, and thermal conductivity, whereas the slightly altered rocks from Site 802 and the lowermost part of Site 801 represent the other extreme in physical properties variations. In order to better establish the relationship between physical properties and alteration of the rocks, the compressional wave velocities were compared with results from major and trace elemental analyses and petrographic examination of select samples. For the Leg 129 basalts, velocity displays a generally consistent decrease with increasing K2O, H2O+, loss on ignition, and Rb contents and the value of Fe3+/FeT and decreasing concentrations of SiO2, FeOT, CaO, MgO, and MnO. These trends are consistent with trends documented for the progressive alteration of oceanic crust and indicate that on a laboratory sample scale, basalt alteration is largely responsible for the variation of the physical properties of basalts sampled at Sites 800, 801, and 802.

Table 1: Technetium-99 in seawater in the West Spitsbergen Current and adjacent areas, sorted from east to west

99Tc levels were measured in seawater samples collected between 2000 and 2002 in the West Spitsbergen Current (WSC) and along the western coast of Svalbard or Spitzbergen and compared with available oceanographic 3-D modelling results for the late 1990s. Additional data from related regions are also presented in order to support the data interpretation. The seawater in the Arctic fjord Kongsfjorden on the western coast of Svalbard is influenced by the WSC, as shown by the 99Tc levels in surface water. By means of the WSC, 99Tc reaches the Eastern Fram Strait, where one branch of the WSC turns west into the East Greenland Current (EGC), and another branch continues northwards into the Arctic Ocean. Surface seawater collected in the central part of the WSC during a cruise on board the R/V "Polarstern" in the summer of 2000, showed higher levels of 99Tc than samples measured in Kongsfjorden in the spring of 2000. However, all levels measured in surface water are of the same order of magnitude. Data from sampling of deeper water in the WSC and EGC provide information pertaining to the lateral distribution of 99Tc. In all vertical profiling surveys (conducted in spring and summer), the highest levels of 99Tc were found in surface water. Comparison with oceanographic 3-D modelling indicates both significant seasonal variations in the lateral stratification of the WSC and variations with depth over shorter vertical distances. This information can be applied in sampling strategies, environmental monitoring, long-range transport of pollutants and physical oceanography.

Trace metals in shells of mussels and clams from deep-sea hydrothermal vent fields of the Mid-Atlantic Ridge and East Pacific Rise

Bioaccumulation of trace metals in carbonate shells of mussels and clams was investigated at seven hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, Rainbow, and Broken Spur) and the Eastern Pacific (9°N and 21°N at the East Pacific Rise and the southern trough of Guaymas Basin, Gulf of California). Mineralogical analysis showed that carbonate skeletons of mytilid mussel Bathymodiolus sp. and vesicomyid clam Calyptogena m. are composed mainly of calcite and aragonite, respectively. The first data were obtained for contents of a variety of chemical elements in bivalve carbonate shells from various hydrothermal vent sites. Analyses of chemical compositions (including Fe, Mn, Zn, Cu, Cd, Pb, Ag, Ni, Cr, Co, As, Se, Sb, and Hg) of 35 shell samples and 14 water samples from mollusk biotopes revealed influences of environmental conditions and some biological parameters on bioaccumulation of metals. Bivalve shells from hydrothermal fields with black smokers are enriched in Fe and Mn by factor of 20-30 relative to the same species from the Menez Gwen low-temperature vent site. It was shown that essential elements (Fe, Mn, Ni, and Cu) more actively accumulated during early ontogeny of the shells. High enrichment factors of most metals (n x 100 - n x 10000) indicate efficient accumulation function of bivalve carbonate shells. Passive metal accumulation owing to adsorption on shell surfaces was estimated to be no higher than 50% of total amount and varied from 14% for Fe to 46% for Mn.

XRD mineralogy of sediments from the Greenland east coast

Quantitative X-Ray Diffraction (qXRD) analysis of the <2 mm sediment fraction from surface (sea floor) samples, and marine sediment cores that span the last 10-12 cal ka BP, are used to describe spatial and temporal variations in non-clay mineral compositions for an area between Kangerlussuaq Trough and Scoresby Sund (?67°-70°N), East Greenland. Bedrock consists primarily of an early Tertiary alkaline complex with high weight% of pyroxene and plagioclase. Farther inland and to the north, the bedrock is dominantly felsic with a high fraction of quartz and potassium feldspars. Principal Component (PC) analysis of the non-clay sediment compositions indicates the importance of quartz and pyroxene as compositional end members, with an abrupt shift from quartz and k-feldspar dominated sediments north of Scoresby Sund to sediments rich in pyroxene and plagioclase feldspars offshore from the early Tertiary basaltic outcrop. Coarse (<2 mm or <1 mm) ice-rafted sediments are largely absent from the trough sediments between ?8 and 5 cal ka BP, but then increase in the last 4 cal ka BP. Compositional unmixing of the sediments in Grivel Basin and Kangerlussuaq Trough indicate the dominance of local over long distance sediment sources, with pulses of sediment from tidewater glaciers in Kangerlussuaq and Nansen fjords reaching the inner shelf during the Neoglaciation. The change in IRD is more dramatic in the sediment grain-size proxies than in the quartz wt%. Forty to seventy percent of the variance in the quartz records from either side of Denmark Strait is explained by low frequency trends, but the data from the Grivel Basin, East Greenland, are distinctly different, with an approximate 2500 yr periodicity.

Mineralogy ol late Quaternary marine sediments from the West and East Greenland shelves offshore from early Tertiary basalt outcrops

We use quantitative X-ray diffraction to determine the mineralogy of late Quaternary marine sediments from the West and East Greenland shelves offshore from early Tertiary basalt outcrops. Despite the similar basalt outcrop area (60 000-70 000 km**2), there are significant differences between East and West Greenland sediments in the fraction of minerals (e.g. pyroxene) sourced from the basalt outcrops. We demonstrate the differences in the mineralogy between East and West Greenland marine sediments on three scales: (1) modern day, (2) late Quaternary inputs and (3) detailed down-core variations in 10 cores from the two margins. On the East Greenland Shelf (EGS), late Quaternary samples have an average quartz weight per cent of 6.2 ± 2.3 versus 12.8 ± 3.9 from the West Greenland Shelf (WGS), and 12.02 ± 4.8 versus 1.9 ± 2.3 wt% for pyroxene. K-means clustering indicated only 9% of the samples did not fit a simple EGS vs. WGS dichotomy. Sediments from the EGS and WGS are also isotopically distinct, with the EGS having higher eNd (-18 to 4) than those from the WGS (eNd = -25 to -35). We attribute the striking dichotomy in sediment composition to fundamentally different long-term Quaternary styles of glaciation on the two basalt outcrops.

Sedimentology of cores from the shelf of Lazarev Sea, East Antarctica

Distinct facies types, classified in radiocarbon-dated sediments from the shelf of the Lazarev Sea, East Antarctica, reveal a detailed history of processes that have controlled sedimentation during the deglaciation over the last 10,000 yr. The ice retreat on this part of the Antarctic shelf started 9500 yr BP, marked by the deposition of laminated sediments, deposited from a floating ice shelf. These laminites, which occur on top of diamictons laid down from a grounded ice sheet, are the basal sediments of the postglacial sequence. The intensity of the Antarctic Coastal Current (ACC), directed by shelf morphology, controlled sedimentation of the postglacial facies. A residual glaciomarine sediment with the fine fraction winnowed by strong currents developed from 9000-8000 yr BP in the western part of the investigation area and from 9000-5000 yr BP in the eastern part, closer to the prominent 'Fenno Deep' trough. Current velocities apparently decreased between 8000 and 2000 yr BP due to a deflection of the ACC by advancing ice tongues to the east of the investigation area during the 'Hypsithermal'. This led to a deposition of fine-grained sediments, and clay mineralogy suggests a continental source, possibly near the grounding line of the Nivl Ice Shelf, rather than a winnowing of sediments near the shelf break or advection from deeper water. Current velocities intensified after 2000 yr BP, removed fine material from these sediments and led to a relict sediment, consisting of coarse bryozoan and molluscan debris.

(Table 2) Chemical composition of bottom sediments from the Equatorial East Pacific

Fe, Mn, Cu, Ni, and Co contents in bottom sediment samples from the Clarion Clipperton fracture zone and Guatemala Basin were studied; maps of their distribution in the upper layer of sediments were prepared. At some stations contents of these elements were also measured in Pleistocene and Oligocene sediments. Elevated contents of five ore elements (except for Zn) were found at the East Pacific Rise and in the Clarion-Clipperton province; and of Mn, Ni, and Cu in the Guatemala basin. Increased zinc contents occur only in sediments of the East Pacific Rise and Guatemala Basin. Enrichment of sediments in these elements results from under¬water hydrothermal activity and high biological productivity.