Sediment constituents and quartz in the sand-sized fraction, West Florida slope and Mississippi Fan sediments

The interval between 488.2 and 513.7 m below seafloor at Deep Sea Drilling Project (DSDP) Site 615 is interpreted as a single carbonate gravity-flow deposit. The deposit has characteristics of both a debris flow and a high-density turbidity current. Comparison of the sedimentary constituents in 15 samples from this site with samples from 26 core tops from the upper West Florida continental slope and eastern Mississippi Fan shows many similarities. Shallow-water indicators, such as mollusk and echinoid fragments, occur in both suites of samples. The West Florida continental margin, therefore, is a potential provenance area. The Yucatan slope is also a possible source, but data from it are limited. The recognition of carbonate gravity-flow deposits intercalated within the Mississippi Fan refines our understanding of Pleistocene sedimentation within the Gulf basin. Deposition in the deep Gulf is dominated by the construction of the Mississippi Fan. However, this marine terrigenous depocenter is located between two large carbonate depocenters, the West Florida continental margin on the east and the Yucatan peninsula on the southwest. Periodically, the carbonate slope in these two regions fails, injecting carbonate gravity flows into the accreting terrigenous deep-sea fan.

Quantitative composition and distribution of the silt fraction in surface sediments of the North-West African continental margin

Surface sediments from the continental slope and rise of North-West Africa between the Canary lslands and the Cape Verde Islands are mainly composed of silt-sized material (2-63 µm). A number of sampling profiles were run normal to the coast and the composition of the silt fraction was determined quantitatively by scanning electron microscope analysis. The carbonate portion of the sediment was found to be nearly exclusively of biogenic origin. The most important contributors are planktonic foraminifers and coccoliths with minor contributions derived from pteropods. Plankton-produced biogenic opal such as diatoms and radiolarians play a very minor role. The high production rates of opal-silica plankton which exists in the surface waters of the NW-African upwelling system does not give rise to corresponding increases of opal accumulation in the bottom sediment. Benthic producers consist mainly of foraminifers and molluscs but the entire input from benthic producers is extremely small. An exception to this occurs in the prodelta sediments of the Senegal river. Downslope particle transport is indicated by the occurrence of shallow-water coralline algae, ascidian sclerites and cliona boring chips and can be traced as far down as the continental rise. The non-carbonate silt fraction mostly consists of quartz which is derived as eolian dust from the Sahara desert by the Harmattan and the NE-Trade-wind system. The percentage of carbonate in the surface sediments directly indicates the relative proportions of autochthonous biogenic components and terrigenous allochthonous quartz particles.

(Table 1) Vertical distribution of relative contents of the main planktonic groups in the water column of the Kuril-Kamchatka region at Station VITYZ5635

Determinations were made of contents of carbon, lipids, nitrogen and, in some material, protein, carbohydrates, elementary composition of lipids and their spectral composition in total plankton samples from different depths (from the surface to 3000 m) and in several species of macroplanktonic deep-water crustaceans (decapods and mysids) living at different depths. Content of organic carbon and lipids in total plankton is high (40 to 60 and 35 to 70% of dry weight, respectively) and it does not change significantly with increasing depth. Deep-water macroplanktonic crustaceans have extremely high content of organic carbon and lipids, but there are no significant differences in this respect between species that live in different layers of the deep-water zone. Elementary composition of lipids indicates that they are highly saturated, with a marked predominance of unsaponifiable fraction, about 20% of which consists of methane hydrocarbons.

Chemical composition of the soluble fraction of oxidate portions of Lakes Shebadnowan ferromanganese concretions

Shebandowan Lakes, Ontario, are the site of at least 49 shallow (2-12 m) ferromanganese concretion deposits, widely distributed throughout the 48 km of the watercourse. X-ray diffraction and Mossbauer methods have revealed the presence of goethite in some of the concretions. Chemical analyses of the acid soluble portions of 72 samples gave an average composition of 43.1% Fe and 5.65% Mn with a low content of trace elements. The Shebandowan concretions are among the richest in iron of lake concretions reported, possibly because only the acid soluble portion was analysed. Their low content of trace elements suggests rapid growth rates and a relatively young age. A positive correlation was found between Mn and K, Ca, Mg, Cu, Ni, and Co and the relationship between the last three and Mn was deemed significant. Zn was independent of association with either Mn and Fe, probably due to the presence locally of zinc sulphide deposits. Analyses of lake bottom and influent waters suggested that frequent resampling of the sites would be required throughout the year to permit meaningful interpretation of the effect of water composition of concretions. Analyses of sediment cores from 20 concretion sites indicated an upward increase in Fe and Mn and in the Mn/Fe ratio, consistent with the model of upward migration of the elements, where Mn is more mobile than Fe. This study concludes that a considerable proportion of the elements have been supplied to the Shebandowan concretions via the diagenetic process; generally a minor fraction of the elements has been abstracted directly from the superjacent water.

Elemental and stable isotopic composition of fine fraction samples of ODP Site 115-709 (Table 1)

Stable isotopic and minor element compositions were measured on the fine fraction of pelagic carbonate sediments from Ocean Drilling Program Site 709 in the central Indian Ocean. This section ranges in age from 47 Ma to the present. The observed compositional variations are the result of either paleoceanographic changes (past oceanic chemical or temperature variations) or diagenetic changes. The CaCO3 record is little affected by diagenesis. From previous work, carbonate content is known to be determined by the interplay of biological productivity, water column dissolution, and dilution. The carbon isotopic record is generally similar to previously published curves. A good correlation was observed between sea-level high stands and high 13C/12C ratios. This supports Shackleton's hypothesis that as the proportion of organic carbon buried in marine sediments becomes larger, oceanic-dissolved inorganic carbon becomes isotopically heavier. This proportion appears to be higher when sea level is higher and organic carbon is buried in more extensive shallow-shelf sediments. The strontium content and oxygen isotopic composition of carbonate sediments are much more affected by burial diagenesis. Low strontium concentrations are invariably associated with high values of d18O, probably indicating zones of greater carbonate recrystallization. Nevertheless, there is an inverse correlation between strontium concentration and sea level that is thought to be a result of high-strontium aragonitic sedimentation on shallow banks and shelves during high stands. Iron and manganese concentrations and, to a lesser extent, magnesium and strontium concentrations and carbon isotopic ratios are affected by early diagenetic reactions. These reactions are best observed in a slumped interval of sediments that occurs between 13.0 and 17.5 Ma. As a result of microbial reduction of manganese and iron oxides and dissolved sulfate, it is hypothesized that small amounts of mixed-metal carbonate cements are precipitated. These have low carbon isotopic ratios and high concentrations of metals.

(Table 1) Content of sand fraction components in sediments of the interglacial MIS 7 optimum from Core LV28-42-4

Distribution of diatoms, radiolarians, planktonic and benthic foraminifers, and sediment components in fraction >0.125 mm was analyzed in a core obtained from the central Sea of Okhotsk within frameworks of the Russian-German KOMEX Project. The core section characterizes the period 190-350 ka, which corresponds to marine-isotopic stages (MIS) 7 to 10. During glacial MIS 10 and MIS 8, the basin accumulated terrigenous material lacking microfossils or containing them in low abundance, which reflects, along with their composition, heavy sea-ice conditions, suppressed bioproductivity, and bottom environment aggressive toward calcium carbonate. Interglacial MIS 9 was characterized by elevated bioproductivity with accumulation of diatomaceous ooze during the climatic optimum (328 to 320 ka). Water exchange with the Pacific was maximal from 328 to 324 ka ago. Environment became moderate and close to the present-day one at the end of the optimum exhibiting possible existence of a dichothermal layer with substantial amounts of surface Pacific water still flowing into the basin. Similar to interglacial MIS 5e and MIS 1, ''old'' Pacific water determined near-bottom environment in the central Sea of Okhotsk during that period, although influx of terrigenous material was higher, probably reflecting more humid climate of the region. Slight warming marked the terminal MIS 8 (approximately 260 ka ago). Paleoceanographic situation during the interglacial MIS 7 was highly variable: from warm-water to almost glacial. The main climatic optimum of MIS 7 occurred within 220-210 ka, when subsurface stratification increased and the dichothermal layer developed. Bottom environment during the studied time interval, except for the optimum of interglacial MIS 9, resembled those characteristic of glacial periods: actively formed ''young'' Okhotsk water displaced ''old'' Pacific deep water.

Radionuclides measured in surface water samples from the South Atlantic

In the nutrient-rich Southern Ocean, Fe is a vital constituent controlling the growth of phytoplankton. Despite much effort, the origin and transport of Fe to the oceans are not well understood. In this study we address the issue with geochemical data and Nd isotopic compositions of suspended particle samples collected from 1997 to 1999 in the South Atlantic Sector of the Southern Ocean. Al, Th, and rare earth element (REE) concentrations as well as 143Nd/144Nd isotopic ratios in acetic acid-leached particle samples representing the lithogenic fraction delineate three major sources: (1) Patagonia and the Antarctic Peninsula provide material with eNd > -4 that is transported toward the east with the polar and subpolar front jets, (2) the south African shelf, although its influence is limited by the circumpolar circulation and wind direction, can account for material with eNd of -12 to -14 adjacent to South Africa, and (3) East Antarctica provides material with eNd of -10 to -15 to the eastern Weddell Sea and adjacent Antarctic Circumpolar Current. For this region we interpret the Nd isotopic evidence in combination with oceanographic/atmospheric constraints as evidence for supply of significant amounts of terrigenous detritus by icebergs.