(Table 2) Comparative growth rates of upper and lower parts of Fe-Mn nodules from the Pacific Ocean

Dates and growth rates of iron-manganese nodules obtained by various direct and indirect methods, including radiometric, micropaleontological, geological and experimental, are discussed. Validity of assumptions, on which the radiometric dating of nodules is based and reliability of results are discussed. The problem of "buoyancy" of slow-growing nodules resting on the surface of faster-accumulating sediments is considered: It may be caused by action of deep-water fauna, bottom currents, or plastic properties of sediments.

Sedimentological and geochemical characteristics of upper Cretaceous and lower Tertiary sediments of ODP Hole 108-661A

Based on sedimentological and geochemical data, the Upper Cretaceous and Tertiary sequence at Ocean Drilling Program Site 661 was subdivided into four intervals: Interval I (Campanian age) is characterized by sediments deposited below the calcite compensation depth (CCD) inside a high-productivity area and well-oxygenated bottom waters, indicated by the absence of carbonate, the major occurrence of zeolites and opal-CT, and intense bioturbation. Very fine-grained siliciclastic sediments and the lack of any erosional features suggest a low-energy environment. The terrigenous fraction was probably supplied by winds from the nontropical areas in South Africa. Interval II (Maestrichtian age) is characterized by high-amplitude variations in the carbonate content indicative of a deposition above the CCD, superimposed by (climate-controlled) short-term fluctuations of the CCD. The absence of both zeolites and opal-CT imply a position of Site 661 outside high-productivity areas. The first occurrence of higher amounts of kaolinite (especially during the middle Maestrichtian) suggests the onset of a terrigenous sediment supply from tropical areas. Interval III (between uppermost Cretaceous to early Tertiary) is characterized by the absence of carbonate and zeolites, interpreted as deposition below the CCD and outside an oceanic high-productivity belt. The kaolinite-over-illite dominance suggests a terrigenous sediment supply from tropical areas. Interval IV (between early Tertiary and Miocene age) is characterized by the occurrence of black manganeserich layers, major nodules/pebbles, and erosional surfaces, indicating phases of extremely reduced sediment accumulation and bottom-current activities. In the lower part of this interval (?Eocene age), higher amounts of zeolites occur, which suggest a higher oceanic productivity caused by equatorial upwelling. The source area of the terrigenous sediment fraction at Site 661 was the tropical region of northwest Africa, as suggested by the kaolinite-over-illite dominance.

Sediment properties, contents and accumulation rates of Hg in cores of Upper Quaternary sediments from the Deryugin Basin, Okhotsk Sea

The Hg distribution and some mineralogical-geochemical features of bottom sediments up to a depth of 10 m in the Deryugin Basin showed that the high and anomalous Hg contents in the Holocene deposits are confined to a spreading riftogenic structure and separate fluid vents within it. The accumulations of Hg in the the sediments were caused by its fluxes from gas and low-temperature hydrothermal vents under favorable oceanological conditions in the Holocene. The two mainly responsible for the high and anomalous Hg contents are infiltration (fluxes of hydrothermal or gas fluids from the sedimentary cover) and plume (Hg precipitation from water plumes with certain hydrochemical conditions forming above endogenous sources). The infiltration anomalies of Hg were revealed in the following environments: (1) near gas vents on the northeastern Sakhalin slope, where high Hg contents are associated only with Se and were caused by the accumulation of gases ascending from beneath the gas hydrate layer; (2) in the area of inferred occasionally operating low-temperature hydrothermal seeps in the central part of the Deryugin Basin, in which massive barite chimneys, hydrothermal Fe-Mn crusts, and anomalous contents of Mn, Ba, Zn, and Ni in sediments develop.

Chemistry of upper Eocene microtektites

Late Eocene microtektites and crystal-bearing microkrystites extracted from DSDP and ODP cores from the Atlantic, Pacific, and Indian oceans have been analyzed to address their provenance. A new analysis of Nd and Sr isotopic compositions confirms previous work and the assignment of the uppermost microtektite layer to the North American tektites, which are associated with the 35.5 Ma, 85 km diameter Chesapeake impact structure of Virginia, USA. Extensive major element and Nd and Sr isotopic analyses of the microkrystites from the lowermost layer were obtained. The melanocratic microkrystites from Sites 216 and 462 in the Indian and Pacific oceans possess major element chemistries, Sr and Nd isotopic signatures and Sm-Nd, T CHUR, model ages similar to those of tagamite melt rocks in the Popigai impact structure. They also possess Rb-Sr, T UR, model ages that are younger than the tagamite TCHUR ages by up to ~1 Ga, which require a process, as yet undefined, of Rb/Sr enrichment. These melanocratic microkrystites are consistent with a provenance from the 35.7 Ma, 100 km diameter Popigai impact structure of Siberia, Russia, while ruling out other contemporaneous structures as a source. Melanocratic microkrystites from other sites and leucocratic microkrystites from all sites possess a wide range of isotopic compositions (epsilon (143Nd) values of -16 to -27.7 and epsilon (87Sr) values of 4.1-354.0), making the association with Popigai tagamites less clear. These microkrystites may have been derived by the melting of target rocks of mixed composition, which were ejected without homogenization. Dark glass and felsic inclusions extracted from Popigai tagamites possess epsilon (143Nd) and epsilon (87Sr) values of -26.7 to -27.8 and 374.7 and 432.4, respectively, and T CHUR and T UR model ages of 1640-1870 Ma and 240-1830 Ma, respectively, which require the preservation of initially present heterogeneity in the source materials. The leucocratic microkrystites possess diverse isotopic compositions that may reflect the melting of supra-basement sedimentary rocks from Popigai, or early basement melts that were ejected prior to homogenization of the Popigai tagamites. The ejection of melt rocks with chemistries consistent with a basement provenance, rather than the surface ~1 km of sedimentary cover rocks, atypically indicates a non-surficial source to some of the ejecta. Microkrystites from two adjacent biozones possess statistically indistinguishable major element compositions, suggesting they have a single source. The occurrence of microkrystites derived from a single impact event, but in different biozones, can be explained by: (1) diachronous biozone boundaries; (2) post-accumulation sedimentary reworking; or (3) erroneous biozonation.