Occurrence of tethyan radiolarian species in early Cretaceous sediments of ODP Hole 123-765C in the Northeast Indian Ocean (Fig. 4)

During ODP Leg 123, abundant and well-preserved Neocomian radiolarians were recovered at Site 765 (Argo Abyssal Plain) and Site 766 (lower Exmouth Plateau). Assemblages are characterized by the numerical dominance of a small number of non-tethyan forms and by the scarcity of tethyan taxa. Remarkable contrasts exist between radiolarian assemblages extracted from claystones of Site 765 and reexamined DSDP Site 261, and faunas recovered from radiolarian sand layers, only found at Site 765. Clay faunas are unusual in their low diversity of apparently ecologically tolerant (or solution resistant?), ubiquist species, whereas sand faunas are dominated by non-tethyan taxa. Comparisons with Sites 766 and 261, as well as sedimentological observations, lead to the conclusion that this faunal contrast resulted from a difference in provenance, rather than from hydraulic sorting or selective dissolution. The ranges of 27 tethyan taxa from Site 765 were compared to the tethyan radiolarian zonation by Jud ( 1992 ) by means of the Unitary Associations Method. This calculation allows to directly date the Site 765 assemblages and to estimate the amount of truncation of ranges for tethyan taxa. Over 70% of the already few tethyan species of Site 765, have truncated ranges during the Valanginian-Hauterivian. Radiolarian assemblages recovered from claystones at Sites 765 and 261 in the Argo Basin apparently reflect restricted oceanic conditions during the latest Jurassic-Barremian. Neither sedimentary facies nor faunal associations bear any resemblance to what we know from typical tethyan sequences. We conclude that the Argo Basin was paleoceanographically separated from the Tethys during the Late Jurassic and part of the Early Cretaceous by its position at higher paleolatitudes and/or by enclosing land masses. Assemblages recovered from radiolarian sand layers are dominated by non-tethyan species that are interpreted as circumantarctic. Their first appearance in the late Berriasian-early Valanginian predates the oceanization of the Indo-Australian breakup (M11, late Valanginian), but coincides with a sharp increase in margin-derived pelagic turbidites. The Indo-Australian rift zone and the adjacent margins must have been submerged deeply enough to allow an intermittent influx of circumantarctic cold water into the Argo Basin, creating increased bottom current activity. Cold-water radiolarians carried into the Argo Basin upwelled along the margin, died, and accumulated in radiolarite layers due to winnowing by bottom currents. High rates of faunal change and the sharp increase of bottom current activity are thought to be synchronous with possible pronounced late Berriasian-early Valanginian lowstands in sea level. Hypothetically, both phenomena might have been caused by a tendency to glaciation on the Antarctic-Australian continent, which was for the first time isolated from the rest of Gondwana by oceanic seaways as a result of Jurassic-Early Cretaceous sea-floor spreading. The absence of most typical tethyan radiolarian species during the Valanginian-Hauterivian is interpreted as reflecting a time of strong influx of circumantarctic cold water following oceanization (M 11) and rapid spreading between Southeast India and West Australia. The reappearance and gradual abundance/diversity increase of tethyan taxa, along with the still dominant circumantarctic species are thought to result from overall more equitable climatic conditions during the Barremian-early Aptian and from the establishment of an oceanic connection with the Tethys Ocean during the early Aptian.

Strontium isotopic composition and chronology of sediments from DSDP Site 90-588

We report 261 strontium isotopic analyses of well-preserved planktonic foraminifers from three Deep Sea Drilling Project Sites (519, 588, and 607). These samples cover the period from 24 Ma to present with an average of approximately one sample per 100 ka. The combination of high sample density and uniformity of analytical procedures has produced a well-defined record of changes in the 87Sr/86Sr of seawater during the Neogene. The record can be viewed as a series of essentially linear segments with slopes ranging from as high as 0.00006/m.y. to as low as 0/m.y. The times associated with major inflections in the curve do not appear to correspond to simple geologic phenomena such as eustatic cycles, but are probably controlled by a combination of tectonic and climatic factors that influenced the abundance and isotopic composition of terrestrial strontium input to the oceans. The strontium isotopic data are consistent with a progressive increase in the chemical weathering rates of the continents during the Neogene, probably related to repeated glaciations, increased exposure of continents by lowered sea level, and increased continental relief resulting from high rates of tectonic uplift.

Compositions of Ni-rich magnesiowüstite and coexisting magnesioferrite spinel from sediments of DSDP Holes 86-577 and 91-596

We have found trace inclusions of Ni-rich magnesiowüstite within grains of magnesioferrite spinel recovered from Cretaceous/Tertiary boundary sediments from DSDP Site 596, South Pacific (23°51.20'S, 169°39.27'W) and DSDP Site 577, North Pacific (3°6.51'N, 157°43.40'E). Measured compositions of these inclusions range from (Mg_0.85Ni_0.74Fe_0.17)O to (Mg_0.74Ni_0.09Fe_0.17)O. Coexisting magnesioferrite and magnesiowüstite can only crystallize from ultramafic, refractory, Mg-rich liquids with Mg/Si > 2 (atom ratio). Such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation. We infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures (probably >2300°C), resulting in fractionation of volatile SiO2 from more refractory MgO. The most plausible source of this high-temperature vapor is in the fireball of the major impact event that terminated the Cretaceous.

Physical properties, mineral and chemical compositions, and accumulation rates of Holocene and Upper Pleistocene bottom sediments from the Sea of Okhotsk

Mineral and chemical compositions and physical properties of diatomaceous clayey-siliceous sediments from the Sea of Okhotsk are studied. Accumulation rates of silica are determined. Their compositional model based on silica content is similar to that of Late Jurassic and Olenekian-Middle Anisian cherts from the Sikhote Alin region. Thickness of Holocene siliceous unit and accumulation rates of siliceous deposits depended on bioproductivity in the upper water layer and seafloor topography. Accumulation rates of amorphous SiO2 (0.05-5.7 g/cm**2/ka) and free SiO2 (0.5-11.6 g/cm**2/ka) are minimal on seamounts and maximal in depressions near foothills. These values match accumulation rates of free SiO2 in Triassic and Late Jurassic basins of the Sikhote Alin region (0.33-3 g/cm**2/ka). Comparison of composition and accumulation rates of silica shows that Triassic and Late Jurassic siliceous sequences of Sikhote Alin could accumulate in a marginal marine basin near a continent.

Chemical composition of bottom sediments from the Deryugin Basin, Sea of Okhotsk

This paper reports data including new analyses of contents of Ni, Co, V, Mo, Fe, Mn, Zn, Ba, Sc, Y, Cd, Rb, Cs, and W in bottom sediments of the Deryugin Basin. Features of chemical element distribution in the bottom area were identified and zones of maximum accumulation of major and trace elements were allocated. A correlation between the elements was shown.

Chemical composition of bottom sediments along the Transpacific section

Distribution of Fe, Mn, Ti, Cu, Ni, Co, V, Cr, Mo, As in bottom sediments of a section from the Hawaiian Islands to the coast of Mexico. In the surface layer and isochronic layers of sediments from biogenic-terrigenous sediments of the Mexico coast to pelagic red clays of the Northeast Basin contents of all studied elements increase, and more sharply for mobile ones - Mn, Mo, Cu, Ni, Co, As. In near Hawaii sediments rich in coarsely fragmented volcanic-terrigenous and pyroclastic material of basalt composition enriched in Ti, Fe, Cr, V, P contents of these elements in surface sediments and in sediment mass increase and contents of Mn, Mo, Ni, Co, Cu, As (for the same reason) decrease compared to red clays. An area of hemipelagic and transition sediments is identified; these sediments have much higher contents of Mn, Fe, Cu, Ni, Mo, As, (Ba) than red clays and similar sediments of the Northwest Pacific Ocean. This is due to hydrothermal activity in the tectonically active zone at the northern extension of the East Pacific Rise. Similar character of distribution of the elements in the surface layer and in the isochrone layers of bottom sediments along the most part of the section is shown. Similarity between distribution of the elements in sediments of the western and the eastern parts of the Transpacific section is established.

CPI, d13C, Corg and diploptene from ODP Hole 169-1034B varved sediments

Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1-2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17alpha(H),21beta(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ~6000 years BP up to present time to about 70 µg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (<51 µg/g Corg) and even slightly decrease to 36 µg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ~6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between ~31.5 and ~39.6 per mil PDB after ~6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic-anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate delta13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at ~31.4 ± 1.1 per mil PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ~6000 years BP, diploptene enriched in 13C of up to -26.3 per mil PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.