Authigenic carbonate of ODP SIte 104-643

Different generations of complex authigenic carbonates formed in siliceous muds (lithologic Unit IV) and hemipelagic clays (lithologic Unit V) of ODP Site 643, Leg 104 Norwegian Sea. The dominant phase in Unit IV is an early diagenetic Mn, Fe-calcite with a strong negative d13C ( -14 to -16 per mil) signature, and slightly negative d180 values. The strong negative d13C results from extensive incorporation of 12C-enriched CO2 derived from bacterial degradation of marine organic matter into early Mn, Fe - calcite cements. Concomitant framboidal pyrite precipitation and abundant SEM microtextures showing excellent preservation of delicate structures of fragile diatom valves by outpourings with early Mn-calcites strongly support their shallow burial formation before the onset of compaction. Later generations of authigenic mineralizations in lithologic Unit IV include minor amounts of a second generation of calcite with platy crystals, possibly precipitated along with opal-A dissolution, and finally opal-CT crystallization in deeper seated environments overgrowing earlier precipitates with films and lepispheres. The last mineralization is collophane (fluor apatite) forming amorphous aggregates and tiny hexagonal crystals. Authigenic mineral assemblages in lithologic Unit V consist of rhodochrosites, transitional rhodochrosite/manganosiderites, and apatite. A negative d13C ( -7.1 to -15.6 per mil) and a fluctuating d18O signal indicates that the micritic to sparitic rhodochrosites, transitional rhodochrosites/manganosiderites were formed at various burial depths. CO2 resulted from organic matter degradation in the lowermost sulfate reduction zone and from biogenic methane generation in the lowermost sediments, resulting in variable and negative d13C signals. The change in carbonate mineralogy reflects major compositional differences compared to sediments in Unit IV. Most prominent is an increase in altered ash as a primary sediment component and a sudden decrease of siliceous microfossils. Upward diffusion of cations, lowered salinities in pore waters, and elevated temperatures provide diagenetic environments favoring increased remobilization processes.

Neogene magneto- and biostratigraphy of ODP Leg 104 holes

Silicoflagellate assemblages of ODP Leg 104 Neogene sequences are the basis of an interpretation of changes in the Neogene paleoenvironment of the Norwegian Sea. Fluctuations in the percentages of temperature and nutrient-sensitive taxonomic groups document major changes in sea-surface conditions. A brief, but distinct, cooling event occurred at 18.0-17.5 Ma which resulted in the disappearance of Naviculopsis. Following this early Miocene cooling a long period of increasing surface-water temperature occurred, leading up to a thermal high in the early middle Miocene (14.0 Ma). The early late Miocene (10.0-9.0 Ma) was distinctly cooler than the middle Miocene, but warmer than the remainder of the Neogene. Conditions between 13.0 and 10.0 Ma are unrecorded because of a regional hiatus, which is the earliest evidence for an end to the more temperate and stable conditions of the early to middle middle Miocene. A major plunge in temperatures occurred between 8.5 and 7.4 Ma and during the remainder of the late Miocene and Pliocene; from 7.4 to 2.65 Ma subpolar conditions prevailed. Silicoflagellates disappeared, except for sporadic occurrences, at 2.64 Ma with the beginning of dominant glacial sedimentation. Biogenic opal is absent in sediments younger than 0.76 Ma, indicating the dominance of glacial conditions with extensive sea ice.

Dinoflagellates of ODP Leg 104 holes

Dinoflagellate cysts, pollen, and spores were studied from 78 samples of the Eocene to Miocene section of ODP Site 643 at the outer Wring Plateau. Dinoflagellate cysts ranging from less than 1,000 to rarely over 30,000 per gram of sediment in the Paleogene, and generally between 50,000 and 100,000 in the Miocene were present. The shift to conspicuously higher cyst frequencies takes place in the lowermost Miocene section and appears to reflect increased cyst recruitment rather than a change in sedimentation rate. Of the 179 dinoflagellate cyst forms whose ranges were recorded, 129 are known species. Fifteen assemblage zones have been recognized, although the upper Eocene is missing and no substantial lower Eocene was recorded at Site 643. Norwegian Sea and Rockall Plateau zonations were compared with this study. Detailed correlation with existing onshore section zonations was difficult because key zonal species are inadequately represented; however, the middle to upper Miocene zonation established for Denmark is applicable. Pollen and spores occur with relatively low frequencies, and palynodebris is generally absent, in contrast to the observations from DSDP Leg 38. Thirty-nine samples from Eocene to Miocene sediments at Site 642 were studied and correlated with Site 643. A lower Eocene cyst assemblage present in Hole 642D is older than the questionably lower Eocene assemblage from Site 643. Site 642 has a lower Eocene to lower Miocene hiatus.

The petrology of the lower series volcanics of ODP Hole 104-642E

Between 1086.6 and 1229.4 m below seafloor at Site 642 on the Outer Vøring Plateau, a series of intermediate volcanic extrusive flow units and volcaniclastic sediments was sampled. A mixed sequence of dacitic subaerial flows, andesitic basalts, intermediate volcaniclastics, subordinate mid-ocean ridge basalt, (MORB) lithologies, and intrusives was recovered, in sharp contrast to the more uniform tholeiitic T-type MORB units of the overlying upper series. This lower series of volcanics is composed of three chemically distinct groups, (B, A2, A1), rather than the two previously identified. Flows of the dacitic group (B) have trace-element and initial Sr isotope signatures which indicate that their source magma derived from the partial melting of a component of continental material in a magma chamber at a relatively high level in the crust. The relative proportions of crustal components in this complex melt are not known precisely. The most basic group (A2) probably represents a mixture of this material with MORB-type tholeiitic melt. A third group (A1), of which there was only one representative flow recovered, is chemically intermediate between the two groups above, and may suggest a repetition of, or a transition phase in, the mixing processes.

Hydrogen, carbon, and oxygen isotope ratios of interstitial fluides of ODP Leg 104 holes

Carbon, hydrogen, and oxygen isotope ratios determined on 32 squeezed interstitial fluid samples show remarkable variations with depth. For the most part these variations are related to diagenetic and alteration reactions taking place in the sediments, and in the underlying basalts. delta13C SumCO2 depth distributions at Sites 642 and 643 are the result of mixing of original SumCO2 of the paleo bottom water with SumCO2 released by remineralization of organic matter. At Site 644, where sulfate exhaustion occurs, the processes of methanogenesis by CO2 reduction and anaerobic methanotrophy strongly influence the delta13C SumCO2 distribution. Hydrogen and oxygen isotopes roughly covary, and become enriched in 16O and1H with depth. This effect is most pronounced at Sites 642 and 643, possibly due to the influence of the directly underlying basalts. Isotope depletions at Site 644 are much lower, corresponding to the greater sediment depth to basement. The alternative, that the O, H isotope shifts are due primarily to autochthonous diagenetic and exchange reactions, is not supported by the data available.

Ice-rafted debris of Late Cenozoic sediments from ODP Leg 104 holes

The abundance and composition of the upper Cenozoic terrigenous coarse-sand fraction (250 µm-2 mm) at ODP Sites 642, 643, and 644 were investigated to date the onset of significant ice-rafting in the Norwegian Sea, establish the regional chronology of ice-rafting, and determine the relative importance of global vs. regional controls on ice-rafting in this area. The first input of ice-rafted debris (IRD) occurs at approximately 2.9 Ma, with significant ice-rafting beginning at about 2.5 Ma. IRD abundances increase significantly in sediments younger than 0.9 Ma at all three holes, indicating climatic deterioration in the late Pleistocene. Differences in the timing of this IRD increase between holes result from regional patterns of IRD supply and surface circulation. Variations in IRD sources and dispersal patterns may also explain the slightly higher background level of IRD abundance at Hole 642B, a seaward site. Major peaks in the generalized IRD records from the Norwegian Sea are tentatively correlated to glacial stages or glacial-to-interglacial transitions in the globally defined oxygen isotope record. This correlation indicates the effect of global conditions on the regional climate of the Norwegian Sea, although the detailed IRD records at these sites are also affected by local/regional processes (e.g., circulation patterns and source area differences).

Hydrocarbon gases in ODP Leg 104 samples

Geochemical studies at three ODP Leg 104 sites on the Wring Plateau help define the distribution of hydrocarbon gases in sediment of this prominent feature of the Norwegian continental margin. Low levels of hydrocarbon gas were encountered in sediment of the outer part of the plateau, but sediment of the inner part of the plateau is very gassy. The molecular composition of inner plateau gases (>99.9% methane) and the carbon isotopic composition of the methane (avg. = -76 per mil relative to the PDB standard) clearly show that the gas is biogenic. Heavier hydrocarbon gases accompany this methane, and their presence is probably a result of both chemical and microbial low-temperature diagenesis. Although these heavier hydrocarbons were not detected in sediment of the outer part of the plateau during shipboard analyses, subsequent shore-based analyses showed that these compounds are present at very low concentrations. Methane in the gassy sediment of the inner part of the plateau may be present as gas hydrates, judging from sedimentological and inorganic geochemical considerations, but no discernible gas hydrates were recovered during drilling.