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.

Mineralogy and inorganic geochemistry of sediments at DSDP Leg 65 Holes

During Leg 65, 15 holes were drilled at four sites located on young crust in the mouth of the Gulf of California. Quaternary to upper Pliocene hemipelagic sediments above and interlayered within the young basaltic basement were cored. The influence of hot lava, high temperature gradients, and hydrothermal activity on the mineralogy and geochemistry of the terrigenous sediments near contacts with basalts might therefore be expected. The purpose of the present study was to determine the mineralogy and inorganic geochemistry of these sediments and to analyze the nature and extent of low temperature alteration. To this end we studied the mineralogy and inorganic geochemistry of 75 sediment samples, including those immediately overlying uppermost basalts and those from layers alternating with basalts within the basement. We separated three size fractions - <2 µm (clay), 2-20 µm (intermediate), and >20 µm (coarse) - and applied the following mineralogical determinations: x-ray diffraction (XRD), infrared spectroscopy, transmission and scanning electron microscopy, and optical microscopy (for coarse fractions, using thin sections and smear slides). We calculated the percentages of clay minerals using Biscaye's (1964) method, and used routine wet chemical analyses to determine bulk composition and quantitative spectral analyses for trace elements.

Mineral and chemical compositions of sedimentary and volcano-sedimentary rocks drilled in DSDP Legs 43 and 44

Sedimentary cover on the bottom of the Northwest Atlantic Ocean is underlain by Late Jurassic - Cretaceous tholeiite-basalt formation. It consists of come sedimentary formations with different lithologic features and age. Their composition, stratigraphic position and, distribution are described on materials of deep-sea drilling. Mineralogical and geochemical studies of DSDP Leg 43 and Leg 44 holes lead to new ideas about composition and genesis of some sediment types of and their associations. High metal contents in the chalk formation of black clays on the Bermuda Rise probably result from exhalations. Connection of red-colored and speckled deposits with hiatuses in sedimentation is shown. Main stages of geological history of the North American Basin are reflected in accumulation of the followed formations: ancient carbonate formation (Late Jurassic - Early Cretaceous), formation of black clays rich in organic matter (Cretaceous), formation of speckled clays (Late Cretaceous), siliceous-clayey turbidite formation (Eocene), hemipelagic and pelagic clayey formation (Neogene), and terrigenous turbidite formation (Pleistocene).

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.

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.

Late Miocene cyclicity and intercore correlations for ODP Sites 127-794 and 127-797

During Leg 127, the formation microscanner (FMS) logging tool was used as part of an Ocean Drilling Program (ODP) logging program for only the second time in the history of the program. Resistivity images, also known as FMS logs, were obtained at Sites 794 and 797 that covered nearly the complete Yamato Basin sedimentary sequence to a depth below 500 mbsf. The FMS images from these two sites at the northeastern and southwestern corners of the Yamato Basin thus were amenable to comparison. A strong visual correlation was noticed between the FMS logs taken in Holes 794B and 797C in an upper Miocene interval (350-384 mbsf), although the two sites are approximately 360 km apart. In this interval, the FMS logs showed a series of more resistive thin beds (10-200 cm) alternating with relatively lower resistivity layers: a pattern that was manifested by alternating dark (low resistivity) and light (high resistivity) banding in the FMS images. We attribute this layering to interbedding of chert and porcellanite layers, a common lithologic sequence throughout Japan (Tada and Iijima, 1983, doi:10.1306/212F82E7-2B24-11D7-8648000102C1865D). Spatial frequency analysis of this interval of dominant dark-light banding showed spatial cycles of period of 1.1 to 1.3 and 0.6 m. This pronounced layering and the correlation between the two sites terminate at 384 mbsf, coincident with the opal-CT to quartz transition at Site 794. We think the correlation in the FMS logs might well extend earlier in the middle Miocene, but the opal-CT to quartz transition obscures this layering below 384 mbsf. Although 34 m is only a small part of the core recovered at these two sites, it is significant because it represents an area of extremely poor core recovery and an interval for which a near-depositional hiatus was postulated for Site 797, but not for Site 794.

Accumulation rates of sediments and main sedimentary components in ODP Leg 121 holes on Broken Ridge

Broken Ridge, in the eastern Indian Ocean,is overlain by about 1600 m of middle Cretaceous to Pleistocene tuffaceous and carbonate sediments that record the oceanographic history of southern hemisphere mid-to high-latitude regions. Prior to about 42 Ma, Broken Ridge formed the northern part of the broad Kerguelen-Broken Ridge Plateau. During the middle Eocene, this feature was split by the newly forming Southeast Indian Ocean Ridge; since then, Broken Ridge has drifted north from about 55° to 31°S. The lower part of the sedimentary section is characterized by Turonian to Santonian tuffs that contain abundant glauconite and some carbonate. The tuffs record a large but apparently local volcanic input that characterized the central part of Broken Ridge into the early Tertiary. Maestrichtian shallow-water(several hundred to 1000 m depth) limestones and cherts accumulated at some of the highest rates ever documented from the open ocean, 4 to 5 g/cm**2/kyr. A complete (with all biostratigraphic zones) Cretaceous-Tertiary boundary section was recovered from site 752. The first 1.5 m.y. of the Tertiary is characterized by an order-of-magnitude reduction in the flux of biogenic sediments, indicating a period of sharply reduced biological productivity at 55°S, following which the carbonate and silica sedimentation rates almost reach the previous high values of the latest Cretaceous. We recovered a complete section through the Paleocene that contains all major fossil groups and is more than 300 m thick, perhaps the best pelagic Paleocene section encountered in ocean drilling. About 42 Ma, Broken Ridge was uplifted 2500 m in response to the intra-plateau rifting event; subsequent erosion and deposition has resulted in a prominent Eocene angular unconformity atop the ridge. An Oligocene disconformity characterized by a widespread pebble layer probably represents the 30 Ma sea-level fall. The Neogene pelagic ooze on Broken Ridge has been winnowed, and thus its grain size provides a direct physical record of the energy of the southern hemisphere drift current in the Indian Ocean for the past 30 m.y.