Geochemical composition and atomic formulas of saponites, celadonites and carbonates from ODP Leg 168 sites

With this study, we investigate the mineralogical variations associated with the low-temperature (<100°C) alteration of normal tholeiitic pillow basalts varying in age from 0.8 to 3.5 Ma. Their alteration intensity varies systematically and is related to several factors, including (1) the aging of the igneous crust, (2) the increase of temperatures from the younger to the older sites, measured at the sediment/basement interface, (3) the local and regional variations in lithology and primary porosity, and (4) the degree of pillow fracturing. Fractures represent the most important pathways that allow significant penetration of fluids into the rock and are virtually the only factor controlling the alteration of the glassy rim and the early stages of pillow alteration. Three different alteration stages have been recognized: alteration of glassy margin, oxidizing alteration through fluid circulation in fracture systems, and reducing alteration through diffusion. All the observed mineralogical and chemical variations occurring during the early stages of alteration are interpreted as the result of the rock interaction with "normal," alkaline, and oxidizing seawater, along preferential pathways represented by the concentric and radial crack systems. The chemical composition of the fluid progressively evolves while moving into the basalt, leading to a reducing alteration stage, which is initially responsible for the precipitation of Fe-rich saponite and minor sulfides and subsequently for the widespread formation of carbonates. At the same time, the system evolved from being "water dominated" to being "rock dominated." No alteration effects in pillow basalts were observed that must have occurred at temperatures higher than those measured during Leg 168 at the basement/sediment interface (e.g., between 15° and 64°C).

Grain size composition and age of ODP Hole 167-1017E sediments

Siliciclastic sedimentation at Ocean Drilling Program Site 1017 on the southern slope of the Santa Lucia Bank, central California margin, responded closely to oceanographic and climatic change over the past ~130 ka. Variation in mean grain-size and sediment sorting within the ~25-m-thick succession from Hole 1017E show Milankovitch-band to submillenial-scale variation. Mean grain size of the "sortable silt" fraction (10-63 µm) ranges from 17.6 to 33.9 µm (average 24.8 µm) and is inversely correlated with the degree of sorting. Much of the sediment has a bimodal or trimodal grain-size distribution that is composed of distinct fine silt, coarse silt to fine sand, and clay-size components. The position of the mode and the sorting of each component changes through the succession, but the primary variation is in the presence or abundance of the coarse silt fraction that controls the overall mean grain size and sorting of the sample. The occurrence of the best-sorted, finest grained sediment at high stands of sea level (Holocene, marine isotope Substages 5c and 5e) reflect the linkage between global climate and the sedimentary record at Site 1017 and suggest that the efficiency of off-shelf transport is a key control of sedimentation on the Santa Lucia Slope. It is not clear what proportion of the variation in grain size and sorting may also be caused by variations in bottom current strength and in situ hydrodynamic sorting.

Composition and properties of rocks and sediments from the Cape Verde fracture zone

The book presents results of comprehensive geological and geophysical studies, carried out in the Cape Verde fault zone in the 3-rd cruise of R/V Akademik Nikolaj Strakhov (1986). Detailed characterization of bottom relief, thickness and structure of the sedimentary cover, magnetic field, crust structure, lithology and stratigraphy of sediments, petrography and geochemistry of magmatic rocks. Conclusions about tectonic layering of the crust and upper mantle in the fault zone, and about a concurrent structural section of large mantle inhomogeneities have been done. The book is the first monographic description of a major fault structure of the ocean floor.

Composition and distribution of silica in sediments of the Broken Ridge

Within a dipping sequence of middle Cretaceous to Eocene sediments on Broken Ridge, opal-A, opal-CT, and quartz occur as minor constituents in carbonate and ash-rich sediments. Biogenic opal-A is mainly derived from diatoms and radiolarians. Opal-A and almost all siliceous microfossils disappear within a narrow (<20-m-thick) transition zone below which authigenic opal-CT and quartz are present. These latter silica polymorphs occur together within a 750-m-thick interval, but the ratio of quartz/opal-CT increases with increasing age and depth within the pre-rift sediment sequence. The boundary between opal-A- and opal-CT-bearing sediments is also a physical boundary at which density, P-wave velocity, and acoustic impedance change. This physical transition is probably caused by infilling of pore space by opal-CT lepispheres.

(Table 1) Lead isotopic composition and concentration at DSDP Leg 82 Holes

The aim of DSDP Leg 82 was to decipher the temporal and spatial evolution of Azores Plume. The Pb-isotopic results of this leg are rather complex, and can be summarized as follows: 1. At a given site (561, 558), variations of Pb isotopic compositions are generally accompanied by major changes in trace-element ratios, indicating significant heterogeneity of the source region. There is a correlation between Pb isotopes and trace elements. 2. In contrast, if all the data (i.e., all studied sites) of Leg 82 are considered together, no correlation can be discerned between Pb isotopes and trace elements. Site 556, especially, shows abnormal behavior. 3. Leg 82 samples not only cover the entire range of Pb isotopic composition previously established for the Atlantic Ocean, but extend this field to more radiogenic values. 4. The data are compatible with the hot-spot model proposed by Schilling (1975), if one considers that the Azores Plume itself is isotopically heterogeneous, and that it has been progressively contaminated to various degrees by upper mantle material.

Mineral composition and geochemistry of sediments from ODP Leg 127 sites

The lithostratigraphy of Neogene hemipelagic sediments recovered from the Japan Sea during Leg 127 was revised to improve intersite consistency and to remove confusion stemming from diagenetic modification of the lithology through the opal-A to opal-CT transformation. Special emphasis was put on the presence and nature of dark-light cycles in revising the lithostratigraphy. Mineral composition analysis was conducted for samples from Sites 794, 795, and 797. In addition, major element chemical composition analysis was conducted for these same sample sets from Site 794. The result of mineral composition analysis suggests that the detrital component, which consists of such minerals as quartz, plagioclase, illite, and kaolinite plus chlorite, is diluted to various degrees by biogenic silica (opal-A) and its diagenetic equivalents (opal-CT and quartz). Smectite, on the other hand, may be a diagenetic or hydrothermal alteration product of volcanic material, although more study is necessary to confirm its origin. As a whole, vertical variation in the sediment composition is consistent with the revised lithostratigraphy and helps to characterize the redefined lithologic units quantitatively.

Biometrics, fatty acid composition and stable isotopic ratios of five seabird species from Kongsfjorden, Svalbard

Marine birds are important predators in the marine ecosystem, and dietary studies can give useful information about their feeding ecology, food webs and oceanographic variability. The aim of this study was to increase our understanding of the diet and trophic level of the seabirds breeding in Kongsfjorden, Svalbard. We have used fatty acids and stable isotopes, both of which integrate diet information over space and time, to determine trophic relationships in marine food webs. Fatty acid compositions of muscle from Little auk (Alle alle), Brünnich's guillemot (Uria lomvia), Black-legged kittiwake (Rissa tridactyla), Northern fulmar (Fulmarus glacialis) and Glaucous gull (Larus hyperboreus) were determined and compared with their prey species. Canonical analysis (CA) showed that fatty acid composition differed among the five seabird species. Little auk, Black-legged kittiwake and Northern fulmar had high levels of the Calanus markers 20:1n9 and 22:1, indicating that these seabirds are a part of the Calanus food chain. Brünnich's guillemot differed from the other species with much lower levels of 20:1n9 and 22:1. Brünnich's guillemot is a pursuit diver feeding on fish and amphipods deeper in the water column, below 30 m. Glaucous gull also differed from the other seabird species, with a larger variation in the fatty acid composition indicating a more diverse diet. Trophic level analysis placed Little auk at the lowest trophic level, Brünnich's guillemot and Black-legged kittiwake at intermediate levels and Glaucous gull and Northern fulmar at the highest trophic level.

Sediment composition and sedimentation rates of four Holes in ODP Leg 166

To date, work on the Great Bahama Bank's western, leeward margin has centred chiefly on seismic-scale expressions of carbonate sequences and systems tracts. However, periplatform, slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres. It is these small-scale, high-frequency cycles within the larger-scale facies successions of the Quaternary which form the main topic of this paper. Previous studies have shown that the small-scale cycles correlate to the orbitally forced, high-frequency sea-level changes. Therefore these cycles should indicate how sea level has affected the slope development and thus platform-margin evolution during this period. Through detailed, high-resolution sequence stratigraphy of the Great Bahama Bank's leeward margin, obtained via delta18O isotope and mineralogical (XRD) analyses, confined by U/Th dating and nannofossil bioevents, a greater understanding of the bedding geometries within the Pleistocene-Holocene seismic sequences and clues as to the nature of the slope development has been achieved. The high-resolution seismic profiles indicate that since the Plio-Pleistocene change in geometry, in which the Great Bahama Bank developed into a rimmed platform, continued steepening and subsequent progradation of the leeward margin has typified slope development during the Quaternary, which is described as an accretionary slope. However, on the basis of our observations we conclude that only the early to lower middle Pleistocene section (isotope stages 45-20) and the Holocene (isotope stage 1) of the leeward margin is accretionary. This indicates that a degree of erosion and/or by-passing has occurred on the leeward margin since the lower middle Pleistocene (isotope stage 19). During the first part of this period (isotope stages 19-12) erosion and/or by-passing occurred in the middle to lower slope regions and toe-of-slope. By the end of the upper middle to late Pleistocene phase (isotope stages 11-2) erosion also occurred on the upper slope. This erosion by currents at the toe-of-slope and oversteepening of the upper and middle slopes have led to back-cutting upslope and resulted in the progressive retreat of the toe-of-slope towards the platform to the east. However, the rise in sea level since the Last Glacial Maximum to its present-day level has allowed high productivity on the platform top during the Holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks. This has led to the redevelopment of an accretionary slope and continued westward progradation of the Great Bahama Bank's western, leeward margin.

Various ratios of C1-C5 hydrocarbons, gas composition and computed temperature at DSDP Leg 64 Holes

I have evaluated shipboard data and preliminary interpretations related to organic geochemistry in light of additional shore-based analyses. Data on interstitial gas, the C/N ratio, and fluorescence indicate that organic matter was altered by sills and that these were all single intrusions except the upper sill complex at Site 481, which was a multiple emplacement. Site 477 had the highest in situ temperature, estimated from interstitial gas composition to be 225°C.

Geochemical composition and radiocarbon ages of cores from Lago Fugnano, Tierra del Fuego

Recent advances in the chronology and the palaeoclimatic understanding of Antarctic ice core records point towards a larger heterogeneity of latitudinal climate fluctuations than previously thought. Thus, realistic palaeoclimate reconstructions rely in the development of a tight array of well-constrained records with a dense latitudinal coverage. Climatic records from southernmost South America are critical cornerstones to link these Antarctic palaeoclimatic archives with their South American counterparts. At 54° S on the Island of Tierra del Fuego, Lago Fagnano is located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (~110km long) and non-ice covered lake at high southern latitudes. A multi-proxy study of selected cores allows the characterisation of a Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete lacustrine laminated sequence reveal variations in major and trace elements, as well as organic content, suggesting high variability in environmental conditions. Comparison of these results with other regional records allows the identification of major known late Holocene climatic intervals and the proposal for a time for the onset of the Southern Westerlies in Tierra del Fuego. These results improve our understanding of the forcing mechanisms behind climate change in southernmost Patagonia.

Mineral composition and 40Ar-39Ar data of samples from Garnet Point, Aurora Peak, Correll Nunatak, Penguin Point and Horn Bluffs, East Antarctica

George V Land (Antarctica) includes the boundary between Late Archean-Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross-Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar-39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar-39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ~1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (~180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro-Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar-39Ar ages from ~530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.