Chemistry of interstitial waters of ODP Leg 112 samples

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13?S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to ~20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87Sr/86Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to -200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.

Niche differentiation among mat-forming, sulfide-oxidizing bacteria in chemosynthetic ecosystems of the Nile Deep Sea Fan (Eastern Mediterranean Sea)

Sulfidic muds of cold seeps on the Nile Deep Sea Fan are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, or (iii) single, spherical cells resembling Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped at the bottom water interface. Filamentous sulfide oxidizers were associated with non-overlapping, steep gradients of oxygen and sulfide. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the Nile Deep Sea Fan, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting.

Mineralogy and geochemistry of Mg-phyllosilicates from Middle Valley, northern Juan de Fuca Ridge

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24 Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of = 0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14 Å with Fe/(Fe+Mg) ratios of ? 0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensiteychlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in d18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300° C ± 30° for chlorite-quartz at 32 m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10-11°/m in the vent region.

Lithium sources in pore fluids of Peru slope

High Li concentrations, up to a maximum of 1155 µM are observed in the pore fluids of the Peru convergent margin slope sediments. At Ocean Drilling Program Sites 683 and 685 (ca. 9°S), the Li concentration depth gradients are twice as steep as at Site 682 and 688 (ca. 11°S). Within the sediments, the most important Li sources are from aluminosilicate minerals. Biogenic opal-A contains little Li and thus dilutes the Li concentration of the bulk sediments. The sediment compositions and the thermal regimes are similar at 9° and 11°S, suggesting there is an additional, non-sedimentary source for the observed high Li concentrations in the northern pore fluids. At 9°S, the 87Sr/86Sr ratios reach a maximum value of 0.709958. The observed radiogenic 87Sr/86Sr values in the pore fluids support the suggestion that the additional Li may derive from exchange reactions with underlying continental crust. The high concentrations of Li at 11°S may derive from basalt alteration at moderate to high temperatures, as suggested by the non-radiogenic 87Sr/86Sr ratios in these pore fluids, which reach a minimum value of 0.707218. Based on (1) Li concentrations in the pore fluids in slope sediments from Peru and several other margins, and (2) an approximate estimate of fluid flux from continental margins into the ocean, continental margins provide an estimated 1 to 3 * 10**10 moles Li/yr to the ocean. This source of oceanic Li, which has not been considered previously, is of the same order of magnitude as some estimates of hydrothermal and river Li fluxes and may have important consequences for the oceanic Li isotope budget. The sink is unknown for this newly discovered and possibly large Li source, but it may be more pervasive low-temperature alteration of oceanic basement than previously estimated, or burial of mineral phases, such as authigenic clay minerals, or metal oxyhydroxides which may be Li-rich.

Mineralogy and geochemistry at DSDP Leg 64 Holes

At DSDP Site 477, late Quaternary diatomaceous muds and delta-derived silty-sand turbidites at 2000 meters water depth have been extensively and progressively altered by a deep-seated heat source beneath a sill. Bulk petrologic and microprobe analyses have identified a crudely zoned paragenesis within 260 meters sub-bottom which ranges from unaltered to slightly altered oozes (0-50 m), anhydrite-dolomite claystones (105-125 m), illite-chlorite-pyrite claystones (125-140 m), chlorite-pyrite-calcite-carbonaceous claystones with traces of K-feldspar, albite, epidote (140-190 m), and chlorite-epidote-quartz-albite-pyrrhotite-sphene sandstone (190-260 m). Several petrologic features suggest rapid processes of ocean floor metamorphism: (1) friable and porous textures, (2) abundant relict grains with overgrowths, (3) idiomorphic habits on epidotes, feldspars, and quartz, and (4) a steep gradient in levels of alteration. Many aspects of this hydrothermal assemblage are similar to hydrothermally metamorphosed sandstones of the Cerro Prieto, Mexico, geothermal area.

Strontium isotope ratios of planktonic foraminifera from late Neogene sediment samples

A curve describing the variation of the strontium isotopic composition of seawater for the late Neogene (9 to 2 Ma) was constructed from 87Sr/86Sr analyses of marine carbonate in five Deep Sea Drilling Project (DSDP) sites: 502, 519, 588, 590, and 593. The strontium isotopic composition of the oceans increased between 9 and 2 Ma with several changes in slope. From 9 to 5.5 Ma, 87Sr/86Sr values were nearly constant at ~0.708925. Between 5.5 and 4.5 Ma, 87Sr/86Sr ratios increased monotonically at a rate of not, vert, similar 1 * 10**-4 per million years. The steep slope during this interval provides the potential for high resolution strontium isotope stratigraphy across the Miocene/Pliocene boundary. The rate of change of 87Sr/86Sr decreases to near zero again during the interval 4.5-2.5 Ma, and ratios average 0.709025. The relatively rapid increase of 87Sr/86Sr between 5.5 and 4.5 Ma must be related to changes in the flux or average 87Sr/86Sr ratios of the major inputs of Sr to the oceans. Quantitative modelling of these inputs suggests that the increase was most probably caused by an increase in the dissolved riverine flux of strontium to the oceans, an increase in the average 87Sr/86Sr composition of river water, or some combination of these parameters. Modelling of this period as a transient-state requires a pulse-like increase in the input of 87Sr to the oceans between 5.5 and 4.5 Ma. Alternatively, the 5.5-4.5 Ma period can be modelled as a simple transition from one steady-state to another if the oceanic residence time of strontium was eight times less than the currently accepted value of 4 Ma. During the time interval of steep 87Sr/86Sr increase, other geochemical and sedimentologic changes also occur including an increase in sediment accumulation rates, a drop in the calcite compensation depth (CCD), and a decrease in the delta13C of dissolved bicarbonate (i.e., "carbon shift"). The simplest mechanism to explain 87Sr/86Sr variation and these related geochemical changes is to invoke an increase in the dissolved chemical fluxes carried by rivers to the oceans. This, in turn, implies increased chemical denudation rates of the continents and shelves during the late Neogene. The increase in chemical weathering rates is attributed to increased exposure of the continents by eustatic regression, intensified glacial/interglacial cycles, and accelerated rates of global tectonism beginning at 5.5 Ma during the latest Miocene.

Aprubt climate change events in the Northeastern Atlantic Ocean

The western Iberian margin has been one of the key locations to study abrupt glacial climate change and associated interhemispheric linkages. The regional variability in the response to those events is being studied by combining a multitude of published and new records. Looking at the trend from Marine Isotope Stage (MIS) 10 to 2, the planktic foraminifer data, conform with the alkenone record of Martrat et al. [2007], shows that abrupt climate change events, especially the Heinrich events, became more frequent and their impacts in general stronger during the last glacial cycle. However, there were two older periods with strong impacts on the Atlantic meridional overturning circulation (AMOC): the Heinrich-type event associated with Termination (T) IV and the one occurring during MIS 8 (269 to 265 ka). During the Heinrich stadials of the last glacial cycle, the polar front reached the northern Iberian margin (ca. 41°N), while the arctic front was located in the vicinity of 39°N. During all the glacial periods studied, there existed a boundary at the latter latitude, either the arctic front during extreme cold events or the subarctic front during less strong coolings or warmer glacials. Along with these fronts sea surface temperatures (SST) increased southward by about 1°C per one degree of latitude leading to steep temperature gradients in the eastern North Atlantic and pointing to a close vicinity between subpolar and subtropical waters. The southern Iberian margin was always bathed by subtropical water masses - surface and/ or subsurface ones -, but there were periods when these waters also penetrated northward to 40.6°N. Glacial hydrographic conditions were similar during MIS 2 and 4, but much different during MIS 6. MIS 6 was a warmer glacial with the polar front being located further to the north allowing the subtropical surface and subsurface waters to reach at minimum as far north as 40.6°N and resulting in relative stable conditions on the southern margin. In the vertical structure, the Greenland-type climate oscillations during the last glacial cycle were recorded down to 2465 m during the Heinrich stadials, i.e. slightly deeper than in the western basin. This deeper boundary is related to the admixing of Mediterranean Outflow Water, which also explains the better ventilation of the intermediate-depth water column on the Iberian margin. This compilation revealed that latitudinal, longitudinal and vertical gradients existed in the waters along the Iberian margin, i.e. in a relative restricted area, but sufficient paleo-data exists now to validate regional climate models for abrupt climate change events in the northeastern North Atlantic Ocean.

Component analysis of three TV-guided grab sampler from the Sahul Shelf (NW Australia)

This paper constitutes a first detailed and systematic facies and biota description of an isolated carbonate knoll (Pee Shoal) in the Timor Sea (Sahul Shelf, NW Australia). The steep and flat-topped knoll is characterized by a distinct facies zonation comprising (A) soft sediments with scattered debris and scarce sponges, hydrozoans and crinoids (320-210 m water depth), (B) hardground outcrops (step-like banks, vertical cliffs) that are mainly colonized by octocorals and sponges (210-75 m), and (C) the summit region (75-21 m) where the slopes merge gently into the flat-topped summit that is densely colonized by massive and encrusting zooxanthellate corals and the octocoral Heliopora coerulea. In contrast, the sediments recovered from the summit are dominated by the green alga Halimeda, subordinate components are corals, benthic foraminifers, mollusks, and coralline red algae. Thus, the sediments are classified as chlorozoan grain assemblage. However, non-skeletal grains (fecal pellets, ooids) are almost completely absent. This discrepancy between the living biota and the sediment composition could reflect a disruption by the severe tropical cyclone Ingrid that hit the northern Australian shelf in March 2005, just before the sampling for this study took place (September 2005).

Analyses of Manganese nodules off Izu Island

Manganese nodules have been found by the author in the shallow waters of the Hyotan-se bank west of Shikime-jima, an island of the Izu archipelago in the Sea of Japan. The slopes around the bank are steep and rocks are exposed; gravels and coarse material cover the broad and flat plain on its top; andesite and basalt, which are very common in the bedrock, are found mingled with liparite gravels together with a number of manganese concretions from the bank.

Physiography of the Orca Seamount in the Bransfield Strait, Antarctic Peninsula

Extract from related chapter 5.5.2 in reference: The Orca Seamount was discovered in the central basin of the Bransfield Strait around the posit 62°26'S and 58°24'W on the west side of the Antarctic Peninsula, the most western area of the south polar continent. Through the discovery was made known in 1987, it was only during three bathymetric surveys with high resolution fan echosounders between 1993 and 1995 that the character and complete shape of a remarkable volcano seamount became evident. The data acquisition and processing revealed a spectacular crater of 350 m depth. The relative hight of this 3 km wide "caldera" rim is 550 m with a basal diameter of the seamount cone of 11 km. Its flanks are about 15° steep but in some places the slope reaches up to 36°. The nearly circular shape of the Orca edifice spreads outh with several pronounced spurs, trending parallel to the basin axis in a northeast-southwest direction. The Bransfield Strait is a trough-shaped basin of 400 km length and 2 km depth between the South Shetland Island Arc and the Antarctic Peninsula, formed by rifting behind the islands. The separation of the South Shetland island chain from the peninsula began possibly several million years ago. The active rifting is still going on however, and has caused recent earthquakes and volcanism along the Bransfield Strait. The Strait hosts a chain of submerged seamounts of volcanic origin with the presently inactive Ora Seamount as the most spectacular one. The South Shelfand Island owe their existence to a subduction related volcanism which is perhaps 5-10 times older than the age of Orca and the other seamounts along the central basin of the Bransfield Strait.

Lithology of sediments from the Arabian Sea

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced >4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5-11.5 km. Curiously <400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (>600°C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting >28 m.y., and induced by the steep continent-ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.

Grains size and components distribution in turbidite layesr of ODP Leg 101

Textural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-ofslope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central basin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 µm) of turbidite samples from Hole 628A, while platform detritus is rare. We interpreted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Exuma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635.

(Table 1) Chloride concentrations in water obtained from observation wells on Helgoland Dune, North Sea

The hydrogeological conditions are unfavourable for a sufficient supply of drinking-water. The small size of the catchment area, the large hydraulic gradient inside the steep 'Buntsandstein'-cliff and the low geodetic level of the 'Dune Island' and the foreshore at the eastern foot of the cliff do not allow the formation and recharge of a sufficiently exploitable geodetic freshwater dome over the underlying saltwater. This means that until recently the provision of sufficient drinking-water for the island's inhabitants, for its garrison as well as for visiting ships was a problem. This problem has now been solved by the desalination of seawater.

Oxygen isotope concentrations of minerals from the upper kilometer of the ocean crust, DSDP Hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in 18O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 have delta 18O values generally ranging from +6.1 to +8.5? SMOW (mean= +7.0?), although minor zeolite-rich samples range up to 12.7?. Rocks depleted in 18O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6?, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4?, respectively. Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10-100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios (~1), and hydrothermal fluids probably contained mantle-derived CO2. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient (~2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and 18O enrichment of fluids, resulting in local increases in delta 18O of rocks which had been previously depleted in 18O during prior axial metamorphism.

Predicting bed form roughness: the influence of lee side angle

Flow transverse bedforms (ripples and dunes) are ubiquitous in rivers and coastal seas. Local hydrodynamics and transport conditions depend on the size and geometry of these bedforms, as they constitute roughness elements at the bed. Bedform influence on flow energy must be considered for the understanding of flow dynamics, and in the development and application of numerical models. Common estimations or predictors of form roughness (friction factors) are based mostly on data of steep bedforms (with angle-of-repose lee slopes), and described by highly simplified bedform dimensions (heights and lengths). However, natural bedforms often are not steep, and differ in form and hydraulic effect relative to idealised bedforms. Based on systematic numerical model experiments, this study shows how the hydraulic effect of bedforms depends on the flow structure behind bedforms, which is determined by the bedform lee side angle, aspect ratio and relative height. Simulations reveal that flow separation behind bedform crests and, thus, a hydraulic effect is induced at lee side angles steeper than 11 to 18° depending on relative height, and that a fully developed flow separation zone exists only over bedforms with a lee side angle steeper than 24°. Furthermore, the hydraulic effect of bedforms with varying lee side angle is evaluated and a reduction function to common friction factors is proposed. A function is also developed for the Nikuradse roughness (k s), and a new equation is proposed which directly relates k s to bedform relative height, aspect ratio and lee side angle.