Composition of minerals and rocks from the Markov Deep, Central Atlantic

The paper presents materials on composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6'-5°32.4'N) in the rift valley of the Mid-Atlantic Ridge. Predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of rocks and consists of Ol_89.8-90.4 + En_90.2-90.8 + Di_91.8 + Chr (Cr#32.3-36.6, Mg#67.2-70.0), while the second generation composes very thin branching veinlets and consists of PlAn_32-47 + Ol_74.3-77.1 + Opx_55.7-71.9 + Cpx_67.5 + Amph_53.7-74.2 + Ilm. Syndeformational olivine neoblasts in recrystallization zones are highly magnesian. Concentrations and covariations of major elements in harzburgites indicate that these rocks are depleted in mantle residues (high Mg# of minerals and whole-rock samples and low in CaO, Al2O3, and TiO2) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). Mineralogy and geochemistry of harzburgites were formed by interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. Mineral composition of veinlets in harzburgites and mineralogical-geochemical characteristics of related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melt residuals after crystallization of gabbronorites. Modern characteristics of harzburgites were shaped by the following processes: (i) partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of maximal stress relief and interaction of this melt with magnesian mantle residue.

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Lithium and strontium isotopic compositions of sediments and pore waters at ODP Sites 152-918 and 152-919

The distribution of Li isotopes in pore waters to a depth of 1157 m below seafloor is presented for ODP Sites 918 and 919 in the Irminger Basin, offshore Greenland. Lithium isotope data are accompanied by strontium isotope ratios to decipher diagenetic reactions in the sediments which are characterized by the pervasive presence of volcanic material, as well as by very high accumulation rates in the upper section. The lowering of the 87Sr/86Sr ratio below contemporaneous seawater values indicates several zones of volcanic material alteration. The Li isotope profiles are complex suggesting a variety of exchange reactions with the solid phases. These include cation exchange with NH4+ and mobilization from sediments at depth, in addition to the alteration of volcanic matter. Lithium isotopes are, therefore, a sensitive indicator of sediment-water interaction. d6Li values of pore waters at these two sites vary between -42 and -25?. At shallow depths (<100 mbsf), rapid decreases in the Li concentration, accompanied by a shift to heavier isotopic compositions, indicate uptake of Li into alteration products. A positive anomaly of d6Li observed at both sites is coincident with the NH4+ maximum produced by organic matter decomposition and may be related to ion exchange of Li from the sediments by NH4+. In the lower sediment column at Site 918, dissolved Li increases with depth and is characterized by enrichment of 6Li. The Li isotopic compositions of both the waters and the solid phase suggest that the enrichment of Li in deep interstitial waters is a result of release from pelagic sediments. The significance of sediment diagenesis and adsorption as sinks of oceanic Li is evaluated. The maximum diffusive flux into the sediment due to volcanic matter alteration can be no more than 5% of the combined inputs from rivers and submarine hydrothermal solutions. Adsorption on to sediments can only account for 5-10% of the total inputs from rivers and submarine hot springs.

Biostratigraphy of benthic and planktonic foraminifera in Paleocene samples of DSDP HOle 93-605

Paleocene benthic and planktonic foraminifers occur throughout a long interval of the sedimentary succession cored at Site 605. A biostratigraphic zonation based on planktonic foraminifers is proposed for this Paleocene section. Zones identified are Subbotina pseudobulloides Zone, Morozovella trinidadensis Zone, M. uncinata Zone, M. pusilla pusilla Zone, Planorotalites pseudomenardii Zone, and M. velascoensis Zone. Fluctuations in the sedimentation rate occurred at Site 605. Rates of deposition were high during the M. pusilla pusilla and P. pseudomenardii zones, and a depositional hiatus may occur at the base of the M. velascoensis Zone. Qualitative and quantitative analysis of benthic foraminiferal assemblages suggests that the Paleocene sediments of Site 605 were deposited near the upper limit of Nuttallides truempyi, that is, approximately in the middle bathyal zone (600 m or more).

Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.

Pore water and sediment geochemistry of the Bothian Sea

Methane is a powerful greenhouse gas and its biological conversion in marine sediments, largely controlled by anaerobic oxidation of methane (AOM), is a crucial part of the global carbon cycle. However, little is known about the role of iron oxides as an oxidant for AOM. Here we provide the first field evidence for iron-dependent AOM in brackish coastal surface sediments and show that methane produced in Bothnian Sea sediments is oxidized in distinct zones of iron- and sulfate-dependent AOM. At our study site, anthropogenic eutrophication over recent decades has led to an upward migration of the sulfate/methane transition zone in the sediment. Abundant iron oxides and high dissolved ferrous iron indicate iron reduction in the methanogenic sediments below the newly established sulfate/methane transition. Laboratory incubation studies of these sediments strongly suggest that the in situ microbial community is capable of linking methane oxidation to iron oxide reduction. Eutrophication of coastal environments may therefore create geochemical conditions favorable for iron-mediated AOM and thus increase the relevance of iron-dependent methane oxidation in the future. Besides its role in mitigating methane emissions, iron-dependent AOM strongly impacts sedimentary iron cycling and related biogeochemical processes through the reduction of large quantities of iron oxides.

Physical, chemical and biological characteristics of disturbed and undisturbed Arctic lakes, Northwest Territories

1. Global warming is predicted to cause changes in permafrost cover and stability in the Arctic. Zones of high ion concentration in regions of ice-rich permafrost are a reservoir of chemicals that can potentially be transferred to fresh waters during thawing. Consequently, input of enriched runoff from the thaw and sediment and vegetation from the landscape could alter lakes by affecting their geochemistry and biological production. 2. Three undisturbed lakes and five lakes disturbed by retrogressive permafrost thaw slumps were sampled during late summer of 2006 to assess the potential effects of thermokarst shoreline slumping on water and sediment chemistry, the underwater light regime, and benthic macrophyte biomass and community structure. 3. Undisturbed lakes had sediments rich in organic material and selected micronutrients, while disturbed lakes had sediments richer in calcium, magnesium and strontium, greater transparency of the water column, and a well-developed submerged macrophyte community. 4. It is postulated that enriched runoff chemistry may alter nutrient availability at the sediment-water interface and also the degradation of organic material, thus affecting lake transparency and submerged macrophytes. The results suggest that retrogressive permafrost slumping can significantly affect food webs in arctic tundra lakes through an increase in macrophyte biomass and development of a more complex benthic habitat.

Concentrations of dissolved and particulate manganese in waters of the Southwest Pacific

Concentrations of dissolved and particulate manganese in relation with organic matter in waters of the Southwest Pacific are under consideration.

Gas content, composition of gas hydrate and volumetric gas/water ratios of ODP and DSDP sites

Gas hydrate samples were recovered from four sites (Sites 994, 995, 996, and 997) along the crest of the Blake Ridge during Ocean Drilling Program (ODP) Leg 164. At Site 996, an area of active gas venting, pockmarks, and chemosynthetic communities, vein-like gas hydrate was recovered from less than 1 meter below seafloor (mbsf) and intermittently through the maximum cored depth of 63 mbsf. In contrast, massive gas hydrate, probably fault filling and/or stratigraphically controlled, was recovered from depths of 260 mbsf at Site 994, and from 331 mbsf at Site 997. Downhole-logging data, along with geochemical and core temperature profiles, indicate that gas hydrate at Sites 994, 995, and 997 occurs from about 180 to 450 mbsf and is dispersed in sediment as 5- to 30-m-thick zones of up to about 15% bulk volume gas hydrate. Selected gas hydrate samples were placed in a sealed chamber and allowed to dissociate. Evolved gas to water volumetric ratios measured on seven samples from Site 996 ranged from 20 to 143 mL gas/mL water to 154 mL gas/mL water in one sample from Site 994, and to 139 mL gas/mL water in one sample from Site 997, which can be compared to the theoretical maximum gas to water ratio of 216. These ratios are minimum gas/water ratios for gas hydrate because of partial dissociation during core recovery and potential contamination with pore waters. Nonetheless, the maximum measured volumetric ratio indicates that at least 71% of the cages in this gas hydrate were filled with gas molecules. When corrections for pore-water contamination are made, these volumetric ratios range from 29 to 204, suggesting that cages in some natural gas hydrate are nearly filled. Methane comprises the bulk of the evolved gas from all sites (98.4%-99.9% methane and 0%-1.5% CO2). Site 996 hydrate contained little CO2 (0%-0.56%). Ethane concentrations differed significantly from Site 996, where they ranged from 720 to 1010 parts per million by volume (ppmv), to Sites 994 and 997, which contained much less ethane (up to 86 ppmv). Up to 19 ppmv propane and other higher homologues were noted; however, these gases are likely contaminants derived from sediment in some hydrate samples. CO2 concentrations are less in gas hydrate than in the surrounding sediment, likely an artifact of core depressurization, which released CO2 derived from dissolved organic carbon (DIC) into sediment. The isotopic composition of methane from gas hydrate ranges from d13C of -62.5 per mil to -70.7 per mil and dD of -175 per mil to -200 per mil and is identical to the isotopic composition of methane from surrounding sediment. Methane of this isotopic composition is mainly microbial in origin and likely produced by bacterial reduction of bicarbonate. The hydrocarbon gases here are likely the products of early microbial diagenesis. The isotopic composition of CO2 from gas hydrate ranges from d13C of -5.7 per mil to -6.9 per mil, about 15 per mil lighter than CO2 derived from nearby sediment.

Mineralogy and chemistry of basalts and secondary minerals from DSDP Site 417, Atlantic Ocean

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H20, increase of Fe 3 +/FeT and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H20, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3 b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe 3 +/FeT and H20 occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H20 , and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

Chemistry of zeolithe and clays from Central Kerguelen Plateau basalts

Ocean Drilling Program Leg 120 recovered basement samples that consisted of zeolite-facies metabasalts at Sites 747, 749, and 750 on the Kerguelen Plateau. These basalts were metamorphosed in the low to intermediate zones of the zeolite facies, as indicated by the presence of diagnostic zeolites and the absence of chlorite, epidote, prehnite, pumpellyite, and wairakite. Chabazite, natrolite, thompsonite, mesolite, stilbite, huelandite, and smectites occur as amygduloidal fillings in basalts from Holes 747C and 750B, whereas only stilbite, laumontite, and pure and mixed-layered smectites were identified in amygduloidal basalts from Hole 749C. In the lower sections of Hole 749C, only laumontite and mixed-layered smectites coexist. Based on calculations with published experimental phase equilibria, the absence of wairakite in basalts from Hole 749C and of laumontite in basalts from Holes 747C and 750B suggests that metamorphic temperatures did not exceed approximately 225° and 120°C, respectively. The presence of well-developed zeolite mineral assemblages and the absence of carbonate and clay mineral assemblages restricts XCO2 in the fluid to approximately <=0.0075. Low- to intermediate-zone zeolite-facies mineral assemblages in basalts from the Kerguelen Plateau can be accounted for by metamorphism in an active geothermal area such as present-day Iceland.

(Table 2) Rates of nitrate and ammonium uptake, concentrations of particulate organic nitrogen and carbon, and dissolved nitrate and ammonium in waters of the Black Sea August-September 1990 and November 1991

To study inorganic nitrogen uptake rates by microplankton in the Black Sea the first 15N-experiments were carried out in August-September 1990 and in November 1991. In surface waters nitrate uptake rates varied from 5.7 to 28.5 nM/l/h in summer and from 1.9 to 7.8 nM/l/h in autumn. In both seasons maximal and minimal rates were observed in frontal zones of shelf/slope areas and in open waters, respectively. In summer average nitrate uptake rate per unit of particulate organic nitrogen was 0.0037 1/h for all stations. In autumn it varied from 0.0007 1/h in the central part of the sea to 0.0033 1/h in the slope near the southeastern Crimean coast. In autumn ammonium uptake rate varied from 7.1 to 22.2 nM/l/h and from 0.0025 to 0.00094 1/h. Ammonium uptake correlated linearly with nitrate uptake, with new production being 22-36% of total summary nitrate and ammonium uptake. There was a linear correlation between nitrogen uptake and chlorophyll a concentrations in the Black Sea. In the water column in autumn both nitrate and ammonium uptake decreased as chlorophyll a concentration diminishes with depth.

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.