C1-C5 hydrocarbons from core gas pockets at DSDP Leg 56 Holes

C1-C5 hydrocarbons from DSDP Legs 56 and 57 sediment gas pockets were analyzed on board ship. Results suggest that the C2-C5 hydrocarbons accompanied biogenic methane and were generated at low temperatures - less than 50° C - either by microorganisms or by low-temperature chemical reactions. Neopentane, a rare constituent of petroleum, is the major C5 component (about 80%) in much of the sediment at Site 438. This compound, which appeared in smaller amounts at Sites 434, 439, 440, and 441, seems to correlate with either fractured or coarse-grained sediments. Scatter in C4 and C5 isomer ratios and generally good correlation between C3, C4 and C5 components suggest local sources for these molecules.

Concentration of organic compounds in the snow-ice cover of the Eastern Antarctic

This paper presents data on concentrations and composition of organic substances, lipids, and hydrocarbons, in the snow-ice cover of fast ices and continental lakes of Antarctic. It was shown that organic compounds were accumulated in layers with the most intense autochthonous processes (mainly at the snow-ice and ice-water boundaries). These zones remain active at a biogeochemical medium even at low temperatures. The maximum content of organic compounds (10-20 times that of the snow-ice cover of other regions) and a sharp change in the proportions of their migration forms in the ice volume were detected in the regions of penguin colonies (fast ice in the Buromsky Island and a lake in the Haswell Island). Contents and composition of hydrocarbons in Antarctic ices were compared with those of Arctic ices.

Isotope characterisation of Villars Cave stalagmites

We present a new mid-latitude speleothem record of millennial-scale climatic variability during OIS3 from the Villars Cave that, combined with former published contemporaneous samples from the same cave, gives a coherent image of the climate variability in SW-France between ~55 ka and ~30 ka. The 0.82 m long stalagmite Vil-stm27 was dated with 26 TIMS U-Th analyses and its growth curve displays variations that are linked with the stable isotopes, both controlled by the climatic conditions. It consists in a higher resolved replicate of the previously published Vil-stm9 and Vil-stm14 stalagmites where Dansgaard-Oeschger (DO) events have been observed. The good consistency between these three stalagmites and the comparison with other palaoeclimatic reconstructions, especially high resolution pollen records (ODP 976 from the Alboran Sea, Monticchio Lake record from southern Italy) and the nearby MD04-2845 Atlantic Ocean record, permits to draw a specific climatic pattern in SW-France during the OIS3 and to see regional differences between these sites. Main features of this period are: 1) warm events corresponding to Greenland Interstadials (GIS) that are characterized by low speleothem d13C, high temperate pollen percentages, warm temperatures and high humidity; among these events, GIS#12 is the most pronounced one at Villars characterized by an abrupt onset at ~46.6 ka and a duration of about 2.5 ka. The other well individualized warm event coincides with GIS#8 which is however much less pronounced and occurred during a cooler period as shown by a lower growth rate and a higher d13C; 2) cold events corresponding to Greenland Stadials (GS) that are clearly characterized by high speleothem d13C, low temperate pollen abundance, low temperature and enhanced dryness, particularly well expressed during GS coinciding with Heinrich events H5 and H4. The main feature of the Villars record is a general cooling trend between the DO#12 event ~45.5 ka and the synchronous stop of the three stalagmites at ~30 ka ±1, with a first well marked climatic threshold at ~41 ka after which the growth rate and the diameter of all stalagmites slows down significantly. This climatic evolution differs from that shown at southern Mediterranean sites where this trend is not observed. The ~30 ka age marks the second climatic threshold after which low temperatures and low rainfalls prevent speleothem growth in the Villars area until the Lateglacial warming that occurred at ~16.5 ± 0.5 ka. This 15 ka long hiatus, as the older Villars growth hiatus that occurred between 67.4 and 61 ka, are linked to low sea levels, reduced ocean circulation and a southward shift of the Polar Front that likely provoked local permafrost formation. These cold periods coincide with both low summer 65°N insolation, low atmospheric CO2 concentration and large ice sheets development (especially the Fennoscandian).

Concentration of organic compounds in slush, under-ice water, and the snow-ice fast-ice cover of the southeast sector of the Antarctic in April 2008

Data are presented on concentration of hydrocarbons (HC) relative to concentrations of suspended matter, lipids, organic carbon, and chlorophyll a in surface waters and snow-ice cover of the East Antarctic coastal areas. It was shown that growth of concentrations of aliphatic HC (AHC) to 30 µg/l in surface waters takes place in frontal zones and under young ice formation. AHC concentration in snow increases with growth of aerosol concentration in the atmosphere. In the lower part of ice, at the boundary with seawater, despite low temperatures, autochthonous processes may provide high AHC concentrations (up to 289 µg/l). Within the snow-ice cover on fast ice, concentration co-variations of all the compounds considered take place.

Chemistry of altered basalts from ODP Hole 123-765D

Volcanic basement recovered at Hole 765D is characterized by nonpervasive, oxidative alteration, typical of seafloor weathering. Chilled margins and the mesostasis of the lavas are variably altered to assemblages of celadonite, Fe-oxyhydroxides, zeolites, and calcite with trace saponite. Plagioclase is partially altered to Ca-Na zeolites and/or albite. Well-developed alteration halos parallel fracture surfaces and extend several centimeters into the surrounding rock. These clay-rich halos are enriched in K2O and Fe2O3 relative to the adjacent clay-poor rock. The halos and adjacent rock are characterized by d18O values 2 per mil-3 per mil higher than those of fresh MORB. The "freshness" of the samples and the scarcity of saponite suggest that the duration of seawater circulation was short-lived. Albitization of plagioclase indicates that the volcanic rocks were altered initially at low temperatures and were subsequently reheated off-axis in a closed environment. Reheating did not result in significant modification of the bulk composition of the crust.

Strontium concentrations and isotopic compositions of anhydrites from the TAG active mound

Sr isotope analyses have been conducted on anhydrite samples from the TAG (Trans-Atlantic Geotraverse) active hydrothermal mound (26°08?N, Mid-Atlantic Ridge) that have previously been shown to exhibit two distinct patterns of REE behavior when normalized to TAG end-member hydrothermal fluid. Despite differences in REE patterns, the Sr isotope data indicate that all the anhydrites precipitated from fluids with a similar range of hydrothermal fluid and seawater components, and all but one were seawater-dominated (52%-75%). Speciation calculations using the EQ3/6 software package for geochemical modeling of aqueous systems suggest that the REE complexation behavior in different fluid mixing scenarios can explain the variations in the REE patterns. Anhydrites that exhibit relatively flat REE patterns [(La_bs)/(Yb_bs) = 0.8-2.0; subscript bs indicates normalization to end-member black smoker hydrothermal fluid] and a small or no Eu anomaly [(Eu_bs)/(Eu*_bs) = 0.8-2.0] are inferred to have precipitated from mixes of end-member hydrothermal fluid and cold seawater. REE complexes with hard ligands (e.g., fluoride and chloride) are less stable at low temperatures and trivalent Eu has an ionic radius similar to that of Ca2+ and the other REE, and so they behave coherently. In contrast, anhydrites that exhibit slight LREE-depletion [(La_bs)/(Yb_bs) = 0.4-1.4] and a distinct negative anomaly [(Eu_bs)/(Eu*_bs) = 0.2-0.8] are inferred to have precipitated from mixes of end-member hydrothermal fluid and conductively heated seawater. The LREE depletion results from the presence of very stable LREE chloro-complexes that effectively limit the availability of the LREE for partitioning into anhydrite. Above 250°C, Eu is present only in divalent form as chloride complexes, and discrimination against Eu2+ is likely due to both the mismatch in ionic radii between Eu2+ and Ca2+, and the strong chloro-complexation of divalent Eu which promotes stability in the fluid and inhibits partitioning of Eu2+ into precipitating anhydrite. These variations in REE behavior attest to rapid fluctuations in thermal regime, fluid flow and mixing in the subsurface of the TAG mound that give rise to heterogeneity in the formation conditions of individual anhydrite crystals.

Geochemistry of basalts from ODP Hole 113-690C

Basalts from Maud Rise, Weddell Sea, are vesicular and olivine-phyric. Major, trace, and rare earth element concentrations are similar to those of alkali basalts from ocean islands and seamounts. The rocks are low in MgO, Cr, Ni, and Sc, and high in TiO2, K2O, P2O5, Zr, and LREE contents. The abundance of "primary" biotite and apatite in the matrix indicates the melting of a hydrous mantle. Prevalence of olivine and absence of plagioclase in the rocks suggests that the volatile in the melt was an H2O-CO2 mixture, where H2O was <0.5. Mantle derived xenocrysts in the basalt include corroded orthopyroxene, chromite, apatite, and olivine. Olivine (Fo90) is too magnesian to be in equilibrium with the basalts, as they contain only 5-6 wt% MgO. Based on the presence of mantle xenocrysts, the high concentration of incompatible elements, the spatial and chemical affinity with other ocean island basalts from the area, and the relative age of the basalt (overlain by late Campanian sediments), it is suggested that Maud Rise was probably generated by hot-spot activity, possible during a ridge crest jump prior to 84 Ma (anomaly 34 time). Iddingsite, a complex intergrowth of montmorillonite and goethite, is the major alteration product of second generation olivine. It is suggested that iddingsite crystallized at low temperatures (<200°C) from an oxidized fluid during deuteric alteration. Vesicles are commonly filled by zeolites which have been replaced by K-feldspars.

Sulfur concentrations and isotopic composition of serpentinized oceanic peridotites

The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200°-400°C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low fO2 and produced a reduced, low-sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low-delta34S sulfide (1.5? to -23.7?). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures(~20°-200°C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher fO2. Microbial reduction of seawater sulfate resulted in addition of low-delta34S sulfide (~15 to ~43?) and formation of higher-sulfur assemblages that include valleriite and pyrite. The high SO4/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high delta34S of total sulfur (mean ~8?). In contrast, Iberian Margin serpentinites gained large amounts of 34S-poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in delta34S of total sulfur (mean ~ -5?). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4-14 * 10**12 g S/yr from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output.

Oxidation rate of thiosulfate and elemental sulfur in the Black Sea

Oxidation rate of 35S-thiosulfate under simulated natural conditions and abundance of thiosulfate-oxidizing bacteria in a redox zone of the Black Sea are lower during winter and spring than in summer, especially in halistatic regions. Oxidation of thiosulfate under natural conditions is performed chiefly by lithotropic thionic bacteria, whose activity is limited by low temperatures. Adding thiosulfate and readily available organic matter to water samples from the redox zone and raising temperature of water stimulated activity of heterotrophic thiosulfate-oxidizing bacteria. Oxidation of elemental sulfur tagged with 35S apparently invovled two stages: abiotic oxidation of thiosulfate and subsequent bacterial oxidation of thiosulfate to sulfate.

Experiments on polar seaweeds

Organisms populating benthic shallow water systems of both polar regions are adapted to a particularly harsh environment. We studied effects of freezing and the combination of high light intensities and low water temperatures on photosynthesis of key macroalgal species from the Arctic intertidal (Fucus distichus) and Antarctic subtidal (Palmaria decipiens). Photosynthetic activity of F. distichus specimens was monitored during the freezing process; there was a marked decrease in quantum yield with decreasing temperatures, and a rapid recovery as soon as temperatures increased again. Thus, under the experimental conditions tested, no indication of photodamage was found. Specimens of Palmaria were exposed to a combination of high light intensities and low water temperatures. A persistent impairment of photosynthetic activity occurred at 0°C at light intensities of 400 µmol photons m-2 s-1. In all treatments, there was a decreasing ratio of phycobiliproteins to chlorophyll a. Overall, the two studies provide baseline data for interpreting physiological responses of two important macroalgal species in an extreme environment, the polar coastal ecosystem.

Age model for the time between 3.45 and 2.45 Ma of ODP Site 189-1172

Calcareous nannoplankton assemblages and benthic d18O isotopes of Pliocene deep-sea sediments of ODP site 1172 (East of Tasmania) have been studied to improve our knowledge of the Southern Ocean paleoceanography. Our study site is located just north of the Subtropical Front (STF), an ideal setting to monitor migrations of the STF during our study period, between 3.45 and 2.45 Ma. The assemblage identified at ODP site 1172 has been interpreted as characteristic for the transitional zone water mass, located south of the STF, based on: (i) the low abundances (< 1%) of subtropical taxa, (ii) relatively high percentages of Coccolithus pelagicus, a subpolar type species, (iii) abundances from 2-10% of Calcidiscus leptoporus, a species that frequently inhabits the zone south of the STF and (iv) the high abundances of small Noelaerhabdaceae which at present dominates the zone south of the STF. Across our interval the calcareous nannoplankton manifests glacial-interglacial variability. We have identified cold events, characterized by high abundances of C. pelagicus which coincide with glacial periods, except during G7. After 3.1 Ma cold events are more frequent, in concordance with global cooling trends. Around 2.75 Ma, the interglacial stage G7 is characterized by anomalous low temperatures which most likely are linked to definite closure of the Central American Seaway (CAS), an event that is believed to have had global consequences. A gradual increase of very small Reticulofenestra across our section marks a significant trend in the small Noelaerhabdaceae species group and has been linked to a general enhanced mixing of the water column in agreement with previous studies. It is suggested that a rapid decline of small Gephyrocapsa after isotopic stage G7 might be related to the cooling observed in our study site after the closure of the CAS.

Thermal diagenesis of clay minerals within volcanogenic material from ODP Site 135-841

Clay minerals are examined in detail in the sediment from the Tonga Trench margin at Site 841 (Leg 135 ODP). The changes in amount and nature of secondary clays with depth provide an alternative explanation for the intensive alteration of volcanogenic material at convergent margins. A characteristic distribution of clay minerals with depth shows four distinct zones unexplainable by simple burial diagenesis processes. These are named the upper, reactive, lower and rhyolitic zones. The reactive zone is intercalated with numerous sills and is characterized by the dominant iron-rich clays such as saponite, corrensite and chlorite associated with analcime. The occurrence of such iron-rich clays, mostly associated with a large amount of analcime, yields chemical and mineralogical evidence for thermal diagenesis. The required heat for the diagenetic process was transferred from recently intruded basaltic andesite sills. In the vicinity of these intrusions, the iron-rich clay minerals may have formed at temperatures up to 200°C. A zoning with respect to clay and zeolite minerals indicates that the influence of the palaeoheat flow decreased with the distance from the intrusion. The formation of interlayered I/S, illite, kaolinite and aluminous chlorite, which are recognized as major secondary minerals within the rhyolitic complex, was mainly controlled by both early diagenesis at moderately elevated temperatures, and since the Eocene by burial diagenesis at low temperatures. The occurrence of a steam zone in an early stage of the intrusion is restricted to Miocene tuffs and has overprinted the early alteration of the volcanogenic material within the tuffs and has changed the originally pristine composition of the pore fluids.

Geochemistry on rocks from Shackleton Range

During the GEISHA expedition (Geologische Expedition in die Shackleton Range 1987/88), the Pioneers Escarpment was visited and sampled extensively for the first time. Most of the rock types encountered represent amphibolite facies metamorphics, but evidence for granulite facies conditions was found in cores of garnet. These conditions must have been at least partly reached during the peak of metamorphism. For the Pioneers Escarpment a varicolored succession of sedimentary and bimodal volcanic origin is typical. It comprises: quartzites muscovite quartzite, sericite quartzite, fuchsite quartzite, garnet-quartz schists etc.; pelites: mica schists and plagioclase or plagioclase-microcline gneisses, aluminous schists; marls and carbonates: grey meta-limestones, carbonaceous quartzites, but also pure white, often fine-grained, saccharoidal marble, or a variety of tremolite marble, olivine (forsterite) marble, diopside-clinopyroxene-tremolite marble, etc.; basic volcanic rocks: amphibole fels, amphibolite schist, garnet amphibolite, and acidic to intermediate volcanic rocks: garnet-biotite schist, epidote-biotite-plagioclase gneiss, microcline gneiss. These rocks are considered to be a supracrustal unit, called the Pioneers Group. In the easternmost parts of the Pioneers Escarpment, e.g. at Vindberget, nonmetamorphic shales, sandstones and greywackes crop out, which are cover rocks of possibly Jurassic age. These metasediments, which represent a quartz-pelite-carbonate (QPC) association, indicate that deposition took place on a stable shelf, i.e. on the submerged rim of a craton. Marine shallow-water sedimentation including marls and aluminous clays form the protoliths. The volcanics may be part of a bimodal volcanics-arkose-conglomerate (BVAC) association. Geochemical analyses support the assumption of volcanic protoliths. This is demonstrated especially by the elevated amounts of the immobile, incompatible high-field-strength elements (HFSE) Nb, Ta, Ti, Y, and Zr encountered in some of the gneisses. Microscopic investigation suggests the existence of ortho-amphibolites. This is confirmed by the geochemistry. A bimodal volcanic association is evident. The amphibolites plot in both the tholeiite and calc-alkaline fields. The acidic volcanics are mainly rhyolitic. The sediments and volcanics were subjected to conditions of 10-11 kbar and 600°C during the peak of metamorphism, i.e. granulite facies metamorphism, which can be deduced from the Fe mole ratios of 0.71-0.73 in the garnet cores. Due to the relatively low temperatures, no anatectic melting took placc. The rims of the garnets show a Fe mole ratio of 0.84-0.86, and the coexisting mineral association garnet-biotite-staurolite-kyanite indicate amphibolite facies. The thermobarometry shows P-T conditions of 5-6 kbar and 570-580°C for this stage. The metamorphic history indicates deep burial at depths down to 35 km (subduction?) i.e. high pressure metamorphism, followed by pressure release due to uplift associated with retrograde metamorphism. This may have happened during a pre-Ross metamorphic event or orogeny. The Ross Orogeny at about 500 Ma probably just led to the weak greenschist facies overprint that is evident in the rocks of the Pioneers Group. Finally, sedimentation resumed in the area of the present Shackleton Range, or at least in the eastern part of the Pioneers Escarpment, probably when detritus from erosion of the basement (Read Group and Pioneers Group) was deposited, forming sandstones and greywackes of possibly Jurassic age. There is no indication that these sediments belong to the former Turnpike Bluff Group.

Lithium concentrations and isotope composition of basalt samples from ODP/DSDP Sites from Costa Rica Rift

Ocean Drilling Program (ODP) Hole 504B near the Costa Rica Rift is the deepest hole drilled in the ocean crust, penetrating a volcanic section, a transition zone and a sheeted dike complex. The distribution of Li and its isotopes through this 1.8-km section of oceanic crust reflects the varying conditions of seawater alteration with depth. The upper volcanic rocks, altered at low temperatures, are enriched in Li (5.6-27.3 ppm) and have heavier isotopic compositions (delta7Li=6.6-20.8?) relative to fresh mid-ocean ridge basalt (MORB) due to uptake of seawater Li into alteration clays. The Li content and isotopic compositions of the deeper volcanic rocks are similar to MORB, reflecting restricted seawater circulation in this section. The transition zone is a region of mixing of seawater with upwelling hydrothermal fluids and sulfide mineralization. Li enrichment in this zone is accompanied by relatively light isotopic compositions (-0.8-2.1?) which signify influence of basalt-derived Li during mineralization and alteration. Li decreases with depth to 0.6 ppm in the sheeted dike complex as a result of increasing hydrothermal extraction in the high-temperature reaction zone. Rocks in the dike complex have variable isotopic values that range from -1.7 to 7.9?, depending on the extent of hydrothermal recrystallization and off-axis low-temperature alteration. Hydrothermally altered rocks are isotopically light because 6Li is preferentially retained in greenschist and amphibolite facies minerals. The delta7Li values of the highly altered rocks of the dike complex are complementary to those of high-temperature mid-ocean ridge vent fluids and compatible to equilibrium control by the alteration mineral assemblage. The inventory of Li in basement rocks permits a reevaluation of the role of oceanic crust in the budget of Li in the ocean. On balance, the upper 1.8 km of oceanic crusts remains a sink for oceanic Li. The observations at 504B and an estimated flux from the underlying 0.5 km of gabbro suggest that the global hydrothermal flux is at most 8*10**9 mol/yr, compatible with geophysical thermal models. This work defines the distribution of Li and its isotopes in the upper ocean crust and provides a basis to interpret the contribution of subducted lithosphere to arc magmas and cycling of crustal material in the deep mantle.

Chemical and isotopical analsyes of DSDP Hole 34-321 basalts

A blue-green smectite (iron-rich saponite) and green mica (celadonite) are the dominant sheet silicates in veins within the 10.5 m of basalt cored during DSDP Leg 34, Site 32l, in the Nazca plate. Oxygen isotopic analyses of these clays, and associated calcite, indicate a formation temperature of <25°C. Celadonite contains appreciable Fe2O3, K2O and SiO2, intermediate MgO, and very little Al2O3. Celadonite is commonly associated with goethite and hematite, which suggests that this phase formed by precipitation within a dominantly oxygenated environment of components leached from basalt and provided by seawater. A mass balance estimate indicates that celadonite formation can remove no more than 15% of the K annually transported to the oceans by rivers. In contrast, iron-rich saponite containing significant Al2O3 appears to have precipitated from a nonoxidizing, distinctly alkaline fluid containing a high Na/K ratio relative to unmodified seawater. Seawater-basalt interaction at low temperatures, resulting in the formation of celadonite and smectite may explain chemical gradients observed in interstitial waters of sediments overlying basalts.