Chemical, isotope and mineral composition of hydrothermally altered upper oceanic crust drilled at ODP Site 129-801C

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100°C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100°C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni. Secondary carbonate veins have 87Sr/86Sr = 0.706337 - 0.707046, and a negative correlation with d18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable d13C values (-11.2? to 2.9?) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the d13C of seawater over time. Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.

Isotopic ratios and geochemical composition of ODP Hole 176-735B minerals

The mineralogy and stable (O and C) and Sr isotopic compositions of low-temperature alteration phases were determined in Hole 735B gabbroic rocks in order to understand the processes of low-temperature alteration in this uplifted block of lower oceanic crust. Phyllosilicates include smectite (saponite, Mg montmorillonite, and nontronite), chlorite/smectite, chlorite, talc, and serpentine. Other phases include prehnite, albite, K-feldspar, analcite, natrolite, thompsonite, pyrite, and titanite. The low-grade mineral assemblages mainly represent zeolite facies and lower-temperature "seafloor weathering" processes. Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress. Alteration temperatures were probably somewhat greater below 1300 meters below seafloor. Mineralogy and isotopic data indicate that conditions were mostly reducing and that seawater solutions were rock dominated. Carbonates formed late from cold and generally oxidizing seawater solution, however, as seawater penetrated downward as the result of fracturing and faulting in the uppermost portion of the uplifted crustal block.

Geochemistry and morphometry on planktonic foraminifera

About one third of the anthropogenic carbon dioxide (CO2) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO2 invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO2 emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer Globigerinoides ruber in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on 14C and d13C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (d13C and d18O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the 14C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens.

Stable isotopes and ages from sediment core FR01/97-12, Tasman Sea

Deep-sea sediment core FR1/97 GC-12 is located 990 mbsl in the northern Tasman Sea, southwest Pacific, where Antarctic Intermediate Water (AAIW) presently impinges the continental slope of the southern Great Barrier Reef. Analysis of carbon (d13C) and oxygen (d18O) isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr. During the Last Glacial Maximum, there was a large d13C offset (1.1 per mil) between the surface-dwelling planktonic foraminifera and benthic species living within the AAIW. In contrast, during the last deglaciation (Termination 1), the d13C(planktonic-benthic) offset reduced to 0.4 per mil prior to an intermediate offset (0.7 per mil) during the Holocene. We suggest that variations in the dominance and direction of AAIW circulation in the Tasman Sea, and increased oceanic ventilation, can account for the rapid change in the water column d13C(planktonic-benthic) offset during the glacial-interglacial transition. Our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics. In this case, climatic variations in the Southern Hemisphere may have led to the rapid ventilation of deep water and AAIW during Termination 1, which contributed to the postglacial rise in atmospheric CO2.

Core alignment and composite depth scale of DSDP Site 86-577

Deep Sea Drilling Project Site 577 on Shatsky Rise (North Pacific Ocean) recovered a series of cores at three holes that contain calcareous nannofossil ooze of latest Cretaceous (late Maastrichtian) through early Eocene age. Several important records have been generated using samples from these cores, but the stratigraphy has remained outdated and confusing. Here we revise the stratigraphy at Site 577. This includes refining several age datums, realigning cores in the depth domain, and placing all stratigraphic markers on a current time scale. The work provides a template for appropriately bringing latest Cretaceous and Paleogene data sets at old drill sites into current paleoceanographic literature for this time interval. While the Paleocene Eocene Thermal Maximum (PETM) lies within core gaps at Holes 577* and 577A, the sedimentary record at the site holds other important events and remains crucially relevant to understanding changes in oceanographic conditions from the latest Cretaceous through early Paleogene.

Mg/Ca and stable isotope record of benthic foraminifera of the Paleocene-Eocene Thermal Maximum

A rapid increase in greenhouse gas levels is thought to have fueled global warming at the Paleocene-Eocene Thermal Maximum (PETM). Foraminiferal magnesium/calcium ratios indicate that bottom waters warmed by 4° to 5°C, similar to tropical and subtropical surface ocean waters, implying no amplification of warming in high-latitude regions of deep-water formation under ice-free conditions. Intermediate waters warmed before the carbon isotope excursion, in association with downwelling in the North Pacific and reduced Southern Ocean convection, supporting changing circulation as the trigger for methane hydrate release. A switch to deep convection in the North Pacific at the PETM onset could have amplified and sustained warming.

Lipid biomarkers in urface sediments of the Southeast Atlantic

Surface sediments from the eastern South Atlantic were investigated for their lipid biomarker contents and bulk organic geochemical characteristics to identify sources, transport pathways and preservation processes of organic components. The sediments cover a wide range of depositional settings with large differences in mass accumulation rates. The highest marine organic carbon (OC) contributions are detected along the coast, especially underlying the Benguela upwelling system. Terrigenous OC contributions are highest in the Congo deep-sea fan. Lipid biomarker fluxes are significantly correlated to the extent of oxygen exposure in the sediment. Normalization to total organic carbon (TOC) contents enabled the characterization of regional lipid biomarker production and transport mechanisms. Principal component analyses revealed five distinct groups of characteristic molecular and bulk organic geochemical parameters. Combined with information on lipid sources, the main controlling mechanisms of the spatial lipid distributions in the surface sediments are defined, indicating marine productivity related to river-induced mixing and oceanic upwelling, wind-driven deep upwelling, river-supply of terrigenous organic material, shallow coastal upwelling and eolian supply of plant-waxes.

Main components and isotopic composition of gases from mineral waters of the Taman Peninsula and Caucasus

Helium isotope composition as an indicator of the mantle-derived component was studied in gases from mineral springs, stratal waters, and mud volcanoes developed west of the Teberda River valley (10 objects) and two springs in the central segment of the Greater Caucasus orogen between the active El'brus and Kazbek volcanoes. In the western segment of the orogen ratios of 3He/4He = R_corr vary from 46x10**-8 to 114x10**-8 (from 0.33 to 0.81 R_atm, where R_atm = 1.4x10**-6 is the atmospheric ratio). They are substantially lower relative to ratios in the vicinity of El'brus and Kazbek and close to those in samples from the central segment (from 70x10**-8 to 134x10**-8 (from 0.50 to 0.96 R_atm), as well as to ratios previously recorded in the Caucasian Mineral Waters (CMW) area. Moreover, concentration of 3He in them is notably higher than its crustal radiogenic level characteristic of mud volcanoes in the Taman Peninsula, where 3He/4He varies from 1.4x10**-8 to 2.8x10**-8 (from 0.01 to 0.02 R_atm). Nitrogen-methane gas from northern piedmonts of the western Caucasus also contains nonatmogenic components including radiogenic 40Ar (40Ar/36Ar = 900), excessive nitrogen (~87% of total N2 concentration in sample) and mantle He. These data specify distribution of mantle derivates along the orogen strike and age of intrusive magmatic activity in its different segments.

Results from field observations and in situ 13C feeding experiments in the Nazaré Canyon conducted during RRS James Cook 10 cruise Leg 2

Submarine canyon systems provide a heterogeneous habitat for deep-sea benthos in terms of topography, hydrography, and the quality and quantity of organic matter present. Enhanced meiofauna densities as found in organically enriched canyon sediments suggest that nematodes, as the dominant metazoan meiobenthic taxon, may play an important role in the benthic food web of these sediments. Very little is known about the natural diets and trophic biology of deep-sea nematodes, but enrichment experiments can shed light on nematode feeding selectivity and trophic position. An in-situ pulse-chase experiment (Feedex) was performed in the Nazaré Canyon on the Portuguese margin in summer 2007 to study nematode feeding behaviour. 13C-labelled diatoms and bacteria were added to sediment cores which were then sampled over a 14-day period. There was differential uptake by the nematode community of the food sources provided, indicating selective feeding processes. 13C isotope results revealed that selective feeding was less pronounced at the surface, compared to the sediment subsurface. This was supported by a higher trophic diversity in surface sediments compared to the subsurface, implying that more food items may be used by the nematode community at the sediment surface. Predatory and scavenging nematodes contributed relatively more to biomass than other feeding types and can be seen as key contributors to the nematode food web at the canyon site. Non-selective deposit feeding nematodes were the dominant trophic group in terms of abundance and contributed substantially to total nematode biomass. The high levels of 'fresh' (bioavailable) organic matter input and moderate hydrodynamic disturbance of the canyon environment lead to a more complex trophic structure in canyon nematode communities than that found on the open continental slope, and favours predator/scavengers and non-selective deposit feeders.

Sediment descriptions, stage, calcium carbonate, organic carbon, delta 13C and geolipids at DSDP Hole 93-603B

Organic matter contents of black shales from the Cretaceous Hatteras and Blake-Bahama formations have been compared to those from surrounding organic-poor strata using C/N ratios, d13C values, and distributions of extractable and nonsolvent-extractable, long-chain hydrocarbons, acids, and alcohols. The proportion of marine and land-derived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbon-poor relative to their organic-carbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this outer continental rise location under generally oxygenated bottom-water conditions.