Molecular and isotopic compositions of hydrocarbons at DSDP Site 76-533

In an investigation of gas hydrates in deep ocean sediments, gas samples from Deep Sea Drilling Project Site 533 on the Blake Outer Ridge in the northwest Atlantic were obtained for molecular and isotopic analyses. Gas samples were collected from the first successful deployment of a pressure core barrel (PCB) in a hydrate region. The pressure decline curves from two of the four PCB retrievals at in situ pressures suggested the presence of small amounts of gas hydrates. Compositional and isotopic measurements of gases from several points along the pressure decline curve indicated that (1) biogenic methane (d13C = -68 per mil; C1/C2 = 5000) was the dominant gas (>90%); (2) little fractionation in the C1/C2 ratio or the C carbon isotopic composition occurred as gas hydrates decomposed during pressure decline experiments; (3) the percent of C3, i-C4, and CO2 degassed increased as the pressure declined, indicating that these molecules may help stabilize the hydrate structure; (4) excess nitrogen was present during initial degassing; and (5) C1/C2 ratios and isotopic ratios of C gases were similar to those obtained from conventional core sampling. The PCB gas also contained trace amounts of saturated, acyclic, cyclic, and aromatic C5-C14 hydrocarbons, as well as alkenes and tetrahydrothiophenes. Gas from a decomposed specimen of gas hydrate had similar molecular and isotopic ratios to the PCB gas (d13C of -68 per mil for methane and a C1/C2 ratio of about 6000). Regular trends in the d13C of methane (about -95 to -60 per mil) and C1/C2 ratios (about 25000 to 2000) were observed with depth. Capillary gas chromatography (GC) and total scanning fluorescence measurements of extracted organic material were characteristic of hydrocarbons dominated by a marine source, though significant amounts of perylene were also present.

(Table 1) Strontium isotopic ratios of interstitial waters from ODP Leg 125 holes

The strontium isotopic data presented here are from interstitial waters squeezed from unconsolidated serpentine, an unusual type of substrate that was recovered from Mariana and Bonin forearc seamounts and has not been previously drilled by the Deep Sea Drilling Project or Ocean Drilling Program. The texture and composition of some of these serpentine deposits from Conical Seamount, located on the Mariana forearc, indicate emplacement as low- or high-viscosity, cold gravitational flows, which are therefore neither sediment nor igneous rock. The strontium isotopic ratios of the interstitial waters from the unconsolidated serpentine range from 0.70912 to 0.70525 and trend toward a relatively less radiogenic composition with increasing sub-bottom depth. These strontium isotopic ratios are derived from at least two strontium sources: seawater and igneous. The strontium isotopic gradients from the interstitial waters from the Leg 125 sites are probably the result of diffusive transport of strontium from an igneous source deep within the lithosphere that may be contaminated with subducted or underplated sediment.

(Table 3) Oxygen and carbon isotopic record of living (Rose Bengal Stained) benthic foraminifera of North Atlantic surface sediments

Variation of the d13C of living (Rose Bengal stained) deep-sea benthic foraminifera is documented from two deep-water sites (~2430 and ~3010 m) from a northwest Atlantic Ocean study area 275 km south of Nantucket Island. The carbon isotopic data of Hoeglundina elegans and Uvigerina peregrina from five sets of Multicorer and Soutar Box Core samples taken over a 10-month interval (March, May, July, and October 1996 and January 1997) are compared with an 11.5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa. Carbon isotopic data of Hoeglundina elegans at 3010 m show 0.3 per mil lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This d13C change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of d13C in the pore waters within the phytodetritus and overlying the sediment surface. Carbon isotopic data of H. elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a d13C enrichment following the spring bloom. This different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the March 1996 cruise. The d13C of Uvigerina peregrina at 2430 m shows variation over the 10 month interval, but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes. The isotopic analyses are grouped into 100 µm size classes on the basis of length measurements of individual specimens to evaluate d13C ontogenetic changes of each species. The data show no consistent patterns between size classes in the d13C of either H. elegans or U. peregrina. These results suggest that variation in organic carbon flux does not preferentially affect particular size classes, nor do d13C ontogenetic changes exist within the >250 to >750 µm size range for these species at this locality. On the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct d13C in paleoceanographic studies. The prediction standard deviation, which is composed of cruise, core, subcore, and residual (replicate) variability, provides an estimate of the magnitude of variability in fossil d13C data; it is 0.27 per mil for H. elegans at 3010 m and 0.4 per mil for U. peregrina at the 2430 m site. Since these standard deviations are based on living specimens, they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses. Most paleoceanographic reconstructions are based on the analysis of multiple specimens, and as a result, the standard error would be expected to be reduced for any particular sample. The reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records.

Geochemistry and isotopic ratios of Neogene basalts from Spitzbergen Island, Svalbard archipelago

Petrological and geochemical data obtained on Neogene magmatism restricted to a deep fault in Andree Land at Spitsbergen Island, which was related to the overall restyling of the Arctic territory at 25-20 Ma, indicate that the derivation of the Neogene magmas was significantly affected by the continental pyroxenite mantle. The Neogene basalts are noted for a radiogenic isotopic composition of Pb (207Pb/204Pb= 15.5-15.55, 206Pb/204Pb = 18.4-18.6, 208Pb/204Pb = 38.4-38.6) and Sr (87Sr/86Sr = 0.7038-0.7048) at low 143Nd/144Nd = 0.5129. Melts of this type are the extremely enriched end member of the isotopic mixing of a depleted and enriched sources and determine a geochemical trend that passes through the compositions of alkaline magmas from Quaternary volcanoes at Spitsbergen and weakly enriched tholeiites of the Knipovich Ridge, which started to develop simultaneously with the onset of Neogene magmatism in the western part of Spitsbergen. The composition of the liquidus olivine (which is rich in NiO) indicates that melting occurred in the olivine-free mantle. Our data thus testify that a significant role in the genesis of the Neogene magmas was played by continental pyroxenite mantle.

(Table 1) Composition of interstitial waters from ODP Leg 125 sites

Pore waters were collected from nine sites during Leg 125 of the Ocean Drilling Program (ODP). The first four sites (778-781) were drilled in the Mariana forearc on and near Conical Seamount, an active serpentine "mud volcano" located about 80 km behind the trench axis and 120 km in front of the active island arc. The last five sites (782-786) were drilled in the Izu-Bonin forearc between the trench and the outer arc high. Pore waters from the five sites from both areas that penetrated serpentine silts (Sites 778,779,780,783, and 784) are discussed in detail by Mottl (this volume). Here we report analyses of the pore waters from all nine sites for Li, Rb, Sr, Ba, Mn, B, and the sulfur isotopic ratio of dissolved sulfate. Sampling methods and results of analyses for major and minor species determined aboard ship were presented by Fryer, Pearce, Stokking, et al. (1990, doi:10.2973/odp.proc.ir.125.1990).

FORAMINIFERAL RECORD OF CHANGES IN SUMMER MONSOON PRECIPITATION AT THE SOUTHEASTERN BRAZILIAN CONTINENTAL MARGIN SINCE THE LAST GLACIAL MAXIMUM

Changes in the oxygen isotopic composition of the planktonic foraminifer Globigerinoides ruber and in the foraminifera faunal composition in a core retrieved from the southeastern Brazilian continental margin were used to infer past changes in the hydrological balance and monsoon precipitation in the western South Atlantic since the Last Glacial Maximum (LGM). The results suggest a first-order orbital (precessional) control on the South American Monsoon precipitation. This agrees with previous studies based on continental proxies except for LGM estimates provided by pollen records. The causes for this disagreement are discussed.

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (delta(15)N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and delta(15)N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar delta(15)N was depleted, varying from -9.6 to 1.6aEuro degrees in the white-sand vegetations. The legume Aldina heterophylla had the highest average delta(15)N values (-1.5aEuro degrees) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted delta(15)N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar delta(15)N among plants, a significant and gradual (15)N-enrichment in foliar isotopic signatures throughout the campina-campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in (15)N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar delta(15)N values throughout the campina-campinarana transect.

Understanding the Influences of Spatial Patterns on N Availability Within the Brazilian Amazon Forest

Nitrogen variations at different spatial scales and integrated across functional groups were addressed for lowland tropical forests in the Brazilian Amazon as follows: (1) how does N availability vary across the region over different spatial scales (regional x landscape scale); ( 2) how are these variations in N availability integrated across plant functional groups ( legume 9 non-legume trees). Leaf N, P, and Ca concentrations as well the leaf N isotope ratios (delta(15)N) from a large set of legume and non-legume tree species were measured. Legumes had higher foliar N/Ca ratios than non-legumes, consistent with the high energetic costs in plant growth associated with higher foliar P/Ca ratios found in legumes than in non-legumes. At the regional level, foliar delta(15)N decreased with increasing rainfall. At the landscape level, N availability was higher in the forests on clayey soils on the plateau than in forests on sandier soils. The isotope as well as the non-isotope data relationships here documented, explain to a large extent the variation in delta(15)N signatures across gradients of rainfall and soil. Although at the regional level, the precipitation regime is a major determinant of differences in N availability, at the landscape level, under the same precipitation regime, soil type seems to be a major factor influencing the availability of N in the Brazilian Amazon forest.

Middle Pleistocene sea surface temperature in the Brazil-Malvinas Confluence Zone: Paleoceanographic implications based on planktonic foraminifera

We reconstructed Middle Pleistocene surface hydrography in the western South Atlantic based on planktonic foraminiferal assemblages, modern analog technique and Globorotalia truncatulinoides isotopic ratios of core SP1251 (38 degrees 29.7`S / 53 degrees 40.7`W / 3400 m water depth). Biostratigraphic analysis suggests that sediments were deposited between 0.3 and 0.12 Ma and therefore correlate to Marine Isotopic Stage 6 or 8. Faunal assemblage-based winter and summer SST estimates suggest that the western South Atlantic at 38 degrees S was 4-6 degrees C colder than at present, within the expected range for a glacial interval. High relative abundances of subantarctic species, particularly the dominance of Neogloboquadrina pachyderma (left), support lower than present SSTs throughout the recorded period. The oxygen isotopic composition of G. truncatulinoides suggests a northward shift of the Brazil-Malvinas Confluence Zone and of the associated mid-latitude frontal system during this Middle Pleistocene cold period, and a stronger than present influence of superficial subantarctic waters and lowering in SSTs at our core site during the recorded Middle Pleistocene glacial.

Nutrient-induced perturbations to delta C-13 and delta N-15 in symbiotic dinoilagellates and their coral hosts

Inorganic nutrients play a critical role in determining benthic community structure in tropical seas. This study examined the impact of adding inorganic nutrients (ammonium and phosphate) on the isotopic composition of 2 reef-building corals, Pocillopora damicornis and Heliofungia actiniformis, on the southern Great Barrier Reef. The addition of elevated nutrients to patch reefs that pond at low tide did not perturb the C:N ratio of either species or their symbiotic dinoflagellates. The C:N ratios were significantly higher in material extracted from the skeleton (14.8 +/- 1.50 and 10.8 +/- 1.42) than either host (7.6 +/- 0.87 and 6.0 +/- 0.71) or symbiotic dinoflagellates (5.7 +/- 0.48 and 6.9 +/- 0.66) (P. damicornis and H. actiniformis respectively; 95 confidence intervals). The ratio of acquired N to background N suggests that the added dissolved inorganic nitrogen (DIN) accounted for 50 to 100% of total nitrogen within the tissues of P. damicornis and H. actiniformis at the end of the experiment. The addition of the isotopically depleted nutrients (delta(15) N = 0parts per thousand) to patch reefs significantly decreased delta(15)N from control values of between 3 and 4 to values to below 1 in the case of all compartments, while delta(13)C values were relatively unresponsive to nutrient treatments. These findings suggest that coral delta(15)N has the potential to provide a historical record of the delta(15)N of dissolved nitrogen surrounding reef-building corals and their symbiotic dinoflagellates.

Natural abundance of stable carbon and nitrogen isotopes in Cannabis sativa reflects growth conditions

Stable carbon and nitrogen isotope signatures (delta C-13 and delta N-15) of Cannabis sativa were assessed for their usefulness to trace seized Cannabis leaves to the country of origin and to source crops by determining how isotope signatures relate to plant growth conditions. The isotopic composition of Cannabis examined here covered nearly the entire range of values reported for terrestrial C-3 plants. The delta C-13 values of Cannabis from Australia, Papua New Guinea and Thailand ranged from -36 to -25 parts per thousand, and delta N-15 values ranged from -1.0 to 15.8 parts per thousand. The stable isotope content did not allow differentiation between Cannabis originating from the three countries, but delta C-13 values of plantation-grown Cannabis differed between well-watered plants (average delta C-13 of -30.0 parts per thousand) and plants that had received little irrigation (average delta C-13 of -26.4 parts per thousand). Cannabis grown under controlled conditions had delta C-13 values of -32.6 and -30.6 parts per thousand with high and low water supply, respectively. These results indicate that water availability determines leaf C-13 in plants grown under similar conditions of light, temperature and air humidity. The delta C-13 values also distinguished between indoor- and outdoor-grown Cannabis; indoor- grown plants had overall more negative delta C-13 values (average -31.8 parts per thousand) than outdoor-grown plants (average -27.9 parts per thousand). Contributing to the strong C-13-depletion of indoor- grown plants may be high relative humidity, poor ventilation and recycling of C-13-depleted respired CO2. Mineral fertilizers had mostly lower delta N-15 values (-0.2 to 2.2 parts per thousand) than manure-based fertilizers (7.6 to 22.7 parts per thousand). It was possible to link delta N-15 values of fertilizers associated with a crop site to soil and plant delta N-15 values. The strong relationship between soil, fertilizer, and plant delta N-15 suggests that Cannabis delta N-15 is determined by the isotopic composition of the nitrogen source. The distinct delta N-15 values measured in Cannabis crops make delta N-15 an excellent tool for matching seized Cannabis with a source crop. A case study is presented that demonstrates how delta C-13 and delta N-15 values can be used as a forensic tool.

Monooxygenase stereoselectivity in the biosynthesis of stereoisomeric spiroacetals in the cucumber fly, Bactrocera cucumis

The stereoselectivity of hydroxylation of alkyltetrahydropyran-2-ols (or their biological equivalents) in the formation of stereoisomers of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecanes in male Bactrocera cucumis has been investigated. Racemic, (6R)-, and (6S)-6-methyl-2-[5-H-2(1)]-n-pentyltetrahydropyran-2-ol was administered under an [O-18(2)]-enriched atmosphere. The stereochemistry and isotopic composition of generated spiroacetals were monitored by combined enantioselective GC-MS. The monooxygenase(s) strongly prefers the (6S)-substrate and furnishes predominantly the (S)-alcohol and then the (2S,6R,8S)-2,8-dimethyl-1,7-dioxaspiro[5.5]undecane. The (2S,6S,8R) and (2R,6S,8S) (E,Z)-isomers appear to be derived in vivo predominantly from (R)-hydroxylation of the (6S)-tetrahydropyranol.

The role of a transcrustal shear zone in orogenic gold mineralization at the Aijanahalli Mine, Dharwar Craton, South India

The Ajjanahalli gold mine is spatially associated with a Late Archean craton-scale shear zone in the eastern Chitradurga greenstone belt of the Dharwar craton, India. Gold mineralization is hosted by an similar to100-m-wide antiform in a banded iron formation. Original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite, pyrite, pyrrhotite, and gold at ca. 300degrees to 350degreesC. Elements enriched in the banded iron formation include Ca, Mg, C, S, An, As, Bi. Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn, Bi, Ag, and An are only slightly enriched. Strontium correlates with CaO, MgO, CO2, and As, which indicates cogenetic formation of arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in the Fe-rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers. The ore fluids, as interpreted from their isotopic composition (delta(18)O = 6.5-8.5parts per thousand; initial Sr-87/Sr-86 = 0.7068-0.7078), formed by metamorphic devolatilization of deeper levels of the Chitradurga greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have delta(34)S values within a narrow range between 2.1 and 2.7 per mil, consistent with a sulfur source in Chitradurga greenstone belt lithologies. Based on spatial and temporal relationships between mineralization, local structure development, and sinistral strike-slip deformation in the shear zone at the eastern contact of the Chitradurga greenstone belt, we suggest that the Ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first-order structure. The ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics. Based on this model of fluid flow, exploration should focus on similar low strain areas or potentially connected higher order splays of the first-order shear zone.

Relació entre fotosíntesi i respiració en cultius sotmesos a estrés hídric mitjançant isòtops estables

Report for the scientific sojourn carried out at the Universite Paris Sud – XI, France, from July until october 2007. The relationship between photosynthesis and respiration were studied in crops using carbon and nitrogen stable isotopes under well watered and water-stressed conditions. The analyses of the 13C isotopic composition (δ13C) of total organic matter (TOM) recently fixed of well-watered plants revealed that it was mainly delivered to apical tissues and tap root. The fact that that the apical leaf and stems together with the inflorescences were d13C depleted, suggests that those tissues were newly formed and had a larger sink strength and metabolic activity. Secondly, the analyses of δ13C of respired CO2 immediately after the labelling (T=0) showed that a significant part of the C respired by leaves and nodules proceeded of the recently fixed CO2. In the following harvests (T=7 and T=14) such percentage tended to decrease, especially in apical leaves. Interestingly, the respiration d13C data also highlighted that even if at T=0 part of the respired proceeded from the CO2 fixed during the labelling, this percentage was even larger at T=7. Finally, the d15N also revealed that, similarly to what described for 12C, immediately after the 15N2 labelling (T=0), apical leaf and stems, together with tap root and in this case the nodules, were the tissues with larger sink strength. It is noteworthy the fact that the largest amount of N2 newly fixed was delivered to the tap roots where it was stored until it was required for the aboveground regrowth period.