Rhenium and osmium concentrations and isotopic compositions in altered ocean crust rocks at ODP Site 185-801

Re and Os concentrations and Os isotopic ratios were determined for composite samples prepared from volcanoclastics (VCL) and basaltic flows (FLO) from Jurassic oceanic crust (Ocean Drilling Program Leg 185, Site 801 in the western Pacific), with the aim of determining the effect of seafloor weathering on the Re-Os budget. A supercomposite sample, prepared from a proportionate mixture of the various composite powders, served to represent the average composition of the altered oceanic crust [Kelley, K.A., Plank, T., Ludden, J. and Staudigel, H., (2003). Composition of altered oceanic crust at ODP Sites 801 and 1149, Geochem. Geophys. Geosyst. 4(6) 8910, doi:10.1029/2002GC000435.]. Re contents vary from 0.2 to 1.3 ng/g, and from 2.2 to 3.1 ng/g in the VCL and FLO composites respectively. Os contents vary from 0.005 to 0.047 ng/g in the VCL, and from 0.008 to 0.027 ng/g in the FLO composites. The FLO composites have much higher Re/Os ratios and thus have more radiogenic Os compositions (187Os/188Os = 1.38 to 8.48) than the VCL composites (187Os/188Os = 0.32 to 4.40). The VCL composite from the upper section of the crust shows evidence for substantial Re loss and Os uptake, consistent with oxidative weathering processes. However, Re uptake during weathering processes under more reducing conditions, evident in the FLO samples from throughout the section and to a lesser extent in the lower VCL samples, more than compensates for this Re loss in the upper VCL. Os concentrations were essentially unchanged by these reductive processes. Model age calculations suggest that Re uptake continued for tens of millions of years after crust formation. Abundant secondary pyrite is found throughout the altered Hole 801C crust in zones of restricted seawater flow, and this may have accommodated an important part of the input Re. The Re content of the supercomposite (~2.2 ng/g) is about 1 ng/g higher than would be expected on the basis of its Yb content. If the results from Hole 801C are typical, they suggest that the Re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration. Such large extents of Re uptake would have a significant effect on the oceanic Re budget. Furthermore, assuming that they survive passage through the subduction zone, these elevated Re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic Os compositions of many ocean island basalts.

Chemical and germanium isotopic compositions of sediments and sedimentary rocks from ODP Sites 129-801 and 185-1149

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICP-MS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of 74Ge/70Ge ratios are expressed as d74Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2? for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4? in d74Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of 74Ge/70Ge, 73Ge/70Ge and 72Ge/70Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of d74Ge in igneous rocks is only 0.25? without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a d74Ge of 1.3+/-0.2? can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher d74Ge values between 2.0? and 3.0?. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the d74Ge of the seawater to +3.0?.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have d74Ge values ranging from 0.7? to 2.0?. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.

Geochemistry of volcanic glass from ODP holes 129-801C and 185-801C in the western Pacific

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma.Glass recovered from Jurassic age (170 Ma) Pacific ocean crust (Bartolini and Larson, 2001, doi:10.1130/0091-7613(2001)029<0735:PMATPS>2.0.CO;2) at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60°C hotter than today.Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma. Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs (Humler et al., 1999, doi:10.1016/S0012-821X(99)00218-6), which may have been a consequence of the Cretaceous superplume event in the Pacific.Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.

Chlorine concentration and isotopic data for sediments and serpentine seamount material from ODP Legs 125, 129, 185, and 195

Chlorine isotope ratios were determined for volcanic gas, geothermal well, ash, and lava samples along the Izu-Bonin-Mariana volcanic front, serpentinite clasts and muds from serpentine seamounts (Conical, South Chamorro, Torishima), basalts from the Guguan cross-chain, and sediments from Ocean Drilling Program (ODP) Sites 800, 801, 802, and 1149. There is no systematic variation in d37Cl values along the volcanic front in either gas or ash samples. In contrast, distinct variations occur across the arc, implying variations in the fluid source at different depths within the subduction zone. Serpentinite clasts and serpentine muds from the seamounts tap a source of ~30 km depth and have d37Cl values of structurally bound chloride of +0.4 per mil +/- 0.4 per mil (n = 24), identical to most seafloor serpentinites, suggesting a serpentinite (chrysotile and/or lizardite to antigorite transition) fluid source. Tapping deeper levels of the subduction zone (~115-130 km depth), volcanic gases and ashes have d37Cl values averaging -1.1 per mil +/- 1.0 per mil (n = 29), precisely overlapping the range measured in sediments from ODP cores (-1.1 per mil +/- +0.7 per mil, n = 11) and limited altered oceanic crust (AOC). Both sediments and AOC are possible Cl sources in the volcanic front. The Guguan cross-chain basalts come from the greatest depths and have an average d37Cl value of +0.2 per mil +/- 0.2 per mil (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at ~200 km depth.

Nitrogen isotopic composition and chemical composition of altered oceanic crust from ODP Legs 129 and 185

Knowledge of the subduction input flux of nitrogen (N) in altered oceanic crust (AOC) is critical in any attempt to mass-balance N across arc-trench systems on a global or individual-margin basis. We have employed sealed-tube, carrier-gas-based methods to examine the N concentrations and isotopic compositions of AOC. Analyses of 53 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Antarctic oceans (with larger numbers of samples from Site 801 outboard of the Mariana trench and Site 1149 outboard of the Izu trench), and 14 composites for the AOC sections at Site 801, give N concentrations of 1.3 to 18.2 ppm and d15N_air of -11.6? to +8.3?, indicating significant N enrichment probably during the early stages of hydrothermal alteration of the oceanic basalts. The N-d15N modeling for samples from Sites 801 and 1149 (n=39) shows that the secondary N may come from (1) the sedimentary N in the intercalated sediments and possibly overlying sediments via fluid-sediment/rock interaction, and (2) degassed mantle N2 in seawater via alteration-related abiotic reduction processes. For all Site 801 samples, weak correlation of N and K2O contents indicates that the siting of N in potassic alteration phases strongly depends on N availability and is possibly influenced by highly heterogeneous temperature and redox conditions during hydrothermal alteration. The upper 470-m AOC recovered by ODP Legs 129 and 185 delivers approximately 800 kg/km N annually into the Mariana margin. If the remaining less-altered oceanic crust (assuming 6.5 km, mostly dikes and gabbros) has MORB-like N of 1.5 ppm, the entire oceanic crust transfers 5100 kg/km N annually into that trench. This N input flux is twice as large as the annual N input of 2500 kg/km in seafloor sediments subducting into the same margin, demonstrating that the N input in oceanic crust, and its isotopic consequences, must be considered in any assessment of convergent margin N flux.

Granulometry of two sediment cores from Maxwell Bay, South Shetland Islands, West Antarctica

The climate evolution of the South Shetland Islands during the last c. 2000 years is inferred from the multiproxy analyses of a long (928 cm) sediment core retrieved from Maxwell Bay off King George Island. The vertical sediment flux at the core location is controlled by summer melting processes that cause sediment-laden meltwater plumes to form. These leave a characteristic signature in the sediments of NE Maxwell Bay. We use this signature to distinguish summer and winter-dominated periods. During the Medieval Warm Period, sediments are generally finer which indicates summer-type conditions. In contrast, during the Little Ice Age (LIA) sediments are generally coarser and are indicative of winter-dominated conditions. Comparison with Northern and Southern Hemisphere, Antarctic, and global temperature reconstructions reveals that the mean grain-size curve from Maxwell Bay closely resembles the curve of the global temperature reconstruction. We show that the medieval warming occurred earlier in the Southern than in the Northern Hemisphere, which might indicate that the warming was driven by processes occurring in the south. The beginning of the LIA appears to be almost synchronous in both hemispheres. The warming after the LIA closely resembles the Northern Hemisphere record which might indicate this phase of cooling was driven by processes occurring in the north. Although the recent rapid regional warming is clearly visible, the Maxwell Bay record does not show the dominance of summer-type sediments until the 1970s. Continued warming in this area will likely affect the marine ecosystem through meltwater induced turbidity of the surface waters as well as an extension of the vegetation period due to the predicted decrease of sea ice in this area.

Calcareous nannofossil abundance in ODP Site 185-1149 sediments

Basal carbonate sediments recovered at Ocean Drilling Program (ODP) Site 1149 lie directly on magnetic Anomaly M12. They contain abundant and moderately well preserved calcareous nannofossils. Two nannofossil zones are recognized: the lower Calcicalathina oblongata Zone and the upper Lithraphidites bollii Zone, indicating a late Valanginian-late Hauterivian age. The close occurrence of two significant bioevents, the first occurrence (FO) of L. bollii and the FO of Rucinolithus terebrodentarius in Core 185-1149B-20R, together with dip data recorded during in situ geophysical logging, suggest the presence of an unconformity that corresponds to the lower Hauterivian sedimentary section. The continuous occurrence of L. bollii is reported for the first time in sediments from the Pacific Ocean. Other marker species regarded as cosmopolitan (e.g., C. oblongata) have a sporadic occurrence. Nannoconids, very useful zonal markers for Tethyan areas, are virtually absent. The presence of an unusually high abundance of Diazomatolithus lehmanii is also recorded and correlates with the Valanginian 13C positive excursion.