Showing papers in "Geochemistry Geophysics Geosystems in 2006"

MPI-DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

Abstract: We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS

Abstract: The Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia has undertaken a systematic analysis of the isotopic (Sr, Nd, and Pb) compositions and concentrations of a broad compositional range of U.S. Geological Survey (USGS) reference materials, including basalt (BCR-1, 2; BHVO-1, 2), andesite (AGV-1, 2), rhyolite (RGM-1, 2), syenite (STM-1, 2), granodiorite (GSP-2), and granite (G-2, 3). USGS rock reference materials are geochemically well characterized, but there is neither a systematic methodology nor a database for radiogenic isotopic compositions, even for the widely used BCR-1. This investigation represents the first comprehensive, systematic analysis of the isotopic composition and concentration of USGS reference materials and provides an important database for the isotopic community. In addition, the range of equipment at the PCIGR, including a Nu Instruments Plasma MC-ICP-MS, a Thermo Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR-ICP-MS, permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the TIMS and MC-ICP-MS methods (e.g., Nd isotopic compositions). For each of the reference materials, 5 to 10 complete replicate analyses provide coherent isotopic results, all with external precision below 30 ppm (2 SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS and MC-ICP-MS, respectively). Our results also show that the first- and second-generation USGS reference materials have homogeneous Sr and Nd isotopic compositions. Nd isotopic compositions by MC-ICP-MS and TIMS agree to within 15 ppm for all reference materials. Interlaboratory MC-ICP-MS comparisons show excellent agreement for Pb isotopic compositions; however, the reproducibility is not as good as for Sr and Nd. A careful, sequential leaching experiment of three first- and second-generation reference materials (BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic compositions, and concentrations, could be directly related to contamination by the steel (mortar/pestle) used to process the materials. Contamination also accounts for the high concentrations of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W) in various USGS reference materials.

A catalogue of deep mantle plumes: New results from finite‐frequency tomography

Abstract: New finite-frequency tomographic images of S-wave velocity confirm the existence of deep mantle plumes below a large number of known hot spots. We compare S-anomaly images with an updated P-anomaly model. Deep mantle plumes are present beneath Ascension, Azores, Canary, Cape Verde, Cook Island, Crozet, Easter, Kerguelen, Hawaii, Samoa, and Tahiti. Afar, Atlantic Ridge, Bouvet(Shona), Cocos/Keeling, Louisville, and Reunion are shown to originate at least below the upper mantle if not much deeper. Plumes that reach only to midmantle are present beneath Bowie, Hainan, Eastern Australia, and Juan Fernandez; these plumes may have tails too thin to observe in the lowermost mantle, but the images are also consistent with an interpretation as “dying plumes” that have exhausted their source region. In the tomographic images, only the Eifel and Seychelles plumes are unambiguously confined to the upper mantle. Starting plumes are visible in the lowermost mantle beneath South of Java, East of Solomon, and in the Coral Sea. All imaged plumes are wide and fail to show plumeheads, suggesting a very weakly temperature-dependent viscosity for lower mantle minerals, and/or compositional variations. The S-wave velocity images show several minor differences with respect to the earlier P-wave results, including plume conduits that extend down to the core-mantle boundary beneath Cape Verde, Cook Island, and Kerguelen. A more substantial disagreement between P-wave and S-wave images reopens the question on the depth extent of the Iceland plume. We suggest that a pulsating behavior of the plume may explain the shape of the conduit beneath Iceland.

Global compilation of variations in slab depth beneath arc volcanoes and implications

Abstract: [1] The location and motion of subducting plates relative to volcanic arcs provide a first-order constraint on theories of arc magmagenesis We compile volcano-specific subduction parameters for 33,000 km of the global arc system at 839 volcanic centers, measuring the depth to the top of the slab (H) beneath each volcano The compilation also includes estimates of slab strike and dip, incoming plate velocity, and age, all available in accompanying auxiliary material The slab geometry is contoured from the top surface of Wadati-Benioff zones (WBZs) for a variety of teleseismic and local seismicity catalogs, which provides a reference surface for evaluating the distribution of seismicity within subducting plates The WBZ thickness exceeds that expected from hypocentral errors in a manner correlating with plate age, indicating that old plates have thicker regions in which earthquakes can occur When averaged over 500-km-long arc segments, H ranges from 72 to 173 km with a global average of 105 km, increasing by 20 km when hypocentral error effects are taken into account These depths correlate poorly with most subduction parameters, but significant correlations exist between H and slab dip (correlation coefficient is 054 for 45 arc segments) The dip correlation can be explained if the melting region is displaced from the Wadati-Benioff zone by a constant-thickness boundary layer For the north Pacific, H varies inversely with descent rate; this trend may reflect the manner in which wedge thermal structure affects arc location Over short distances some arc segments exhibit abrupt variations in arc location but not slab geometry, indicating that upper-plate tectonic processes also exert control on H These along-strike trends in H also correlate with geochemical proxies for the degree of melting, at least in one test case Thus slab geometry and kinematics provide an important control on the melting that produces arc volcanoes

Trace element composition of mantle end‐members: Implications for recycling of oceanic and upper and lower continental crust

Abstract: [1] Recycling of oceanic crust together with different types of marine sediments has become somewhat of a paradigm for explaining the chemical and isotopic composition of ocean island basalts. New high-precision trace element data on samples from St. Helena, Gough, and Tristan da Cunha, in addition to recent data from the literature, show that the trace element and isotope systematics in enriched mantle (EM) basalts are more complex than previously thought. EM basalts have some common characteristics (e.g., high Rb/La, Ba/La, Th/U, and Rb/Sr and low Nb/La and U/Pb) that distinguish them from HIMU basalts (high μ = 238U/204Pb). The isotopically distinct EM-1 and EM-2 basalts, however, cannot be clearly distinguished on the basis of incompatible trace element ratios. Ultimately, each suite of EM basalts carries its own specific trace element signature that must reflect different source compositions. In contrast, HIMU basalts show remarkably uniform trace element ratios, with a characteristic depletion in incompatible trace elements (Rb, Ba, Th, U, and Pb) and enrichment in Nb and Ta relative to EM basalts. Compositional similarities between HIMU and EM basalts (e.g., Nb/U, La/Sm, La/Th, Sr/Nd, Ba/K, and Rb/K) suggest that their sources share a common precursor, most likely recycled oceanic lithosphere. The compositional differences between HIMU and EM basalts, on the other hand, can only be explained if the EM sources contain an additional heterogeneous component. Parent-daughter ratios in subducted marine sediments have a unimodal distribution. Recycling of sediments alone can therefore not account for the isotopic bimodality of EM basalts. The upper and lower continental crust have similarly variable trace elements ratios but are systematically distinct in their Rb/Sr, U/Pb, Th/Pb, and Th/U ratios. Thus the upper and lower continental crust evolve along two distinct isotopic evolution paths but retain their complex trace element characteristics, similar to what is observed in EM basalts. We therefore propose that recycling of oceanic lithosphere together with variable proportions of lower and upper continental crust, which are introduced into the mantle together with the oceanic lithosphere via subduction erosion and/or subduction of marine sediments, respectively, provides a plausible explanation for the trace element and isotope systematics in ocean island basalts.

Long-term volumetric eruption rates and magma budgets

Abstract: [1] A global compilation of 170 time-averaged volumetric volcanic output rates (Qe) is evaluated in terms of composition and petrotectonic setting to advance the understanding of long-term rates of magma generation and eruption on Earth. Repose periods between successive eruptions at a given site and intrusive:extrusive ratios were compiled for selected volcanic centers where long-term (>104 years) data were available. More silicic compositions, rhyolites and andesites, have a more limited range of eruption rates than basalts. Even when high Qe values contributed by flood basalts (9 ± 2 × 10−1 km3/yr) are removed, there is a trend in decreasing average Qe with lava composition from basaltic eruptions (2.6 ± 1.0 × 10−2 km3/yr) to andesites (2.3 ± 0.8 × 10−3 km3/yr) and rhyolites (4.0 ± 1.4 × 10−3 km3/yr). This trend is also seen in the difference between oceanic and continental settings, as eruptions on oceanic crust tend to be predominately basaltic. All of the volcanoes occurring in oceanic settings fail to have statistically different mean Qe and have an overall average of 2.8 ± 0.4 × 10−2 km3/yr, excluding flood basalts. Likewise, all of the volcanoes on continental crust also fail to have statistically different mean Qe and have an overall average of 4.4 ± 0.8 × 10−3 km3/yr. Flood basalts also form a distinctive class with an average Qe nearly two orders of magnitude higher than any other class. However, we have found no systematic evidence linking increased intrusive:extrusive ratios with lower volcanic rates. A simple heat balance analysis suggests that the preponderance of volcanic systems must be open magmatic systems with respect to heat and matter transport in order to maintain eruptible magma at shallow depth throughout the observed lifetime of the volcano. The empirical upper limit of ∼10−2 km3/yr for magma eruption rate in systems with relatively high intrusive:extrusive ratios may be a consequence of the fundamental parameters governing rates of melt generation (e.g., subsolidus isentropic decompression, hydration due to slab dehydration and heat transfer between underplated magma and the overlying crust) in the Earth.

Species-specific responses of calcifying algae to changing seawater carbonate chemistry

Abstract: Uptake of half of the fossil fuel CO2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO2-related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO2 levels (∼360 ppm CO2). With particulate organic carbon (POC) remaining constant over the range of CO2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO2, POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.

Controls on back-arc basin formation

Abstract: The relationship between subduction and back-arc spreading has been well known since the early days of plate tectonics. However, the reasons why back-arc basins are associated with some subduction systems but not all has remained elusive. We examine the kinematic controls on subduction and back-arc basins for both the present-day and Cenozoic to differentiate between the major competing hypotheses for back-arc basin formation and to explain their temporal and spatial distribution. Our new data set of subduction and back-arc basin parameters uses a new set of paleo-oceanic age grids (Muller et al., 2005) associated with a moving Atlantic-Indian Ocean hot spot reference frame (O'Neill et al., 2005). The plate model includes detailed reconstructed spreading histories of back-arc basins based on marine geophysical and satellite gravity data. Our combined rotation and oceanic paleo-age model provides the age distribution of subducting lithosphere through space and time, convergence rates, and the absolute motion of the downgoing and overriding plates. We find that back-arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years. Additionally, we establish an age-dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30° with back-arc spreading. Our results suggest that back-arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge, thereby creating accommodation space between the overriding and subducting plates. Once back-arc extension is established, it continues regardless of overriding plate motion, indicating back-arc spreading is not a simple consequence of overriding plate behaviour. The landward migration of the overriding plate as a precursor to back-arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle. However, once back-arc extension is established, rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space. Our analysis indicates the driving mechanism for back-arc extension is a combination of surface kinematics, properties of the downgoing slab, the effect of lateral mantle flow on the slab, and mantle wedge dynamics.

Influence of trench width on subduction hinge retreat rates in 3‐D models of slab rollback

Abstract: Subduction of tectonic plates limited in lateral extent and with a free-trailing tail, i.e., “free subduction,” is modeled in a three-dimensional (3-D) geometry. The models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength, width, and thickness. We investigate the time evolution of this progressive rollback subduction, measure the accompanying return flow in the upper mantle, and quantify the plate kinematics. Due to the 3-D geometry, flow is allowed to accompany slab rollback around the lateral edges of the slab (the toroidal component), as opposed to 2-D geometry, where material is forced to flow underneath the slab tip (the poloidal component). A simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate. Our results indicate most of the gravitational energy of the system (i.e., the negative buoyancy of the subducting slab) is converted into a toroidal flow (∼69%), a much smaller amount goes into weakening the plate (∼18%), and the remaining amount goes into driving flow parallel to displacement of the slab (∼13%). For the trench widths (W) we investigate (≤1500 km), a maximum trench retreat rate occurs for trenches 600 km wide, which is attributed to the interaction between a plate of finite width and the induced flow (which has a lengthscale in the horizontal direction). These numerical results quantitatively agree with comparable 3-D laboratory experiments using analogue models with a purely viscous plate material (Schellart, 2004a, 2004b), including correlations between increasing retreat rate with increasing plate thickness, trench width for maximum retreat rate (500 km), and estimated amount of slab buoyancy used to drive rollback-induced flow (∼70%). Several implications for plate tectonics on Earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle (and endothermic phase transition) prior to a flushing event into the lower mantle (mantle avalanche).

Global temperature calibration of the alkenone unsaturation index (UK′37) in surface waters and comparison with surface sediments

Abstract: In this paper, we compile the current surface seawater C37 alkenone unsaturation (UK′37) measurements (n = 629, −1 to 30°C temperature range) to derive a global, field-based calibration of UK′37 with alkenone production temperature. A single nonlinear “global” surface water calibration of UK′37 accurately predicts alkenone production temperatures over the diversity of modern-day oceanic environments and alkenone-synthesizing populations (T = −0.957 + 54.293(UK′37) − 52.894(UK′37)2 + 28.321(UK′37)3, r2 = 0.97, n = 567). The mean standard error of estimation is 1.2°C with insignificant bias in estimated production temperature among the different ocean regions sampled. An exception to these trends is regions characterized by strong lateral advection and extreme productivity and temperature gradients (e.g., the Brazil-Malvinas Confluence). In contrast to the surface water data, the calibration of UK′37 in surface sediments with overlying annual mean sea surface temperature (AnnO) is best fit by a linear model (AnnO = 29.876(UK′37) − 1.334, r2 = 0.97, n = 592). The standard error of estimation (1.1°C) is similar to that of the surface water production calibration, but a higher degree of bias is observed among the regional data sets. The sediment calibration differs significantly from the surface water calibration. UK′37 in surface sediments is consistently higher than that predicted from AnnO and the surface water production temperature calibration, and the magnitude of the offset increases as the surface water AnnO decreases. We apply the global production temperature calibration to the coretop UK′37 data to estimate the coretop alkenone integrated production temperature (coretop IPT) and compare this with the overlying annual mean sea surface temperature (AnnO). We use simple models to explore the possible causes of the deviation observed between the coretop temperature signal, as estimated by UK′37, and AnnO. Our results indicate that the deviation can best be explained if seasonality in production and/or thermocline production as well as differential degradation of 37:3 and 37:2 alkenones both affect the sedimentary alkenone signal.

Size sorting in marine muds: Processes, pitfalls, and prospects for paleoflow-speed proxies

Abstract: The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice-rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.

U‐Pb dating of zircon by LA‐ICP‐MS

Abstract: In this study we used LA-ICP-MS (laser ablation–inductively coupled plasma–mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age (∼1800 Ma to ∼50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double-focusing ICP-MS coupled with a New Wave Research UP-213 laser system. The laser ablation pit sizes ranged from 30 to 40 μm in diameter. Laser-induced time-dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

Modeling open system metamorphic decarbonation of subducting slabs

Abstract: Fluids derived from the devolatilization of subducting slabs play a critical role in the melting of the mantle wedge and global geochemical cycles. However, in spite of evidence for the existence and mobility of an aqueous fluid phase during subduction metamorphism, the effect of this fluid on decarbonation reactions in subducting lithologies remains largely unquantified. In this study we present results from thermodynamic modeling of metamorphic devolatilization of subducted lithologies for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach in which fluid flow is an intrinsic component of the chemical model offers an alternative to closed system models of subduction zone decarbonation. In general, our models simulating pervasive fluid flow in subducting lithologies predict CO2 fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H2O-rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO2 is returned to the deep mantle at convergent margins, particularly in cool and intermediate subduction zones. Our results demonstrate that for most subduction zones, a significant proportion of the CO2 derived from the slab is lost beneath the fore arc, and therefore CO2 flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab-derived CO2 flux for individual margins. Nevertheless, our predicted global slab-derived CO2 flux from convergent margins of 0.35–3.12 × 1012 mols CO2/yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab-derived CO2 flux is significantly less than atmospheric CO2 drawdown by chemical weathering, significant CO2 emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.

Microstructural and geochemical perspectives on planktic foraminiferal preservation: “Glassy” versus “Frosty”

Abstract: In recent years it has become apparent that the “cool tropic paradox” of Paleogene and Cretaceous “greenhouse” climates arises because of the diagenetic alteration of tropical planktic foraminiferal calcite near the seafloor, yielding artificially high δ 18O values. Because the Mg/Ca compositions of foraminiferal and inorganic calcite are thought to be quite different, Mg/Ca measurements should be a sensitive way of monitoring diagenetic alteration. Here we examine the extent of diagenetic alteration of Eocene planktic foraminiferal calcite using scanning electron microscope imaging of foraminiferal test microstructures and geochemical (δ 18O and Mg/Ca) analyses. We compare microstructural and geochemical characteristics between given species exhibiting two contrasting states of preservation: those that appear “frosty” under reflected light and those that appear “glassy.” Microstructural evidence reveals extensive diagenetic alteration of frosty foraminiferal tests at the micron scale, while δ 18O analyses document consistently higher δ 18O (and therefore lower paleotemperatures) in this material. Yet we find that δ 18O offsets between species in these frosty foraminiferal assemblages appear to be generally preserved, suggesting that frosty foraminifera remain valuable for generating relatively short (approximately ≤1 Myr) paleoceanographic time series that do not demand absolute estimates of paleotemperature. We also find that the observed increase in Mg/Ca for planktic foraminifera exhibiting diagenetic alteration (compared to glassy taphonomies) is far smaller than would be expected from the addition of inorganic calcite based on laboratory-derived Mg2+ partition coefficients. Our findings imply that a much lower Mg2+ partition coefficient controls inorganic calcite formation in deep sea sedimentary sections, in accordance with the findings of Baker et al. (1982).

Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif (MAR 30°N)

Abstract: The Atlantis Massif (Mid-Atlantic Ridge, 30°N) is an example of an oceanic core complex (OCC) exposed by a major fault system. Our integrated field and analytical study of mafic and ultramafic rocks exposed on the south wall of the massif demonstrates the complex interplay of fluids, mass transfer, and metamorphism in strain localization associated with the evolution of a major detachment shear zone and development of this OCC. Extensive talc-amphibole-chlorite metasomatism as well as heterogeneous, crystal-plastic to cataclastic deformation characterize a strongly foliated, 100-m-thick zone of detachment faulting. The metasomatic fault rocks are key elements of this OCC and record a deformation and metamorphic history that is distinct from the underlying basement rocks. Talc-rich fault rocks preserve textural and geochemical characteristics of their ultramafic protoliths. Although primary textures and mineral parageneses are commonly obliterated in rocks dominated by amphibole, bulk rock data point to a mafic protolith. Major and trace elements indicate a complex mutual interaction between gabbroic and ultramafic rocks during metasomatism, which together with microstructures suggest localized circulation of oxidizing, Si-Al-Ca-rich fluids and mass transfer in high strain deformation zones. This type of flow was distinct from the more pervasive circulation that led to strongly serpentinized domains in the south wall. In contrast, cataclastic microfracturing is associated with a dominantly static metasomatism in less deformed domains, suggesting that a significant amount of metasomatism was controlled by diffuse flow and mass transfer associated with fractures that lack a strong preferred orientation. Distinct differences in lithologies, metamorphic overprinting, and degree of deformation between the south wall and central dome of the Atlantis Massif demonstrate the complex lateral and vertical heterogeneity in composition, alteration, and structure of this OCC.

Impact of biomineralization processes on the Mg content of foraminiferal shells: A biological perspective

Abstract: The Mg/Ca ratio in foraminiferal shells is widely used as a proxy for paleotemperatures. Nevertheless, it seems that the basic Mg content of foraminifera is determined by biological factors, as can be concluded from the large inter species and intrashell variability and the frequent deviations from inorganic behavior. This paper discusses three possible ways by which foraminifera can control or modify the Mg content in their shell: (1) involvement of organic matrix in the precipitation process that may alter the partition coefficient of Mg in biogenic calcite, (2) controlled conversion of transient amorphous phases to calcite, and (3) modification of the Mg concentration in the parent solution from which the crystals precipitate. The first two mechanisms are probably responsible for the precipitation of high-Mg calcite phases (whole shell or sublayers), while the third mechanism leads to the formation of low-Mg calcite phases. We propose a model adapted from epithelial cells that allows massive Mg2+ removal from the biomineralization site. This model is especially relevant to the planktonic and deep benthic low-Mg foraminifera that are frequently used for paleotemperature reconstructions. We discuss the possible biological roles of Mg in the shell in terms of the calcite polymorph conservation, the in vivo chemical stability of the shell, the functions of Mg as a stabilizer of transient phases and as a controlling agent of the precipitation process. Several temperature sensitive biological processes that may influence the Mg/Ca ratio of the shell are suggested and a model that combines biogenic and inorganic considerations is presented. The model uses Mg heterogeneity in the shell together with temperature response (biologic and inorganic) of biomineralization processes, to account for the deviation of planktonic foraminifera from inorganic calcite at equilibrium with seawater.

Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise

Abstract: Dust aerosol samples were collected across the western North Pacific Ocean during May–June 2002. Samples were analyzed for soluble aerosol Fe(II), Fe(II) + Fe(III), and Al as well as major cations and anions. The aerosol samples were leached using a 10 second exposure to either filtered surface seawater or ultrapure deionized water yielding a measure of the “instantaneous” soluble fraction. A variety of analytical methods were employed, including 57Fe isotope dilution high-resolution ICP-MS, energy dispersive X-ray fluorescence, graphite furnace AAS, ion chromatography, and the FeLume chemiluminescent technique. Fe was found to be more soluble in ultrapure deionized water leaches, especially during periods of higher dust concentrations. Fe solubility averaged 9 ± 8% in ultrapure water leaches and 6 ± 5% in seawater leaches. Significant correlations were found between both soluble aerosol FeT and soluble Fe(II) concentrations and aerosol acidity; however, the percentages of soluble aerosol FeT and Fe(II) did not correlate with aerosol acidity We also did not observe significant correlations between total and soluble aerosol Fe concentrations and the concentrations of either particulate Fe or dissolved Fe in surface waters.

Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

Abstract: [1] Hf isotope compositions, coupled with Pb-Sr-Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back-arc opening (23–14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest ɛHf (+9) in the early Miocene to the highest ɛHf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back-arc opening. In the early stage of the back-arc opening, when injection of hot asthenospheric mantle induced high-temperature conditions in the mantle wedge, slab melting contributed to low ɛHf coupled with low ɛNd, relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high ɛHf, decoupling of ɛHf and ɛNd, high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.

Deep water mass geometry in the glacial Atlantic Ocean: A review of constraints from the paleonutrient proxy Cd/Ca

Abstract: [1] Paleonutrient proxies currently provide the strongest constraints on the past spatial distribution of deep water masses. We review the state of knowledge derived from the trace metal proxy Cd/Ca for the Atlantic Ocean during the Last Glacial Maximum (LGM). We compile published benthic foraminiferal Cd/Ca data, supplemented with new data, to reconstruct meridional Cd sections through the Holocene and LGM Atlantic. Holocene Cd/Ca reflects the modern water masses North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW), and Antarctic Intermediate Water (AAIW) reasonably well, except for anomalously high values in the intermediate to mid-depth far North Atlantic. LGM Cd/Ca clearly shows that NADW was replaced by the shallower Glacial North Atlantic Intermediate Water (GNAIW), with significant northward expansion of AABW. The boundary level between GNAIW and AABW was very abrupt, occurring between � 2200 and 2500 m modern water depth. Combined Cd/Ca and Cibicidoides d 13 C data allow us to also calculate the air-sea signature of d 13 C( d 13 Cas). The LGM Atlantic can be explained entirely by mixing between high-d 13 Cas GNAIWand low-d 13 Cas AABW. Negative d 13 Cas values in glacial AABW were likely caused by poor ventilation during formation, probably associated with extensive sea ice coverage. Overall, our knowledge base would benefit from improved data coverage in the South Atlantic and Southern Ocean, and a better understanding of ancillary effects on the Cd/Ca proxy itself.

Neogene overflow of Northern Component Water at the Greenland‐Scotland Ridge

Abstract: In the North Atlantic Ocean, flow of North Atlantic Deep Water (NADW), and of its ancient counterpart Northern Component Water (NCW), across the Greenland-Scotland Ridge (GSR) is thought to have played an important role in ocean circulation. Over the last 60 Ma, the Iceland Plume has dynamically supported an area which encompasses the GSR. Consequently, bathymetry of the GSR has varied with time due to a combination of lithospheric plate cooling and fluctuations in the temperature and buoyancy within the underlying convecting mantle. Here, we reassess the importance of plate cooling and convective control on this northern gateway for NCW flow during the Neogene period, following Wright and Miller (1996). To tackle the problem, benthic foraminiferal isotope data sets have been assembled to examine δ13C gradients between the three major deep water masses (i.e., Northern Component Water, Southern Ocean Water, and Pacific Ocean Water). Composite records are reported on an astronomical timescale, and a nonparametric curve-fitting technique is used to produce regional estimates of δ13C for each water mass. Confidence bands were calculated, and error propagation techniques used to estimate %NCW and its uncertainty. Despite obvious reservations about using long-term variations of δ13C from disparate analyses and settings, and despite considerable uncertainties in our understanding of ancient oceanic transport pathways, the variation of NCW through time is consistent with independent estimates of the temporal variation of dynamical support associated with the Iceland Plume. Prior to 12 Ma, δ13C patterns overlap and %NCW cannot be isolated. Significant long-period variations are evident, which are consistent with previously published work. From 12 Ma, when lithospheric cooling probably caused the GSR to submerge completely, long-period δ13C patterns diverge significantly and allow reasonable %NCW estimates to be made. Our most robust result is a dramatic increase in NCW overflow between 6 and 2 Ma when dynamical support generated by the Iceland Plume was weakest. Between 6 and 12 Ma a series of variations in NCW overflow have been resolved.

Detachment shear zone of the Atlantis Massif core complex, Mid‐Atlantic Ridge, 30°N

Abstract: Near-bottom investigations of the cross section of the Atlantis Massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex. Integrated direct observations, sampling, photogeology, and imaging define a mylonitic, low-angle detachment shear zone (DSZ) along the crest of the massif. The shear zone may project beneath the nearby, corrugated upper surface of the massif. The DSZ and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme, localized tectonic extension during seafloor spreading over the past 2 m.y. The DSZ is characterized by strongly foliated to mylonitic serpentinites and talc-amphibole schists. It is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction. The DSZ foliation arches over the top of the massif in a convex-upward trajectory mimicking the morphology of the top of the massif. Kinematic indicators show consistent top-to-east (toward the MAR axis) tectonic transport directions. Foliated DSZ rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop-scale foliation. The DSZ and underlying basement rocks are cut by discrete, anastomosing, normal-slip, shear zones. Widely spaced, steeply dipping, normal faults cut all the older structures and localize serpentinization-driven hydrothermal outflow at the Lost City Hydrothermal Field. A thin (few meters) sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the DSZ and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present, gently dipping orientation during lateral spreading and uplift.

Tectonic discrimination diagrams revisited

Abstract: The decision boundaries of most tectonic discrimination diagrams are drawn by eye. Discriminant analysis is a statistically more rigorous way to determine the tectonic affinity of oceanic basalts based on their bulk-rock chemistry. This method was applied to a database of 756 oceanic basalts of known tectonic affinity ( ocean island, mid-ocean ridge, or island arc). For each of these training data, up to 45 major, minor, and trace elements were measured. Discriminant analysis assumes multivariate normality. If the same covariance structure is shared by all the classes (i.e., tectonic affinities), the decision boundaries are linear, hence the term linear discriminant analysis (LDA). In contrast with this, quadratic discriminant analysis (QDA) allows the classes to have different covariance structures. To solve the statistical problems associated with the constant-sum constraint of geochemical data, the training data must be transformed to log-ratio space before performing a discriminant analysis. The results can be mapped back to the compositional data space using the inverse log-ratio transformation. An exhaustive exploration of 14,190 possible ternary discrimination diagrams yields the Ti-Si-Sr system as the best linear discrimination diagram and the Na-Nb-Sr system as the best quadratic discrimination diagram. The best linear and quadratic discrimination diagrams using only immobile elements are Ti-V-Sc and Ti-V-Sm, respectively. As little as 5% of the training data are misclassified by these discrimination diagrams. Testing them on a second database of 182 samples that were not part of the training data yields a more reliable estimate of future performance. Although QDA misclassifies fewer training data than LDA, the opposite is generally true for the test data. Therefore LDA is a cruder but more robust classifier than QDA. Another advantage of LDA is that it provides a powerful way to reduce the dimensionality of the multivariate geochemical data in a similar way to principal component analysis. This procedure yields a small number of "discriminant functions,'' which are linear combinations of the original variables that maximize the between-class variance relative to the within-class variance.

GeoFramework: Coupling multiple models of mantle convection within a computational framework

Abstract: Solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems. A software package has been developed to couple geophysical solvers, demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework (Pyre). Pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations. We restructured and ported CitcomS, a finite element code for mantle convection, into the Pyre framework. Two CitcomS solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution. A comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models.

Determination of temperature‐dependent stable strontium isotope (88Sr/86Sr) fractionation via bracketing standard MC‐ICP‐MS

Abstract: Stable strontium isotopes (here 88Sr/86Sr) are introduced as a new member of the nontraditional stable isotopes. We have developed a bracketing standard method for the determination of δ88/86Sr using an AXIOM MC-ICP-MS and normalizing to strontium SRM NBS987. For individual measurements the external reproducibility is better than about 25 ppm (1σ RSD). For the IAPSO seawater standard a δ88/86Sr value of 0. 381 ± 0.010‰ (2SEM) was determined. For the first time a temperature-dependent strontium isotope fractionation during calcium carbonate precipitation could be shown. Aragonite samples inorganically precipitated under temperature control between 10 and 50°C revealed a δ88/86Sr/temperature dependency of 0.0054(5)‰/°C (R2 = 0.987). In contrast, for natural coral samples (Pavona clavus) from a proxy calibration study (23 to 27°C) we did determine 0.033(5)‰/°C (R2 = 0.955). The processes causing this sixfold stronger temperature dependency for the natural coral samples have to be studied in more detail in future studies. In a first approach the different slopes can be interpreted as effects of kinetic fractionation of strontium ions with or without a hydrate shell of 22 to 29 water molecules.

Geochemical controls on hydrothermal fluids from the Kairei and Edmond Vent Fields, 23°–25°S, Central Indian Ridge

Abstract: Prior to 2000, no mid-ocean ridge hydrothermal systems were identified and sampled in the Indian Ocean. The Kairei field, Central Indian Ridge, was discovered and sampled in 2000 (Gamo et al., 2001; Hashimoto et al., 2001). This paper reports the chemical composition of hydrothermal fluids collected in 2001 from both the Kairei and newly discovered Edmond hydrothermal systems. Data are used to infer subsurface processes and conditions at the two sites, as well as to place them within the global context of known hydrothermal sites on the mid-ocean ridge system. While both sites are located on the intermediate spreading Central Indian Ridge, their hydrologic systems have affinities with those observed on slow spreading ridges. Although the compositions of fluids from the four individual vents sampled at Edmond and the three individual vents sampled at Kairei vary respectively, our interpretation is that a single source fluid at each site is subsequently modified by processes including phase separation, subsurface mixing with seawater, and deposition and/or remobilization of metal-sulfide deposits to generate the observed range of compositions. Both fields are located ≥6 km from the neovolcanic zone, on steps on the east wall of the axial valley, with the Edmond field ∼160 km north of Kairei and almost 1000 m deeper (3300 versus 2450 m). The Edmond fluids are extremely hot, with maximum measured temperatures of 382°C compared to 365°C for Kairei. All of the fluids sampled have chlorinities greater than local ambient seawater due to phase separation of seawater at supercritical conditions. Visual observations suggest both sites have been active for a substantial period of time, again similar to observations of slow spreading ridges. Our chemical data suggest that the substrate underlying Kairei is highly altered, while that at Edmond is not. Significant albitization appears to be occurring below the Edmond field, but not at Kairei. The Edmond fluids are by far the hottest brines yet observed venting from a mid-ocean ridge hydrothermal site, resulting in unusually high concentrations of several transition metals.

Lithium isotopic composition of marine sediments

Abstract: To understand the role of subducted sediment in arc magmatism and the formation of the continental crust, we have determined the Li isotopic composition of marine sediments subducting at several trenches, as well as from nonconvergent margin settings. The bulk isotopic composition (δ7Li) of global sediments varies widely (from −4.3 to 14.5‰), reflecting sediment type, provenance, and diagenetic processes. Among detrital sediments (−1.5 to 5‰), clay-rich variants are generally isotopically lighter than sands owing to isotopic fractionation during continental weathering. Sediments derived from mature continental crust are especially light. Volcanogenic sediments can have either heavier or lighter isotopic compositions than the mantle depending on alteration effects. Biogenic carbonate and silica are susceptible to recrystallization, which results in heavier isotopic compositions (6 to 14.5‰). The lightest composition was observed in hydrothermally leached sediments (−4.3‰). Slowly accumulated metalliferous sediments display anomalously high δ7Li (6 to 10‰) values due to incorporation of seawater-derived Li. On the basis of this and previous studies, integrated δ7Li values for subducted sediment vary from arc to arc, ranging from −0.4 to 9‰. Calculated fluxes of subducted sediment-derived Li also vary, from 1.4 to 18.2 g/yr per cm arc length. Although Li isotopic compositions of marine sediments largely overlap with those of the upper mantle and altered oceanic crust, controls on Li isotope composition of arc lavas are complex and must be understood in terms of the primary compositions and relative contributions of subducted sediments and oceanic crust, fractionation during metamorphic dehydration, and equilibration of slab-derived fluids with the subarc mantle. Finally, although clay-rich sediments can contribute to the light isotopic composition of continental crustal rocks, our results suggest diverse compositions of sedimentary rocks. Incorporation of sedimentary components in the formation of granitic rocks may give rise to greater heterogeneity in the composition of the upper continental crust than is currently estimated.

Third generation of the Potsdam Magnetic Model of the Earth (POMME)

Abstract: The Potsdam Magnetic Model of the Earth (POMME) is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields. The internal field is represented to spherical harmonic (SH) degree 90, while the secular variation and acceleration are given to SH degree 16. Static and time-varying magnetospheric fields are parameterized in Geocentric Solar-Magnetospheric (GSM) and Solar-Magnetic (SM) coordinates and include Disturbance Storm-Time (Dst index) and Interplanetary Magnetic Field (IMF-By) dependent contributions. The model was estimated from five years of CHAMP satellite magnetic data. All measurements were corrected for ocean tidal induction and night-side ionospheric F-region currents. The model is validated using an independent model from a combined data set of Orsted and SAC-C satellite measurements. For the core field to SH degree 13, the root mean square (RMS) vector difference between the two models at the center of the model period is smaller than 4 nT at the Earth's surface. The RMS uncertainty increases to about 100 nT for the predicted field in 2010, as inferred from the difference between the two models.

East Asian monsoon variability over the last seven glacial cycles recorded by a loess sequence from the northwestern Chinese Loess Plateau

Abstract: A 180-m-thick loess-paleosol sequence from the northwestern Chinese Loess Plateau was investigated to construct a high-resolution record of the East Asian monsoon variability over the last seven glacial-interglacial cycles. The low-field magnetic susceptibility (?, mass-specific) and the mean grain size are used as proxies for changes in the intensity of the East Asian summer and winter monsoon, respectively. Because of the weaker pedogenesis at the northwestern Chinese Loess Plateau compared to the central Chinese Loess Plateau, our ? and mean grain size records show a muted glacial-interglacial contrast for the Asian summer monsoon but an enhanced contrast for the Asian winter monsoon. Although better resolved, most orbital-scale East Asian monsoon variations captured by our ? and grain size records are similar to those reported from the central Chinese Loess Plateau. Nevertheless, variations in ? exhibit clear precessional cycles in three paleosol layers (i.e., S1, S2, and S3), corresponding with solar insolation maxima. Furthermore, unlike ? records at the central Chinese Loess Plateau where ? is dramatically enhanced at paleosol S5SS1 (corresponding to marine isotope stage 13), our new ? record shows a major enhancement at paleosol S4 (corresponding to marine isotope stage 11), which indicates geographic differences in the timing of local monsoon precipitation in the two regions.

Submarine venting of liquid carbon dioxide on a Mariana Arc volcano

Abstract: Although CO2 is generally the most abundant dissolved gas found in submarine hydrothermal fluids, it is rarely found in the form of CO2 liquid. Here we report the discovery of an unusual CO2-rich hydrothermal system at 1600-m depth near the summit of NW Eifuku, a small submarine volcano in the northern Mariana Arc. The site, named Champagne, was found to be discharging two distinct fluids from the same vent field: a 103°C gas-rich hydrothermal fluid and cold (<4°C) droplets composed mainly of liquid CO2. The hot vent fluid contained up to 2.7 moles/kg CO2, the highest ever reported for submarine hydrothermal fluids. The liquid droplets were composed of ∼98% CO2, ∼1% H2S, with only trace amounts of CH4 and H2. Surveys of the overlying water column plumes indicated that the vent fluid and buoyant CO2 droplets ascended <200 m before dispersing into the ocean. Submarine venting of liquid CO2 has been previously observed at only one other locality, in the Okinawa Trough back-arc basin (Sakai et al., 1990a), a geologic setting much different from NW Eifuku, which is a young arc volcano. The discovery of such a high CO2 flux at the Champagne site, estimated to be about 0.1% of the global MOR carbon flux, suggests that submarine arc volcanoes may play a larger role in oceanic carbon cycling than previously realized. The Champagne field may also prove to be a valuable natural laboratory for studying the effects of high CO2 concentrations on marine ecosystems.

Tracking the extent of the South Pacific Convergence Zone since the early 1600s

Abstract: (1) The South Pacific Convergence Zone (SPCZ) is the largest and most persistent spur of the Intertropical Convergence Zone. At the southeastern edge of the SPCZ near 170� W and 15� -20� S a surface ocean salinity frontal zone exists that separates fresher Western Pacific Warm Pool water from saltier and cooler waters in the east. This salinity front is known to shift east and west with the phase of the El Nino Southern Oscillation. We have generated subannually resolved and replicated coral oxygen isotopic time series from Fiji (17� S, 179� E) and Rarotonga (21.5� S, 160� W) that have recorded interannual displacements of the salinity front over the last 380 years and also indicate that at lower frequencies the decadal mean position of the salinity front, and eastern extent of the SPCZ, has shifted east-west through 10� to 20� of longitude three times during this interval. The most recent and largest shift began in the mid 1800s as the salinity front progressively moved eastward and salinity decreased at both sites. Our results suggest that sea surface salinity at these sites is now at the lowest levels recorded and is evidence for an unprecedented expansion of the SPCZ since the mid 1800s. The expansion of the SPCZ implies a gradual change in the South Pacific to more La Nina-like long-term mean conditions. This observation is consistent with the ocean thermostat mechanism for the Pacific coupled ocean-atmosphere system, whereby exogenous heating of the atmosphere would result in greater warming in the western Pacific and a greater east-west surface temperature gradient. Components: 8034 words, 7 figures, 2 tables.