Showing papers in "Geochemistry Geophysics Geosystems in 2010"

Geochemistry of oceanic anoxic events

Abstract: [1] Oceanic anoxic events (OAEs) record profound changes in the climatic and paleoceanographic state of the planet and represent major disturbances in the global carbon cycle. OAEs that manifestly caused major chemical change in the Mesozoic Ocean include those of the early Toarcian (Posidonienschiefer event, T-OAE, ∼183 Ma), early Aptian (Selli event, OAE 1a, ∼120 Ma), early Albian (Paquier event, OAE 1b, ∼111 Ma), and Cenomanian–Turonian (Bonarelli event, C/T OAE, OAE 2, ∼93 Ma). Currently available data suggest that the major forcing function behind OAEs was an abrupt rise in temperature, induced by rapid influx of CO2 into the atmosphere from volcanogenic and/or methanogenic sources. Global warming was accompanied by an accelerated hydrological cycle, increased continental weathering, enhanced nutrient discharge to oceans and lakes, intensified upwelling, and an increase in organic productivity. An increase in continental weathering is typically recorded by transient increases in the seawater values of 87Sr/86Sr and 187Os/188Os ratios acting against, in the case of the Cenomanian-Turonian and early Aptian OAEs, a longer-term trend to less radiogenic values. This latter trend indicates that hydrothermally and volcanically sourced nutrients may also have stimulated local increases in organic productivity. Increased flux of organic matter favored intense oxygen demand in the water column, as well as increased rates of marine and lacustrine carbon burial. Particularly in those restricted oceans and seaways where density stratification was favored by paleogeography and significant fluvial input, conditions could readily evolve from poorly oxygenated to anoxic and ultimately euxinic (i.e., sulfidic), this latter state being geochemically the most significant. The progressive evolution in redox conditions through phases of denitrification/anammox, through to sulfate reduction accompanied by water column precipitation of pyrite framboids, resulted in fractionation of many isotope systems (e.g., N, S, Fe, Mo, and U) and mobilization and incorporation of certain trace elements into carbonates (Mn), sulfides, and organic matter. Sequestration of CO2 in organic-rich black shales and by reaction with silicate rocks exposed on continents would ultimately restore climatic equilibrium but at the expense of massive chemical change in the oceans and over time scales of tens to hundreds of thousands of years.

Improved laser ablation U‐Pb zircon geochronology through robust downhole fractionation correction

Abstract: Elemental fractionation effects during analysis are the most significant impediment to obtaining precise and accurate U-Pb ages by laser ablation ICPMS. Several methods have been proposed to minimize the degree of downhole fractionation, typically by rastering or limiting acquisition to relatively short intervals of time, but these compromise minimum target size or the temporal resolution of data. Alternatively, other methods have been developed which attempt to correct for the effects of downhole elemental fractionation. A common feature of all these techniques, however, is that they impose an expected model of elemental fractionation behavior; thus, any variance in actual fractionation response between laboratories, mineral types, or matrix types cannot be easily accommodated. Here we investigate an alternate approach that aims to reverse the problem by first observing the elemental fractionation response and then applying an appropriate (and often unique) model to the data. This approach has the versatility to treat data from any laboratory, regardless of the expression of downhole fractionation under any given set of analytical conditions. We demonstrate that the use of more complex models of elemental fractionation such as exponential curves and smoothed cubic splines can efficiently correct complex fractionation trends, allowing detection of spatial heterogeneities, while simultaneously maintaining data quality. We present a data reduction module for use with the lolite software package that implements this methodology and which may provide the means for simpler interlaboratory comparisons and, perhaps most importantly, enable the rapid reduction of large quantities of data with maximum feedback to the user at each stage. Copyright 2010 by the American Geophysical Union.

Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system

Abstract: Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas These ''fore-arc basalts'' (FAB) underlie boninites and overlie diabasic and gabbroic rocks Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events

Boron and magnesium isotopic composition of seawater

Abstract: The isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes. The isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard. Given the long residence times of both elements (∼10 7 years) it is commonly assumed that seawater is isotopically homogenous for these systems, yet no systematic studies currently exist. Here we present the B and Mg isotopic composition of 26-28 seawater samples from a number of ocean basins that encompass a significant range in salinity (32 to 38 psu), temperature (-0.3 to +25.9C) and water depth (0 to 1240 m). We find no significant or systematic variation for either system in accordance with their long residence times. We recommend that the mean values we report (δ 11 B = 39.61 0.04 ‰ (2 s.e.; n = 28), δ 25 Mg = -0.43 0.01 ‰ (2 s.e.; n = 26), δ 26 Mg = -0.82 0.01 ‰ (2 s.e.; n = 26)) be used in future studies involving Mg and B isotopes. Copyright © 2010 by the American Geophysical Union.

Detection of noninteracting single domain particles using first-order reversal curve diagrams

Abstract: We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

High‐precision U‐Pb zircon age calibration of the global Carboniferous time scale and Milankovitch band cyclicity in the Donets Basin, eastern Ukraine

Abstract: [1] High-precision ID-TIMS U-Pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic, cyclostratigraphic, and biostratigraphic framework of the Carboniferous Donets Basin of eastern Europe. Chemical abrasion of zircons, use of the internationally calibrated EARTHTIME mixed U-Pb isotope dilution tracer, and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of <0.05%, or ∼100 ka, for these Carboniferous volcanics. This precision allows the resolution of time in the Milankovitch band and confirms the long-standing hypothesis that individual high-frequency Pennsylvanian cyclothems and bundles of cyclothems into fourth-order sequences are the eustatic response to orbital eccentricity (∼100 and 400 ka) forcing. Tuning of the fourth-order sequences in the Donets Basin to the long-period eccentricity cycle results in a continuous age model for the Middle to Late Pennsylvanian (Moscovian-Kasimovian-Ghzelian) strata of the basin and their record of biological and climatic changes through the latter portion of the late Paleozoic Ice Age. Detailed fusulinid and conodont zonations allow the export of this age model to sections throughout Euramerica. Additional ages for Mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations.

A geologically constrained Monte Carlo approach to modeling exposure ages from profiles of cosmogenic nuclides: An example from Lees Ferry, Arizona

Abstract: We present a user-friendly and versatile Monte Carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides (TCNs). Our program (available online at http://geochronology.earthsciences.dal.ca/downloads-models.html) permits the incorporation of site-specific geologic knowledge to calculate most probable values for exposure age, erosion rate, and inherited nuclide concentration while providing a rigorous treatment of their uncertainties. The simulator is demonstrated with 10Be data from a fluvial terrace at Lees Ferry, Arizona. Interpreted constraints on erosion, based on local soil properties and terrace morphology, yield a most probable exposure age and inheritance of 83.9−14.1+19.1 ka, and 9.49−2.52+1.21 × 104 atoms g−1, respectively (2σ). Without the ability to apply some constraint to either erosion rate or age, shallow depth profiles of any cosmogenic nuclide (except for nuclides produced via thermal and epithermal neutron capture, e.g., 36Cl) cannot be optimized to resolve either parameter. Contrasting simulations of 10Be data from both sand- and pebble-sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with TCN depth profiles and, when possible, sand—not pebbles—should be used for depth profile exposure dating.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Abstract: Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods ( 20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3-D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves

Abstract: [1] The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide-angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near-vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift-parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (SH) and Rayleigh (SV) waves vary in similar fashions with azimuth for LPO- and OMP-induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost ∼10 km, OMP between ∼20–75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning.

Reconciling astrochronological and 40Ar/39Ar ages for the Matuyama-Brunhes boundary and late Matuyama Chron

Abstract: When five Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the δ18O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773.1 ka (standard deviation = 0.4 kyr), ∼7 kyr younger than the presently accepted astrochronological age for this polarity reversal (780–781 ka). Two recently proposed revisions of the age of the 40Ar/39Ar Fish Canyon sanidine (FCs) standard to 28.201 ± 0.046 Ma and 28.305 ± 0.036 Ma would adjust 40Ar/39Ar ages applicable to the M-B boundary (and other reversals and excursions back to 1.2 Ma) to ages older than the new astrochronological ages by 8–24 kyr. The variables used to construct the ice volume models cannot account for the discrepancy. The FCs standard age that best fits the astrochronological ages is 27.93 Ma, which is within the uncertainty associated with the commonly used value of 28.02 (±0.16) Ma but younger than the recently proposed FCs ages. The EDC2 and EDC3 age models in the Dome C (Antarctic) ice core yield ages of 771.7 ka and 766.4 ka, respectively, for the 10Be flux peak that denotes the paleointensity minimum at the reversal boundary, implying that the EDC2 (rather than EDC3) age model is consistent with the observations from marine sediments, at least close to the M-B boundary.

Duration, magnitude, and frequency of subaerial volcano deformation events: New results from Latin America using InSAR and a global synthesis

Abstract: [1] We combine new observations of volcano deformation in Latin America with more than 100 previous deformation studies in other areas of the world to constrain the frequency, magnitude, and duration of subaerial volcano deformation events. We discuss implications for eruptive hazards from a given deformation event and the optimum repeat interval for proposed InSAR satellite missions. We use L band (23.6 cm wavelength) satellite-based interferometric synthetic aperture radar (InSAR) to make the first systematic search for deformation in all volcanic arcs of Latin America (including Mexico, Central America, the Caribbean, and the northern and southern Andes), spanning 2006–2008. We combine L and C band (5.6 cm wavelength) InSAR observations over the southern Andes volcanoes to extend the time series from 2002 to 2008 and assess the capabilities of the different radars: L band gives superior results in highly vegetated areas. Our observations reveal 11 areas of volcano deformation, some of them in areas that were thought to be dormant. There is a lack of observed deformation at several erupting volcanoes, probably due to temporal aliasing. The total number of deforming volcanoes in the central and southern Andes now totals 15 (from observations between 1992 and 2008), comparable to the Alaska/Aleutian arc. Globally, volcanoes deform across a variety of time scales (from seconds to centuries) often without eruption and with no apparent critical observation time scale, although observations made every minute are sometimes necessary to see precursors to eruption.

Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256

Abstract: [1] ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ∼100 m into plutonic rocks We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes The lavas are slightly altered at low temperatures ( 350°C up to ∼600°C) Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ∼800°C down to 450°C from hydrothermal fluids that were affected by supercritical phase separation Fluids had variable salinities and were enriched in 18O (+04‰ to +35‰) relative to seawater, similar to seafloor vent fluids Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system Low-temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture-controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface

Constraints on lithosphere net rotation and asthenospheric viscosity from global mantle flow models and seismic anisotropy

Abstract: Although an average westward rotation of the Earth's lithosphere is indicated by global analyses of surface features tied to the deep mantle (e.g., hot spot tracks), the rate of lithospheric drift is uncertain despite its importance to global geodynamics. We use a global viscous flow model to predict asthenospheric anisotropy computed from linear combinations of mantle flow fields driven by relative plate motions, mantle density heterogeneity, and westward lithosphere rotation. By comparing predictions of lattice preferred orientation to asthenospheric anisotropy in oceanic regions inferred from SKS splitting observations and surface wave tomography, we constrain absolute upper mantle viscosity (to 0.5–1.0 × 1021 Pa s, consistent with other constraints) simultaneously with net rotation rate and the decrease in the viscosity of the asthenosphere relative to that of the upper mantle. For an asthenosphere 10 times less viscous than the upper mantle, we find that global net rotation must be <0.26°/Myr (<60% of net rotation in the HS3 (Pacific hot spot) reference frame); larger viscosity drops amplify asthenospheric shear associated with net rotation and thus require slower net rotation to fit observed anisotropy. The magnitude of westward net rotation is consistent with lithospheric drift relative to Indo-Atlantic hot spots but is slower than drift in the Pacific hot spot frame (HS3 ≈ 0.44°/Myr). The latter may instead express net rotation relative to the deep mantle beneath the Pacific plate, which is moving rapidly eastward in our models.

Precise determination of the open ocean 234U/238U composition

Abstract: Uranium has a long residence time in the open oceans, and therefore, its salinity-normalized U concentration and 234U/238U activity ratio (expressed herein as δ234U, the ‰ deviation from secular equilibrium) are assumed to be uniform. The marine 234U/238U activity ratio is currently in radioactive disequilibrium and shows a ∼15% excess of 234U with respect to the secular equilibrium value due to continuous input from riverine sources. Knowledge of the marine δ234U, and how it has evolved through the Quaternary, is important for validating age accuracy in the U series dating of marine carbonates, which is increasingly relied upon for providing a chronological basis in paleoclimate research. However, accurate and precise measurements of δ234U are technically difficult. Thus, existing compilations of the open ocean δ234U value vary by up to ∼10‰, and the assumed uniformity in the oceanic δ234U remains to be confirmed. Using MC-ICPMS techniques and a suite of multiple Faraday cups instead of the typical configurations based on a combined Faraday cup–multiplier array, a long-term reproducibility of better than ±0.3‰ (2σ) is achieved for δ234U measurements. Applying these very high precision techniques to open ocean seawater samples, an average δ234U of 146.8 ± 0.1‰ (2σm, n = 19) is obtained. These high-precision seawater measurements yield an external reproducibility of better than ±0.4‰ (2σ) and show that the open oceans have a uniform δ234U on the sub-‰ level. These new data constrain the vertical mixing time of the open oceans to less than 1000 years.

Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes

Abstract: As part of the 2007 Marnaut cruise in the Sea of Marmara, an investigation of the pore fluid chemistry of sites along the Main Marmara Fault zone was conducted. The goal was to define the spatial relationship between active faults and fluid outlets and to determine the sources and evolution of the fluids. Sites included basin bounding transtensional faults and strike-slip faults cutting through the topographic highs. The basin pore fluids are dominated by simple mixing of bottom water with a brackish, low-density Pleistocene Lake Marmara end-member that is advecting buoyantly and/or diffusing from a relatively shallow depth. This mix is overprinted by shallow redox reactions and carbonate precipitation. The ridge sites are more complex with evidence for deep-sourced fluids including thermogenic gas and evidence for both silicate and carbonate diagenetic processes. One site on the Western High displayed two mound structures that appear to be chemoherms atop a deep-seated fluid conduit. The fluids being expelled are brines of up to twice seawater salinity with an exotic fluid chemistry extremely high in Li, Sr, and Ba. Oil globules were observed both at the surface and in cores, and type II gas hydrates of thermogenic origin were recovered. Hydrate formation near the seafloor contributes to increase brine concentration but cannot explain their chemical composition, which appears to be influenced by diagenetic reactions at temperatures of 75°C–150°C. Hence, a potential source for fluids at this site is the water associated with the reservoir from which the gas and oil is seeping, which has been shown to be related to the Thrace Basin hydrocarbon system. Our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes.

Organized melt, seismic anisotropy, and plate boundary lubrication

Abstract: Based on observations in both the laboratory and the Earth, we develop the hypothesis that plate boundaries are lubricated by networks of melt-rich shear zones. Such lubrication would serve to reduce effective strength and focus deformation at plates boundaries. This idea emerges from two sets of observations: (1) stress-driven melt segregation and organization in experimentally deformed mantle rocks and (2) seismic anisotropy patterns as observed at three divergent plate boundaries (the Ethiopian Rift, the Reykjanes Ridge, and the East Pacific Rise). In all three tectonic settings, the magnitude of anisotropy is greatest at the probable locations of the lithosphere-asthenosphere boundary within the plate boundary (“marginal LAB”). Seismic anisotropy in the upper mantle is controlled by the lattice preferred orientation (LPO) of predominant olivine and the alignment of melt structures. The observed patterns of anisotropy are controlled by the dip angle of the marginal LAB. When steeply dipping, shear wave splitting in vertically traveling waves (e.g., SKS phases) is most sensitive to the alignment of melt, and surface waves should reveal faster Rayleigh wave velocities than Love wave velocities (VSV > VSH). When shallowly dipping, shear wave splitting in vertically traveling body waves is controlled by olivine LPO, and surface waves show faster Love wave velocities than Rayleigh wave velocities (VSV < VSH). The formation of melt-rich networks by stress-driven segregation should be most effective where strain rates are highest. These melt-lubricated shear zones will reduce effective viscosity relative to the direct extrapolation of viscosity values derived from laboratory creep experiments on homogenous samples. A composite model of anisotropic seismic properties is developed to test the hypothesis that melt segregates along the LAB, incorporating olivine fabrics with oriented and segregated melt over a range of length scales. This model is applied to observations from the three example plate boundaries, leaving the reader to speculate on the implications for interpretation of anisotropy patterns at other geodynamic settings.

Late Cretaceous arc development on the SW margin of the Caribbean Plate: Insights from the Golfito, Costa Rica, and Azuero, Panama, complexes

Abstract: The processes of arc initiation at the margin of an oceanic plateau are remarkably well preserved along the southern coastline of eastern Costa Rica and western Panama. We present new results of a combined tectonostratigraphic and petrologic study with which protoarc initiation (75–73 Ma) at the margin of an oceanic plateau (89–85 Ma) is documented. Dykes of protoarc igneous rocks within the plateau and occurrences of protoarc igneous rocks are widely distributed. These types of field observations, geochemical data, and paleontologic ages for Late Cretaceous to Eocene fore-arc rocks of the Golfito Complex and Azuero Marginal Complex (southern Costa Rica and western Panama) provide the first direct evidence that a Coniacian–early Santonian oceanic plateau forms the arc basement. Stratigraphic and geochemical constraints from Golfito and Azuero indicate subduction initiation in south Central America, associated with geochemically distinctive suprasubduction igneous rocks, occurred in the late Campanian along the margin of the newly defined Azuero Plateau. Overall, the Golfito Complex and Azuero Marginal Complex provide a significant opportunity for exploration of petrologic mechanisms linking some oceanic plateaus to the growth of continents. The Azuero Plateau may extend further toward the Colombian Basin and forms thickened Caribbean crust. It served as a nucleus for accretion of additional oceanic plateaus, seamounts, and oceanic islands of Pacific origins.

Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab, revisited

Abstract: New Sr, Nd, Hf, and Pb isotope and trace element data are presented for basalts erupted in the Izu back arc. We propose that across-arc differences in the geochemistry of Izu-Bonin arc basalts are controlled by the addition of aqueous slab fluids to the volcanic front and hydrous partial melt of the slab to the back arc. The volcanic front has the lowest concentrations of incompatible elements, the strongest relative enrichments of fluid-mobile elements, and the most radiogenic Sr, Nd, Hf, and Pb, suggesting the volcanic front is the result of high degrees of partial melting of a previously depleted mantle source caused by an aqueous fluid flux from the slab. Relative to the volcanic front, the back arc has higher concentrations of incompatible elements and elevated La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less depleted or even enriched mantle source. Positive linear correlations between fluid-immobile element concentrations and the estimated degree of mantle melting suggest the slab contribution added to the mantle wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf isotopes and Th/La systematics of back-arc basalts are consistent with a slab melt composed of >90% altered oceanic crust and <10% sediment; that is, altered oceanic crust, not subducted sediment, dominates the slab contribution. High field strength element systematics require supercritical melts to be in equilibrium with residual rutile and zircon.

A multiscale approach to estimating topographically correlated propagation delays in radar interferograms

Abstract: When targeting small amplitude surface deformation, using repeat orbit Interferometric Synthetic Aperture Radar (InSAR) observations can be plagued by propagation delays, some of which correlate with topographic variations These topographically-correlated delays result from temporal variations in vertical stratification of the troposphere An approximate model assuming a linear relationship between topography and interferometric phase has been used to correct observations with success in a few studies Here, we present a robust approach to estimating the transfer function, K, between topography and phase that is relatively insensitive to confounding processes (earthquake deformation, phase ramps from orbital errors, tidal loading, etc) Our approach takes advantage of a multiscale perspective by using a band-pass decomposition of both topography and observed phase This decomposition into several spatial scales allows us to determine the bands wherein correlation between topography and phase is significant and stable When possible, our approach also takes advantage of any inherent redundancy provided by multiple interferograms constructed with common scenes We define a unique set of component time intervals for a given suite of interferometric pairs We estimate an internally consistent transfer function for each component time interval, which can then be recombined to correct any arbitrary interferometric pair We demonstrate our approach on a synthetic example and on data from two locations: Long Valley Caldera, California, which experienced prolonged periods of surface deformation from pressurization of a deep magma chamber, and one coseismic interferogram from the 2007 Mw 78 Tocapilla earthquake in northern Chile In both examples, the corrected interferograms show improvements in regions of high relief, independent of whether or not we pre-correct the data for a source model We believe that most of the remaining signals are predominately due to heterogeneous water vapor distribution that requires more sophisticated correction methods than those described here

Origin of cross‐chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes

Abstract: [1] We present major, trace element, and Pb-Sr-Nd-Hf isotope data for Quaternary basalt and basaltic andesite lavas from cross-chain volcanoes in the northern Izu (N-Izu) arc. Lavas from Izu-Oshima, Toshima, Udonejima, and Niijima islands show consistent chemical changes with depth to the Wadati-Benioff zone, from 120 km beneath Izu-Oshima to 180 km beneath Niijima. Lavas from Izu-Oshima at the volcanic front (VF) have elevated concentrations of large ion lithophile elements (LILEs), whereas rear-arc (RA) lavas are rich in light rare earth elements (LREEs) and high field strength elements (HFSEs). VF lavas also have more radiogenic Pb, Nd, Sr, and Hf isotopic compositions. We have used the Arc Basalt Simulator version 3 (ABS3) to examine the mass balance of slab dehydration and melting and slab fluid/melt-fluxed mantle melting and to quantitatively evaluate magma genesis beneath N-Izu. The results suggest that the slab-derived fluids/melts are derived from ∼20% sediment and ∼80% altered oceanic crust, the slab fluid is generated by slab dehydration for the VF magmas at 3.3–3.5 GPa/660°C–700°C, and slab melt for RA magmas is supplied at 3.4–4.4 GPa/830°C–890°C. The degree of fluxed melting of the mantle wedge varies between 17% and 28% (VF) and 6% and 22% (RA), with a slab flux fraction of 2%–4.5% (VF fluid) to 1%–1.5% (RA melt), and at melting depths 1–2.5 GPa (VF) and 2.4–2.8 GPa (RA). These conditions are consistent with a model whereby shallow, relatively low temperature slab fluids contribute to VF basalt genesis, whereas deeper and hotter slab melts control formation of RA basalts. The low-temperature slab dehydration is the cause of elevated Ba/Th in VF basalt due mainly to breakdown of lawsonite, whereas deeper breakdown of phengite by slab melting is the cause of elevated K and Rb in RA basalts. Melting in the garnet stability field, and at lower degrees of partial melting, is required for the elevated LILEs, LREEs, and HFSEs observed in the RA basalts. Less radiogenic Sr, Nd, Hf, and Pb in RA basalts are all attributable to lesser slab flux additions. The low H2O predicted for RA basalt magmas (<1.5 wt %) relative to that in VF basalt magmas (5–8 wt %) is also due to melt addition rather than fluid. All these conclusions are broadly consistent with existing models; however, in this study they are quantitatively confirmed by the geochemical mass balance deduced from petrological ABS3 model. Overall, the P-T-X(H2O) structure of the slab and the mantle wedge exert the primary controls on arc basalt genesis.

Three‐dimensionality of slab detachment due to ridge‐trench collision: Laterally simultaneous boudinage versus tear propagation

Abstract: The detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide. In particular, the propagation of slab detachment (“tearing”) of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift. However, there is little understanding of the causes and dynamics associated with three-dimensional (3-D) slab tearing, especially in the case of ridge-trench collision. Here we show using fully dynamic 3-D numerical models that the process of detachment due to ridge-trench collision depends on the geometry of the ridge segments approaching the trench. For a finite laterally symmetric slab, the 3-D detachment process occurs nearly simultaneously along strike by way of boudinage-type necking and opening of holes central to the slab. For a case involving the approach of two offset ridge segments to the trench, slab tearing occurs in the form of (1) a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and (2) horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments. However, lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone. Whether detachment occurs simultaneously along strike or propagates laterally, the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull.

An ellipsoidal harmonic representation of Earth's lithospheric magnetic field to degree and order 720

Abstract: [1] While high-degree models of the Earth's gravity potential have been inferred from measurements for more than a decade, corresponding geomagnetic models are difficult to produce. The primary challenge lies in the estimation of the magnetic potential, which is not completely determined by available field intensity measurements and cannot be computed by direct integration. Described here is the methodology behind the third generation of the National Geophysical Data Center's degree 720 magnetic model. Key issues are (1) the ellipsoidal harmonic representation of the magnetic potential, (2) the reduction of ambiguities by a suitable penalty function, and (3) the use of an iterative method to estimate the model coefficients. The NGDC-720 model provides the lithospheric magnetic field vector at any desired location and altitude close to and above the Earth's surface. Anticipated uses are in geological and tectonic studies of the lithosphere, as a local correction for magnetic navigation and heading systems, and the calibration of ground, marine, airborne, and spaceborne magnetometers. The NGDC-720 model is available at http://geomag.org/models/ngdc720.html and for long-term archive at http://earthref.org/cgi-bin/er.cgi?s=erda.cgi?n=989.

Fossil clams from a serpentinite‐hosted sedimented vent field near the active smoker complex Rainbow, MAR, 36°13′N: Insight into the biogeography of vent fauna

Abstract: Hydrothermal circulation at ultramafic-hosted sites supports a large variety of high-and low-temperature hydrothermal vents and associated ecosystems. The discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest, approximately 2.5 km east of the active Rainbow vent field on the Mid-Atlantic Ridge (MAR, 36 degrees 13'N), increased our knowledge regarding the diversity of vent communities at slow spreading ridges. Bivalve molluscs of the family Vesicomyidae were represented by the genus Phreagena. Here we present the first record of this genus in the Atlantic Ocean. This second vesicomyid species known from the MAR, Phreagena sp., was found to be associated with a Thyasira species that is affiliated with T. southwardae (at the Logatchev vent field on the MAR) and with T. vulcolutre (in the Gulf of Cadiz). These two clams have close relationships with seep taxa along the continental margin, and were likely associated with sedimented vent fields. delta O-18 and delta C-13 analyses of the shells suggested that the burrowing bivalve Thyasira could incorporate isotopically light carbon, derived from the oxidation of methane in the sediment, while the signature of Phreagena sp. shells denoted a different carbonate source. C-14 dating of the shells denoted that the hydrothermal activity in the Rainbow area began at least similar to 25.5 kyr BP, which is similar to the model of the hydrothermal vent field distribution that was proposed for the Logatchev hydrothermal site. The results provide new insight regarding the diversity of chemosynthetic fauna on the MAR over geologic time. Ultramafic-hosted, on-axis sedimented vent fields extend the range of habitats for chemosynthetic communities, underlying the need to further explore the geology of these types of environments on slow-spreading ridges and to determine their role in the ecology of deep-sea vent communities.

In situ stress state in the Nankai accretionary wedge estimated from borehole wall failures

Abstract: We constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures (borehole breakouts and drilling-induced tensile fractures) detected in four vertical boreholes (C0002, C0001, C0004, and C0006 from NW to SE) drilled in the Nankai accretionary wedge. The directions of the maximum horizontal principal stress (SHmax), indicated by the azimuths of borehole wall failures, are consistent in individual holes, but those in C0002 (margin-parallel SHmax) are nearly perpendicular to those in all other holes (margin-normal SHmax). Constrained stress magnitudes in C0001 and C0002, using logged breakout widths combined with empirical rock strength derived from sonic velocity, as well as the presence of the drilling-induced tensile fractures, suggest that the stress state in the shallow portion of the wedge (fore-arc basin and slope sediment formations) is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike-slip faulting or possible reverse faulting. Thus, the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults. The margin-perpendicular tectonic stress components in the two adjacent sites, C0001 and C0002, are different, suggesting that tectonic force driven by the plate pushing of the Philippine Sea plate does not uniformly propagate. Rather, the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation-driven extension in the fore arc and thrusting and bending within individual geologic domains.

Global distribution and long‐term fate of anthropogenic 129I in marine and surface water reservoirs

Abstract: [1] Since the advent of the nuclear age in the mid-1940s, the mass of radioactive 129I (t1/2 = 15.7 Myr) circulating in the Earth's hydrosphere has increased nearly fortyfold from its natural background level of 140 kg. Nuclear fuel reprocessing has been by far the major contributor, responsible for releasing 5400 kg of 129I, primarily into the North Atlantic Ocean. Regional and global trends in the distribution of the 129I inventory are elucidated from an examination of more than 600 determinations of 129I in environmental samples from around the world. Because the major point sources are located in Europe and the United States, more than 99% of the present 129I reservoir is distributed in the Northern Hemisphere, where both 129I concentrations and 129I/I ratios in rivers, lakes, and shallow seawater are several orders of magnitude above the preanthropogenic background. Downwelling in the North Atlantic presently provides a major sink for marine 129I; however, marine upwelling along the margins of the Pacific will eventually return part of this anthropogenic input to the ocean surface, where it will find its way back into surface waters and the atmosphere. Iodine-129 has a long half-life (15.7 Myr), and consequently, there is also the possibility that climate change will influence the dynamics of iodine transfer in surface reservoirs. We model the effect of a collapse in thermohaline circulation and project a concentration increase of more than 3 orders of magnitude in shallow oceans over the 10,000 years that follow if nuclear reprocessing is to continue at the present rate.

Characterizing the 410 km discontinuity low-velocity layer beneath the LA RISTRA array in the North American southwest

Abstract: [1] Receiver functions recorded by the 54-station 920 km long Program for Array Seismic Studies of the Continental Lithosphere–Incorporated Research Institutions for Seismology Colorado Plateau/Rio Grande Rift Seismic Transect Experiment (LA RISTRA) line array display a pervasive negative polarity P to S conversion (Pds) arrival preceding the positive polarity 410 km discontinuity arrival These arrivals are modeled as a low-velocity layer atop the 410 km discontinuity (410-LVL) and are inverted for a velocity profile via a grid search using a five-parameter linear gradient velocity model Model parameter likelihood and correlations are assessed via calculation of one- and two-dimensional marginal posterior probability distributions The maximum likelihood model parameter values found are top velocity gradient thickness of 00 km with a 46% (−022 km/s) shear velocity reduction, a 198 km constant velocity layer, and bottom gradient thickness of 250 km with a 35% (+017 km/s) shear velocity increase The estimated mean thickness of the 410-LVL is 323 km The top gradient of the 410-LVL is sharp within vertical resolution limits of P to S conversion (<10 km), and the diffuse 410 km velocity gradient is consistent with hydration of the olivine-wadsleyite phase transformation The 410-LVL is interpreted as a melt layer created by the Transition Zone Water Filter model Two secondary observations are found: (1) the 410-LVL is absent from the SE end of the array and (2) an intermittent negative polarity P525s arrival is observed We speculate that upper mantle shear velocity anomalies above the 410 km discontinuity may manifest Rayleigh-Taylor instabilities nucleated from the 410-LVL melt layer that are being shed upward on time scales of tens of millions of years

Rapid, continuous streaking of tremor in Cascadia

Abstract: Nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move. Its spatiotemporal distribution, however, is complex and varies over different time scales, and the causal physical mechanisms remain unclear. Here we use a beam backprojection method to show rapid, continuous, slip-parallel streaking of tremor over time scales of several minutes to an hour during the May 2008 episodic tremor and slip event in the Cascadia subduction zone. The streaks propagate across distances up to 65 km, primarily parallel to the slip direction of the subduction zone, both updip and downdip at velocities ranging from 30 to 200 km/h. We explore mainly two models that may explain such continuous tremor streaking. The first involves interaction of slowly migrating creep front with slip-parallel linear structures on the fault. The second is pressure-driven fluid flow through structurally controlled conduits on the fault. Both can be consistent with the observed propagation velocities and geometries, although the second one requires unlikely condition. In addition, we put this new observation in the context of the overall variability of tremor behavior observed over different time scales.

Movement of Amazon surface water from time‐variable satellite gravity measurements and implications for water cycle parameters in land surface models

Abstract: [1] The large-scale observations of terrestrial water storage from GRACE satellites over the Amazon are analyzed with land surface model (LSM) outputs of runoff and soil moisture. A simple yet effective runoff routing method based on a continuity equation is implemented to model horizontal transport of surface water within the Amazon basin. The GRACE observations are analyzed separately for soil moisture and surface water storages (generated from runoff), relying on their distinct spatial patterns, being disperse for soil moisture and localized for surface water. Various effective velocities for storage transport are tested against the GRACE observations. When the model runoff is routed with an uniform velocity of 30 cm/s, the annual variation of the resulting surface water storage is generally found to be larger than the satellite measurements and ground gauge data by a factor of 1.5 or higher. The peak annual anomaly of surface water storage is observed around the midstream of the Amazon main stem. However, the runoff routing simulations present the peak amplitude consistently around the delta (downstream), unless the increasing velocity in a downstream region is used. As complements to the ground gauge data, the satellite observations provide unique ‘spatial’ information of water cycle parameters. Our analysis indicates possible shortcomings in the certain LSM mass transport scheme between atmosphere and land surface, particularly the production of too large seasonal variations in runoff (and maybe too little variations in evapotranspiration), and the dynamic characteristics of surface water transport within the Amazon basins.

Seismic heterogeneity and small‐scale convection in the southern California upper mantle

Abstract: We invert traveltime residuals of teleseismic P and S phases for 3-D perturbations in VP, VS, and VP/VS structures in the southern California upper mantle. The tomographic inversion uses frequency-dependent 3-D sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals. In addition to separate VP and VS tomography, we jointly invert the P and S data sets for VP/VS perturbations by imposing a smoothness constraint on the δlnVS/δlnVP field. The regional upper mantle is very heterogeneous with the greatest amplitude of velocity and VP/VS perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern California. Our imaging improves the resolution of the geometry and amplitude of these features. In addition to the major structures imaged and discussed in previous papers, we find a large high-velocity anomaly at depths between ∼340 and 500 km beneath the Borderland, Transverse Ranges, and northern Peninsular Ranges. This anomaly is separate from the well-known uppermost mantle Transverse Ranges anomaly, which extends no deeper than ∼175–200 km. The strongest low-velocity, high VP/VS anomaly is found beneath and west of the Salton Trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning. The magnitudes of VP, VS, and VP/VS perturbations and knowledge of regional 1-D velocity and attenuation lead us to conclude that the asthenosphere contains up to ∼1% partial melt extending to depths as great as 150–200 km.

Trends in the solubility of iron in dust-dominated aerosols in the equatorial Atlantic trade winds: importance of iron speciation and sources.

Abstract: [1] We present measurements of the solubility of Fe(II) and Fe(III) extracted from bulk and size-fractionated African dust collected in summer trade winds at Barbados, West Indies Iron solubilities typically ranged from 1% to 3% Fe(III) dominates the iron solubility over the entire range of particle sizes At mineral dust concentrations below about 5 ug/m3 Fe(II), believed to be largely derived from anthropogenic and biomass burning sources, becomes increasingly important Samples containing large soluble iron fractions and high Fe(II)/Fe(III) ratios are associated with South Atlantic back trajectories; the gray coloration of the filters suggests that the source may be biomass burning in southern Africa In general, much of the variability in Fe solubility is linked to Fe(II) concentration changes Vanadium is often used as a tracer of anthropogenic impacts Although many of our samples yielded V/Ti ratios much greater than average crustal abundances, we could find no relationship between the enrichment of V and Fe solubility Our Fe solubility results are quite similar to those obtained by others despite the fact that the measurements were made in diverse ocean regions and the protocols used were quite different in all cases This uniformity implies that the factors controlling aerosol iron solubility are largely inherent in the properties of the aerosols themselves and not the procedures used to extract the iron Our results suggest that dust transport models that focus on the role of iron in ocean biogeochemistry must take into account aerosol origin in order to better model the solubility of iron