Earth and Planetary Science Letters (v.259, #1-2)
Editorial Board (iii).
Quantitative study of the deformation at Southern Explorer Ridge using high-resolution bathymetric data by Anne Deschamps; Maurice Tivey; Robert W. Embley; William W. Chadwick (1-17).
We present preliminary results of a morphological study of the summit of the Southern Explorer Ridge (SER). The SER is an inflated intermediate-rate spreading center located in the northeast Pacific off the West coast of Canada, that hosts a large hydrothermal vent complex known as “Magic Mountain”. A quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high-resolution, near-bottom, bathymetric data. These data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor. Structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures. The ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults, highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer. Populations of small faults exhibit an exponential size-frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain (3.7 to 18.4%) accommodated by the normal faults. We propose that the 500 to 600-m wide and ∼ 60-m deep asymmetric axial summit graben of the SER formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben. We find a well-defined geographic distribution in the types of lava flows, which indicates a general decrease of the eruption rate through time. We also find that the “Magic Mountain” hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile-transition depth. This fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature.
Keywords: midocean ridge; faulting; Explorer; seafloor spreading; high-resolution bathymetry; autonomous underwater vehicle;
Sound velocity of MgSiO3 post-perovskite phase: A constraint on the D″ discontinuity by Motohiko Murakami; Stanislav V. Sinogeikin; Jay D. Bass; Nagayoshi Sata; Yasuo Ohishi; Kei Hirose (18-23).
The discovery of a post-perovskite phase transition in MgSiO3 has significant implications for seismological observations in the D″ region at the bottom of Earth's mantle. The D″ discontinuity, which is manifested as a sharp positive seismic-wave velocity jump 200–300 km above the core–mantle boundary (at pressure of 119∼ 125 GPa), is one of the most enigmatic seismic features in this region. Whether this velocity increase may be due to the formation of a post-perovskite phase at the D″ discontinuity has not, however, been directly addressed by experiments. Here we present the results of aggregate sound velocity measurements of the MgSiO3 post-perovskite phase by Brillouin spectroscopy in the diamond anvil cell (DAC) up to a pressure of 172 GPa, in combination with infrared laser annealing of the sample. Based on these results and our recent high-pressure velocity measurements on perovskite, the aggregate shear wave velocity contrast across the perovskite to post-perovskite phase transition is at most 0.5%. This contrast is much smaller than typically observed across the D″ discontinuity, indicating that the formation of an isotropic aggregate of the post-perovskite phase provides an insufficient velocity increase to explain the D″ discontinuity. Lattice preferred orientation (LPO) of post-perovskite is likely to be crucial for explaining the D″ discontinuity.
Keywords: MgSiO3 post-perovskite; D″ discontinuity; sound velocity; Brillouin scattering; elastic properties; high pressure;
Crustal deformation associated with the 1996 Gjálp subglacial eruption, Iceland: InSAR studies in affected areas adjacent to the Vatnajökull ice cap by Carolina Pagli; Freysteinn Sigmundsson; Rikke Pedersen; Páll Einarsson; Thóra Árnadóttir; Kurt L. Feigl (24-33).
Crustal deformation signals associated with the September 30–October 13, 1996 Gjálp subglacial eruption, Vatnajökull ice cap, Iceland, have been identified using interferometric analysis of Synthetic Aperture Radar images (InSAR) in areas outside the ice cap. On September 29, 1996 an M w 5.6 earthquake occurred at the nearby Bárdarbunga volcano and on September 30 seismicity propagated 20 km southwards where the Gjálp eruption occurred. Analysis of interferograms spanning different times from 1992 to 2000 allows us to separate two distinct co-eruptive deformation periods in areas outside the ice cap. Diking at the Bárdarbunga caldera rim appears to be responsible for deformation during the first week of eruption while significant deflation occurred at Bárdarbunga only after October 6 when most of the magma had already been erupted at Gjálp. A pressure connection between the Bárdarbunga volcano and the Gjálp eruptive fissure is inferred. Fault slip in three areas up to 30 km from the center of the Bárdarbunga volcano was triggered by the deflation. Local deformation signals there are consistent with small fault movements.
Keywords: magma intrusion; Gjálp eruption; multiple magma chambers; fault slip triggering;
Rapid passage of a small-scale mantle heterogeneity through the melting regions of Kilauea and Mauna Loa Volcanoes by Jared P. Marske; Aaron J. Pietruszka; Dominique Weis; Michael O. Garcia; J. Michael Rhodes (34-50).
Recent Kilauea and Mauna Loa lavas provide a snapshot of the size, shape, and distribution of compositional heterogeneities within the Hawaiian mantle plume. Here we present a study of the Pb, Sr, and Nd isotope ratios of two suites of young prehistoric lavas from these volcanoes: (1) Kilauea summit lavas erupted from AD 900 to 1400, and (2) 14C-dated Mauna Loa flows erupted from ∼ 2580–140 yr before present (relative to AD 1950). These lavas display systematic isotopic fluctuations, and the Kilauea lavas span the Pb isotopic divide that was previously thought to exist between these two volcanoes. For a brief period from AD 250 to 1400, the 206Pb/204Pb and 87Sr/86Sr isotope ratios and ε Nd values of Kilauea and Mauna Loa lavas departed from values typical for each volcano (based on historical and other young prehistoric lavas), moved towards an intermediate composition, and subsequently returned to typical values. This is the only known period in the eruptive history of these volcanoes when such a simultaneous convergence of Pb, Sr, and Nd isotope ratios has occurred. The common isotopic composition of lavas erupted from both Kilauea and Mauna Loa during this transient magmatic event was probably caused by the rapid passage of a small-scale compositional heterogeneity through the melting regions of both volcanoes. This heterogeneity is thought to have been either a single body (∼ 35 km long based on the distance between the summits of these volcanoes) or the plume matrix itself (which would be expected to be present beneath both volcanoes). The time scale of this event (centuries) is much shorter than previously noted for variations in the isotopic composition of Hawaiian lavas due to the upwelling of heterogeneities within the plume (thousands to tens of thousands of years). Calculations based on the timing of the isotopic convergence suggest a maximum thickness for the melting region (and thus, the heterogeneity) of ∼ 5–10 km. The small size of the heterogeneity indicates that melt can be extracted from small regions within the Hawaiian plume with minimal subsequent chemical modification (beyond the effects of crystal fractionation). This would be most effective if melt transport in the mantle beneath Hawaiian shield volcanoes occurs mostly in chemically isolated channels.
Keywords: Hawaii; volcanoes; Kilauea; Mauna Loa; mantle plumes; isotopes; geochemistry;
On the anelastic contribution to the temperature dependence of lower mantle seismic velocities by Jan Matas; Mark S.T. Bukowinski (51-65).
There is little agreement regarding the importance of anelastic contributions to the temperature dependence of seismic wave velocities. Some studies consider the anelastic contribution to be negligible, whereas others argue that it may be as important as the purely elastic contribution. In order to evaluate these claims, we review physically plausible models for mantle relaxation processes and construct a model of activation enthalpy based on solid state physics as well as on recent experimental results. We then deduce self-consistent 1-D Q profiles and evaluate the anelastic contribution to the temperature dependence of V s and V p . Physically plausible values of activation enthalpies yield anelastic contributions that increase with depth and account for at least 20% of the velocity temperature derivatives at 2600 km. We show that anelasticity may significantly moderate temperature variations in hot (slow velocity) zones. This suggests that inferred lateral changes in temperature, as well as arguments regarding the presence of chemical heterogeneity, and/or temperature induced phase transformations, are likely to be significantly affected by anelastic effects in the mantle. We also compute values of the dimensionless parameters R s/p , R ϕ/s , and R ρ/s . We find that anelasticity can either increase or decrease R s/p , but the magnitude of the correction, of the order of 15%, is smaller than the uncertainty in the elastic value. Anelasticity cannot change the sign of R ϕ/s and R ρ/s , but it can decrease their values by as much as 50%. However, given current uncertainties in seismic and mineral data, the effects of anelasticity on the dimensionless parameters do not bring any quantitative constraints on lateral variations of temperature, chemical composition and/or mineral phase.
Keywords: seismic attenuation; mantle relaxation processes; thermodynamics;
Deep upper-mantle melting beneath the Tasman and Coral Seas detected with multiple ScS reverberations by Anna M. Courtier; Justin Revenaugh (66-76).
Multiple ScS reverberations are used to search for mantle reflectors beneath the Tasman and Coral Seas with a hierarchical waveform-inversion/migration method. In addition to the major transition zone discontinuities, a low-velocity layer above the 410-km discontinuity is detected. The top of the low-velocity layer lies at an average depth of 352 km, indicating that the layer could be more than 70-km thick if it persists to the 410-km discontinuity, which occurs at an average depth of 420 km along paths containing the low-velocity layer. We attribute the low velocities to partial melt resulting from volatile-induced melting. The considerable thickness of the partial melt layer may require thin films of a hydrous melt with a zero-degree dihedral angle surrounding grains or the combined effect on melting of the addition of both water and carbon to the deep upper mantle via subduction. Although the depths of the transition zone discontinuities do not indicate that the transition zone itself is rich in water, the impedance contrasts do contain a subtle signature that could be related to transition zone water, namely a decrease in the impedance contrast across the 410-km discontinuity and a relatively strong 520-km discontinuity.
Keywords: mantle; transition zone; discontinuities; partial melting; water; low-velocity zone;
Uranium–lead systematics of phosphates in lunar basaltic regolith breccia, Meteorite Hills 01210 by Kentaro Terada; Yu Sasaki; Mahesh Anand; Katherine H. Joy; Yuji Sano (77-84).
Chronological studies of brecciated lunar meteorites have proved difficult, because they are mixtures of materials from various sources and the radiometric “clocks” are sometimes affected by the subsequent impact events on the Moon. Here, we report the in-situ U–Pb dating of phosphates in lunar meteorite, Meteorite Hills (MET) 01210, which is a regolith breccia consisting of low-Ti mare basalt clasts and mineral fragments with a minor anorthositic component. In-situ analyses of four merrillite and four apatite grains in MET 01210, which are resistant to secondary events, resulted in a 207Pb/206Pb–204Pb/206Pb isochron age of 3904 ± 85 Ma (95% confidence limit). This phosphate formation age, when considered as the crystallisation age of this low-Ti basalt, is similar to crystallization ages of 3.8–3.9 Ga for unbrecciated low-Ti basalt meteorites, Asuka 881757 and Yamato 793169. This result reinforces the hypothesis that all these three meteorites originated from the same area on the Moon and were launched by a single impact event, consistent with the similarity of launch ages, mineralogical and geochemical signatures.
Keywords: U–Pb dating; lunar meteorite; mare basalt; SHRIMP;
Numerical modeling of the Cenozoic geomorphic evolution of the southern Sierra Nevada, California by Jon D. Pelletier (85-96).
Recent geomorphic studies suggest that significant (∼ 1.5 km) late Cenozoic surface uplift occurred in the southern Sierra Nevada, a conclusion that is difficult to reconcile with recent stable-isotopic paleoaltimetry studies. Numerical modeling can play an important role in resolving this dispute. In this paper I use two models of bedrock channel erosion, the stream-power model and a sediment-flux-driven model, to test hypotheses for the fluvial Cenozoic geomorphic evolution and surface uplift history of the southern Sierra Nevada. Cosmogenic data for upland erosion and river incision rates allow each model parameter to be uniquely constrained. Numerical experiments using the sediment-flux-driven model suggest that the modern southern Sierra Nevada was constructed from a 1.0-km pulse of range-wide surface uplift in the latest Cretaceous (∼ 60 Ma) and a 0.5-km pulse in the late Miocene (∼ 10 Ma). The persistent geomorphic response to latest Cretaceous uplift in this model is the result of limited “cutting tools” supplied from the upland low-relief Boreal Plateau. This uplift history correctly predicts the modern topography of the range, including the approximate elevations and extents of the Chagoopa and Boreal Plateaux and their associated river knickpoints. Numerical experiments using the stream-power model are most consistent with a 1-km pulse of uplift in the late Eocene (∼ 30 Ma) and a 0.5-km pulse in the late Miocene (∼ 7 Ma). Both models suggest that the remaining rock uplift required to produce the 4-km peak elevations of the modern southern Sierra Nevada was produced by flexural-isostatic uplift in response to river incision. The balance of evidence, including the dominance of sediment-flux-driven erosion in granitic rocks, previous paleoaltimetry studies, and the timing of sediment accumulation in the Great Valley, support the conclusions of the sediment-flux-driven model, i.e. that the Sierra Nevada experienced range-wide surface uplift events in the latest Cretaceous and late Miocene. More broadly, these results indicate that nonequilibrium landscapes can persist for long periods of geologic time, and hence low-relief upland landscapes do not necessarily indicate late Cenozoic surface uplift.
Keywords: Sierra Nevada; paleoaltimetry; bedrock channel erosion; numerical modeling;
The Central Metasedimentary Belt (Grenville Province) as a failed back-arc rift zone: Nd isotope evidence by A.P. Dickin; R.H. McNutt (97-106).
Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a 150 km-wide belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust (containing the Elzevir pluton) which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block within the CMB has profound implications for its structure and tectonic evolution, because it implies that juvenile Grenvillian crust, apparently forming a wide NE–SW belt, is in fact distributed in two narrower segments with approximately N–S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments, created by back-arc spreading behind a continental margin arc. These rift segments extend southwards (in the subsurface) into the northeastern Unites States. The rift segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a ‘Marble domain’ of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB in Ontario and Quebec as the northern termination of a failed back-arc rift zone.
Keywords: Grenville; rift; Nd model age;
Magnetic proxy for the deep (Pacific) western boundary current variability across the mid-Pleistocene climate transition by Alessandra Venuti; Fabio Florindo; Elisabeth Michel; Ian R. Hall (107-118).
The Deep Western Boundary Current (DWBC) inflow to the SW Pacific is one of the largest, transporting ∼ 40% of the total input of deep water to the world's oceans. Here we use a sedimentary record from the giant piston core MD97-2114 collected on the northern flank of the Chatham Rise located at 1935 m water depth, east of New Zealand, to investigate DWBC variability during the Pleistocene epoch when the period of glacial cycles changed progressively from a 41 kyr to 100 kyr rhythm. Magnetic grain-size may be directly related to orbitally forced fluctuations in the strength of the upper circumpolar deep water (UCDW) through its interaction with terrigenous sediments supplied from the south and west. The long-term trends in magnetic properties are characterized by two main perturbations centered at 870 ka (Marine Isotope Stage, MIS 22) 450 ka (MIS 12), which is broadly consistent with the inferred perturbation during the mid-Pleistocene climate transition based on sedimentological paleocurrent reconstruction from Ocean Drilling Program Site 1123 located at 3290 m water depth in the main core of the DWBC flow on the North Chatham Drift. This similarity suggests that both the upper and middle CDW are modulated by similar processes and fluctuations of Antarctic Bottom Water production could be directly responsible for this deep Pacific Ocean inflow variability over the past 1.2 Ma.
Keywords: DWBC; Chatham Rise; New Zealand; Pleistocene; magnetostratigraphy; environmental magnetism;
Partial melting and melt percolation in the mantle: The message from Fe isotopes by Stefan Weyer; Dmitri A. Ionov (119-133).
High precision Fe isotope measurements have been performed on various mantle peridotites (fertile lherzolites, harzburgites, metasomatised Fe-enriched peridotites) and volcanic rocks (mainly oceanic basalts) from different localities and tectonic settings. The peridotites yield an average δ56Fe = 0.01‰ and are significantly lighter than the basalts (average δ56Fe = 0.11‰). Furthermore, the peridotites display a negative correlation of δ56Fe with Mg# indicating a link between δ56Fe and degrees of melt extraction. Taken together, these findings imply that Fe isotopes fractionate during partial melting, with heavy isotopes preferentially entering the melt.The slope of depletion trends (δ56Fe versus Mg#) of the peridotites was used to model Fe isotope fractionation during partial melting, resulting in α mantle-melt ≈ 1.0001–1.0003 or lnα mantle-melt ≈ 0.1–0.3‰. In contrast to most other peridotites investigated in this study, spinel lherzolites and harzburgites from three localities (Horoman, Kamchatka and Lherz) are virtually unaffected by metasomatism. These three sites display a particularly good correlation and define an isotope fractionation factor of lnα mantle-melt ≈ 0.3‰. This modelled value implies Fe isotope fractionation between residual mantle and mantle-derived melts corresponding to Δ 56Femantle–basalt ≈ 0.2–0.3‰, i.e. significantly higher than the observed difference between averages for all the peridotites and the basalts in this study (corresponding to Δ 56Femantle–basalt ≈ 0.1‰). Either disequilibrium melting increased the modelled α mantle-melt for these particular sites or the difference between average peridotite and basalt may be reduced by partial re-equilibration between the isotopically heavy basalts and the isotopically light depleted lithospheric mantle during melt ascent. The slope of the weaker δ56Fe–Mg# trend defined by the combined set of all mantle peridotites from this study is more consistent with the generally observed difference between peridotites and basalts; this slope was used here to estimate the Fe isotope composition of the fertile upper mantle (at Mg# = 0.894, δ56Fe ≈ 0.02 ± 0.03‰).Besides partial melting, the Fe isotope composition of mantle peridotites can also be significantly modified by metasomatic events, e.g. melt percolation. At two localities (Tok, Siberia and Tariat, Mongolia) δ56Fe correlates with iron contents of the peridotites, which was increased from about 8% to up to 14.5% FeO by post-melting melt percolation. This process produced a range of Fe isotope compositions in the percolation columns, from extremely light (δ56Fe = − 0.42‰) to heavy (δ56Fe = + 0.17‰). We propose reaction with isotopically heavy melts and diffusion (enrichment of light Fe isotopes) as the most likely processes that produced the large isotope variations at these sites. Thus, Fe isotopes might be used as a sensitive tracer to identify such metasomatic processes in the mantle.
Keywords: Fe isotopes; mantle; partial melting; diffusion; melt percolation; heavy stable isotopes; high temperature isotope fractionation;
Ancient recycled crust beneath the Ontong Java Plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands by Akira Ishikawa; Takeshi Kuritani; Akio Makishima; Eizo Nakamura (134-148).
We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic garnet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-garnet clinopyroxenite, whose major element compositions remain intact, has lower 206Pb/204Pb–143Nd/144Nd and higher 87Sr/86Sr–207Pb/204Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf–Nd isotopic decoupling and the good coincidence of Pb isotopes to the Stacey–Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5–1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau.
Keywords: xenoliths; pyroxenite; recycled crust; Ontong Java Plateau; mantle plumes;
Reflection of seismic surface waves at the northern Apennines by Daniel Stich; Andrea Morelli (149-158).
Seismic surface waves with periods of 15 to 20 s are reflected laterally at the northern Apennines. For earthquakes originated and recorded in the wider Alpine area, a few hundred kilometres north of the Apennines, we can directly observe late arrivals of reflected regional surface waves. These have a characteristic polarization of particle motion and often dominate the intermediate-period surface wave coda. Love waves are the most prominent coda arrivals, while reflected Rayleigh waves show smaller amplitudes, and become clear only after rotation of the recordings to the reflection incidence direction. We can track the development of the Love wave reflection along a temporary broad-band transect across the northern Apennines, indicating the location of the reflector near to the highest topography. From spectral element simulations of three-dimensional wave propagation, we attribute the reflection to a continuous, ∼ 270 km long offset in the crust-mantle boundary under the mountain belt with vertical throw of ∼ 20 km, thus supporting a deep crustal root under the outer side of the Apennines fold and thrust belt, and significant crustal thinning on the inner side. Amplitude reflection coefficients for near-normal incidence can be estimated as ∼ 7% for both Love and Rayleigh waves.
Keywords: seismogram coda; surface waves; multipathing; 3D waveform modelling; deep crustal structure;
Gas/aerosol–ash interaction in volcanic plumes: New insights from surface analyses of fine ash particles by Pierre Delmelle; Mathieu Lambert; Yves Dufrêne; Patrick Gerin; Niels Óskarsson (159-170).
The reactions occurring between gases/aerosols and silicate ash particles in volcanic eruption plumes remain poorly understood, despite the fact that they are at the origin of a range of volcanic, environmental, atmospheric and health effects. In this study, we apply X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique, to determine the chemical composition of the near-surface region (2–10 nm) of nine ash samples collected from eight volcanoes. In addition, atomic force microscopy (AFM) is used to image the nanometer-scale surface structure of individual ash particles isolated from three samples. We demonstrate that rapid acid dissolution of ash occurs within eruption plumes. This process is favoured by the presence of fluoride and is believed to supply the cations involved in the deposition of sulphate and halide salts onto ash. AFM imaging also has permitted the detection of extremely thin (< 10 nm) coatings on the surface of ash. This material is probably composed of soluble sulphate and halide salts mixed with sparingly soluble fluoride compounds. The surface approach developed here offers promising aspects for better appraising the role of gas/aerosol–ash interaction in dictating the ability of ash to act as sinks for various volcanic and atmospheric chemical species as well as sources for others.
Keywords: volcanic ash; volcanic plume; x-ray photoelectron spectroscopy; atomic force microscopy;
The tracing of riverine U in Arctic seawater with very precise 234U/238U measurements by M.B. Andersen; C.H. Stirling; D. Porcelli; A.N. Halliday; P.S. Andersson; M. Baskaran (171-185).
The riverine flux of U that enters the deep oceans is not well constrained since the net losses during estuarine mixing are difficult to quantify. Riverine-dissolved U normally has a higher 234U/238U activity ratio (234U/238Uar) than the uniform value that characterizes open ocean seawater and could be used as a tracer of riverine inputs if one could resolve subtle variations in seawater composition. Using new mass spectrometry techniques we achieve a long-term reproducibility ± 0.3‰ on 234U/238Uar which permits the tracing of riverine U in seawater samples from the Arctic – a partially restricted basin that is ideal for such a study. We find that surface waters from the Arctic basins carry elevated 234U/238Uar when compared with deep ocean seawater. Samples from the Canada Basin have a significant freshwater component and provide evidence that the Mackenzie River loses ∼ 65% of its U in the Mackenzie shelf/estuary zone before entering the deeper basin. This is in contrast to samples from the Makarov Basin, which provide evidence that all of the freshwater input is derived from the major Yenisey River alone, despite the proximity of the Lena and Ob Rivers. The differing behaviour of U between the Mackenzie and Yenisey Rivers is most likely a consequence of the strong binding of U to dissolved organic matter (DOC) or secondary phases in these rivers. The Yenisey River appears to transport the majority of the DOC through the shelf and into the Makarov Basin. In contrast, the Mackenzie River appears to lose a significant amount of DOC (> 50%) in the estuary/shelf zone, which may lead to loss of associated U. These findings offer a more detailed picture of the fresh riverine water flow patterns in the Arctic Ocean when compared to other geochemical proxies. The non-conservative behaviour of U in the Mackenzie River through the shelf/estuaries has important implications for U input into oceans and the total marine budget.
Keywords: Arctic Ocean; U-series; radionuclides; uranium; non-conservative; uranium disequilibrium;
The peculiar geochemical signatures of São Miguel (Azores) lavas: Metasomatised or recycled mantle sources? by Christoph Beier; Andreas Stracke; Karsten M. Haase (186-199).
The island of São Miguel, Azores consists of four large volcanic systems that exhibit a large systematic intra-island Sr–Nd–Pb–Hf isotope and trace element variability. The westernmost Sete Cidades volcano has moderately enriched Sr–Nd–Pb–Hf isotope ratios. In contrast, lavas from the easternmost Nordeste volcano have unusually high Sr and Pb and low Nd and Hf isotope ratios suggesting a long-term evolution with high Rb/Sr, U/Pb, Th/Pb, Th/U and low Sm/Nd and Lu/Hf parent–daughter ratios. They have trace element concentrations similar to those of the HIMU islands, with the exception of notably higher alkali element (Cs, Rb, K, Ba) and Th concentrations. The time-integrated parent–daughter element evolution of both the Sete Cidades and Nordeste source matches the incompatibility sequence commonly observed during mantle melting and consequently suggests that the mantle source enrichment is caused by a basaltic melt, either as a metasomatic agent or as recycled oceanic crust. Our calculations show that a metasomatic model involving a small degree basaltic melt is able to explain the isotopic enrichment but, invariably, produces far too enriched trace element signatures. We therefore favour a simple recycling model. The trace element and isotopic signatures of the Sete Cidades lavas are consistent with the presence of ancient recycled oceanic crust that has experienced some Pb loss during sub-arc alteration. The coherent correlation of the parent–daughter ratios (e.g. Rb/Sr, Th/U, U/Pb) and incompatible element ratios (e.g. Nb/Zr, Ba/Rb, La/Nb) with the isotope ratios in lavas from the entire island suggest that the Sete Cidades and Nordeste source share a similar genetic origin. The more enriched trace element and isotopic variations of Nordeste can be reproduced by recycled oceanic crust in the Nordeste source that contains small amounts of evolved lavas (∼ 1–2%), possibly from a subducted seamount. The rare occurrence of enriched source signatures comparable to Nordeste may be taken as circumstantial evidence that stirring processes in the Earth's mantle are not able to homogenise material within the size of seamounts over timescales of mantle recycling.
Keywords: enriched mantle; radiogenic isotopes; incompatible elements; subduction; recycling; Ocean Island Basalts;
Consequences of moderate ∼ 25,000 yr lasting emission of light CO2 into the mid-Cretaceous ocean by Thomas Wagner; Klaus Wallmann; Jens O. Herrle; Peter Hofmann; Isabel Stuesser (200-211).
Future warming is predicted to shift the Earth system into a mode with progressive increase and vigour of extreme climate events possibly stimulating other mechanisms that invigorate global warming. This study provides new data and modelling investigating climatic consequences and biogeochemical feedbacks that happened in a warmer world ∼ 112 Myr ago. Our study focuses on the Cretaceous Oceanic Anoxic Event (OAE) 1b and explores how the Earth system responded to a moderate ∼ 25,000 yr lasting climate perturbation that is modelled to be less than 1 °C in global average temperature. Using a new chronological model for OAE 1b we present high-resolution elemental and bulk carbon isotope records from DSDP Site 545 from Mazagan Plateau off NW Africa and combine this information with a coupled atmosphere–land–ocean model. The simulations suggest that a perturbation at the onset of OAE 1b caused almost instantaneous warming of the atmosphere on the order of 0.3 °C followed by a longer (∼ 45,000 yr) period of ∼ 0.8 °C cooling. The marine records from DSDP Site 545 support that these moderate swings in global climate had immediate consequences for African continental supply of mineral matter and nutrients (phosphorous), subsequent oxygen availability, and organic carbon burial in the eastern subtropical Atlantic, however, without turning the ocean anoxic. The match between modelling results and stratigraphic isotopic data support previous studies [summarized in Jenkyns, H.C., 2003. Evidence for rapid climate change in the Mesozoic–Palaeogene greenhouse world. The Royal Society, 361: 1885–1916.] in that methane emission from marine hydrates, albeit moderate in dimension, may have been the trigger for OAE 1b, though we can not finally rule out alternative mechanisms. Following the hydrate mechanism a total of 1.15 × 1018 g methane carbon (δ 13C = − 60 ‰), equivalent to about 10% to the total modern gas hydrate inventory, generated the δ 13Ccarb profile recorded in the section. Modelling suggests a combination of moderate-scale methane pulses supplemented by continuous methane emission at elevated levels over ∼ 25,000 yr. The proposed mechanism, though difficult to finally confirm in the geological past, is arguably more likely to occur in a warmer world and apparently perturbs global climate and ocean chemistry almost instantaneously. This study shows that, once set-off, this mechanism can maintain Earth's climate in a perturbed mode over geological time leading to pronounced changes in regional climate.
Keywords: Cretaceous greenhouse; oceanic anoxic events; marine black shale; Mazagan Plateau; biogeochemical modelling; nutrient cycle;
Modeling the evolution of continental subduction processes in the Pamir–Hindu Kush region by Ana M. Negredo; Anne Replumaz; Antonio Villaseñor; Stéphane Guillot (212-225).
Several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the India–Eurasia collision zone, with steep northward subduction of Indian lithosphere beneath the Hindu Kush and southward subduction of Asian lithosphere under the Pamir. We investigate the geometry and the timing of these subduction processes. Seismic tomography images are used to constrain the geometry of the two slabs under the Pamir and the Hindu Kush. By measuring the tomographically inferred length of the Indian slab under the Hindu Kush region and by comparison with paleomagnetic reconstructions, we estimate that the process of Indian slab break-off most likely occurred at the early stage of the collision, at ca. 44–48 Ma. We infer that after slab break-off, western India continued its northward motion until its northern boundary reached the present Hindu Kush region, where it began to subduct. We estimate an age for the initiation of subduction in this region of about 8 Ma. We apply 2D thermo-kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the Hindu Kush and Pamir regions. For the Hindu Kush we introduce parameters reproducing fast and near vertical subduction, whereas in the Pamir region we use the results from published studies to reproduce slower subduction. Modeling results indicate that faster subduction under the Hindu Kush results in a deeper brittle region, which is consistent with observed deeper seismicity than in the Pamir region.
Keywords: Hindu Kush; Pamir; continental subduction; numerical modeling; temperature; rheology;
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