bluuebunny/crossref_metadata_2025 huggingface dataset:
Neodymium isotopic composition of Ordovician conodonts as a seawater proxy: Testing paleogeography
Authors: Wright Cynthia A. , Barnes Christopher R. , Jacobsen Stein B.
Published: October 2002

To evaluate recent Ordovician paleogeographic reconstructions, εNd values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates. Isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography. The isotopic patterns show a consistent picture of the changes in the Ordovician oceans. The εNd values for water masses associated with Laurentia are strongly negative (−28 to −18) in the Early Ordovician, evolving over time to higher values in the range −13 to −5. The Early Ordovician signature of Laurentia is in marked contrast to other cratons and microplates, which have a range of values from −10 to −5. Samples from South China show interesting signals that reflect a greater similarity to Laurentia than other peri‐Gondwana terranes. The isotopic variations are a function of both regional geology and global tectonic processes, the most obvious being the Taconic Orogeny and onset of the closure of the Iapetus Ocean. Regional and global models of paleogeography are considered in light of these proxy signals. This study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses. The use of the Nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models. By integrating such information, a more multidisciplinary approach is possible.
Does the oxidation of methane leave an isotopic fingerprint in the geologic record?
Authors: Stott Lowell D. , Bunn Tonya , Prokopenko Maria , Mahn Chris , Gieskes Joris , Bernhard Joan M.
Published: October 2002

Pore water ΣCO2 and δ13C data are presented for the dysoxic Santa Barbara and Santa Monica Basins in an attempt to constrain the influence of methane oxidation on the pore water gradients and benthic foraminiferal δ13C. Although these two basins are adjacent to one another Santa Barbara Basin has steeper pore water ΣCO2 and δ13C gradients. Methane oxidation could be contributing to the higher gradients in this basin. The isotope mass balance for pore water ΣCO2 indicates, however, that the pore water gradients in the upper 2 cm of both basins are controlled only by the rate of photosynthate carbon oxidation and the rate of diffusion. Methane derived CO2 has no discernable influence on pore water or carbonate δ13C values at these sediment depths in either basin. Benthic foraminiferal species inhabiting different depth horizons in the top centimeter incorporate these steep isotopic gradients in their calcite tests and can be used therefore to reconstruct the history of carbon oxidation within the basins. On the basis of these modern geochemical constraints, a more likely explanation for the apparent δ13C excursions recorded in the late Pleistocene sediments from Santa Barbara Basin involves enhanced carbon rain and carbon oxidation in the basin sediments rather than clathrate destabilization.
D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: Water from old D‐rich recycled crust or primordial water from the lower mantle?
Authors: Kingsley Richard H. , Schilling Jean‐Guy , Dixon Jacqueline E. , Swart Peter , Poreda Robert , Simons Kyla
Published: October 2002

Deuterium/Hydrogen isotope ratios were measured in 22 fresh basalt glasses dredged from young seamounts along the Easter‐Salas y Gomez seamount chain (ESC) and from the spreading centers of the Easter Microplate (EMP). The δD values decrease regularly from −36‰ near Salas y Gomez to −63‰, 800 km west on the East Pacific Rise (EPR) and the west rift of the EMP. Similar gradients are observed in H2O, Pb and Sr radiogenic isotope ratios, trace element concentrations, and in the ratios of incompatible to moderately incompatible trace elements. Fractionation of the D/H ratios by recent low‐temperature near surface processes during the emplacement of these basalts is discounted on the basis of (1) degassing and hydrothermal alteration modeling, (2) the observed correlation of H2O with refractory incompatible elements such as Ce and Th, and (3) the young age and freshness of the basalt glasses studied. The δD variation shows a strong geographical coherence with the long‐lived Pb and Sr isotope ratios, suggesting long‐lived upper mantle source heterogeneities in D/H and H2O as well. Binary mixing of the δD and H2O‐rich Salas y Gomez mantle plume with the depleted upper mantle low in δD and H2O is clearly evident in the covariation of Pb radiogenic‐bearing isotope ratios, δD, and H2O along the ESC‐EMP. The positive correlations of δD with Pb isotope ratios indicate that the Salas y Gomez mantle plume with HIMU‐affinity is rich in deuterium and in H2O. A mantle plume model with two possible variations, not mutually exclusive, namely recycling of old hydrothermally altered and dehydrated crust, with or without entrainment of primordial water from the lower mantle is proposed and evaluated.
Consequences of diffusive reequilibration for the interpretation of melt inclusions
Authors: Cottrell E. , Spiegelman M. , Langmuir C. H.
Published: October 2002

Trace element abundances in melt inclusions are commonly used to interpret melting and melt extraction processes. These interpretations, however, often assume that the chemical compositions of melt inclusions are identical to the liquid from which the host crystal grew, even though driving forces for postentrapment diffusion and modification are demonstrable. This paper begins to quantify the effects of diffusion on melt inclusions using a numerical model. The model calculates the compositional evolution of a spherical inclusion which initially is in equilibrium with a crystal host out to some distance rjump and out of equilibrium beyond. In particular we consider the end‐member scenario, whereby the trapped melt is initially out of equlibrium with the neighboring crystal as this sets the minimum time for reequilibration. A package of numerical codes is provided that allows the user to explore other initial conditions. The model calculates the change in inclusion composition and also the structure of diffusion halos that grow around the inclusion as it reequilibrates with the surrounding crystal. The detection of these profiles in naturally occurring inclusions may allow the time since entrapment and the initial inclusion concentration to be estimated. The extent of reequilibration is most strongly influenced by the partition coefficient, diffusivity, and the inclusion radius. Fast‐diffusing elements with high mineral/melt partition coefficients are modified rapidly, particularly in small inclusions. Because minerals have very different Dmineral/melt for the various elements, the effects of diffusive reequilibration differ substantially from one mineral to another. For example, the higher partition coefficients of the heavy rare earth elements (HREE) in olivine make HREE concentrations easier to modify than light rare earth elements (LREE) concentrations. In contrast, Sr, Eu, and Ba in plagioclase hosted inclusions equilibrate more rapidly than the other trace elements. Examination of published trace element concentrations of olivine hosted inclusions show little evidence for reequilibration, at least for the light REE and other highly incompatible elements. It is difficult, however, to provide firm constraints due to the uncertainties in olivine diffusivities and the initial condition. In contrast, trace element diffusivities in plagioclase have been determined experimentally [Cherniak, 2001], and the trace element concentrations of published plagioclase hosted inclusions show evidence for extensive diffusive exchange with the host in a manner consistent with model predictions. Postentrapment modification therefore is likely an important factor in the interpretation of some melt inclusion data.
A major dissolution event at the close of MIS 5e in the western equatorial Atlantic
Authors: Broecker Wallace S. , Clark Elizabeth
Published: October 2002

Measurements of the CaCO3 content and the CaCO3 size index on a core from the equatorial Atlantic's Ceara Rise reveal a pronounced dissolution event associated with the transition between MIS 5e and 5d. We tentatively correlate this event with the dramatic drop in atmospheric CO2 content that occurred at the end of MIS 5e. Attempts to quantify the carbonate ion concentration drop associated with this event failed because the shells in this zone had been largely destroyed by dissolution. By leaving out the sonification step in our cleaning procedure, we were able to “rescue” enough shells to weigh, but they showed no dramatic weight loss. We attribute this to the survival of the hardiest shells.
Effects of variable sedimentation rates and age errors on the resolution of sedimentary paleointensity records
Authors: Guyodo Yohan , Channell James E. T.
Published: October 2002

Synthetic, u‐channel records of relative paleointensity have been generated with a numerical model simulating the recording process in sediments, variable quality age control and variable sedimentation rates, over a time interval of 500 ky. Simulation results indicate that paleointensity records with mean sedimentation rates up to 15 cm/ky can reflect geomagnetic dipole intensity fluctuations, with some amplitude differences between individual records of nongeomagnetic origin. This study confirms that relative paleointensity records have great potential as a stratigraphic tool and that a stratigraphic precision of a few thousand years can be achieved with records characterized by high sedimentation rates. Spectral analyses suggest that caution should be used when interpreting the power spectra of individual records and that stacked records should be favored. Stacked u‐channel records with mean sedimentation rates of 1 cm/ky do not provide reliable spectral information on the dipole intensity for wavelengths shorter than 25–50 ky, and their utility is limited to long‐term trends in paleointensity. For higher sedimentation rates, the range of spectral information depends on the stack resolution (sedimentation rates) and the age model. The best results are, predictably, obtained with high sedimentation rates and excellent age control. In these cases, the power spectra are reliable for wavelengths as short as 4 ky.
A simple way to extract baddeleyite (ZrO2)
Authors: Söderlund Ulf , Johansson Leif
Published: October 2002

Baddeleyite is a reliable geochronometer for dating the crystallization of mafic and other silica‐undersaturated intrusive rocks. Its high U and negligible initial Pb content enable precise age determinations with statistical errors of a few million years or less. The widespread use of baddeleyite for dating has, however, been limited by the low content of baddeleyite in many samples and by difficulties in isolating baddeleyite. We have developed a new separation technique that utilizes differences in transport velocity among grains of varying size as they move across the deck of a water‐shaking table. A small sample portion is loaded instantly on the table, and only the finest and densest material remaining after ∼120 s is collected. Repeated recovery of sample portions yields a concentrate strongly enriched in baddeleyite and opaque minerals: the latter is easily removed by a hand magnet. The “water‐based” separation technique will improve yields of baddeleyite from samples processed on a water‐shaking table and is capable of recovering very fine grains. As the entire separation is made in water, problems arising from adhesive forces between minerals are minimized, as is the risk of cross contamination. No heavy liquids are necessary, making separation quick, uncomplicated and inexpensive.
The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa
Authors: Ghiorso Mark S. , Hirschmann Marc M. , Reiners Peter W. , Kress Victor C.
Published: October 2002

We describe a newly calibrated model for the thermodynamic properties of magmatic silicate liquid. The new model, pMELTS, is based on MELTS [Ghiorso and Sack, 1995] but has a number of improvements aimed at increasing the accuracy of calculations of partial melting of spinel peridotite. The pMELTS algorithm uses models of the thermodynamic properties of minerals and the phase equilibrium algorithms of MELTS, but the model for silicate liquid differs from MELTS in the following ways: (1) The new algorithm is calibrated from an expanded set of mineral‐liquid equilibrium constraints from 2439 experiments, 54% more than MELTS. (2) The new calibration includes mineral components not considered during calibration of MELTS and results in 11,394 individual mineral‐liquid calibration constraints (110% more than MELTS). Of these, 4924 statements of equilibrium are from experiments conducted at elevated pressure (200% more than MELTS). (3) The pMELTS model employs an improved liquid equation of state based on a third‐order Birch‐Murnaghan equation, calibrated from high‐pressure sink‐float and shockwave experiments to 10 GPa. (4) The new model employs a revised set of end‐member liquid components. The revised components were chosen to better span liquid composition‐space. Thermodynamic properties of these components are optimized as part of the mineral‐liquid calibration. Comparison of pMELTS to partial melting relations of spinel peridotite from experiments near 1 GPa indicates significant improvements relative to MELTS, but important outstanding problems remain. The pMELTS model accurately predicts oxide concentrations, including SiO2, for liquids from partial melting of MM3 peridotite at 1 GPa from near the solidus up to ∼25% melting. Compared to experiments, the greatest discrepancy is for MgO, for which the calculations are between 1 and 4% high. Temperatures required to achieve a given melt fraction match those of the experiments near the solidus but are ∼60°C high over much of the spinel lherzolite melting interval at this pressure. Much of this discrepancy can probably be attributed to overstabilization of clinopyroxene in pMELTS under these conditions. Comparison of pMELTS calculations to the crystallization and partial melting experiments of Falloon et al. [1999] shows excellent agreement but also suffers from exaggerated calculated stability of clinopyroxene. Finally, comparison of pMELTS calculations to the garnet peridotite experiments of Walter [1998] at 3–7 GPa reveals disparities between calculations and experiments that increase with pressure. The most prominent of these disparities is manifest as overprediction of the stability of garnet and underprediction of that of olivine. Part of this problem may be attributed to inadequacies in the Birch‐Murnaghan equation of state in reproducing the behavior of highly compressible liquids at high pressures and temperatures.
On the initiation of a snowball Earth
Authors: Schrag Daniel P. , Berner Robert A. , Hoffman Paul F. , Halverson Galen P.
Published: October 2002

The Snowball Earth hypothesis explains the development of glaciation at low latitudes in the Neoproterozoic, as well as the associated iron formations and cap carbonates, in terms of a runaway ice‐albedo feedback leading to a global glaciation followed by an extreme greenhouse climate. The initiation of a snowball glaciation is linked to a variety of unusual perturbations of the carbon cycle operating over different timescales, as evidenced by unusual patterns in the carbon isotopic composition of marine carbonate. Thus a theory for why multiple glaciations happened at this time, and not in the Phanerozoic nor earlier in the Proterozoic, requires a reexamination of the carbon cycle and the controls on climate stability. We propose that the concentration of continental area in the tropics was a critical boundary condition necessary for the onset of glaciation, both because the existence of substantial continental area at high latitudes may prevent atmospheric carbon dioxide from getting too low and because a tropical concentration of continental area may lead to more efficient burial of organic carbon through increased tropical river discharge. Efficient organic carbon burial sustained over tens of millions of years, required by the high carbon isotopic compositions of preglacial carbonate, may lead to the buildup of enormous quantities of methane, presumably in hydrate reservoirs. We examine how the slow release of this methane may explain the drop in δ13C values immediately before the glaciation. Moreover, the accumulation of methane in the atmosphere coupled with the response of silicate weathering to the additional greenhouse forcing can lead to a climate with methane as the major greenhouse gas. This situation is unstable because methane is not buffered by a large ocean reservoir like carbon dioxide, and so the collapse of the methane source may provide a trigger for the onset of a runaway ice‐albedo feedback. A simple model of the carbon cycle is used to explore the boundary conditions that would allow this to occur.
Timescales for the evolution of seismic anisotropy in mantle flow
Authors: Kaminski Édouard , Ribe Neil M.
Published: October 2002

We study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it, using the plastic flow/recrystallization model of Kaminski and Ribe [2001]. In this model, a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization, by nucleation and grain boundary migration. Numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages: (1) for small times , recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid; (2) for intermediate times , the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system; (3) for , the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation (the “infinite strain axis”.) In more realistic nonuniform flows, lattice preferred orientation evolution depends on a dimensionless “grain orientation lag” parameter Π(x), defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow. Explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume‐ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where Π ≪ 1. Calculation of Π provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost.
Kinematics of mantle flow beneath a fossil Overlapping Spreading Center: The Wuqbah massif case, Oman ophiolite
Authors: Girardeau Jacques , Monnier Christophe , Launeau Patrick , Quatrevaux Frédéric
Published: October 2002

The Wuqbah massif (central Oman Ophiolite) comprises a well preserved <2 km thick crustal unit over a 6–8 km thick mantle sequence. The mantle comprises harzburgites and few dunites cut by various types of dykes. With the exception of basal and late fault zones, peridotites display granoblastic and porphyroclastic textures, acquired by high‐temperature plastic deformation. They locally suffered important recrystallization as shown by neoblastic shapes and arrangements. Macroscopically, the peridotites have well‐defined foliations and lineations, whose features allow the massif to divide into three structural domains. It comprises a N110° trending central zone that suffered an anticlockwise rotation, with an upwelling zone in its southwestern part, bound between two NS areas sheared in right‐lateral conditions. This peculiar geometry is thought to reflect an overlapping spreading center (OSC) where the east and west bordering zones would define the trace, in the mantle, of two ridge segment terminations and the oblique N110° central zone the axial part of the overlap area. It is proposed that the northward migration of the southwest branch and its concomitant EW spreading caused the deformation and rotation of the overlap zone. In our OSC model, flow in the mantle would be continuous from one spreading center branch to the other, being curved in the overlap area to accommodate the offset. The mantle flow would therefore be more and more decoupled with the brittle crust when approaching the overlap zone, where extensional tectonics can occur. We consider that some mantle upwelling is possible due to its specific location, in between two ridge segments and over the main mantle flow. The distance between the two ridge axes is ∼15 km, which means that this paleo‐OSC can be ranked in the class of intermediate OSCs, illustrating a third‐order ridge segmentation. At the scale of the ophiolite, the Wuqbah OSC is located northwest of a major domain marked by the presence of a NW‐SE trending propagating ridge responsible for the formation of a new oceanic crust. It is suggested that the Wuqbah OSC formed earlier and underlines the trace of a ridge that had later migrated to the East.
Carbonate ion concentration in glacial‐age deep waters of the Caribbean Sea
Authors: Broecker Wallace S. , Clark Elizabeth
Published: October 2002

On the basis of the whole foraminifera shell weights for G. sacculifer and N. dutertrei from Caribbean cores, it appears that the carbonate ion concentration of water at 1800 m depth in the open Atlantic (i.e., the depth of the Caribbean sill) was ∼14 μmol/kg greater during glacial time than during the Holocene. The sense of this difference is consistent with the observation that the 13C/12C ratio for benthic foraminifera from glacial‐age Atlantic upper deep water was higher than that for their Holocene equivalents. It is also consistent with fragment to whole foraminifera results. However, as several questions regarding the assumptions involved in converting shell weights to paleocarbonate ion concentrations remain unanswered, this reconstruction must be considered tentative. Critical in this regard are the assumptions that shell‐wall thicknesses were the same as today's at any given site during glacial time and that the 0.3 μg per μmol/kg relationship between shell weight and carbonate ion concentration is applicable over the entire deep water column. The records reported here reveal no evidence for an early Holocene preservation event. However, that for core V28‐122 documents a dissolution event close to the MIS 5‐4 boundary.
Temporal variability of the hydrothermal plume above the Kairei vent field, 25°S, Central Indian Ridge
Authors: Rudnicki Mark D. , German Christopher R.
Published: October 2002

Continuous profiling of the nonbuoyant plume above a hydrothermal vent site can be used to determine the interaction between the plume formed by venting high‐temperature fluids and changing background temperature, salinity, and current fields over a tidal cycle. We have conducted a 12‐hour monitoring of the nonbuoyant plume above the Kairei vent field, 25°S, Central Indian Ridge. The depth of the plume particle maximum varies significantly in the water column, from 2150 m to >2350 m, but remains strongly associated with the σ2 = 36.987 potential density isopycnal. We have used the interaction between two unrelated physical oceanographic phenomena to isolate the section of the time series record most suitable for determining the heat flux of the Kairei source on the basis of the rise height of the plume to neutral buoyancy. At the start of the time series, internal waves are observed with a period similar to the calculated buoyancy period N−1, about 3 hours; no consistent relationship is observed between the rise height of the plume and the calculated heat flux. For the last 6 hours of the time series, isopycnal surfaces are compressed above a strongly developed nonbuoyant plume such that the calculated buoyancy period is less than that of the internal waves. A more constant heat flux can be calculated, on the basis of the plume rise height, from this part of the record.
A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: Prelude to snowball Earth?
Authors: Halverson Galen P. , Hoffman Paul F. , Schrag Daniel P. , Kaufman Alan J.
Published: October 2002

A large (11–15‰) negative shift in δ13C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as ∼0.6 × 106 years from a simple thermal subsidence model. Similar or larger δ13C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative δ13C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation.
Fossil ridge‐transform intersection in the Troodos ophiolite: New perspectives from rock magnetism in the gabbro suite and fracture mechanics analysis
Authors: Abelson Meir , Baer Gidon , Agnon Amotz
Published: October 2002

The “missing link” in the comprehensive view of the ridge‐transform intersection (RTI) frozen in Troodos is that portion of the extinct spreading axis between the Solea graben and the fossil Arakapas transform. Directional indicators of magma flow verified by paleomagnetic tests along with tectonic indicators permit tracing the extinct spreading axis through the gabbro suite at the continuation of the Solea graben axis. These indicators of axial deformation include measurements of the anisotropy of magnetic susceptibility (AMS) and petrofabric determinations in the gabbro, dikes, major normal faults, and fabric of mantle rocks. The inferred axis is curved as it approaches the fossil transform. The sense of curvature is in agreement with dextral slip along the fossil transform, assuming curvature has resulted from transform‐induced shear stresses. Fracture mechanics analysis predicts the control on the axis shape by the ratio between driving stresses of spreading (SH) and resistance for sliding along the fossil transform (T). Accordingly, the curved shape of the spreading axis frozen in Troodos indicates T/SH ∼ 0.1–0.2, whereas for straight axis and orthogonal RTI, the analysis predicts T/SH < 0.01. These results suggest that the fossil curvature reflects a strong transform relative to curvature‐free RTIs. A strong Arakapas transform is also in agreement with previous evidence of diffused deformation north of the transform, such as block rotation and sigmoidal structure of dikes and faults.
Seismic anisotropy in the upper mantle 2. Predictions for current plate boundary flow models
Authors: Blackman Donna K. , Kendall J.‐Michael
Published: October 2002

The anisotropic seismic structure due to flow‐induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine:enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite‐strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two‐dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.
Seismic anisotropy of the upper mantle 1. Factors that affect mineral texture and effective elastic properties
Authors: Blackman Donna K. , Wenk H.‐Rudolf , Kendall J. Michael
Published: October 2002

Flow‐induced mineral alignment in the Earth's mantle affects the nature of seismic wave propagation. Since measurements of seismic travel time and shear wave splitting are a key means by which the structure of the upper mantle can be imaged, it is important to understand the factors that contribute to variability in elastic properties. Seismic anisotropy associated with lattice‐preferred orientations of minerals in peridotite is the focus of this study. One way to better understand anisotropy in a convecting upper mantle is to simulate texture evolution based on certain assumptions. Simulations of the development of olivine and orthopyroxene alignment along streamlines of a mantle flow field illustrate how continuously varying strain conditions affect the resulting orientation distribution. There are various uncertainties in rock texture simulations, and the aim here is to investigate how much model assumptions may influence the results. A model of upper mantle flow in the vicinity of an oceanic spreading center is used to illustrate these points. First we assess how sensitive results are on assumptions of different polycrystal plasticity models, specifically lower bounds and viscoplastic self‐consistent approaches. We also investigate how recrystallization during deformation might affect the texture that develops along a streamline. The effects of grain growth and nucleation produce, as expected, textures that are different from the deformation‐only case. However, the basic P wave structure of the predicted anisotropy is similar between models for streamlines in a slow‐spreading, passive flow model as is commonly used for simulating flow near this type of plate boundary. Shear wave splitting patterns are more complex and differ somewhat more between the models in the off‐axis region. We also compare predictions for texture development of olivine‐only aggregates to that of mixed composition with 70% olivine and 30% orthopyroxene. Although we consider only one aspect of such polyphase deformation, i.e., plastic deformation on specified crystal slip systems, our results are consistent with field observations of the orientation distributions in ophiolitic peridotites. Finally, we determine how simplifying assumptions about the symmetry of the elastic anisotropy can bias interpretations of seismic travel time and shear wave splitting. Whereas a hexagonal approximation can lead to underestimates of the degree of anisotropy, an orthorhombic approximation is found to closely match the results predicted for a general elastic tensor corresponding to an orientation distribution whose symmetry is as low as monoclinic or triclinic.
Temporal‐compositional trends in intraplate basalt eruptions: Implications for mantle heterogeneity and melting processes
Authors: Reiners Peter W.
Published: October 2002

Because primitive basaltic magmas travel from their mantle sources to the surface without significant cooling and crystallization en route, compositional variations within them provide direct information on the sources and processes of partial melting in the mantle. Most studies of basalt magmagenesis have focused on chemical variations in magmas erupted from multiple vents in regional complexes or on long timescale (105–106years) variations in magmas from single shield volcanoes; few studies have aimed to measure changes in melt chemistry with time in individual eruptions. Available observations of individual basaltic eruption sequences, however, show remarkably similar temporal‐compositional trends in the form of decreasing incompatible elements and MgO, with increasing SiO2, as the eruption (or sequence of related, closely‐spaced eruptions) continued. Both trace element and isotopic variations indicate that this does not reflect fractional crystallization nor can it be explained simply by varying extents of partial melting of a single source. Instead, these trends are consistent with systematically changing proportions of mixing between melts produced by different degrees of partial melting of two compositionally distinct but apparently common sources in the mantle. One potential explanation for this is that a relatively deep mantle source melts to large degrees, producing melts that ascend through and mix with small‐degree melts of the lithosphere. The small‐degree lithospheric melts are then progressively exhausted during melt migration and eruption, so that late‐erupting melts contain the smallest proportion of the shallow, small‐degree melt. This cannot explain Os isotopic correlations with melt composition and radiogenic Os isotope signatures of the late‐erupting melts in several eruption sequences, however. An alternative scenario is that systematic changes in erupted melt chemistry with time reflect sequential eruption of melt from increasing depths in mantle melting regions that are compositionally zoned because of different solidii and melt productivities of distinct peridotite and pyroxenite lithologic domains. In either case, compositional variation in many eruptions of primitive, intraplate basalts is dominated by mixing of melts from distinct lithologies, and temporal‐compositional changes in such eruptions reflect the distribution and composition of melt with depth in mantle melting and melt migration regions.
Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard Island, Indian Ocean
Authors: Weis Dominique , Frey Frederick A. , Schlich Roland , Schaming Marc , Montigny Raymond , Damasceno Dimitri , Mattielli Nadine , Nicolaysen Kirsten E. , Scoates James S.
Published: October 2002

The gravity and bathymetric highs on the Kerguelen Plateau in the southern Indian Ocean between the Kerguelen Archipelago and Heard Island are seamounts formed of Miocene alkalic basalts similar to those found on the islands. Dredging during the Kerimis survey cruise recovered >1 ton of mostly basaltic rocks. One of the dredges (D6) yielded a large volume of in situ alkalic picritic pillow basalts, the first picritic lavas recovered that are related to the Kerguelen plume. K‐Ar and 40Ar‐39Ar ages are between 18 and 21 Ma for all but one sample, and these ages are only slightly younger than the main phase of volcanism on the archipelago. The dredged lavas form three distinct groups based on chemical and isotopic compositions. Incompatible element abundance ratios overlap with compositional groups defined by lavas from both the Kerguelen Archipelago and Heard Island indicating that alkalic volcanism in this region of the Kerguelen Plateau has been spatially diverse. Olivine and picritic basalts have Sr and Nd isotopic characteristics similar to most of the lavas exposed on the archipelago and those proposed for the Kerguelen plume. However, compared to Kerguelen Archipelago lavas, the picritic basalts have relatively low 206Pb/204Pb which is a characteristic of Cretaceous basalt forming some parts of the Kerguelen Plateau. We propose that the apparent age trend of the lavas from 34 Ma in the Northern Kerguelen Plateau (ODP Leg 183, Site 1140) to 24–30 Ma on the Kerguelen Archipelago to 18–21 Ma on the dredged submarine volcanoes, and even possibly to recent volcanism on Heard and McDonald Islands, may correspond to the Tertiary hot spot track of the Kerguelen plume.
Application of maximum likelihood and bootstrap methods to nonlinear curve‐fit problems in geochemistry
Authors: Sohn R. A. , Menke W.
Published: October 2002

We apply maximum likelihood and bootstrap methods to the assess parameter estimates and confidence intervals in nonlinear geochemical data analysis. We focus primarily on the regression of “double error” data, or x−y data pairs, where both variables are measurements subject to error (e.g., isochron fitting), but the methods we discuss are general and can be applied to any nonlinear regression. We review the differences between least squares and maximum likelihood methods and describe how maximum likelihood can be naturally extended to permit robust regressions that are relatively insensitive to outliers. We describe the bootstrap technique for the estimation of the standard error and confidence intervals of a model and show how it can be used to sidestep the mathematical intractability of estimating uncertainty in nonlinear regressions. We illustrate the application of these methods through examples using synthetic and real data and demonstrate how they can influence statistical perception of nonlinear parameter estimates. In particular, we demonstrate how the linearized methods that are commonly used to make uncertainty estimates can substantially underestimate model error, and we show how this problem can be addressed with bootstrap confidence intervals that automatically adjust to account for intrinsic asymmetry and non‐Gaussian behavior in a regression. We include computer code to implement the calculations we describe as supplements to this article.
Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges
Authors: Böhm F. , Haase‐Schramm A. , Eisenhauer A. , Dullo W.‐C. , Joachimski M. M. , Lehnert H. , Reitner J.
Published: October 2002

Coralline sponge skeletons are excellent tools for reconstructing the carbon isotope history of dissolved inorganic carbon (DIC) in tropical surface waters. Carbon isotope records from coralline sponges clearly reflect the industrial 12C increase in atmospheric CO2 with a precision that permits quantitative interpretations. We find from a set of δ13C records of four Caribbean sponge specimens that the isotopic response of surface water DIC to the changing isotopic composition of atmospheric CO2 varied dynamically during the last century, depending on the rate of atmospheric change. Three of our sponges provide 600 year long δ13C records. For the first time, we can reconstruct surface water δ13CDIC for the full history of the industrial CO2 release as well as the preceding preindustrial period back to the beginning of the Little Ice Age. This provides a well‐founded estimate of the anthropogenically uninfluenced, preindustrial background level of surface water 13C/12C ratios. Our records show small but systematic variations that appear to be linked to the climate fluctuations of the Little Ice Age.
A quadrupole‐based mass spectrometric system for the determination of noble gas abundances in fluids
Authors: Kulongoski Justin T. , Hilton David R.
Published: October 2002

We present details of the operation and performance of a quadrupole mass spectrometer‐based system developed for the extraction, separation, and high‐precision measurement of noble gases dissolved in fluid samples. The relatively inexpensive quadrupole mass spectrometer system, employed with the isotope dilution technique, achieves a precision for He, Ne, Ar, Kr, and Xe abundances of ±0.4%, ±0.7%, ±0.1%, ±0.8%, and ±0.9% respectively, as determined from analyses of mixed spike‐air aliquots. The system has an overall accuracy of ±0.6°C on the noble gas temperature of a water sample, as determined by analyses of water samples air‐equilibrated under controlled conditions. We describe improvements in calibration procedures and sample processing protocols that have led to these high‐quality measurements.
Ocean biology could control atmospheric δ13C during glacial‐interglacial cycle
Authors: Brovkin Victor , Hofmann Matthias , Bendtsen Jørgen , Ganopolski Andrey
Published: October 2002

Estimates of changes in the global carbon budget are often based on the assumption that the terrestrial biosphere controls the isotopic composition of atmospheric CO2 since terrestrial plants discriminate against the 13C isotope during photosynthesis. However, this method disregards the influence of 13C fractionation by the marine biota. Here an interpretation of the glacial‐interglacial shifts in the atmospheric CO2 concentration and δ13CO2 measured in the Taylor Dome ice core [Smith et al., 1999] is given by accounting for possible changes in the ocean biology based on sensitivity simulations undertaken with the intermediate complexity model CLIMBER‐2. With a combined scenario of enhanced biological and solubility pumps, the model simulates glacial atmospheric CO2 and δ13CO2 similar to those inferred from the ice core. The simulations reveal that a strengthening of the oceanic biological carbon pump considerably affects the atmospheric δ13CO2.
Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E
Authors: Bach Wolfgang , Banerjee Neil R. , Dick Henry J. B. , Baker Edward T.
Published: October 2002

We report the discovery of active and fossil hydrothermal systems during R/V Knorr Cruise 162, Legs VII and IX along a 400 km long segment of the ultra‐slow Southwest Indian Ridge (SWIR) between 10 and 16°E, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr−1 full rate). Eight of forty‐one optical/temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others. Fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias, four deposits ofsepiolite and silica, as well as Mn‐oxide and nontronite cemented breccias. The massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep‐seated normal fault bounding the southern rift valley wall. Silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low‐ to moderate‐temperature ultramafic‐hosted hydrothermal activity. The abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates, and hence the magmatic heat input, along this section of the SWIR are lower than on any other explored segment of the global mid‐ocean ridge system. This observation suggests that high mantle upwelling and magma supply rates are not required to drive mid‐ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra‐slow end of the ridge spreading spectrum. The frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation, with hydrothermal vent sites being preferentially associated with long‐lived faults that provide fluid pathways. Our results suggest the contribution of magma‐starved ridge segments to the global ocean‐lithosphere geochemical budget is potentially much larger than previously thought.
Ocean ventilation and sedimentation since the glacial maximum at 3 km in the western North Atlantic
Authors: Keigwin L. D. , Schlegel M. A.
Published: October 2002

Stable isotope, sedimentological, and radiocarbon data from cores at ∼3 km water depth on the Blake Ridge, western subtropical North Atlantic, reveal the history of deep water ventilation since the last glacial maximum (LGM). Bulk sediment accumulation rates varied locally by a factor of 2 under the influence of bottom currents in this sediment drift environment, but the sand flux, mostly foraminifera, was nearly identical at a given time. This suggests that the rain rate of foraminifera (mostly planktonic) was constant, that transport of foraminifera was negligible, and that current‐controlled differences in clay and silt transport drive bulk accumulation. In two of the cores, flux peaks in the benthic foraminifera Cibicidoides and Uvigerina peregrina occurred during the Younger Dryas (YD) cold event, and at 18.2, 19.6, 21.1, 25.0, and 28.1 ka. Radiocarbon measurements on those benthic foraminifera show the ventilation age of bottom waters was ∼1000 years during the YD, and for older events it was as great as 2000 years. These results contrast with Holocene ventilation, which was ∼500 years and 700 years at 1500 years and ∼7100 years before present, respectively.
Calibrating 3D tomograms of a granitic pluton
Authors: Martí D. , Carbonell R. , Tryggvason A. , Escuder J. , Pérez‐Estaún A.
Published: October 2002

First arrival‐time P‐waves from offset and azimuth variable vertical seismic reflection records are used to tomographically reconstruct the 3D P‐wave velocity structure around a 500 m deep vertical borehole. Comparisons with sonic and fracture index logs and core samples indicates that low velocity anomalies in the tomograms correlate with surface mapped fracture zones. Results also show that low velocity zones intersect the borehole in areas characterized by relatively low values for Vp and Vs determined from sonic logs. These zones are characterized by Poisson's ratio (σ) close to 0.3, which are indicative of altered or fractured rock and supported by the fracture index and core samples. The high velocity anomalies intersect the well in areas characterized by low values of the fracture index and σ = 0.25 or lower. This suggests that the relatively high Vp velocities are indicative of unfractured rock. This study reveals that high resolution tomography produces results consistent with direct observations in a well‐sampled borehole.
Carbon isotope ratios of methyl bromide and methyl chloride emitted from a coastal salt marsh
Authors: Bill Markus , Rhew Robert C. , Weiss Ray F. , Goldstein Allen H.
Published: October 2002

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) play important roles in stratospheric ozone depletion, but their atmospheric budgets have large uncertainties. The analysis of stable isotope composition of methyl halides may provide useful independent information for further constraining their budgets. Here we report the first measurements of CH3Br and CH3Cl stable carbon isotope ratios emitted from a biogenic source under in situ conditions. CH3Br and CH3Cl emissions from the salt marsh plant Batis maritima showed a strong diurnal variation in δ13C, from −65‰ during the daytime to −12‰ at night. The minimum δ13C values were observed at midday, coinciding with the time of greatest emissions and ambient temperature. At night, when the emissions were much smaller, the stable carbon isotopic ratios of CH3Br and CH3Cl became enriched in 13C. The daily mean δ13C of CH3Br and CH3Cl emissions, weighted by emission rate, were −43‰ and −62‰ respectively.