bluuebunny/crossref_metadata_2025 huggingface dataset:
Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate
Authors: Sigman D. M. , Robinson R. , Knapp A. N. , van Geen A. , McCorkle D. C. , Brandes J. A. , Thunell R. C.
Published: May 2003

Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15N/14N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15N/14N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18O/16O suggest that the lower‐than‐expected 15N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low‐15N/14N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.
East Asian monsoon forcing of suborbital variability in the Sulu Sea during Marine Isotope Stage 3: Link to Northern Hemisphere climate
Authors: Dannenmann Stefanie , Linsley Braddock K. , Oppo Delia W. , Rosenthal Yair , Beaufort Luc
Published: May 2003

We have generated a new high‐resolution record of variations in planktonic foraminiferal oxygen isotopes (δ18O) and Mg/Ca from a sediment core (IMAGES 97‐2141) in the Sulu Sea located in the Philippine archipelago of western tropical Pacific. This record reveals distinct, suborbital‐scale δ18O changes, most notably during Marine Isotope Stage 3 (MIS3) (∼30,000 to 60,000 years B.P.). The amplitudes of these δ18O fluctuations (0.4 to 0.7‰) exceed that which can be attributed to sea level changes and must be due to changes in sea surface conditions. In the same interval, variations in planktonic foraminifera Mg/Ca suggest that suborbital surface ocean temperature variations of 1 to 1.5°C in the Sulu Sea were not in phase with δ18O. Combined, this evidence indicates that the MIS3 millennial δ18O events in the Sulu Sea were primarily the result of changes in surface water salinity, which today is directly related to the East Asian Monsoon (EAM) and its influence on the balance between surface water contributions from the South China Sea and Western Pacific Warm Pool (WPWP). Within dating uncertainties the MIS3 Sulu Sea δ18O suborbital variability indicates that times of fresher surface conditions in the Sulu Sea coincide with similar conditions in the WPWP [Stott et al., 2002] and also with intensifications of the summer EAM as recorded in the U‐Th dated Chinese (Hulu Cave) speleothem δ18O record [Wang et al., 2001] and thus by inference with interstadials in the Greenland Ice core records. Combined, these results indicate that pronounced suborbital variability in the summer EAM and Intertropical Convergence Zone (ITCZ) during MIS3 was tightly coupled with climate conditions in the northern high latitudes.
Relationship between seismic P‐wave velocity and the composition of anhydrous igneous and meta‐igneous rocks
Authors: Behn M. D. , Kelemen P. B.
Published: May 2003

This study presents a new approach to quantitatively assess the relationship between the composition and seismic P‐wave velocity of anhydrous igneous and meta‐igneous rocks. We perform thermodynamic calculations of the equilibrating phase assemblages predicted for all igneous composition space at various pressure and temperature conditions. Seismic velocities for each assemblage are then estimated from mixing theory using laboratory measurements of the elastic parameters for pure mineral phases. The resultant velocities are used to derive a direct relationship between Vp and major element composition valid to ±0.13 km/s for pressure and temperature conditions along a normal crustal geotherm in the depth range of 5–50 km and equilibration pressures ≤12 kbar. Finally, we use the calculated velocities to invert for major element chemistry as a function of P‐wave velocity assuming only the in situ temperature and pressure conditions are known. Compiling typical velocity‐depth profiles for the middle and lower continental and oceanic crust, we calculate compositional bounds for each of these geologic environments. We find that the acceptable compositional range for the middle (15–30 km) and lower continental (≥35 km) crust is broad, ranging from basaltic to dacitic compositions, and conclude that P‐wave velocity measurements alone are insufficient to provide fundamental constraints on the composition of the middle and lower continental crust. However, because major oxides are correlated in igneous rocks, joint constraints on Vp and individual oxides can narrow the range of acceptable crustal compositions. In the case of the lower oceanic crust (≥2 km), observed velocities are 0.2–0.3 km/s lower than velocities calculated based on the average bulk composition of gabbros in drill cores and exposed ophiolite sequences. We attribute this discrepancy to a combination of residual porosity at crustal depths less than ∼10 km and hydrous alteration phases in the lower crust, and suggest caution when inferring mantle melting parameters from observed velocities in the lower oceanic crust.
Buoyancy of the continental upper mantle
Authors: Kelly Robyn K. , Kelemen Peter B. , Jull Matthew
Published: May 2003

The thermal boundary layer beneath continental cratons extends into the Earth's mantle to depths of at least 200 km. It has been proposed that chemical depletion of the lithospheric mantle during partial melting offsets the effect of increased density from conductive cooling, resulting in neutral buoyancy with respect to the underlying asthenosphere. Mineral compositions of garnet peridotite xenoliths in the Kaapvaal craton give equilibration temperatures and pressures that define a continental conductive geotherm intersecting a mantle adiabat with a potential temperature of 1300°C at ∼60 kbar. We calculated normative densities for a “low‐temperature” garnet and spinel peridotite xenolith suite using Mg#. At their temperatures and pressures of equilibration, all the low‐temperature peridotites are positively buoyant with respect to the convecting mantle, which is inconsistent with the hypothesis of a neutrally buoyant thermal boundary layer. To account for the possibility that pressure, temperature, and mineral proportions may have varied over time, equilibrium solidus mineral assemblages for the low‐temperature xenoliths over a range of pressures and temperatures were generated by free energy minimization using the program Perplex (http://www.perplex.ethz.ch). The equilibrium solidus densities for xenolith compositions along a 40 mW/m2 conductive geotherm were compared with the density of pyrolite along a mantle adiabat with a potential temperature of 1300°C. These density calculations show that most of the xenoliths are positively buoyant with respect to asthenospheric mantle (“pyrolite”) at their temperatures and pressures of equilibration, confirming the results from the normative density calculations. Also, at the onset of the accretion of the Kaapvaal craton, when the thermal boundary layer was thinner and hotter than today, these Kaapvaal peridotites would have been positively buoyant with respect to the convecting mantle at shallower pressures. Therefore we propose that peridotite at the base of the Kaapvaal “plate,” in the mantle thermal boundary layer, was and is positively buoyant. Combined with evidence from the geoid and geomorphology, which suggests that cratons are isostatically compensated and neutrally buoyant, our results imply that there must be dense layers within cratonic crust or upper mantle that offset the positive buoyancy of depleted cratonic mantle peridotites.
Aftermath of a snowball Earth
Authors: Higgins John A. , Schrag Daniel P.
Published: May 2003

Using a simple 3‐box model of the ocean‐atmosphere system, we simulate the cycling of carbon and strontium in the aftermath of a global glaciation. Model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pCO2 conditions as well as ocean mixing, air‐sea gas exchange, and biological productivity. The δ13C of the first carbonate precipitated after the glaciation depends on the pCO2, temperature, the saturation state of the surface ocean, and kinetic effects associated with mineral precipitation. With no biological productivity, the model produces δ13C values between +1‰ and −3‰, consistent with observations. This is in direct contradiction with arguments by Kennedy et al. [2001a], who suggest that the δ13C value of dissolved carbon in a snowball ocean (and directly afterward) must be −5‰. Kennedy et al. assume the carbon isotope cycle is in steady state, which does not apply to a global glaciation, and also neglect any effect of high pCO2 on the carbonate chemistry of seawater. A major difference between our findings and the qualitative predictions of Hoffman et al. [1998] is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves. As a result, the ∼2‰ drop in the δ13C observed in the cap dolostone is unlikely to be the product of Rayleigh distillation of atmospheric CO2 via silicate weathering. Instead, we interpret the ∼2‰ drop in the δ13C values as indicative of an increase in sea surface temperature which lowers the fractionation between CO2 and carbonate. Kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important. Rayleigh distillation of atmospheric CO2 via silicate weathering is a viable explanation for the continued drop in the δ13C values in the limestone sequence above the cap dolostone, with biological productivity and carbonate weathering driving a slow increase in δ13C values once pCO2 levels decline. Our study also simulates the cycling of strontium in seawater. In contrast to the finding of Jacobsen and Kaufman [1999] and Kennedy et al. [2001a], model simulations show a drop in 87Sr/86Sr of less than 0.001 during 5 million years of global glaciation and an increase of less than 0.001 over the entire episode of silicate weathering. Our calculations emphasize the importance of considering the changes in seawater chemistry due to high pCO2 in evaluating the Snowball Earth hypothesis.
Low‐temperature alteration of mesozoic oceanic crust, Ocean Drilling Program Leg 185
Authors: Talbi El Hassan , Honnorez José
Published: May 2003

Basalts drilled from the Mesozoic crust at Ocean Drilling Program (ODP) Sites 801 and 1149 during Leg 185 have been affected by a succession of submarine, variably oxidative alteration processes by cold seawater. The low‐temperature alteration resulted in various combinations of secondary minerals both as replacements of igneous phases, and as void and crack fillings. Mg‐smectite, calcite, celadonite and Fe‐oxyhydroxide are the dominant secondary minerals. Quartz, chalcedony and phillipsite are always very minor phases that fill open spaces. The rocks generally exhibit concentric haloes, 0.5 to 1 cm in thickness, around gray inner cores and usually adjacent to exposed surfaces. Color zonations result from the abundance of various secondary mineral assemblages present in each zone. The green rocks are dominated by celadonitic phyllosilicates, whereas the dusky red haloes are characterized by abundant iddingsite and Fe‐oxyhydroxides replacing the fine clinopyroxene. Finally Mg‐smectite ± calcite, are the most common secondary phases in the inner gray cores. The most unusual effects of the low‐temperature alteration of the study rocks are the pervasive, pale green to gray green color and the relatively abundant dusky red haloes surrounding inner reddish gray cores. Alteration of Mesozoic crust cored during Leg 185 is comparable with low‐temperature alteration of 310 m thick uppermost volcanic section of the 5.9 My old crust of Hole 504B. Except the renewed precipitation of calcite in veins, the aging of oceanic crust does not depend on time probably because it occurs during the first few millions of years after the accretion, and ceases when the crust is sealed off.
Magmato‐tectonic cyclicity at the ultra‐slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern
Authors: Mendel V. , Sauter D. , Rommevaux‐Jestin C. , Patriat P. , Lefebvre F. , Parson L. M.
Published: May 2003

On‐axis deep tow side scan sonar data are used together with off‐axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra‐slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off‐axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato‐tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato‐tectonic cycle is a pseudoperiodic process that lasts ∼0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long‐lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.
Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B – Implications for seawater‐crust exchange budgets and Sr‐ and Pb‐isotopic evolution of the mantle
Authors: Bach Wolfgang , Peucker‐Ehrenbrink Bernhard , Hart Stanley R. , Blusztajn Jerzy S.
Published: May 2003

This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater‐ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open‐ocean sites 417/418 likely reflects prolonged fluid‐rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1–2 Ga, if on the order of 80–90% Pb, 40–55% Rb, 40% Sr, and 35–40%U are removed during partial dehydration in subduction zones.
Optically Stimulated Luminescence dating supports central Arctic Ocean cm‐scale sedimentation rates
Authors: Jakobsson Martin , Backman Jan , Murray Andrew , Løvlie Reidar
Published: May 2003

This paper presents new results from Optically Stimulated Luminescence (OSL) dating on a sediment core raised from the crest of the Lomonosov Ridge in the central Arctic Ocean. There has been much debate about dating sediment cores from the central Arctic Ocean and by using an independent absolute dating technique we aim to test whether or not relatively fast, cm‐scale/ka, sedimentation rates were typical of Arctic's Pleistocene depositional mode. On the basis of mainly paleomagnetic reversal stratigraphy, many previous studies suggest mm‐scale/ka sedimentation rates. A common feature in these studies is that the first down core paleomagnetic negative inclination is consistently interpreted as the Brunhes/Matuyama boundary at about 780 ka. Our OSL dating results indicate that this assumption is not generally valid, and that the first encountered negative inclination represents younger age excursions within the Brunhes Chron, implying reinterpretation of many published core studies where paleoenvironmental reconstructions have been made for the central Arctic Ocean. Our dating results furthermore corroborates a correlation of the uppermost 2–3 m of the Lomonosov Ridge cores to a well‐dated core located off the Barents‐Kara Sea margin that in turn is correlated to cores in the Fram Strait. Valuable information on the paleoceanographical evolution in the Arctic Ocean from MIS 6 to the Holocene is given through this correlation of records from the central Arctic Ocean to records off the Eurasian continental margin.
Does depleted mantle form an intrinsic part of the Iceland plume?
Authors: Fitton J. Godfrey , Saunders Andrew D. , Kempton Pamela D. , Hardarson Björn S.
Published: May 2003

Icelandic basalt ranges in composition from voluminous tholeiite, erupted in the rift zones, to small‐volume, mildly alkaline basalt erupted off‐axis. In addition, small‐volume flows of primitive basalt, highly depleted in incompatible elements, are sometimes found in the actively spreading rift axes. Relative incompatible‐element depletion or enrichment in Icelandic basalt is correlated with variation in radiogenic isotope ratios, implying that the mantle beneath Iceland is heterogeneous and that the relative contribution of the various mantle components relates to eruption environment (on‐ or off‐axis) and hence to degree of melting. Thus small‐degree off‐axis melting preferentially samples an enriched and more fusible mantle component, whereas more extensive melting beneath the rift axes produces magma that more closely represents the bulk Iceland plume mantle composition. The small‐volume flows of depleted basalt represent melts that have preferentially sampled a depleted and more refractory mantle component. A debate has arisen over the nature of the depleted component in the Iceland plume. Some authors [e.g., Hanan and Schilling, 1997] argue that the depleted component is ambient upper mantle, the source of normal mid‐ocean ridge basalt (NMORB) in this region. Others [e.g., Thirlwall, 1995; Kerr et al., 1995; Fitton et al., 1997], however, have used various lines of evidence to suggest that the plume contains an intrinsic depleted component that is distinct from the NMORB source. Hanan et al. [2000] attempt to refute the existence of a depleted Iceland plume (DIP) component through a critical evaluation of the Nb‐Zr‐Y arguments advanced by Fitton et al. [1997] and the Hf‐Nd‐isotopic evidence presented by Kempton et al. [1998]. In this paper we examine the case presented by Hanan et al. [2000] and conclude that their arguments are flawed. Firstly, their trace‐element data set excludes data from depleted Icelandic basalt samples and so it is not surprising that they find no evidence for a DIP component. Secondly, they present two new Hf‐isotope analyses of a single depleted Icelandic basalt sample and show that the data plot in their NMORB field on an εHf versus εNd diagram. However, new data allow the resolution of distinct NMORB and depleted Icelandic basalt fields on this diagram. We conclude that trace‐element and radiogenic isotope data from Iceland require the existence of a DIP component.
Empirical partition coefficients for Sr and Ca in marine barite: Implications for reconstructing seawater Sr and Ca concentrations
Authors: Averyt Kristen B. , Paytan Adina
Published: May 2003

A robust record of fluctuations in seawater Sr and Ca concentrations is critical for understanding the long‐term global carbon cycle as it is influenced by the history and location of carbonate precipitation, chemical weathering, and hydrothermal activity. Such a record is also necessary for interpretation of paleoceanographic records (temperature, productivity) derived from carbonate sources (e.g. Sr/Ca, Mg/Ca, Li/Ca). Marine barite, an inorganic phase preserved in oxic, deep‐sea sediments, may record seawater Sr and Ca concentrations. Using core top barite samples we have derived the partition coefficients for Sr (DSr = 2.9 × 10−5), Ca (DCa = 1.9 × 10−8), and Sr/Ca (DSr/Ca = 1.6 × 103) in barite. The natural variability of core top marine barite Sr/Ca, Sr/Ba, and Ca/Ba ratios, selected from different ocean basins, is 10.1%, 15.0%, and 16.3%, respectively. Since estimates of Cenozoic fluctuations in seawater Sr/Ca ratios are large (possibly greater than 80%) relative to the variability recorded in core tops, marine barite may be used to reconstruct seawater Sr/Ca ratios, and Sr and Ca concentrations, using empirically derived partition coefficients.
The potential of ion microprobe analysis in detecting geochemical variations across individual foraminifera tests
Authors: Allison Nicola , Austin William E. N.
Published: May 2003

Trace element chemistry variations within individual foraminifera tests may provide valuable information on past ocean conditions. However, the combination of thin test walls and the presence of surface contaminants, not held within the carbonate structure, provide a stringent test to microanalytical methods. We analysed two tests of the benthic foraminifera Ammonia batavus by ion microprobe and compared two sets of analytical conditions. Using apertures to restrict the size of the analysed area (to ca. 10 μm diameter) reduced the effects of sample contamination. Sr concentrations were unaffected by any contamination encountered and Mg could be determined provided checks were made for silicate contamination using Al, Si and K intensities. Measured Sr concentrations (1.012–1.379 mmol mol−1) were in reasonable agreement with previously reported values. Mg concentrations (0.691–1.412 mmol mol−1) were significantly lower than values reported previously for calcitic benthic species. Significant variations in Sr concentrations occurred within individual test chambers and may reflect the juxtapositioning of calcite of different ages as the test is deposited. Significant variations in Mg and Sr concentrations also occurred between test chambers.
The upper end of the UK′37 temperature calibration revisited
Authors: Pelejero Carles , Calvo Eva
Published: May 2003

Being able to decipher paleo sea surface temperatures for warm oceanic waters is of the utmost importance to further evaluate the role of the tropics in global climate change. The UK′37 index is one of such paleothermometers, although the warm range of temperatures often poses serious analytical challenges. This paper discusses and reviews several important aspects of the UK′37 index calibration for waters warmer than 26°–27°C. Amongst them, insights into the possible existence of a limit above which the UK′37 index is not able to accurately record temperature variations are presented. Some warnings are also given on the misleading practice and expected consequences of reporting UK′37 index values of unity in the cases when abundances of the minor triunsaturated alkenone below detection limit are encountered. Future directions toward an improved knowledge of the behaviour of the UK′37 calibration for warm waters are suggested.
Tuff life: Bioalteration in volcaniclastic rocks from the Ontong Java Plateau
Authors: Banerjee Neil R. , Muehlenbachs Karlis
Published: May 2003

We report microscopic textural, geochemical, isotopic, and biomolecular evidence for microbial alteration of glass shards in a 337.7 m thick sequence of poorly sorted vitric and lithic tuffs recovered during Leg 192 of the Ocean Drilling Program on the Ontong Java Plateau. Petrographic analysis has revealed the highest density and variety of exceptionally preserved microbial alteration textures in the glass shards, when compared to previous studies of glassy pillow basalt margins from ocean crust and ophiolites. Two textural types of microbial alteration are commonly observed: tubular and granular. Tubular textures are characterized by well‐preserved, micron‐scale, tubular to vermicular, channel‐like features with both smooth and scalloped walls that commonly extend from a granular alteration interface rimmed by clay into unaltered glass. These channels are highly convoluted or twisted and in some instances bifurcate. Detailed scanning electron microscopy (SEM) images reveal the presence of delicate filaments and desiccated thin films with morphologies suggestive of a biogenic origin within the channels. Granular textures appear as solid bands, semicircles, or irregular patches of individual and/or coalesced spherical bodies with irregular protrusions into fresh glass. Microprobe X‐ray element maps show elevated levels of carbon, nitrogen, phosphorous, and potassium associated with the microbial alteration features. Bulk‐rock carbon isotope ratios of disseminated carbonates in tuffs preserving fresh glass are depleted (<−9‰), suggesting biologic fractionation. The presence of nucleic acids within the microbial alteration features has been confirmed though staining with ethidium bromide, a stain that specifically binds to double‐stranded DNA and RNA. The presence of DNA/RNA suggests that the biogenic features may be relatively recent and that microbes may be currently active.
Assessment of temperature gradients in multianvil assemblies using spinel layer growth kinetics
Authors: van Westrenen Wim , Van Orman James A. , Watson Heather , Fei Yingwei , Watson E. Bruce
Published: May 2003

We present an empirical equation to link the thickness of a MgAl2O4 spinel layer growing at the interface between MgO and Al2O3 multianvil high‐pressure assembly pieces to pressure (P), temperature (T), and time (t), extending the recent study of Watson et al. [2002] to the pressure range 6–16 GPa and temperature range 1673–2273 K. For the spinel thickness ΔX we obtain

urn:x-wiley:15252027:media:ggge349:ggge349-math-0001
This equation can be used to assess the thermal gradient in any multianvil assembly where MgO and Al2O3 filler pieces are in contact at P‐T conditions within the MgAl2O4 spinel stability field. As an illustration, we show that the central hot spot, in which temperatures can be considered constant, is close to 1 mm long in a common octahedral multianvil assembly with 8 mm edge length and rhenium furnace, while it extends to 3 mm long in an octahedron with 18 mm edge length using a straight graphite heating element. In addition, we present the results of a spinel growth experiment performed at 2273 K and 15 GPa with 18O enriched MgO, which shows that oxygen is mobile at a length scale exceeding that of the spinel layer. This finding raises the possibility that under some circumstances growth of spinel (and of reaction‐product layers in other oxide systems) might be accomplished by concurrent fluxes of oxygen and cations. This “mobile oxygen” model differs from the more conventional Mg‐Al exchange model proposed by Watson and Price [2002] for the lower‐pressure experiments and might explain the observed differences in systematics between the high‐ and low‐pressure data sets.
South East tropical Atlantic warm events and southern African rainfall
Authors: Rouault Mathieu , Florenchie Pierre , Fauchereau Nicolas , Reason Chris J. C.
Published: May 2003

Intrusions of warm equatorial water in the South East Atlantic Ocean off Angola and Namibia may be linked with above average rainfall along the coast of those countries but sometimes also with inland areas of southern Africa e.g. Zambia. During the 1984, 1986, 1995 and 2001 warm events, above average rainfall occurred near the sea surface temperature (SST) anomalies and extended inland from the coast to an extent that appeared to depend on the intensity of the regional moisture convergence and atmospheric circulation anomalies. Rainfall over western Angola/Namibia is greatest for those events for which the local circulation anomalies act to strengthen the climatological westwards flux of Indian Ocean sourced moisture across low latitude southern Africa and which flow anticyclonically over the warmest SST off the coast thereby weakening the mean southeasterly moisture flux away from Africa over the SE Atlantic. The significance of the warm events occurring during the February to April period is that this is the time when SST reaches its maximum in the annual cycle (up to 28°C off northern Angola) and this favours more intense local evaporation and convection and a greater impact on late austral summer rainfall. Better understanding of these warm events is necessary for assessing impacts on regional rainfall, agriculture and fisheries and for improving seasonal forecasting in this region.
Modeling CFC inventories and formation rates of Labrador Sea Water
Authors: Böning C. W. , Rhein M. , Dengg J. , Dorow C.
Published: May 2003

A high‐resolution model of the North Atlantic Ocean is used to examine the potential of chlorofluorocarbon (CFC) inventories for calculating the rate of Labrador Sea Water (LSW) formation. While the simulated CFC‐11 inventory and its geographical distribution in 1997 is fairly similar to observations, the model indicates pronounced variations in the history of CFC uptake, reflecting pulsations in LSW renewal in response to changes in wintertime atmospheric conditions. The LSW formation rate based on the volume of newly homogenized water during a winter season varies between 0 Sv and 11 Sv, and it is correlated (with a lag of 1 year) with the North Atlantic Oscillation (NAO) Index. The CFC‐based estimate of the mean LSW formation rate is 3.5–4.4 Sv, approximately representing the mean volumetric formation rate (4.3 Sv) for the period 1970–1997.
Coherent isotope history of Andean ice cores over the last century
Authors: Hoffmann G. , Ramirez E. , Taupin J. D. , Francou B. , Ribstein P. , Delmas R. , Dürr H. , Gallaire R. , Simões J. , Schotterer U. , Stievenard M. , Werner M.
Published: May 2003

Isotope records from Andean ice cores provide detailed and high‐resolution climate information on various time scales. However, the relationship between these valuable isotope records and local or regional climate remains poorly understood. Here we present results from two new drillings in Bolivia, from the Illimani and the Sajama ice caps. All four high altitude isotope signals in the Andes now available (Huascarán, Quelccaya, Illimani and Sajama) show near identical decadal variability in the 20th century. Comparison with general circulation model results and meteorological data suggest that the Andean high altitude records are primarily controlled by precipitation variability over the Amazon basin.
Hydrologic properties and structure of the Mozumi Fault, central Japan
Authors: Forster Craig B. , Evans James P. , Tanaka Hidemi , Jeffreys Ronald , Nohara Tsuyoshi
Published: May 2003

Analyses of rocks from the Active Fault Survey Tunnel (AFST) provides insight into the structure and hydrogeology of the northeast‐trending Mozumi‐Sukenobu fault, an active strike‐slip fault with 125 to 500 m of right‐lateral slip in central Japan. Interlayered regions of sub‐vertical permeability zones formed by cataclasis and slip on clay‐rich foliated zones. Core samples range from 10−19 m2 to almost 10−13 m2. CFC analyses of waters from the fault zone show that water entering the tunnel is 27–36 years old, yielding a bulk fault permeability of 10−14 to 10−15 m2. The data support a fault zone model of a fluid‐saturated, low‐velocity zone with a sharp velocity contrasting with the protolith. Pore‐fluid pressures could build and dissipate in isolated high permeability pods 10's–100's m in dimension.
An inverse estimate of the dynamic topography of the ocean
Authors: LeGrand Pascal , Schrama Ernst J. O , Tournadre Jean
Published: May 2003

The mean dynamic topography of the surface of the ocean is estimated using a finite difference inverse model of the ocean circulation constrained by a prior TOPEX/Poseidon‐EGM96 estimate and an ocean density climatology. The EGM96 geoid uncertainties are represented by their full covariance matrix. The model finds a solution that yields a new 1°‐resolution dynamic topography estimate with a 5 cm precision. The existence of this solution suggests that the uncertainties in EGM96, despite being much smaller than in previous geopotential models, are nonetheless realistic. The geodetic information constrains the inverse estimate of the ocean circulation, particularly at high latitudes.
Recurrence interval of big earthquakes along the Atotsugawa fault system, central Japan: Results of seismo‐geological survey
Authors: Takeuchi Akira , Ongirad Hasbaator , Akimitsu Takebe
Published: May 2003

The Atotsugawa fault system (AFS), characterized by high seismicity of micro‐earthquakes, is an active zone of right‐lateral strike‐slip faults in central Japan. We carried out seismo‐geological studies of three sites on the system. One is in the middle of the master Atotsugawa fault (AGF), one is at the end, and another is on a branching Mozumi‐Sukenobe fault (MSF). Trenching surveys demonstrated a maximum recurrence interval of 3,000 years, which is similar to that of the central AGF (2,500 years). This might indicate a uniform behavior in the seismogenic zone, despite the apparent segmentation in the shallower AGF. Another geo‐slicing survey at MSF depicted a scarce interval of 13,500 years, about six times longer than that of the master fault. Two simultaneous events on both AGF and MSF occurred in 1858 A.D. and at about 17,300 years ago, suggesting an intermittent cooperation between the master and branch faults of AFS.
Dense GPS Array observations across the Atotsugawa fault, central Japan
Authors: Hirahara Kazuro , Ooi Yoichi , Ando Masataka , Hoso Yoshinobu , Wada Yasuo , Ohkura Takahiro
Published: May 2003

A cross‐fault GPS array is established to reveal the crustal motion around the Atotsugawa fault located in a large strain rate zone, in central Japan. The array has 7 sites along a baseline of 30 km. Four‐year observations delineate a characteristic rate field; the eastward and the westward motions of 5 mm/yr at the sites 25 km apart from the fault gradually decrease and the directions of the motion change towards the ENE‐WSW fault strike, as approaching the fault. These observations are explained by a simple model; two 15‐km thick elastic blocks are obliquely colliding along the Atotsugawa fault with a rate of 20 mm/yr in an EW direction. However, in the zone sandwiched by the Atotsugawa and the Ushikubi faults, the rates are small and the rate vectors have the fault‐normal components, suggesting that the collision boundary forms a zone with lower elasticity or inelastic property.
Middle ultraviolet emission from ionized iron
Authors: Dymond K. F. , Wolfram K. D. , Budzien S. A. , Nicholas A. C. , McCoy R. P. , Thomas R. J.
Published: May 2003

We present middle ultraviolet spectra between 2300 and 2700 Å showing emissions due to singly ionized iron and magnesium. The spectra were observed by the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) instrument on the United States Air Force's Advanced Research and Global Observing Satellite (ARGOS). The emission from ionized magnesium has been previously observed and studied; however, the ionized iron lines near 2400 and 2600 Å are newly identified in the Earth's middle ultraviolet dayglow spectrum. Newly calculated fluorescence efficiencies for excitation of the optical emissions are also presented.
Modes of tropical Atlantic climate variability observed by PIRATA
Authors: Servain Jacques , Clauzet Gabriel , Wainer Ilana C.
Published: May 2003

The geography of the Pilot moored Research Array in the Tropical Atlantic (PIRATA) was especially designed to observe the two main modes of climate variability in the tropical Atlantic: the equatorial mode (equivalent to the warm events in the Pacific Ocean), and the meridional mode, also known as the dipole mode. This paper is an attempt, using the first available PIRATA data, to evaluate the presence of the two modes, as well as their relationship with the current array configuration.
On the origin of inner‐source pickup ions
Authors: Wimmer‐Schweingruber Robert F. , Bochsler Peter
Published: May 2003

In situ measurements of pickup ions (PUI) exhibit a component that has nearly thermalized with the solar wind. This implies an origin close to the Sun and is generally ascribed to interaction of the solar wind with interplanetary dust particles (IDPs). We propose a scenario for the origin of inner‐source PUIs in which a population of very small IDPs serves as the neutralizing agent for solar wind ions. The size of these IDPs is less than or comparable to the penetration range, of solar wind ions in IDP material. The interaction of the solar wind with such particles results in a net charge exchange in which solar wind ions exit the IDPs as predominantly neutral or singly‐charged ions. When the neutralized solar wind is reionized, it is picked up and can then be measured as a PUI.
Long‐term hydrographic variability in the Northwest Pacific Ocean
Authors: Joyce Terrence M. , Dunworth‐Baker Jane
Published: May 2003

Hydrographic changes in the Northwest Pacific Ocean are examined using data for two time periods: 1945: 1975 and 1976:1998. The largest changes in T/S (2°C,0.2 psu) are within the interfrontal zone between the Kuroshio Extension (KE) and the subarctic (or Oyashio) front near the Shatsky Rise, and are consistent with a southward shift of the Kuroshio Bifurcation Front (KBF). Other major changes seen are a freshening of ∼0.04 psu of the newly formed North Pacific Intermediate Water (NPIW); a shoaling over time of the halocline in the center of the Western Subarctic Gyre (WSAG); and a southward shift of the subarctic front between the dateline and 150°W. Because long‐term T/S changes near the Shatsky Rise are well‐correlated, we have examined subsurface thermal data at 100 and 200‐m depth in the region and found that shifts in front locations show a high degree of correlation with the PDO index on an interannual basis, and lagging it by ∼1 year at 200‐m depth.
Are the effects of large scale flow conditions really lost through the turbulent cascade?
Authors: Katul Gabriel G. , Angelini Claudia , De Canditiis Daniela , Amato Umberto , Vidakovic Brani , Albertson John D.
Published: May 2003

The conceptual framework for modeling the inertial subrange is strongly influenced by the Kolmogorov cascade phenomena, which is nowadays the subject of significant reinterpretation. It has been argued that the effects of boundary conditions influence large‐scale motion and direct interaction between large and small scales is possible by means other than passing sequentially through the full cascade. Using longitudinal (u) and vertical (w) velocity and temperature (T) time series measurements collected in the atmospheric surface layer (ASL), we evaluate whether the inertial subrange multifractral function (f(α)) of all three flow variables is influenced by atmospheric stability (ξ), which is a bulk measure of the effect of boundary conditions on large scale flow properties for ASL turbulence. This study is the first to demonstrate that ξ significantly influences f(α) for all three flow variables. Here, statistical significance is evaluated using a novel wavelet‐based Functional Analysis of Variance (FANOVA) approach that explicitly considers different classes of ξ, the flow variable type, and possible interactions between ξ and the three flow variables.
Tropical Atlantic and rainfall variability in Côte d'Ivoire
Authors: Kouadio Yves K. , Ochou Delfin A. , Servain Jacques
Published: May 2003

A statistical analysis is performed between the rainfall anomalies in Côte d'Ivoire (West Africa) and those of sea surface temperature and pseudo‐wind stress in the tropical Atlantic. The correlation structures show that the coastal and northern regions of Côte d'Ivoire are differently related to the meteo‐oceanic variables. While the main rainy seasons of these two Ivorian regions are both related to a negative SST/rain correlation pattern, itself strongly linked to the abnormal position of the intertropical convergence zone, the dry season of the littoral zone is related to a positive SST/rain correlation pattern which remains offshore of Côte d'Ivoire. Our study illustrates the inverse climatic behaviour observed between the northern and the littoral zones during the two “Atlantic's El Niños” of 1968 and 1984.
Earth tide forcing of glacier drainage
Authors: Kulessa Bernd , Hubbard Bryn , Brown Giles H. , Becker Julia
Published: May 2003

Fourier analysis reveals that winter electrical self potential (SP), water pressure (PW), and electrical conductivity (EC) time series collected beneath Haut Glacier d'Arolla, Switzerland, are forced by earth and atmospheric tides. Forcing is dominant during periods of expanding bedrock, consistent with glacier substrate deformation periodically driving water from the ice body into the bed. This may modify the strength of subglacial sediments, and could therefore influence glacier drainage and motion on a global scale.
Triggered fault slip on June 17, 2000 on the Reykjanes Peninsula, SW‐Iceland captured by radar interferometry
Authors: Pagli Carolina , Pedersen Rikke , Sigmundsson Freysteinn , Feigl Kurt L.
Published: May 2003

Dynamically triggered seismicity followed shortly after a Ms 6.6 earthquake in Iceland on June 17, 2000. Smaller earthquakes occurred on the Reykjanes Peninsula up to 100 km from the mainshock rupture. Using interferometric analysis of Synthetic Aperture Radar images (InSAR), we measure crustal deformation associated with three triggered deformation events. The largest of these occurred at Lake Kleifarvatn, 85 km west of the mainshock epicenter. Modeling of the InSAR data reveals strikeslip on a north‐striking fault, with a geodetic moment of 6.2 × 1017 Nm, equivalent to magnitude Mw 5.8 earthquake. A seismological estimate of the moment is not yet available, because the seismic signature of this event is partly hidden by the mainshock waveform. The paucity of aftershocks on the triggered rupture plane suggests some aseismic slip there, compatible with a thin seismogenic crust, high heat‐flow, hydrothermal alteration and the presence of fluids in the area.
How do high‐latitude North Atlantic climate signals the crossover between the Deep Western Boundary Current and the Gulf Stream?
Authors: Yang Jiayan , Joyce Terrence M.
Published: May 2003

A simple model is used to examine how the presence of a continental slope along the western boundary, and wind‐driven gyres, especially the Gulf Stream (GS) jet, affect the pathways of water‐mass flow and wave propagation in the abyssal ocean. Two types of forcing: an interior source and a boundary inflow (to mimic the Labrador Sea Water and the marginal‐sea overflow), are considered here. Both topography and wind‐driven gyres considerably alter how the water mass spreads from the source (in the steady state case) and how transient signals propagate to the unforced regions. Sloping bathymetry near the western boundary allows for meridional propagation of topographic waves. The GS jet creates a strong meridional PV gradient which could partially block the southward water‐mass transport and wave propagation. The effectiveness of the crossover to disrupt the lower layer response depends on the type of high latitude water mass forcing.
Evidence for fault lubrication during the 1999 Chi‐Chi, Taiwan, earthquake (Mw7.6)
Authors: Ma Kuo‐Fong , Brodsky Emily E. , Mori Jim , Ji Chen , Song Teh‐Ru A. , Kanamori Hiroo
Published: May 2003

The ground motion data of the 1999 Chi‐Chi, Taiwan, earthquake exhibit a striking difference in frequency content between the north and south portions of the rupture zone. In the north, the ground motion is dominated by large low‐frequency displacements with relatively small high‐frequency accelerations. The pattern is opposite in the south, with smaller displacements and larger accelerations. We analyze the fault dynamics in light of a fault lubrication mechanism using near‐field seismograms and a detailed rupture model. The fault zone contains viscous material (e.g., gouge), in which pressure increases following the Reynolds lubrication equation. When the displacement exceeds a threshold, lubrication pressure becomes high enough to widen the gap, thereby reducing the area of asperity contact. With less asperity contact, the fault slips more smoothly, suppressing high‐frequency radiation.