bluuebunny/crossref_metadata_2025 huggingface dataset:
Late Cenozoic basaltic magmatism in the Western Great Basin, California and Nevada
Authors: Rogers N. W. , Hawkesworth C. J. , Ormerod D. S.
Published: February 2004

Alkali‐rich basaltic rocks have been erupted in small volumes throughout the Western Great Basin (WGB) since 17 Ma. SiO2 ranges from 46 to 53% in samples with >4% MgO, whereas Fe2O3 ranges from 8 to 10% and TiO2 from 1 to 2% in the most magnesian rocks. They have high incompatible element contents, except for relatively low high field strength elements abundances, and thus their minor and trace element patterns closely resemble subduction‐related rather than intraplate magmas. Their radiogenic isotope ratios are also enriched with 87Sr/86Sr = 0.7040–0.7078, 143Nd/144Nd = 0.5129–0.5120 and variable Pb isotopes (206Pb/204Pb = 18.0–19.2), generally displaced above the Northern Hemisphere Reference Line in both the 207Pb/204Pb and 208Pb/204Pb diagrams. The WGB straddles the isotopic and tectonic boundary between continental North America, underlain by Proterozoic basement, and the younger accreted terrain that forms the basement to most of California. There are no discernible differences in the major and trace element contents of the basalts either side of this boundary except that those erupted through Proterozoic basement tend to lower silica contents and have higher 87Sr/86Sr and lower 143Nd/144Nd ratios. Nd model ages and a secondary Pb isochron from basalts east of the boundary suggest an age of 1.6–1.8 Ga, significantly younger than the age of crust generation inferred from Nd model ages of granites. Correlated trace element and isotope ratios in basaltic rocks to the west of the boundary indicate a contribution from recent subduction, recognized by a decrease in 87Sr/86Sr as Nb/Sr and Zr/Sr decrease. A second, less well defined trend to higher Nb/Sr and Zr/Sr and lower 87Sr/86Sr is interpreted to reflect a minor asthenospheric contribution to contemporary magmatism. Relative to basalts from the Basin and Range, the basaltic rocks from the WGB have higher SiO2 and lower TiO2 and Fe2O3 at similar degrees of fractionation. These differences are interpreted to reflect different pressures of segregation from their mantle source regions, the WGB basalts being derived from shallower depths than the Basin and Range basalts, within the lithospheric mantle. This conclusion is consistent with their lithospheric trace element and isotope characteristics and trace element models that allow minimal garnet in the source region. By contrast the lower silica contents of the Basin and Range alkali basalts are more consistent with derivation from depths of 90 km or more, which may place their source regions in the asthenosphere, also consistent with their trace element and isotope variations.
Near‐axis seamounts on the flanks of the East Pacific Rise, 8°N to 17°N
Authors: Scheirer Daniel S. , Macdonald Ken C.
Published: February 2004

With complete SeaBeam and SeaMARCII bathymetry and nearly complete SeaMARCII side scan coverage of the northern East Pacific Rise and its flanks from 8° to 17°N, we characterize the entire population of seamounts on young seafloor (0 to ∼2 Ma) formed along this fast spreading center. There are 179 seamounts in this area taller than 200 m, most of which belong to one of 21 seamount chains. These chains are oriented between the relative and absolute plate motion directions of the Pacific and Cocos Plates, and most of the larger chains are more voluminous near their centers than near their ends. Several of the seamount chains contain bends in orientation which are not coincident with each other. The two largest seamount chains, at 9°55′N (Lamont Seamounts) and 15°40′N, are aligned with portions of the East Pacific Rise crest which are among the shallowest and broadest of this area. Overall, the maximum abundance and size of seamounts increase with shallower and broader ridge axes. No near‐axis seamounts form on the summit of the axial ridge; nonetheless, within 15 km (∼0.3 Ma) of the axis, the abundance of seamounts achieves its overall average. Observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near‐axis seamounts are formed in a narrow zone on crust aged ∼0.1–0.3 Ma (5–15 km from the axis), with significant growth in a zone several times wider. Side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model. Two exceptions to this model, however, are seamount chains at 15°40′N and 8°25′N which show evidence for activity over a broader region; this may be due to the presence of fracture zones within about 30 km of these chains. A compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge. The variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results, although these influences cannot be distinguished in this analysis.
Petrogenesis of basalts from the Blanco Trough, northeast Pacific: Inferences for off‐axis melt generation
Authors: Gaetani Glenn A. , DeLong Stephen E. , Wark David A.
Published: February 2004

Basaltic lavas recovered in a single dredge haul from the Blanco Trough, in the western portion of the Blanco Fracture Zone, demonstrate that off‐axis volcanism can produce significant compositional variability in lavas that are closely related spatially. Glass compositions range from depleted in incompatible elements (0.07 wt % K2O; 35 ppm Zr) to strongly enriched (0.43 wt % K2O; 214 ppm Zr) with SiO2 contents from 48. 1 to 50.9 wt % over a limited range of MgO (9.22 to 6.82 wt %). These basalts are distinct from mid‐ocean ridge basalts sampled globally at spreading centers in that they are characterized by high FeO* and Na2O at a given MgO content and low normative SiO2, a compositional signature consistent with relatively low extents of partial melting at high pressures. This results from the presence of thick lithosphere that forces melting to stop at a significantly greater depth than is the case at ridge axes. Forward models of low‐pressure liquid lines of descent demonstrate that the major and trace element variations observed in these lavas cannot be explained by crystal fractionation from a single parent magma. After correcting the glass compositions to 8 wt % MgO to eliminate the effects of low‐pressure crystallization, the Fe8.0‐Na8.0 and Si8.0‐Na8.0 variations display characteristics of both Atlantic‐ and Pacific‐type local trends. Variable extents of partial melting and interaction with the mantle as the magmas ascended through the lithosphere can explain these systematics. A subset of the samples shows evidence for crystallization of olivine + plagioclase + high‐Ca clinopyroxene at upper mantle pressures (∼400 MPa). Modeling of incompatible element variations suggests that two distinct mantle sources were involved in producing the Blanco Trough lavas. The first resembles the primitive upper mantle of Hart and Zindler (1986) with a small initial depletion (∼0.1 wt % melt extraction). The second is characterized by a significant depletion in incompatible elements (K2O, TiO2, Zr, Sr).
A history of the Easter microplate, 5.25 Ma to present
Authors: Rusby Ruth I. , Searle Roger C.
Published: February 2004

GLORIA and other geophysical data are used to interpret the morphology and tectonic pattern of the Easter microplate. We model magnetic anomaly data and calculate spreading rates for use in Euler pole calculations. We calculate three‐plate closure and best fit poles for Nazca‐Pacific, Nazca‐Easter, and Pacific‐Easter plate motions and compare the results with earthquake focal mechanisms and seismicity. We use the three‐plate closure poles to rotate digital GLORIA and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate's East Rift first started to propagate northward at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca‐Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At some time after 2.5 Ma the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific‐Easter transform. A convergent northern boundary formed involving southward thrusting of the Nazca plate over the microplate. No further Nazca plate lithosphere was transferred from north of the microplate to the microplate interior, and the Pito Deep region opened by pervasively rifting apart preexisting Nazca plate lithosphere.
Scaling invariance of crack surfaces
Authors: Schmittbuhl Jean , Schmitt François , Scholz Christopher
Published: February 2004

The morphology of fractured rock surfaces is studied in terms of their scaling invariance. Fresh brittle fractures of granite and gneiss were sampled with a mechanical laboratory profilometer, and (1 + 1)‐dimensional parallel profiles were added to build actual maps of the surfaces. A first step in the scaling invariance description is a self‐affine analysis using three independent methods. The root‐mean‐square and the maximum‐minimum difference of the height are shown to follow a power law with the sample length. The return probability and the Fourier spectrum are also computed. All these approaches converge to a unique self‐affine exponent: ζ = 0.80. Analysis over a broad statistical set provides a reproducibility error of ±0.05. No significant differences between the isotropic granite and the markedly anisotropic gneiss appear for the scaling exponents. An analysis of the profilometer shows that the two main drawbacks of the setup are not significant in these analyses. The systematic errors of the scaling analysis are estimated for the different methods. Isotropy of the scaling invariance within the mean fracture plane is shown either with the result obtained from different fracture orientations or with the two‐dimensional Fourier spectrum of the surface topography itself. The analysis is brought further into the multifractal framework. The structure functions are shown to have power law behavior, and their scaling exponent varies nonlinearly with the moment order. Finally, the corresponding conserved process belongs to a universal multifractal class with α = 1.5 for the Levy index and C1 = 0.3 for the fractal codimension of the mean singularities. The three indices (ζ, α and C1) completely characterize the scale invariance. The multifractal behavior is significant for physical properties which depend on high‐order moments like contact. According to this study and that of other groups, the self‐affine exponent ζ is constant over a large range of scales and for different fracture modes and various materials. This opens the possibility that there exists a form of universality in the cracking process. It appears that only the prefactor of the roughness is dependent on material and mode.
Why there are no earthquakes on the Marquesas Fracture Zone
Authors: Jordahl Kelsey A. , McNutt Marcia K. , Webb Helen F. , Kruse Sarah E. , Kuykendall Martha G.
Published: February 2004

Although the plate tectonic paradigm does not predict relative horizontal motion between lithosphere on opposing sides of a fracture zone, the fact that younger, more rapidly subsiding seafloor lies adjacent to older seafloor implies relative vertical motion. The observation that fracture zones are notably aseismic has led to the proposition of high strength along fracture zones, such that the differential subsidence is accommodated by flexure across a locked fault. This model predicts that a ridge develops on the young side of the fracture zone flanked by a foredeep trough on the old side, with parallel warping of the Moho and large associated gravity anomalies. Previous analyses of satellite altimetric passes over Pacific fracture zones have shown that the amplitude and shape of the gravity anomalies frequently do not conform to the predictions of this simple model. One of the most notable departures has been the Marquesas Fracture Zone (MFZ), where only one limited section was determined to be “high strength.” Curiously, the only earthquake to rupture the MFZ in the last 35 years is located in this locked region. Elsewhere, where we would predict vertical slip must be occurring, the fault is aseismic. To better understand the history of vertical motion on this fault, we have analyzed geophysical data obtained during the recent survey EW9106 aboard the R/V Maurice Ewing. Our detailed Hydrosweep, gravity, and seismic data resolve this paradox by showing no evidence for vertical slip along the fault. Shear stresses caused by differential thermal subsidence do not exceed the strength of this fault. Rather, the failure of the altimetry signal to conform to the predictions of the high‐strength model along much of the MFZ is caused by changes in the Pacific‐Farallon pole of rotation. Reorientation of the plate boundary was accommodated by propagating rifts, intratransform tension or compression, and changes in transform offset that complicated the signal from differential subsidence across a locked fault. For example, a counterclockwise rotation of the transform in the Cretaceous caused overthrusting in the transform and thus compensation of the depth differential by flexural loading of a very young plate. A clockwise rotation of this right‐stepping transform fault at about 35 Ma led temporarily to a crenulated plate boundary and later to the development of intra‐transform spreading centers. After accounting for these complications, the fracture zone appears capable of sustaining at least 20 MPa of shear stress and remains locked along the entire length of the fault except perhaps locally where it passed over the Tuamotu and Society hot spots.
Ultradeep metamorphic rocks: The retrospective viewpoint
Authors: Schreyer Werner
Published: February 2004

Ultradeep, or ultra‐high‐pressure (UHP), metamorphic rocks, formed from crustal protoliths within the stability field of coesite at pressures >2.5–3.0 GPa corresponding to depths >80–120 km, occur locally though regionally distributed in at least five continental areas. Their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high‐pressure studies. Detailed petrographic and microprobe work, especially on mineral inclusions, in favorable cases allows the derivation of prograde PT paths during subduction and of retrograde ones during exhumation. Commonly, the gneisses adjacent to the UHP rocks do not exhibit signs of ultradeep metamorphism, apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower PT conditions during the retrograde path. It is also possible, however, that UHP equilibria were not attained throughout the rock volumes subducted, but only along zones of shearing and fluid introduction. If it is true that not all UHP metamorphic rocks return to the crustal orogenic belts, but some continue to be subducted to greater mantle depths, the classical geochemical pattern of a one‐way mass transfer from mantle to crust throughout the Earth's history is at stake. The assumed gradual growth of continents may have had a counterpart of continent destruction during collision events. Most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous (Mg)A1‐silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subduction zones, thus extending the water retentivity of subducting slabs to greater, and hitherto unexpected, depths.
The March 25, 1990 (Mw = 7.0, ML = 6.8), earthquake at the entrance of the Nicoya Gulf, Costa Rica: Its prior activity, foreshocks, aftershocks, and triggered seismicity
Authors: Protti Marino , McNally Karen , Pacheco Javier , González Victor , Montero Carlos , Segura Juan , Brenes Jorge , Barboza Vilma , Malavassi Eduardo , Güendel Federico , Simila Gerald , Rojas Daniel , Velasco Aaron , Mata Antonio , Schillinger Walter
Published: February 2004

On March 25, 1990 a large earthquake (Mw = 7.0, ML = 6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, producing considerable damage in central Costa Rica and generating much interest about whether or not the Nicoya seismic gap (Nishenko, 1989) had broken. The local country‐wide seismographic network recorded 6 years of activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by the Costa Rica Volcanological and Seismological Observatory at the National University (OVSICORI‐UNA), and the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz (CFRSL‐UCSC). We obtained high resolution locations from this network and located the mainshock at 9°38.5′N, 84°55.6′W (depth is 20.0 km) and the largest foreshock (Mw = 6.0, March 25, 1990, at 1316:05.8 UTC) at 9°36.4′N, 84°57.1′W (depth is 22.4 km). We find that the aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the installation of the local network in April 1984 to March 24, 1990, nearly 1900 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occurred at the northwest edge of this region, where a sequence of foreshocks began 16 hours prior to the mainshock. The spatial‐temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture. The mainshock ruptured an asperity of approximately 600 km2 of area, with this area expanding to 4000 km2 after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband and long‐period P and SH waves yields a thrust faulting mechanism with the shallow plane striking 292°, dipping 26°, and with a rake of 88°, in agreement with the subduction of the Cocos plate under the Caribbean plate. Local first motions for the largest foreshock and the mainshock agree with this solution. We also present evidence suggesting that the March 25, 1990, earthquake triggered and reactivated several seismic swarms in central Costa Rica and temporally decreased the activity in the epicentral area of the July 3, 1983 (Ms = 6.2), Pérez Zeledón earthquake.
Multistage magma mingling and the origin of flow banding in the Aliso lava dome, Tumacacori Mountains, southern Arizona
Authors: Seaman Sheila J. , Scherer Erik E. , Standish Jared J.
Published: February 2004

Flow banded rocks of the middle Tertiary Aliso lava dome, in the Tumacacori Mountains of southern Arizona, contain light‐colored dacitic flow bands, dark‐colored rhyolitic flow bands, and dark‐colored ellipsoidal enclaves of rhyolite. Groundmass glass of the light‐ and dark‐colored flow bands is trachyte and high‐silica rhyolite, respectively. Enclaves are fine grained, holocrystalline, and phenocryst‐poor. A two‐stage sequence of mixing and mingling of discrete magmas can account for the compositional and textural characteristics of the Aliso rocks: (1) enclave magma mixed with dark band magma and (2) the enclave/dark band magma composite mingled with alkalic trachyte magma, now represented by the light bands. Both mingling events may have happened in the magma chamber, or partially in the magma chamber and partially in the volcanic conduit. Deformation of the magmas and the development of flow bands are likely to have occurred in the conduit. During flow, the highly crystalline enclave magma was slightly stretched and deformed, contaminating the dark band magma. The dark band magma deformed into thin bands, decoupling in some instances from the rhyolitic enclaves which it hosted. Enclaves and phenocrysts from the dark band magma were locally captured by the light band magma. Calculations of viscosities of the three magmas across fanges of equilibration temperature, strain rate, water concentration, and crystallinity indicate that magma viscosities are more strongly affected by water concentration and crystal abundance than by temperature or strain rate.
Ridge offsets, normal faulting, and gravity anomalies of slow spreading ridges
Authors: Escartín Javier , Lin Jian
Published: February 2004

We develop a model relating mid‐ocean ridge normal faulting and crustal structure by examining recently available high‐resolution gravity and multibeam bathymetric data of the Mid‐Atlantic Ridge and other spreading centers. Results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major Atlantic offsets studied, especially along the inside corner side of the offsets. Individual residual gravity anomaly spikes (local peaks), which have amplitudes of up to 20 mGal and typical across‐axis spacing of 10–30 km, often coincide with individual major fault scarps, suggesting crust thinned by normal faulting. Theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive, ridge‐parallel low‐angle faults that originate episodically at inside corners of ridge offset intersections. Fault scarp heights and the amounts of crustal thinning (as inferred from gravity anomalies) are consistently larger at inside corners than at outside corners, supporting a model in which tectonic extension near ridge offsets is asymmetric with low‐angle faults occurring preferentially at inside corners. These results on spatial variations in seafloor morphology and gravity anomalies further support a three‐dimensional tectonic faulting model at oceanic spreading centers with three major characteristics: (1) Low‐angle faults form preferentially at inside corners, where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets; (2) low‐angle faults decrease in throw toward midpoints of long ridge segments, where large low‐angle faults may not be sustained by a weak lithosphere; and (3) the residual gravity peaks are statistically larger at transform than near nontransform offsets, indicating that the length of a ridge offset and its tectonic style control the development of low‐angle faults.
Discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions
Authors: Horiuchi Shigeki , Rocco Guillermo , Hasegawa Akira
Published: February 2004

It is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area. Considering earthquake‐generating stress to be uniform in a studied area, we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of P waves. First, the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution, with intervals of 10°. This calculation is made for all the events used. Then, four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum. A numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 × 25 readings. It is found that a stable solution of the stress tensor is determined by the inversion and about 60% of fault planes are distinguished from auxiliary planes. There are no events whose auxiliary planes are incorrectly determined as fault planes. This method is used to estimate the earthquake‐generating stress in the aftershock area of the 1984 western Nagano prefecture earthquake, central Japan. The results obtained show that the maximum principal stress is horizontal in the direction of N80°W‐S80°E. There are strike‐slip events having fault planes almost perpendicular to the fault plane of the main shock.
Experimental deformation of partially molten Westerly granite under fluid‐absent conditions, with implications for the extraction of granitic magmas
Authors: Rutter E. H. , Neumann D. H. K.
Published: February 2004

The mechanical behavior of partially molten Westerly granite was investigated in the temperature range 800°–1100°C, 250 MPa confining pressure, by means of constant strain rate, creep, and stress relaxation tests. The only water in the samples came from the breakdown of hydrous phases, biotite, minor chlorite and muscovite and alteration products of feldspars. Thus the amount of melt was controlled by the test temperature and ranged from 3% at 800°C to 50% at 1100°C. Over that temperature range, strength decreased from ≈500 MPa to less than 1 MPa, and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates. The comparative viscosity of the melt alone was estimated at 950° and 1000°C from the distance it could be made to penetrate into a porous sand under a known pressure gradient. Under all conditions, deformation of the matrix of solid grains was by brittle fracture only. Samples containing up to 10 vol % melt failed with the formation of a shear fault zone. At higher melt fractions, melt‐filled “pores” collapsed by shear‐enhanced compaction, squeezing the melt into axial cracks. Above 40 vol % melt, unfractured solid grains were carried about passively in the flowing liquid. There was no evidence of a “rheologically critical melt percentage” in this system. By analogy with the uniaxial compaction of water‐saturated soils, a simple model is erected to describe a two‐stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress. Shear‐enhanced compaction is inferred to drive melt into a network of melt‐filled veins, whereupon porous flow through the high‐permeability vein network allows rapid drainage of melt to higher crustal levels.
Footprints of storms on the sea: A view from spaceborne synthetic aperture radar
Authors: Atlas David
Published: February 2004

Synthetic aperture radar (SAR) on board Seasat observed images of stormlike echoes on the sea in 1978. The core of these images is usually an echo‐free hole which is attributed to the damping of the short (30‐cm) radar detectable gravity waves by the intense rain in the storm core. Although “the beating down of waves by rain” is consistent with observations by seafarers and with the first scientific explanation of the phenomenon by Reynolds (1875), neither theory nor experiment has provided definitive support. One experiment appears to provide the key; it shows that the kinetic energy of the rain produces sufficient turbulence in a thin fresh water layer to damp 30‐cm waves in 10–20 s, thus producing the echo‐free hole. A sequence of positive feedbacks then serves to damp the longer waves. The angular dependence of the sea surface echo cross sections seen by Seasat SAR outside the echo‐free hole indicates winds diverging from the downdraft induced by the intense rain core. The wind‐generated waves and associated echoes extend out to a sharply defined gust front. The sea surface footprint thus mimics the features of a storm microburst. The variations in surface radar cross section due to a combination of rain and wind effects impacts spaceborne measurements of surface winds by scatterometry and rainfall measurements by radar. Portions of this synthesis remain speculative but serve as hypotheses for further research.
PCO2, chemical properties, and estimated new production in the equatorial Pacific in January–March 1991
Authors: Lefèvre Nathalie , Andrié Chantal , Dandonneau Yves , Reverdin Gilles , Rodier Martine
Published: February 2004

Measurements of the partial pressure of CO2 (PCO2) at the sea surface, dichlorodifluoromethane (F12), salinity, temperature, oxygen, nutrients, wind, and current velocities were made during a cruise (January–March 1991) in the equatorial Pacific from Panama to Nouméa via Tahiti. In the western Pacific (140°W to 165°E) the westward South Equatorial Current is well established. Distributions of tracers show extrema near the equator in the eastern Pacific (from 95°W to 140°W), indicating that the upwelling is especially active in this area. The zonal distribution of chemical tracers is not regular because of intrusions of warmer water from the north associated with equatorial long waves. The temporal changes in PCO2 result from thermodynamic changes, biological activity, and gas exchange with the atmosphere. In order to compare the magnitude of these processes, we assess the variations of PCO2 (dPCO2) between two stations as the sum of thermodynamic changes driven by temperature and salinity changes, air‐sea exchange computed from observed wind and difference of PCO2 between the sea and the atmosphere, and the biological activity estimated from the nitrate decrease and C:N ratio (106:16). The resulting assessed change in PCO2 is in agreement with the observed change for 42 pairs of stations. Each of these pairs of stations is thus considered as representing a simple water mass advected by the measured currents between the two stations so that daily fluxes can be estimated. The contribution of CO2 outgassing to dPCO2 is low, between −0.2 to −0.0 μatm d−1. The thermodynamical dPCO2 averages 0.7±0.2 μatm d−1 in the mixed layer. The biological dPCO2 (‐1.5±0.5 μatm d−1) is the highest in absolute value implying an average value of new production along the equator of 72±25 mmolC m−2 d−1 (0.9±0.3 gC m−2 d−1) for the equatorial Pacific (130°W‐165°E). This value is very high and the overestimation could result from the simplistic description of the advection and mixing of water. An attempt to account for these processes by constraining the net heat flux to 100 W m−2 [Weare et al., 1981] reduces the estimate of new production to 58 mmolC m−2 d−1 (0.7 gC m−2 d−1 ). A mean upwelling velocity of 0.5±0.1 m d−1 east of 140°W is calculated, based on F12 undersaturations.
The Amazon River Plume during AMASSEDS: Subtidal current variability and the importance of wind forcing
Authors: Lentz Steven J.
Published: February 2004

Current variability over the north Brazilian shelf, particularly within the Amazon Plume, is characterized using observations from a moored array deployed about 300 km north of the Amazon River mouth from February to June 1990 as part of A Multidisciplinary Amazon Shelf SEDiment Study (AMASSEDS). The moored array consisted of a transect perpendicular to the coast with inner shelf (18 m depth) and midshelf (65 m) moorings and a third mooring near the shelf break, in 100 m of water. To our knowledge, the AMASSEDS moored array includes the first long‐term (2 months) moored observations made within the Amazon Plume. The current variability is dominated by two components, semidiurnal cross‐shelf currents with peak velocities of 50 to 200 cm/s and vertically sheared subtidal (timescales days to weeks) along‐shelf currents. This study focuses on the subtidal flow within the Amazon Plume which is strong and variable with along‐shelf currents ranging from −50 cm/s to over 150 cm/s. The variability in the along‐shelf current within the plume is wind driven. The relatively weak (<0.5 dyn/cm2) along‐shelf wind stresses in this region drive strong along‐shelf current variability because the Amazon Plume is thin and the interfacial drag is weak. However, in the absence of an along‐shelf wind stress the plume flow is northwestward at 40 to 80 cm/s, indicating that direct wind forcing does not account for the mean northwestward flow of the Amazon Plume. In the cross‐plume direction there is a tendency for the onshore wind stress due to the persistent trade winds to balance the typically offshore buoyancy force and the Coriolis force associated with the wind‐driven along‐shelf flow. Thus the wind stress is a key factor influencing variability in both the flow and cross‐shelf structure of the Amazon Plume in the vicinity of the moored array.
Eddy fluxes of heat and salt from the southwest Indian Ocean into the southeast Atlantic Ocean: A case study
Authors: van Ballegooyen Roy C. , Gründlingh Marten L. , Lutjeharms Johann R. E.
Published: February 2004

Hydrographic data collected during a dedicated research cruise in the Agulhas Retroflection region in February/March 1987 are analyzed in conjunction with Geosat altimetry from December 1986 to December 1988. Estimates of the heat and salt anomalies of eddies and rings observed in this hydrographic survey, are combined with the altimetric tracking of these features to provide insight into and quantification of the climatically important interbasin exchanges occurring in this region. Over the 2‐year period a total of 14 anticyclonic eddies were generated by the Agulhas Current, approximately 50% of these were observed in the satellite altimetry analysis not to migrate farther than 1500 km west or northwest of the Agulhas Retroflection region. Altimetry data show a number of eddies being shed south of the Agulhas Retroflection; all were observed ultimately to move westward rather than southward across the Subtropical Convergence. The estimated net heat fluxes (300 W m−2) and evaporative losses (1 cm day−1) to the atmosphere from eddies within the Agulhas Retroflection region in February–March 1987 were appreciably larger than the austral summer climatological means (25 to 100 W m−2 and 0.6 cm day−1, respectively) for this region. From the combined altimetric and hydrographic analyses, the volume flux of Indian Ocean water warmer than 10°C into the South Atlantic Ocean via the Agulhas eddy field is estimated to be 6.3 Sv (1 Sv = 106 m3 s−1) and 7.3 Sv for water warmer than 8°C. Similarly, estimated fluxes of heat and salt into the Atlantic Ocean via the Agulhas eddy field are 0.045 PW and 78×1012kg per annum, respectively.
Dynamically consistent hydrography and absolute velocity in the eastern North Atlantic Ocean
Authors: Wunsch Carl
Published: February 2004

The problem of mapping a dynamically consistent hydrographic field and associated absolute geostrophic flow in the eastern North Atlantic between 24° and 36°N is related directly to the solution of the so‐called thermocline equations. A nonlinear optimization problem involving Needler's P equation is solved to find the hydrography and resulting flow that minimizes the vertical mixing above about 1500 m in the ocean and is simultaneously consistent with the observations. A sharp minimum (at least in some dimensions) is found, apparently corresponding to a solution nearly conserving potential vorticity and with vertical eddy coefficient less than about 10−5 m2/s. Estimates of “residual” quantities such as eddy coefficients are extremely sensitive to slight modifications to the observed fields. Boundary conditions, vertical velocities, etc., are a product of the optimization and produce estimates differing quantitatively from prior ones relying directly upon observed hydrography. The results are generally insensitive to particular elements of the solution methodology, but many questions remain concerning the extent to which different synoptic sections can be asserted to represent the same ocean. The method can be regarded as a practical generalization of the β spiral and geostrophic balance inverses for the estimate of absolute geostrophic flows. Numerous improvements to the methodology used in this preliminary attempt are possible.
Century/millennium internal climate oscillations in an ocean‐atmosphere‐continental ice sheet model
Authors: Birchfield Edward G. , Wang Huaxiao , Rich Jonathan J.
Published: February 2004

We demonstrate in a simple climate model that there exist nonlinear feedbacks between the atmosphere, ocean, and ice sheets capable of producing century/millennium timescale internal oscillations resembling those seen in the paleoclimate record. Feedbacks involve meridional heat and salt transports in the North Atlantic, surface ocean freshwater fluxes associated with melting and growing continental ice sheets in the northern hemisphere and with Atlantic to Pacific water vapor transport. The positive feedback between the production of North Atlantic Deep Water (NADW) and the meridional salt transport by the Atlantic thermohaline circulation tends to destabilize the climate system, while the negative feedback between the freshwater flux, either to or from the continental ice sheets, and meridional heat flux to the high‐latitude North Atlantic, accomplished by the thermohaline circulation, stabilizes the system. The thermohaline circulation plays a central role in both positive and negative feedbacks because of its transport of both heat and salt. Because of asymmetries between the growth and melt phases the oscillations are, in general, accompanied by a growing or decreasing ice volume over each cycle, which in the model is reflected by increasing or decreasing mean salinity. The magnitude of the oscillations can be as large as the fluctuations observed in the melt water fluxes during the last deglaciation, suggesting that the feedback mechanisms themselves may have been actively involved in the deglaciation process. The feedbacks could potentially play a causal role in the Dansgaard‐Oeschger events. They are also candidates for driving the apparent major ice sheet rapid collapses (in approximately 300 yrs.) that appear to have occurred at intervals of 7 ky to 10 ky, identified in the North Atlantic sediment record as Heinrich events.
TOPEX/POSEIDON microwave radiometer performance and in‐flight calibration
Authors: Ruf Christopher S. , Keihm Stephen J. , Subramanya Bala , Janssen Michael A.
Published: February 2004

Results of the in‐flight calibration and performance evaluation campaign for the TOPEX/POSEIDON microwave radiometer (TMR) are presented. Intercomparisons are made between TMR and various sources of ground truth, including ground‐based microwave water vapor radiometers, radiosondes, global climatological models, special sensor microwave imager data over the Amazon rain forest, and models of clear, calm, subpolar ocean regions. After correcting for preflight errors in the processing of thermal/vacuum data, relative channel offsets in the open ocean TMR brightness temperatures were noted at the ≈1 K level for the three TMR frequencies. Larger absolute offsets of 6–9 K over the rain forest indicated a ≈5% gain error in the three channel calibrations. This was corrected by adjusting the antenna pattern correction (APC) algorithm. A 10% scale error in the TMR path delay estimates, relative to coincident radiosondes, was corrected in part by the APC adjustment and in part by a 5% modification to the value assumed for the 22.235 GHz water vapor line strength in the path delay retrieval algorithm. After all in‐flight corrections to the calibration, TMR global retrieval accuracy for the wet tropospheric range correction is estimated at 1.1 cm RMS with consistent performance under clear, cloudy, and windy conditions.
Effects of sill processes and tidal forcing on exchange in eastern Long Island Sound
Authors: Valle‐Levinson Arnoldo , Wilson Robert E.
Published: February 2004

Temperature and salinity data from Long Island Sound (LIS) show that the pycnocline rises seaward of Mattituck sill and that the magnitude of the longitudinal density gradient over the area of the sill remains relatively constant throughout the year. This reflects the influence of internal hydraulic processes and vertical mixing on hydrography and volume exchange. Records of moored instruments indicate intratidal and fortnightly variations of water column stratification and current velocity in eastern LIS. During spring tides, thick bottom mixed layers reveal vertically homogeneous temporal changes in temperature, salinity, and sigma‐t, which arise from the oscillatory advection of a longitudinal density gradient. Intratidal variations translate into barotropic rectification of the residual flow and vertical homogeneity of low‐passed temperature, salinity, and sigma‐t. During neap tides the flow may be subject to hydraulic control. Near‐surface temperature and salinity oscillations of higher amplitude than near‐bottom fluctuations are a reflection of vertical excursions of the halocline. The halocline is raised in flood periods and depressed in ebb periods. Intratidal stratification changes reflect the corresponding low‐passed temperature, salinity, and sigma‐t behavior with depth. Near‐bottom residual flow favors salt intrusion as it accelerates down the salinity gradient due to the adjustment of the longitudinal density gradient established on spring tides. The residual density and flow fields reflect a change in dynamical balances from spring periods, dominated by frictional stresses, to neap periods, when inertial and rotation effects increase.
Mediterranean water structure in the central Atlantic: Results of remote acoustic and conductivity‐temperature‐depth measurements
Authors: Berezutskii V. , Maximov S. E. , Rodionov V. B. , Sklyarov V. E.
Published: February 2004

In March 1990, combined acoustic and conductivity‐temperature‐depth measurements were carried out in the central Atlantic (29°–35°N, 20°–26°W) to study volume sound backscattering (VSB) at depths of Mediterranean Intermediate Water (MIW). Spatial variability of VSB in the presence of MIW was found. The influence of the intermittent character of the MIW structure on VSB at depths of 700–1900 m was revealed and examined. The possibility of acoustic detection and monitoring of both meddies and thermohaline fine structure features directly from the sea surface was discovered. This paper presents the first acoustic imaging of the MIW structure and dynamics in the central Atlantic.
On the nature of transverse variability in a coastal plain estuary
Authors: Wong Kuo‐Chuin
Published: February 2004

Even though estuarine processes are inherently three dimensional, most of the previous studies have focused primarily on the axial (longitudinal and vertical) variabilities. Recent surveys in the lower Delaware Bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern. Conceptual models are developed to examine the mechanisms which might explain the observed transverse structure. Results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections.
Grazing by microzooplankton and mesozooplankton in the high‐latitude North Atlantic Ocean: Spring versus summer dynamics
Authors: Gifford Dian J. , Fessenden Lynne M. , Garrahan Peter R. , Martin Elena
Published: February 2004

Grazing on chlorophyll by microzooplankton (<200 μm) and copepods was measured in the mixed layer of the high‐latitude North Atlantic Ocean during May and August 1991. No significant grazing by microzooplankton occurred in May during a spring bloom dominated by colonial Phaeocystis pouchettii and Nitzschia spp. As the bloom declined, the size distribution of chlorophyll shifted from dominance by the >20 μm chlorophyll fraction to dominance by the < 20 μm chlorophyll fraction. The impact of grazing by microzooplankton increased as the bloom declined, with microzooplankton consuming 100% of potential daily chlorophyll production following the bloom. In August, when the phytoplankton was dominated by the <20 μm chlorophyll fraction, microzooplankton consumed 37–53% (mean = 41% ±11% s.d.) of potential daily chlorophyll production. Averaged over all experiments, microzooplankton grazing accounted for 81% of daily chlorophyll production. The grazing impact of Calanus finmarchicus stages C4 and C5, which dominated mesozooplankton biomass in the upper euphotic zone in both spring and late summer, was concentrated on chlorophyll >20 μm in both seasons; C. finmarchicus did not consume significant amounts of chlorophyll <20 μm in either season. Compared to the microzooplankton, copepods did not consume a significant fraction of total chlorophyll in either season, accounting for only ∼ 1% of daily chlorophyll production.
Primary production, water column changes, and the demise of a Phaeocystis bloom at the Marine Light‐Mixed Layers site (59°N, 21°W) in the northeast Atlantic Ocean
Authors: Marra John , Langdon Christopher , Knudson Carol A.
Published: February 2004

Primary production in incubations is compared with daily changes in water column oxygen inventories and particulate organic carbon (POC, and estimated from the beam attenuation coefficient) at a station in the northeast Atlantic (the Marine Light‐Mixed Layers (MLML) site) in the spring of 1991. Drifters were followed over three periods. The first is characterized by a bloom of Phaeocystis pouchetii. Prior to the second and third deployments, there were storms that mixed the water column to >150 m over a period of 3 days, and consequently, the Phaeocystis bloom dissipated. After deployment of the third drifter, the water column restratified, followed by high rates of primary production. All incubation experiments for carbon assimilation were greater than that calculated from changes in oxygen or POC. The 14C estimates agreed with the prediction of a bio‐optical model for primary production. During the early days of the cruise, 14C estimates were high, while water column changes indicated negative net community production. In addition to the usual overestimate of net primary production by the 14C estimates, the differences are ascribed to mixing losses for the first part of the cruise, and to grazing losses after restratification.
Modes of subsurface, intermediate, and deep water renewal in the Red Sea
Authors: Eshel Gidon , Cane Mark A. , Blumenthal M. Benno
Published: February 2004

A linear box model of the Red Sea is presented. The model, which inverts hydrographic and 3He data for the main features of the width‐integrated circulation, is able to describe a large part of the seasonal and annual mean circulation. By combining and reconciling major elements of current theories of the Red Sea circulation, available flow measurements, and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the Red Sea. The model forms deep water in the extreme north in two modes, as first suggested by Cember (1988). The convective mode renews the bottom water at an annual mean rate of ∼0.04 Sv. In the winter the isopycnal mode water renews the uppermost part of the deep water mass. It flows to the south along the bottom of the pycnocline at an annual mean rate of 0.02–0.04 Sv. A new feature in the model's near‐surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of ∼0.04 Sv, also from the extreme north. There is also a middepth return flow to the north at an annual mean rate of ∼0.07 Sv. This flow has been suggested but never rigorously quantified before.