bluuebunny/crossref_metadata_2025 huggingface dataset:
Locating the spreading axis along 80 km of the Mid‐Atlantic Ridge south of the Atlantis Transform
Authors: Smith Deborah K. , Tivey Maurice A. , Schouten Hans , Cann Johnson R.
Published: September 2002

The zone of active eruptive fissuring (the “spreading axis”) is investigated at an accretionary segment of the Mid‐Atlantic Ridge. High‐resolution side‐scan images are used to produce a detailed geologic map of the median valley floor of Segment 18, located immediately south of the Atlantis Transform (30°N). The spreading axis is defined by distinguishing between primary eruptive vents (seamounts and hummocky ridges fed by underlying dikes) and secondary vents (seamounts and terraces fed from lava tubes or channels) and is found to be about 2 km wide or less along the entire segment. It is impossible to narrow the width further with the data we have in hand. For comparison, at faster spreading ridges such as the East Pacific Rise, the spreading axis is generally equated to the width of the axial summit trough, typically, 50–100 m wide. Within the Brunhes anomaly, the high‐resolution structure of the central anomaly magnetic high (CAMH) obtained from several cross‐axis, deep‐tow tracks is used to define regions of recent, major volcanism. The locations of the CAMH peaks generally conform to our pick of the spreading axis. In some places the peaks are located several kilometers away from the spreading axis, possibly indicating that large volumes of lava have been deposited there. Our interpretation of many volcanic structures as secondary vents at Segment 18 leads to the conclusion that lava channels and tubes commonly develop to transport lava to these off‐axis deposition sites.
Intensive gas sampling of noble gases and carbon at Vulcano Island (southern Italy)
Authors: Tedesco Dario , Scarsi Paolo
Published: September 2002

The helium isotopic ratio of crater, beach and submarine fumaroles, water wells, and soil gases at Vulcano Island has been, since 1987, repeatedly measured. The 3He/4He from crater fumaroles (F5 and FA) oscillates biennially between 4.9 and 6.0–6.2 R/Ra. The periodicity of the 3He/4He oscillation may be linked to pressure variations in a deep gas reservoir. Chemical and carbon isotope variations during the same period closely reflect trends shown by helium. These simultaneous variations, chemical and isotopic, suggest a close relationship between these species and reflect one dominant process, which belong to the routine activity of the volcano. The 40Ar/36Ar ratio up to 1200 is not consistent with the presence of an air‐rich source and suggests the addition of radiogenic argon. Neon data (20Ne/22Ne and 21Ne/22Ne) indicates that a crustal component is present beneath the volcano. Heavy carbon, 0‰ ∼ −2‰, also supports the existence of a crustal component. The isotopic data here obtained do not indicate that the recorded variations (chemical and isotopic) indicate a “period of unrest” at Vulcano Island. The present phase of activity is explained with a two‐state model of the feeding sources of the volcano: (a) a gas release from intermediate and surficial sources and (b) a magmatic pulse, 3He‐rich, from a deep gas reservoir (not migration of magma) which continuously mixes with more surficial fluids. Additions of surficial (air saturated waters) or atmospheric fluids are minor and may occur during the ascent of the gas phase to the surface or, more likely, be added at the time of the gas collection.
Vredefort shatter cones revisited
Authors: Nicolaysen L. O. , Reimold W. U.
Published: September 2002

Shatter cones have been described from a number of circular and polygonal structures worldwide, the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes. Despite their association with enigmatic, catastrophic processes, the nature of shatter cones and the physics involved in their formation have not been comprehensively researched. Results of detailed field and laboratory studies of shatter cones from three areas in the collar of the Vredefort Dome in South Africa are presented. Vredefort shatter cones are directly related to a widely displayed fracture phenomenon, termed “multiply striated joint sets (MSJS)”. MSJs are planar to curviplanar fractures occuring at spacings of < 1 to several millimeters. The joint sets have a fractal character. When a new measurement protocol is used in the field, involving study of all joint surfaces and all steps and striae exposed on these surfaces, new information is gained on the genesis and significance of the MSJS and on their relationship to striated conical fractures. The internal constitution of a rock specimen with MSJS was examined in detail, by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen. The steps and striae on shatter cone surfaces have the characteristics of displacement fractures (microfaults), along which evidence of melting is observed. Shatter cone and MSJS surfaces are often covered with glassy films; we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic (friction) melt. Our field and petrographic observations can be interpreted as consistent with the generation of shatter cones/MSJS relatively late in the formation of the Vredefort structure. This scenario contrasts sharply with the widely held view that shatter cones are formed during the early “compression” phase of a shock event that affected horizontal strata.
How well constrained are well‐constrained T, B, and P axes in moment tensor catalogs?
Authors: Frohlich Cliff , Davis Scott D.
Published: September 2002

The T, B and P axes of earthquake moment tensors (MT) are often used to evaluate regional stress directions and other tectonic parameters; we here undertake three comparisons to assess the uncertainty in the orientations of these axes. These are (1) a direct comparison of common MT in the Harvard, U.S. Geological Survey, or Earthquake Research Institute (ERI) catalogs; (2) a comparison of MT slip vectors and plate motion vectors in several tectonically straightforward regions; and (3) an analysis of the axial variability in the Harvard and ERI catalogs implied by the reported uncertainties in individual MT components. All three comparisons indicate that there is considerable variability within the catalog concerning the axial orientation of MT, but all suggest that axis orientations of the majority of Harvard MT have uncertainties of 15° or less. For compensated linear vector dipole (CLVD) components among the three catalogs, the correlation is very low. For the Harvard catalog, three statistics are especially useful for selecting better constrained MT; these are (1) the relative error Erel, which is the ratio of the scalar moments of the reported error tensor and of the MT itself; (2)ƒCLVD, a measure of the strength of the CLVD component; and (3) nfree, the number of MT elements not fixed at zero in the inversion. For selecting better constrained MT, the appropriate statistical cutoffs chosen depend on the problem of interest, the data available, arid personal preference. However, for analysis of shallow earthquakes we have used Erel ≤ 0.15, ƒCLVD ≤ 0.20, and nfree = 6. While this eliminates 53% of the catalog, our calculations suggest that nearly all the remaining events have T, B, and P axes with azimuth and inclination angle uncertainties of 5°–10° or less.
Multiple plume events in the genesis of the peri‐Caribbean Cretaceous oceanic plateau province
Authors: Lapierre Henriette , Bosch Delphine , Dupuis Vincent , Polvé Mireille , Maury René C. , Hernandez Jean , Monié Patrick , Yeghicheyan Delphine , Jaillard Etienne , Tardy Marc , de Lépinay Bernard Mercier , Mamberti Marc , Desmet Alain , Keller Francine , Sénebier François
Published: September 2002

The oceanic crust fragments exposed in central America, in north‐western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean‐Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) ‐enriched and display (relative to normal mid‐ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island‐type mantle source, the composition of which includes the HIMU (high 238U/204Pb) component characteristic of the Galapágos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapágos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge‐centered or near‐ridge hotspot somewhere in the SE Pacific.
Magnetic fabric evidence for conduit‐fed emplacement of a tabular intrusion: Dinkey Creek Pluton, central Sierra Nevada batholith, California
Authors: Cruden Alexander R. , Tobisch Othmar T. , Launeau Patrick
Published: September 2002

Anisotropy of magnetic susceptibility measurements in the Cretaceous Dinkey Creek Pluton (DCP), central Sierra Nevada batholith, reveal a well‐defined structural pattern and zonation in bulk susceptibility (K). Outward decrease in K within the pluton is due to a decrease in ferrimagnetic contribution, corresponding to a compositional zoning from tonalite to granite. Magnetic fabrics in the pluton formed during supersolidus flow of a crystal‐rich magma, except in the eastern domain where subvertical magnetic lineations and NW striking magnetic foliations are attributed to deformation associated with a regional shear zone. Magnetic foliations in the central domain of the DCP define a NNW trending synformal structure, passing structurally upward into a dome beneath a small roof pendant, interpreted to represent different structural levels in a zone of magma upwelling. Arcuate foliation trajectories in the pluton's SW lobe resemble deformed passive markers in two‐dimensional, NW to SW, horizontal, channel flow of non‐Newtonian magma. Analysis of the fabric pattern in cross‐section estimates the preerosion thickness of the lobe to between 915 and 3660 m. The DCP was emplaced as a tabular body with vertical sides and a gently inclined roof. Magma with increasingly more mafic compositions entered the chamber continuously or episodically via a NNW trending conduit. Space for the pluton was created by vertical inflation, probably accommodated by depression of its floor. These results suggest that the mid crust to upper crust of magmatic arcs may be constructed by vertical stacking of tabular granitic plutons with high width/thickness ratios like the DCP.
P and S velocity structure of the Yellowstone volcanic field from local earthquake and controlled‐source tomography
Authors: Miller Douglas S. , Smith Robert B.
Published: September 2002

The three‐dimensional P and S velocity (from Vp/Vs ratios) distribution and improved hypocenters for the Yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled‐source explosions. The P velocity model has an rms residual of ±0.09 s, whereas the Vp/Vs ratio model (calculated from 511 earthquakes) has an rms residual of ±0.29 s. High P and S velocities outside the Yellowstone caldera represent thermally undisturbed basement and sedimentary rocks. A caldera‐wide 15% decrease from regional P velocities at depths of 6 to 12 km is coincident with a −60 mGal gravity anomaly and is interpreted as a hot, subsolidus, granitic batholith with a quasi‐plastic rheology. Localized 30% reductions from regional seismic velocities and higher Vp/Vs ratios 8 km beneath Yellowstone's resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful (less than 2 Ma) silicic volcanism. Additional low seismic velocities and Vp/Vs ratios and a 20 mGal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt. Hypocenters relocated with the three‐dimensional P velocity model show subparallel alignment with NW‐SE trending postcaldera volcanic vents and normal faults northwest of the caldera. Focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera, reflecting thinning and heating of the seismogenic crust.
Paleointensity variations across the last geomagnetic reversal at La Palma, Canary Islands, Spain
Authors: Valet Jean‐Pierre , Brassart Jacques , Quidelleur Xavier , Soler Vicente , Gillot Pierre‐Yves , Hongre Lionel
Published: September 2002

A detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal, from a succession of 69 distinct lava flows in two adjacent sections on the island of La Palma. The correlation between the two sections relies on the positions of the Brunhes‐Matuyama boundary and on the flow chronology obtained by a refined potassium/argon (K/Ar) dating technique. Several pretransitional flows exhibit very shallow paleomagetic inclinations, but they cannot be considered as really transitional. Paleointensity experiments conducted on 288 specimens provided 168 successful determinations (14 were obtained after correction) for 50 lava flows that were considered to represent 45 distinct units. The evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows. The period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 μT which decrease down to 7 μT prior to the transition. The 100 kyr‐long interval following the reversal is associated with field intensities as high as 60 μT and depict a strong and rapid restoration of the dipole field. The virtual dipole moments (VDMs) have been compared with sedimentary VDMs generated from the record of relative paleointensity from Ocean Drilling Program site 851, to which have been added non dipole components similar to the present geomagnetic field. A reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the K‐Ar age determinations. Thus these results can support but do not fully demonstrate the existence of asymmerical saw‐toothed variations in field intensity.
Partial‐melt electrical conductivity: Influence of melt composition
Authors: Roberts Jeffery J. , Tyburczy James A.
Published: September 2002

The electrical conductivity of a partial melt is influenced by many factors, including melt conductivity, crystalline conductivity, and melt fraction, each of which is influenced by temperature. We have performed measurements of bulk conductivity as a function of temperature of an Fo80‐basalt partial melt between 684° and 1244°C at controlled oxygen fugacity. Melt fraction and composition variations with temperature calculated using MELTS [Ghiorso and Sack, 1995] indicate that the effect on melt conductivity of changing melt composition is balanced by changes in temperature (T). Thus bulk conductivity as a function of T or melt fraction in this system can be calculated assuming a constant melt conductivity. The bulk conductivity is well modeled by simple parallel calculations, by the Hashin‐Shtrikman upper bound, or by Archie's law (σpartial melt/σmelt = C1Xmn). We estimate apparent values of the Archie's law parameters between 1150° and 1244°C as C1 = 0.73 ± 0.02 and n = 0.98 ±0.01. Estimates of the permeability of the system are obtained by using an electrical conductivity‐critical scale length relationship and range from ∼10−14 to 10−18 m2, comparing favorably with previously published values.
Heat flow in the Trans‐Hudson Orogen of the Canadian Shield: Implications for Proterozoic continental growth
Authors: Mareschal J. C. , Jaupart C. , Cheng L. Z. , Rolandone F. , Gariépy C. , Bienfait G. , Guillou‐Frottier L. , Lapointe R.
Published: September 2002

Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo‐Proterozoic Trans‐Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 42±11 mW m−2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc‐related volcanic rocks. In the Flin Flon‐Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 km) surface layer overlying the mid and lower crust depleted in radioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of ≈40 km. The coincidence between this “cold spot” and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans‐Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming “normal” continental crust with average thickness, which require crustal flow and soft crustal rheology.
Plankton community composition, production, and respiration in relation to dissolved inorganic carbon on the West Florida Shelf, April 1996
Authors: Hitchcock Gary L. , Vargo Gabriel A. , Dickson Mary‐Lynn
Published: September 2002

In April 1996 the Florida Shelf Lagrangian Experiment examined dissolved inorganic carbon (DIC) dynamics on the West Florida Shelf. DIC concentrations increased over 2 weeks at an average rate of 1 μmol kg−1 d−1 in a patch of the intentionally released tracers sulfur hexafluoride (SF6) and helium 3 (3He). Approximately 20% of the increase was due to air‐sea exchange with the remaining 80% attributed to plankton respiration [Wanninkhof et al., 1997]. Here we present particulate matter concentrations, phytoplankton production, and community respiration rates from the tracer patch that suggest that heterotrophs dominated the community after the termination of a spring bloom. During the experiment, chlorophyll a and phaeopigment concentrations declined from >1.5 to <0.5 μg L−1, with 75–85% of total chlorophyll a in the < 5μm size fraction. Particulate matter composition, with mean ratios of particulate organic carbon:chlorophyll a >200 and particulate organic nitrogerr.chlorophyll a >100, suggests that phytoplankton were a minor component of the plankton biomass. Rates of daily gross primary production estimated by the H2 18O method averaged 69±5 mmol C m−2 d−1 (n = 3) while dark respiration rates, estimated from dark bottle incubations, were approximately ‐ 40±3 mmol C m−2 d−1. Net community production rates (6±6 mmol C m−2 d−1) were much lower than respiration rates. Thus respiration rates nearly balanced phytoplankton production. Light respiration rates were estimated from gross production minus net community production (−51±8 mmol C m−1 d−1) and exceeded dark respiration. Plankton community respiration rates, corrected for autotrophic carbon fixation, were more than sufficient to account for the observed increase of DIC within the tracer patch.
Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle
Authors: Raimbault Patrick , Slawyk Gerd , Boudjellal Benyahia , Coatanoan Christine , Conan Pascal , Coste Bernard , Garcia Nicole , Moutin Thierry , Pujo‐Pay Mireille
Published: September 2002

Biomass, inorganic carbon and nitrogen uptake, ammonium regeneration, nitrification, and vertical flux of particulate matter were measured in the equatorial Pacific at 21 daily productivity stations occupied on a meridional transect (150°W) between 1°N and 16°S. Three areas could be distinguished along the transect: (1) the equatorial area between 1°N and 6°S, where nitrate concentrations were typically eutrophic, reaching up to 3 μg‐at L−1 in surface waters; (2) an intermediate mesotrophic area between 6° and 10°S, where surface nitrate concentrations decreased from 1 μg‐at L−1 to zero; and (3) the oligotrophic area beyond 10°S, characterized by warm and nitrate poor waters. Although nitrate was the main form of inorganic nitrogen available for phytoplankton growth (70%–100% of total), its uptake was severely retarded in the equatorial sector. This lack of nitrate depletion in the equatorial sector between 0 and 6°S may in part result from the important ammonium supply (100 ng‐at L−1 d−1) which could sustain up to 85% of total inorganic nitrogen (nitrate + ammonium) utilization by phytoplankton. In addition, regenerated production also resulted from in situ nitrification (20–80 ng‐atN L−1 d−1) which can fuel 20%–100% of the nitrate uptake. Sinking particles represented <10% of total carbon fixation and ∼10%–50% of new production in terms of carbon and nitrogen. From these discrepancies it was suggested that (1) new production rates were overestimated because of the high level of nitrification that provided “regenerated nitrate” and (2) advection of dissolved organic carbon and nitrogen played an important role in export. The specific hydrodynamical circulation, a conveyor belt generated by upwelling at the equator and downwelling some degrees south, associated with biological in situ remineralization of ammonium and nitrate as well, appeared to be a very efficient system for recycling inorganic nitrogen in the euphotic layer and thus for supporting the high regenerated production levels. On the other hand, the high nitrate/silicate ratios (>1.5) observed in the upwelling waters seemed to indicate that silicate is not efficiently recycled in this specific circulation system because of its low regeneration rate as well as its sink via rapidly sedimenting diatoms cell walls; this may be also true for iron. This reinforces the idea of Si and/or Fe limitation which was put forward earlier to explain low new production levels in the equatorial Pacific.
Eulerian drift induced by progressive waves above rippled and very rough beds
Authors: Davies A. G. , Villaret C.
Published: September 2002

A simple, practical, analytical model is developed of the drift induced by weakly asymmetrical progressive waves in the bottom boundary layer above rippled and very rough beds. Above such beds, momentum transfer is dominated not by random turbulence, but by the well‐organized process of vortex shedding, which is characterized here by a strongly time‐varying “convective” eddy viscosityK. The one‐dimensional wave boundary layer model proposed is consistent with the presence of Stokes' second‐order waves in the free‐stream flow. The solution for the Eulerian drift is characterized by a pronounced near‐bed jet in the direction of wave advance, beneath a layer extending to the edge of the boundary layer in which the drift is in the opposite direction. An empirical approach involving five published laboratory data sets is used to estimate the coefficient defining the asymmetrical time‐varying component ofKabove rippled and very rough beds. The resulting “standard” model is then compared both with measurements of the drift at the edge of the wave boundary layer and also with measured drift profiles. Good agreement is found for ranges of wave steepness and relative bed roughness of practical importance.
A semi‐analytical model of the influence of phytoplankton community structure on the relationship between light attenuation and ocean color
Authors: Ciotti Áurea M. , Cullen John J. , Lewis Marlon R.
Published: September 2002

A model was developed to examine the influence of phytoplankton community structure on the relationship between diffuse attenuation and ratios of upwelling radiance. Shifts in phytoplankton communities were represented by changing mean optical properties as a function of chlorophyll (C, mg m−3), consistent with large data sets from the field and laboratory. The product of cell size and internal pigment concentration, dci, governs pigment packaging, which alters the specific absorption coefficients of phytoplankton (
, m2 mgChl−1). Pigment packaging was parameterized as a function of C by combining the relationship between dci and
from phytoplankton cultures with that between
and C from the field, using data for 675 nm, where absorption by accessory pigments is low. Changes in accessory pigmentation were approximated by quantifying residual variability in
at other wavelengths, as functions of C, once the variability with dci was taken into account. Absorption by colored dissolved organic matter (CDOM), detrital absorption, and scattering by particles were also parameterized as functions of C, so that bio‐optical relationships could be modeled as functions of trophic status. The model thus reconciled recognized relationships between optical properties and C with ecologically interpretable shifts in phytoplankton communities. Empirical relationships between diffuse attenuation and ocean color were well reproduced at low (0.5 mg m−3) to medium (10 mg m−3) C. Analysis of variability imposed by a range of dci suggests that it may be possible to recognize phytoplankton communities with cell sizes and intracellular pigment concentration different from the central tendency, given a set of wavelengths which minimizes the influence of CDOM and detrital absorption.
Revised ocean backscatter models at C and Ku band under high‐wind conditions
Authors: Donnelly William J. , Carswell James R. , McIntosh Robert E. , Chang Paul S. , Wilkerson John , Marks Frank , Black Peter G.
Published: September 2002

A series of airborne scatterometer experiments designed to collect C and Ku band ocean backscatter data in regions of high ocean surface winds has recently been completed. More than 100 hours of data were collected using the University of Massachusetts C and Ku band scatterometers, CSCAT and KUSCAT. These instruments measure the full azimuthal normalized radar cross section (NRCS) of a common surface area of the ocean simultaneously at four incidence angles. Our results demonstrate limitations of the current empirical models, C band geophysical model function 4 (CMOD4), SeaSat scatterometer 2 (SASS 2), and NASA scatterometer 1 (NSCAT) 1, that relate ocean backscatter to the near‐surface wind at high wind speeds. The discussion focuses on winds in excess of 15 m s−1 in clear atmospheric conditions. The scatterometer data are collocated with measurements from ocean data buoys and Global Positioning System dropsondes, and a Fourier analysis is performed as a function of wind regime. A three‐term Fourier series is fit to the backscatter data, and a revised set of coefficients is tabulated. These revised models, CMOD4HW and KUSCAT 1, are the basis for a discussion of the NRCS at high wind speeds. Our scatterometer data show a clear overprediction of the derived NRCS response to high winds based on the CMOD4, SASS 2, and NSCAT 1 models. Furthermore, saturation of the NRCS response begins to occur above 15 m s−1. Sensitivity of the upwind and crosswind response is discussed with implications toward high wind speed retrieval.
Optical backscattering by calcifying algae: Separating the contribution of particulate inorganic and organic carbon fractions
Authors: Balch William M. , Drapeau David T. , Cucci Terry L. , Vaillancourt Robert D. , Kilpatrick Katherine A. , Fritz Jennifer J.
Published: September 2002

Light scattering properties of biogenic CaCO3 particles [particulate inorganic carbon (PIC)] were determined on cultured calcifying algae and field‐derived CaCO3 particles. The particles were separated from particulate organic carbon (POC) with a flow cytometer, volume‐scattering functions were measured with a laser light‐scattering photometer, and particle composition was measured using atomic absorption spectrometry. Small calcite coccoliths were best sorted by gating on the ratio of horizontally polarized forward light scattering and vertically polarized forward light scattering; plated coccolithophores could be sorted by gating on side scattering and forward angle light scattering. Normalized volume‐scattering functions for the culture‐derived calcite particles varied by a factor of 2 for the different species. Backscattering cross sections (m2 particle−1) for calcite particles varied by ∼35 times and were generally a function of size. Backscattering efficiencies were ∼2–4 times higher for cells with CaCO3 than without it. CaCO3‐specific backscattering showed much less variability across various species; the calcite‐specific backscattering coefficient varied by only ∼38% for both cultured coccolithophores and field‐derived CaCO3 particles. Organic carbon‐specific backscattering of “naked” coccolithophores was highly consistent within all coccolithophores used in our experiments, as well as with values in the literature. Our results suggest that both POC and PIC can be optically estimated, the former by measuring backscattering of decalcified phytoplankton as well as their size distribution, and the latter is proportional to acid‐labile backscattering. These results show the feasibility of a rapid optical technique for measuring two biogeochemically important carbon fractions in the sea.
Motional induction effect on the planetary‐scale geoelectric potential in the eastern North Pacific
Authors: Fujii Ikuko , Chave Alan D.
Published: September 2002

Geoelectric potentials induced by seawater motion on a planetary scale were studied using voltage differences measured on the Hawaii‐1 (HAW‐1) submarine cable between Hanauma Bay, Oahu, Hawaii, and Point Arena, California. The motionally induced field was extracted from a 3.6‐year segment of the HAW‐1 voltage by cancelling the component that is coherent with the externally produced geomagnetic field at Honolulu. The corrected voltage shows significant coherence with atmospheric variables (wind stress curl, wind stress, and surface pressure) estimated from the European Centre for Medium‐Range Weather Forecasts product at points throughout the Pacific basin at periods from 5 to 133 days. This suggests that the voltage reflects regional to basin‐wide depth‐averaged (barotropic) water currents that are directly wind driven. Contour maps of the squared coherence reveal that (1) peak coherence as large as 0.6–0.8 is seen at points both near the cable and nonlocally, (2) higher coherence is seen at periods where the cable voltage power spectrum peaks, and (3) spatial patterns of the coherence vary with period. Comparison of power spectra or coherence maps for individual years shows that the wind‐driven flow fluctuates temporally owing to long‐term variations of the wind field. The main features of these observations are consistent with those from point electric field measurements, suggesting that planetary‐scale voltages measured with cables can serve as a useful tool to monitor large‐scale water currents at long periods.
An iron‐based ecosystem model of the central equatorial Pacific
Authors: Leonard Carrie L. , McClain Charles R. , Murtugudde Ragu , Hofmann Eileen E. , Harding Lawrence W.
Published: September 2002

The central and eastern equatorial Pacific region is characterized by lower than expected phytoplankton biomass and primary production given the relatively high ambient nitrate concentrations. These unusual conditions have spawned several field programs and laboratory experiments to determine why this high nitrate‐low chlorophyll pattern persists in this region. To synthesize the results from these field programs, as well as providing additional evidence in support of the iron hypothesis, we developed a one‐dimensional, nine‐component ecosystem model of 0°N 140°W. The model components include two phytoplankton size fractions, two zooplankton size fractions, two detrital size fractions, dissolved iron, nitrate, and ammonium. The model was run for 5 years (1990–1994) and was forced using an atmospheric radiative transfer model, an ocean general circulation model (GCM), and in situ data. To our knowledge, this is the first ecosystem model at 0°N 140°W to synthesize the Joint Global Ocean Flux Study Equatorial Pacific Process Study (JGOFS EqPac) data set, as well as to use both in situ and modeled physical data to drive the model. Modeled phytoplankton, zooplankton, and iron all varied on interannual timescales due to El Niño events. Total phytoplankton biomass increased by as much as 40% from early 1992 (El Niño warm) to 1993 (normal). The results also indicate that the biomass increase during a cool period is not constant for each phytoplankton component, but instead the increase is most evident in the netphytoplankton (>10 μm). Netphytoplankton increase from a low of 0.1% of the total chlorophyll in 1992 to a high of 30% of the total in 1993. Microzooplankton grazing rates fluctuated in response to changes in nanophytoplankton growth rates, whereas mesozooplankton grazing was unrelated to netphytoplankton growth rates. The magnitude and temporal variability of phytoplankton chlorophyll agreed well with in situ data collected during 1992. Modeled primary production was lower than measured during El Niño but agreed with observations during normal conditions. The low primary productivity was probably a result of downwelling produced by the physical model. New production was calculated from total and recycled iron rather than nitrate‐based production and was more variable in general and almost 3 times the nitrate‐based new production during non‐El Niño conditions.
A new approximation of the equation of state for seawater, suitable for numerical ocean models
Authors: Brydon David , Sun Shan , Bleck Rainer
Published: September 2002

An approximation to the UNESCO equation of state for seawater is obtained in the form of a polynomial that is cubic in potential temperature, quadratic in pressure, and linear in salinity. The polynomial represents a reasonable compromise between accuracy and computational economy; it also allows potential temperature to be obtained analytically from given values of density, salinity, and pressure.
Diffuse venting and background contributions to chemical anomalies in a neutrally buoyant ocean hydrothermal plume
Authors: Lavelle J. W. , Wetzler M. A.
Published: September 2002

Plumes originating in high‐temperature hydrothermal discharges entrain background water and often entrain diffuse‐source fluids during convective ascent. The mixing of fluid from those three different sources determines the chemical composition of the ensuing neutrally buoyant plumes. Here convection models and aluminum data from the Trans‐Atlantic Geotraverse (TAG) site are used to examine relative contributions of focused, diffuse, and background sources to anomalies in the plume. Our analysis shows that anomalously high Al values observed just above the seafloor at TAG, which we assume stem from diffuse flows, can contribute as much as 50% to the Al anomaly in the neutrally buoyant plume found >350 m above bottom. Background water contributes nearly an equal fraction of Al to the anomaly. More generally, entrained contributions depend on gradients in profiles external to the rising plume. Entrainment also depends on effective source area; distributed high‐temperature vents entrain more fluid than a single, composite high‐temperature vent. Larger effective source area is particularly significant for entrainment of diffuse‐source or other tracers trapped within a few tens of meters of the seafloor. Downstream, cross‐stream, and temporal variabilities exhibited in the three‐dimensional model plume also underscore problems of sampling and interpreting actual plume data.
Semianalytic Moderate‐Resolution Imaging Spectrometer algorithms for chlorophyll a and absorption with bio‐optical domains based on nitrate‐depletion temperatures
Authors: Carder K. L. , Chen F. R. , Lee Z. P. , Hawes S. K. , Kamykowski D.
Published: September 2002

This paper describes algorithms for retrieval of chlorophyll a concentration and phytoplankton and gelbstoff absorption coefficients for the Moderate‐Resolution Imaging Spectrometer (MODIS) or sensors with similar spectral channels. The algorithms are based on a semianalytical, bio‐optical model of remote sensing reflectance, Rrs(λ). The Rrs(λ) model has two free variables, the absorption coefficient due to phytoplankton at 675 nm, aϕ(675), and the absorption coefficient due to gelbstoff at 400 nm, ag(400). The Rrs model has several parameters that are fixed or can be specified based on the region and season of the MODIS scene. These control the spectral shapes of the optical constituents of the model. Rrs(λi) values from the MODIS data processing system are placed into the model, the model is inverted, and aϕ(675), ag(400), and chlorophyll a are computed. The algorithm also derives the total absorption coefficients a(λi) and the phytoplankton absorption coefficients aϕ(λi) at the visible MODIS wavelengths. MODIS algorithms are parameterized for three different bio‐optical domains: (1) high photoprotective pigment to chlorophyll ratio and low self‐shading, which for brevity, we designate as “unpackaged”; (2) low photoprotective pigment to chlorophyll ratio and high self‐shading, which we designate as “packaged”; and (3) a transitional or global‐average type. These domains can be identified from space by comparing sea‐surface temperature to nitrogen‐depletion temperatures for each domain. Algorithm errors of more than 45% are reduced to errors of less than 30% with this approach, with the greatest effect occurring at the eastern and polar boundaries of the basins.
Intercomparison of backscatter maps over Arctic sea ice from NSCAT and the ERS scatterometer
Authors: Ezraty Robert , Cavanié Alain
Published: September 2002

C band and Ku band Arctic Ocean sea ice backscatter maps at 40° incidence angle, produced from the ERS 2 active microwave instrument operating in scatterometer mode and Advanced Earth Observing System NASA scatterometers, respectively, are presented and compared. The noise level on these maps is estimated from the comparison of successive maps and is found comparable, although larger at C band than at Ku band. In both cases the relative noise level increases as backscatter level decreases. Backscatter shows a greater dynamic range at Ku band than at C band, going from first‐year to multiyear ice; moreover, the Ku band to C band backscatter ratio varies considerably as a function of sea ice type. Surface thawing, in June, leads to a brutal decrease in backscatter in both frequency bands, as well as in the Ku band to C band ratio. The study of the time evolution of backscatter over five different regions of the Arctic Ocean demonstrates the influence of both advection and local evolution on this parameter.
The accuracy of the NSCAT 1 vector winds: Comparisons with National Data Buoy Center buoys
Authors: Freilich Michael H. , Dunbar R. Scott
Published: September 2002

The overall accuracies of vector wind measurements from the NASA scatterometer (NSCAT) are quantified by comparisons with collocated data from operational U.S. National Data Buoy Center ocean buoys. A vector correlation statistic is used to examine the geographical distribution of full‐mission NSCAT‐buoy differences. The dependences of the vector correlation on collocation radius and scatterometer errors such as wind speed offset and random component errors are quantified. For purposes of NSCAT validation, 30 ocean moored buoys having high full‐mission sample correlations are considered. When applied to this set of ocean buoys, a nonlinear analysis shows that the 25‐km resolution NSCAT wind speeds have unity gain, an offset of −0.3 m s−1, an rms error of 1.3 m s−1, and component standard deviations of ∼1.3 m s−1. For wind speeds greater than 6 m s−1, fewer than 3% of all collocated solutions have significant ambiguity removal errors. The rms directional difference for the remaining vectors is less than 17°. At lower wind speeds the frequency of ambiguity removal errors increases and the directional accuracy of the “correct” vectors decreases with decreasing wind speed, consistent with the assumptions of the random error model underlying the nonlinear wind speed regression approach. While the primary emphasis of this paper is on determining the quantitative accuracy of the NSCAT wind velocity measurements, the analysis techniques and interpretations have more general application to validation studies involving vector quantities.
Chaos and predictability in ocean water levels
Authors: Frison Ted W. , Abarbanel Henry D. I. , Earle Marshall D. , Schultz John R. , Scherer Wolfgang D.
Published: September 2002

The classical problem of characterizing and classifying ocean water levels (all fluctuations that are greater than a few minutes duration) is examined using methods derived from studies of nonlinear dynamical systems. The motivation for this study is the difficulty of characterizing coastal water level dynamics and tide zones with existing methods. There is also long‐standing evidence that coastal water levels are not a simple linear superposition of astronomical tides and other influences. Thus it can be appropriate to view water levels as a single, nonlinear, dynamical system. We show that it is appropriate to treat water levels as chaotic by virtue of the existence of a positive Lyapunov exponent for the seven data sets studied. The integer embedding space (the number of state space coordinates) needed to reconstruct an attractor for data collected from sensors exposed to the open ocean is five. Four dynamical degrees of freedom appear to be required to describe the observed dynamics in a state space reconstructed solely from the observations themselves. Water levels in a complex estuary (Chesapeake Bay) have a global dimension of six and have five dynamical degrees of freedom. The largest global Lyapunov exponents, a measure of predictability, vary from 0.57 h−1 for a station relatively well exposed to the ocean (Charleston, South Carolina) to 4.6 h−1 for a station well inside a complex estuary (Baltimore, Maryland). The larger values are generally associated with stations that are less predictable, which is consistent with the errors of the astronomical estimator currently used by the U.S. government to generate tide predictions. Lower values are associated with water levels where the estimator errors are smaller. These results are consistent with the interpretation of the Lyapunov exponents as a measure of dynamical predictability. The dynamical characteristics, notably the Lyapunov exponents, are shown to be good candidates for characterizing water level variability and classifying tide zones.
Mass, heat, and salt transport in the southeastern Pacific: A Circumpolar Current inverse model
Authors: Gille Sarah T.
Published: September 2002

A box inverse model for the southeastern Pacific is used to combine hydrographic measurements with synoptic information from floats, acoustic Doppler surface velocity measurements, altimeter‐derived surface geostrophic velocities, and wind measurements. The southeastern Pacific is marked by substantial changes in eddy kinetic energy along the path of the Antarctic Circumpolar Current (ACC), associated with the Eltanin and Udintsev Fracture Zones in the East Pacific Rise. The results of the inverse model show that the heat and salt carried by the ACC are essentially unchanged as the current passes through regions of high eddy variability. Thus, within the error bars of the available measurements the ACC appears not to exchange significant water properties with its surroundings in the southeastern Pacific Ocean. (The system is geographically variable, and elsewhere in the Southern Ocean, meridional exchanges may be substantial.) Meridional flow through the north side of the inverse model domain is consistent with an overturning circulation that carries surface intermediate water northward and returns deep water southward into the ACC. Diapycnal fluxes identified in the inverse solution indicate that water mass transformation occurs predominantly in the outcropping upper layers of the ocean.
How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures?
Authors: Enfield David B. , Mestas‐Nuñez Alberto M. , Mayer Dennis A. , Cid‐Serrano Luis
Published: September 2002

Several kinds of analysis are applied to sea surface temperature anomalies (SSTA) (1856–1991) to determine the degree to which SSTA of opposite sign in the tropical North and South Atlantic occur. Antisymmetric (“dipole”) configurations of SSTA on basin scales are not ubiquitous in the tropical Atlantic. Unless the data are stratified by both season and frequency, inherent dipole behavior cannot be demonstrated. Upon removing the global El Niño‐Southern Oscillation signal in SSTA (which is symmetric between the North and South Atlantic) from the data, the regions north or south of the Intertropical Convergence Zone have qualitatively different temporal variabilities and are poorly correlated. Dipole configurations do occur infrequently (12–15% of the time), but no more so than expected by chance for stochastically independent variables. Nondipole configurations that imply significant meridional SSTA gradients occur much more frequently, nearly half of the time. Cross‐spectral analysis of seasonally averaged SSTA indices for the North and South Atlantic show marginally significant coherence with antisymmetric phase in two period bands: 8–12 years for the boreal winter‐spring and 2.3 years for the boreal summer‐fall. Antisymmetric coherence is optimal for a small subregion west of Angola in the South Atlantic, with respect to SSTA of basin scale in the tropical North Atlantic. Dipole variability, even where optimal, explains only a small fraction of the total variance in tropical Atlantic SSTA (<7%).