bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic support by the Icelandic plume and vertical tectonics of the northeast Atlantic continental margins
Authors: Clift Peter D. , Turner Jonathan
Published: February 2004

Late Paleocene‐early Eocene continental rifting in the northeast Atlantic differs significantly from earlier episodes of margin formation in the Central Atlantic. At a nonvolcanic margin, rifting occurs over a wide area, with little associated magmatism. Postrift subsidence decreases in a predictable, exponential pattern with time. In contrast, subsidence analysis of Ocean Drilling Program and Deep Sea Drilling Project drill sites from the Vøring Plateau, Hatton Bank, and East Greenland Margin show that in these areas the continent‐ocean transition (COT) is very sharp, with ß increasing over a horizontal distance of 30–50 km from values of 1.1–1.50 on the continent side to ∞ at the COT. Drilling penetrated the thick seaward dipping basaltic sequences that typify the East Greenland Margin at Site 917. Fluvial sandstones underlying the basalts show that the area was subaerially exposed prior to continental breakup, but the amount and timing of any uplift are presently unconstrained. Sediment backstripping techniques allow a comparison between the reconstructed and predicted subsidence histories and thus an estimate of the thermal anomaly through time. Anomalous slow subsidence in the early postrift period at 63°N offshore East Greenland is attributed to support by the Icelandic plume. Dynamic support is weaker on the Hatton Bank and on the Vøring Plateau. Variations in the strength and duration of the support suggest that the plume was a large, 1000‐km‐radius structure that lay under the Greenland craton at the time of breakup. Current data suggest that it may have crossed the East Greenland coast at 40 Ma. Discrepancies between uniform stretching models and the reconstructed subsidence for sites on the East Greenland shelf and the Vøring Plateau allow the amount of igneous underplating at the time of breakup to be estimated. Calculations suggest a maximum of around 8.8 km of gabbroic underplating occurred at the time of breakup under the East Greenland shelf and 2.4 km under the Vøring Plateau, although both these bodies thin rapidly away from the COT.
Effect of three‐dimensional topography on seismic motion
Authors: Bouchon Michel , Schultz Craig A. , Toksöz M. Nafi
Published: February 2004

We present a semianalytical, seminumerical method to calculate the diffraction of elastic waves by an irregular topography of arbitrary shape. The method is a straightforward extension to three dimensions of the approach originally developed to study the diffraction of SH waves [Bouchon, 1985] and P‐SV waves [Gaffet and Bouchon, 1989] by two‐dimensional topographies. It relies on a boundary integral equation scheme formulated in the frequency domain where the Green functions are evaluated by the discrete wavenumber method. The principle of the method is simple. The diffracted wave field is represented as the integral over the topographic surface of an unknown source density function times the medium Green functions. The Green functions are expressed as integrals over the horizontal wavenumbers. The introduction of a spatial periodicity of the topography combined with the discretization of the surface at equal intervals results in a discretization of the wavenumber integrals and in a periodicity in the horizontal wavenumber space. As a result, the Green functions are expressed as finite sums of analytical terms. The writing of the boundary conditions of free stress at the surface yields a linear system of equations where the unknowns are the source density functions representing the diffracted wave field. Finally, this system is solved iteratively using the conjugate gradient approach. We use this method to investigate the effect of a hill on the ground motion produced during an earthquake. The hill considered is 120 m high and has an elliptical base and ratios of height‐to‐half‐width of 0.2 and 0.4 along its major and minor axes. The results obtained show that amplification occurs at and near the top of the hill over a broad range of frequencies. For incident shear waves polarized along the short dimension of the hill the amplification at the top reaches 100% around 10 Hz and stays above 50% for frequencies between 1.5 Hz and 20 Hz. For incident shear waves polarized along the direction of elongation of the topography, the maximum amplification occurs between 2 Hz and 5 Hz with values ranging from 50% to 75%. The results also show that the geometry of the topography exerts a very strong directivity on the wave field diffracted away from the hill and that at some distance from the hill this diffracted wave field consists mostly of Rayleigh waves.
Deconvolution of long‐core paleomagnetic data of Ocean Drilling Program by Akaike's Bayesian Information Criterion minimization
Authors: Oda Hirokuni , Shibuya Hidetoshi
Published: February 2004

Deconvolution of long‐core paleomagnetic data of the Ocean Drilling Program (ODP) was developed based on Bayesian statistics. Samples of deep‐sea sediments obtained by drilling with an advanced piston corer are subjected to pass‐through measurements by a cryogenic magnetometer aboard the ODP's research vessel (Joides Resolution) which provide continuous paleomagnetic records. Deconvolution of the magnetometer output was developed in order to obtain more detailed variations of the magnetization than permitted by the spatial resolution of the pickup coil (approximately 11 cm). The magnetization vector was modeled as a smoothly changing sequence parametrized by L2 norm of second‐order difference and then deconvolved using a matrix calculation as a smoothness constrained least squares method. The optimum smoothness was obtained by minimizing Akaike's Bayesian Information Criterion which is a measure of the logarithm of the likelihood. This deconvolution scheme was applied to pass‐through data artificially produced by convolving synthetic reversal record and adding a Gaussian noise. The amplitude of the noise was estimated and the variations shorter than the spatial resolution of the sensor were obtained. The deconvolution also was applied to the real pass‐through data from Holes 769A and 769B measured at intervals of 5 mm. The deconvolution revealed variations of the magnetizations with a maximum spatial resolution of about 2 cm. The magnetizations after deconvolution were in good agreement with magnetization values measured separately on cube samples taken at intervals of 5 or 10 cm.
Recognizing and dating prehistoric liquefaction features: Lessons learned in the New Madrid seismic zone, central United States
Authors: Tuttle Martitia P. , Schweig Eugene S.
Published: February 2004

The New Madrid seismic zone (NMSZ), which experienced severe liquefaction during the great New Madrid, Missouri, earthquakes of 1811 and 1812 as well as during several prehistoric earthquakes, is a superb laboratory for the study of world‐class, earthquake‐induced liquefaction features and their use in paleoseismology. In seismically active regions like the NMSZ, frequent large earthquakes can produce a complex record of liquefaction events that is difficult to interpret. Lessons learned studying liquefaction features in the NMSZ may help to unravel the paleoseismic record in other seismically active regions. Soil characteristics of liquefaction features, as well as their structural and stratigraphic relations to Native American occupation horizons and other cultural features, can help to distinguish prehistoric liquefaction features from historic features. In addition, analyses of artifact assemblages and botanical content of cultural horizons can help to narrow the age ranges of liquefaction features. Future research should focus on methods for defining source areas and estimating magnitudes of prehistoric earthquakes from liquefaction features. Also, new methods for dating liquefaction features are needed.
Hikurangi Plateau: A Cretaceous large igneous province in the southwest Pacific Ocean
Authors: Mortimer Nick , Parkinson Dave
Published: February 2004

The first dredge haul of basement rocks obtained from the Hikurangi Plateau in the southwest Pacific Ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre–Late Cretaceous age. All samples have undergone extensive seafloor weathering to phyllosilicate‐ and zeolite‐bearing assemblages. Petrography, mineral chemistry, and whole rock element concentrations and ratios of the least altered lavas (e.g., TiO2 = 1.3–1.6 wt%, Cr = 132–224 ppm, Zr/Y = 2.9–3.6, Zr/Nb = 19–29, Ti/V = 22–29) suggest a restricted igneous compositional range broadly comparable with normal to enriched mid‐ocean ridge basalt. However, isotopic compositions of leached samples (87Sr/86Sri = 0.70361–0.70374, εNdi = 5.7–6.2) are comparable with oceanic island basalt suites. Collectively, these petrological characteristics are similar to rocks from large igneous provinces of Cretaceous age in the western Pacific Ocean (e.g., Manihiki and Ontong Java Plateaus). Consequently, we interpret the Hikurangi Plateau as another of these Early Cretaceous basaltic oceanic plateaus. Speculatively, parts of the obducted Tangihua and Matakaoa igneous complexes of the North Island may have been derived from the Hikurangi Plateau as the plateau collided with the New Zealand continent in the early Miocene.
How thick is the magmatic crust at slow spreading oceanic ridges?
Authors: Cannat Mathilde
Published: February 2004

Thermal models based on the geometry of axial segmentation, and mechanical models of axial valley formation, suggest that the axial lithosphere of slow spreading ridges can be thicker than the crust modeled from gravity data, or measured from seismic data. This is particularly the case in thin crust ridge regions that are near axial discontinuities. The temperature field modeled in these thin crust ridge regions is such that some of the melts extracted from the asthenosphere should crystallize in the mantle, before they reach the crust. This prediction is consistent with observations made on gabbroic and ultramafic samples collected in thick lithosphere/thin crust regions of the Mid‐Atlantic Ridge, and along the ultra‐slow Southwest Indian Ridge. A geological model is proposed for the lithosphere created in these thick lithosphere/thin crust ridge regions. This model suggests that crustal thicknesses measured in seismic surveys of these regions do not directly reflect the melt production in the asthenosphere beneath the ridge: some of the melt gets trapped in the mantle, and part of the crust is made of variably fractured and serpentinized residual ultramafics. Potential consequences on the way we interpret along‐axis crustal thickness variations at slow spreading ridges are discussed and evaluated.
Seismic reflection analysis of the Manson Impact Structure, Iowa
Authors: Keiswetter D. , Black R. , Steeples D.
Published: February 2004

Our combined interpretation of new, high‐resolution seismic reflection data and reprocessed, but previously published, industrial Vibroseis data indicates that the Manson Impact Structure, Iowa, has an apparent crater diameter of 35 km, an annular trough diameter of around 21 km, a shallow floor (0.6–0.7 km), and a central uplift that has a minimum diameter of 7.5 km. The two reflection lines are coincidentally located along an east‐west radial transect and are constrained by shallow drill information. Results from the two data sets are correlative; both data sets were instrumental to the final interpretation due to the trade‐off between resolution and depth of energy penetration. Based on the combined interpretation, structural uplift of the central peak is estimated to be around 2.8 km. Onlapping seismic sequences are present at the eastern edge of the central uplift. These seismic packages, observed only in the high‐resolution line, are interpreted to represent impact breccia or debris material that was shed from the central peak or dynamically transported from outside of the crater.
Geomagnetic field intensity over the last 42,000 years from core SOH‐4, Big Island, Hawaii
Authors: Garnier Florence , Herrero‐Bervera Emilio , Laj Carlo , Guillou Hervé , Kissel Catherine , Thomas Don M.
Published: February 2004

A record of the absolute geomagnetic field intensity spanning the last 42 kyr has been obtained from 100 lava flows (187 successful Thellier experiments) recovered in the top 468 m of core from the SOH‐4 well on the Island of Hawaii. Assuming a linear extrusion rate between the present and the 42 kyr date obtained with a refined K/Ar technique, this corresponds to an average of one flow every 420 years. Rock magnetic analysis identifies low‐Ti content and high‐Ti content magnetites in 75% and 25% of the samples, respectively, as the main carriers of magnetization. The low‐Ti magnetites are very stable while some transformations occur upon heating in the other samples, but both groups allow reliable paleointensity determinations. The directional record documents inclinations consistent with a geocentric dipole field for the last 16 kyr and between 38 and 42 kyr. Between 16 and 38 kyr shallower and, in some cases, negative inclinations, were observed. Because of the high extrusion rate and the exceptional percentage of reliable intensity determinations, the record of the geomagnetic field intensity over the last 42 kyr has a resolution comparable to the best sedimentary records. The geomagnetic field intensity at Hawaii appears to have varied between 18 and 79 µT and is characterized by large fluctuation with peak‐to‐peak amplitudes of 20–25 µT. The observed values are quite consistent with other values obtained from Hawaii for the last 31 kyr (Mankinen and Champion, 1993a, b; Tanaka and Kono, 1991; Coe et al., 1978). The mean intensity over the explored time interval, 45 µT, is slightly higher than the present value of the geomagnetic field in Hawaii.
Signal processing of soil gas radon, atmospheric pressure, moisture, and soil temperature data: A new approach for radon concentration modeling
Authors: Pinault Jean‐Louis , Baubron Jean‐Claude
Published: February 2004

The variability in space and in time of gases (He, 222Rn, CO2) in soils might be used for volcano or seismic fault surveillance or in search of hidden mineral deposits. The gases measured in soils or in the weathered layer of the substratum can, however, be strongly altered by environmental conditions, such as atmospheric pressure, soil temperature, or moisture. An accurate knowledge of the influence of environmental conditions is required to decipher information from deeper phenomena in the earth. Variations in radon concentration, used as a gas flow tracer, are modeled using a new approach based on signal processing techniques in order to express impulse responses from multivariable time series. A general formulation is proposed and the inverse problem is solved by calculating the impulse responses of the parameter to be explained versus the variables responsible for the variability of the parameter. Such problems are generally ill posed and regularization methods must be used to develop the pattern. Due to the block‐Toeplitz structure of the correlation matrix, large orders can be used, enabling signal processing of time series controlled by the superposition of fast and slow phenomena. Two examples are processed to illustrate this approach. In the first example (222Rn in soil gas monitoring over a hidden sulphide deposit), radon concentration is controlled only by moisture and temperature, not by atmospheric pressure. The result seems to reveal a diffusive behavior. The impulse response of 222Rn concentration versus rainfall exhibits a great variability, possibly due to the opening of cracks in the surface, the effect being still noticeable 20 days after the rainfall. In the second example (222Rn in a thermal anomaly at summit of Etna volcano), where radon concentration is controlled by soil temperature and atmospheric pressure, the result indicates a convective/advective flow. Radon flux collapsed for 1 day probably due to a break in the rising gas column. In 1993, dramatic increases of signal were recorded for a few hours; such increases are interpreted as gas pulses not related with local seismicity.
Age and character of basaltic rocks of the Yucca Mountain region, southern Nevada
Authors: Fleck R. J. , Turrin B. D. , Sawyer D. A. , Warren R. G. , Champion D. E. , Hudson M. R. , Minor S. A.
Published: February 2004

Volcanism in the Yucca Mountain region of southern Nevada in the last 5 m.y. is restricted to moderate‐to‐small volumes of subalkaline basaltic magmas, produced during at least 6 intervals, and spanning an age range from 4.6 Ma to about 125 ka. Where paleomagnetic evidence is available, the period of volcanism at individual eruptive centers apparently was geologically short‐lived, even where multiple eruptions involved different magma types. K‐Ar studies are consistent with most other geochronologic information, such as the minimum ages of exposure‐dating techniques, and show no evidence of renewed volcanism after a significant quiescence at any of the centers in the Yucca Mountain region. A volcanic recurrence interval of 860 ± 350 kyr is computed from a large K‐Ar data set and an evaluation of their uncertainties. Monte Carlo error propagations demonstrate the validity of uncertainties obtained for weighted‐mean ages when modified using the goodness of fit parameter, MSWD. Elevated 87Sr/86Sr initial ratios (Sri) in the basalts, nearly constant at 0.707, combined with low SiO2 and Rb/Sr ratios indicate a subcontinental, lithospheric mantle source, previously enriched in radiogenic Sr and depleted in Rb. Beginning with eruptions of the most voluminous eruptive center, the newly dated Pliocene Thirsty Mountain volcano, basaltic magmas have decreased in eruptive volume, plagioclase‐phenocryst content, various trace element ratios, and TiO2, while increasing in light rare earth elements, U, Th, P2O5, and light REE/heavy REE ratios. These time‐correlated changes are consistent with either increasing depths of melting or a decreasing thermal gradient in the Yucca Mountain region during the last 5 m.y.
Garnet geotherms: Pressure‐temperature data from Cr‐pyrope garnet xenocrysts in volcanic rocks
Authors: Ryan Chris G. , Griffin William L. , Pearson Norman J.
Published: February 2004

The temperatures and pressures of equilibration of single peridotitic garnet xenocrysts are estimated using a combination of major‐ and trace‐element data, determined using electron microprobe (EMP) and proton‐induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The “Ni thermometer” of Griffin et al. [1989], based on the strong temperature dependence of the partitioning of Ni between garnet and olivine, is refined using an expanded database. Pressure is calculated from garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and Köhler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Cr‐saturated peridotite (chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic garnets provided they were in equilibrium with chromite; garnets from Cr‐undersaturated rocks will produce underestimates of pressure. Therefore, the locus of maximum PCr at a given TNi defines the “garnet geotherm”, and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of garnet grains in concentrates. The assumption of coexisting chromite is tested by comparing the temperature distributions of garnets and chromites from the same concentrate. Chromite equilibration temperature is estimated using the “Zn thermometer”, based on the strong temperature dependence of the partitioning of Zn between chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of garnet‐chromite intergrowths. The garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith‐derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent.
Mauna Loa lava accumulation rates at the Hilo drill site: Formation of lava deltas during a period of declining overall volcanic growth
Authors: Lipman Peter W. , Moore James G.
Published: February 2004

Accumulation rates for lava flows erupted from Mauna Loa, as sampled in the uppermost 280 m of the Hilo drill hole, vary widely for short time intervals (several thousand years), but overall are broadly similar to those documented elsewhere on this volcano since 100 ka. Thickness variations and accumulation rates for Mauna Loa lavas at the Hilo drill site have been strongly affected by local paleotopography, including funneling and ponding between Mauna Kea and Kilauea. In addition, gentle submerged slopes of Mauna Kea in Hilo Bay have permitted large shoreline displacements by Mauna Loa flows. Ages of eruptive intervals have been determined from published isotopic data and from eustatic sea level curves modified to include the isostatic subsidence of the island of Hawaii at 2.2–2.6 mm/yr. Prior to 10 ka, rates of Mauna Loa lava accumulation at the drill site varied from 0.6 to 4.3 mm/yr for dateable intervals, with an overall rate of 1.8 mm/yr. Major eruptive pulses at about 1.3 and 10 ka, each probably representing a single long‐lived eruption based on lack of weathering between flow units, increase the overall accumulation rate to 2.4 mm/yr. The higher rate since 10 ka reflects construction of thick near‐shoreline lava deltas as postglacial sea levels rose rapidly. Large lava deltas form only along coastal segments where initially subaerial slopes have been submerged by the combined effects of eustatic sea level rise, isostatic subsidence, or spreading of volcano flanks. Overall accumulation of 239 m of lava at the drill site since 100–120 ka closely balances submergence of the Hilo area, suggesting that processes of coastal lava deposition have been modulated by rise in sea level. The Hilo accumulation rate is slightly higher than average rates of 1–2 mm/yr determined elsewhere along the Mauna Loa coast, based on rates of shoreline coverage and dated sea cliff and fault scarp exposures. Low rates of coastal lava accumulation since 100 ka, near or below the rate of island‐wide isostatic subsidence, indicate that Mauna Loa is no longer growing vigorously or even maintaining its size above sea level.
Southeastern Caribbean sea level variability and viscoelastic relaxation
Authors: Kozuch Michael J. , Yu Ting‐To , Rundle John B.
Published: February 2004

Viscoelastic modeling of earthquakes in the southeast Caribbean shows that distant dip‐slip earthquakes in the Lesser Antilles and the geometry of the subduction zone play a significant role in vertical deformation observed along the southern Caribbean plate margin. We find that dip‐slip earthquakes yield vertical displacements that are 1–2 orders of magnitude greater than those generated by strike‐slip earthquakes along the Caribbean‐South American plate boundary. In addition, postseismic vertical displacements are at least as great or greater at distances of several hundred kilometers than the coseismic counterpart depending upon the geometry of the source. Tide gage data from Venezuela suggest that these vertical displacement rates may still be observed several decades after the occurrence of large subduction zone earthquakes (Ms 7 or greater). In addition, the uplift rate observed at the tide gages largely follows the anticipated flexure of the Caribbean plate due to the subduction process.
Poisson's ratio and crustal seismology
Authors: Christensen Nikolas I.
Published: February 2004

New measurements of compressional and shear wave velocities to hydrostatic pressures of 1 GPa are summarized for 678 rocks. Emphasis was placed on obtaining high‐accuracy velocity measurements, which are shown to be critical in calculating Poisson's ratios from velocities. The rocks have been divided into 29 major groups for which velocities, velocity ratios, and Poisson's ratios are presented at several pressures. Observed Poisson's ratios for the monomineralic rocks compare favorably with theoretical Poisson's ratios calculated from single‐crystal elastic constants. Plagioclase feldspar composition is important in understanding rock Poisson's ratios, since Poisson's ratio of albite increases from 0.28 to a predicted value of 0.31 for anorthite. Fe substitution for Mg in pyroxene and olivine also increases Poisson's ratio. Plotting rock compressional wave velocities versus Poisson's ratios reveals a triangular distribution bounded by quartzite with low compressional wave velocity and low Poisson's ratio, dunite with high compressional wave velocity and intermediate Poisson's ratio, and serpentinite with low compressional wave velocity and high Poisson's ratio. For common plutonic igneous rocks, there is a clear trend relating Poisson's ratio to composition, in which Poisson's ratio for granitic rocks increases from 0.24 to 0.29 as composition changes to gabbro and then decreases with decreasing plagioclase and increasing olivine contents to 0.25 in dunite. Changes in Poisson's ratio with progressive metamorphism of mafic and pelitic rocks correlate reasonably well with mineral reactions. There is no simple correlation between Poisson's ratio and felsic and mafic rock compositions; however, a linear correlation of increasing Poisson's ratio with decreasing SiO2 content is observed for rocks with 55 to 75 wt % SiO2. Average Poisson's ratios for continental and oceanic crusts are estimated to be 0.265 and 0.30, respectively.
Global displacements caused by point dislocations in a realistic Earth model
Authors: Sun Wenke , Okubo Shuhei , Vaníček Petr
Published: February 2004

We define dislocation Love numbers [hnmij, lnmij, knmij, lnmt,ij] and Green's functions to describe the elastic deformation of the Earth caused by a point dislocation and study the coseismic displacements caused in a radially heterogeneous spherical Earth model. We derive spherical harmonic expressions for the shear and tensile dislocations, which can be expressed by four independent solutions: a vertical strike slip, a vertical dip slip, a tensile opening in a horizontal plane, and a tensile opening in a vertical plane. We carry out calculations with a radially heterogeneous Earth model (1066A). The results indicate that the dominating deformations appear in the near field and attenuate rapidly as the epicentral distance increases. The shallower the point source, the larger the displacements. Both the Earth's curvature and vertical layering have considerable effects on the deformation fields. Especially the vertical layering can cause a 10% difference at the epicentral distance of 0.1°. As an illustration, we calculate the theoretical displacements caused by the 1964 Alaska earthquake (mW = 9.2) and compare the results with the observed vertical displacements at 10 stations. The results of the near field show that the vertical displacement can reach some meters. The far‐field displacements are also significant. For example, the horizontal displacements (uψ) can be as large as 1 cm at the epicentral distance of 30°, 0.5 cm at about 40°, magnitudes detectable by modern instruments, such as satellite laser ranging (SLR), very long baseline interferometry (VLBI), or Global Positioning System (GPS). Globally, the displacement (ur) caused by the earthquake is larger than 0.25 mm.
Influence of the tectosphere upon plate motion
Authors: Stoddard Paul R. , Abbott Dallas
Published: February 2004

This paper tests two hypotheses for the influence of thick lithosphere (the tectosphere) upon plate motion. The first is that the tectosphere reaches down into high‐viscosity regions of the asthenosphere, effectively slowing the motion of continental plates. The second is that the tectosphere reaches down below the slow‐moving convective boundary layer into regions of rapid mantle convection and effectively speeds (or slows) the motion of continental plates. Our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis.
Geochemical stratigraphy of lava flows sampled by the Hawaii Scientific Drilling Project
Authors: Rhodes J. M.
Published: February 2004

Geochemical discriminants are used to place the boundary between Mauna Loa flows and underlying Mauna Kea flows at a depth of about 280 m. At a given MgO content the Mauna Kea flows are lower in SiO2 and total iron and higher in total alkali, TiO2, and incompatible elements than the Mauna Loa lavas. The uppermost Mauna Kea lavas (280 to 340 m) contain alkali basalts interlayered with tholeiites and correlate with the postshield Hamakua Volcanics. In addition to total alkalis, the alkali basalts have higher TiO2, P2O5, Sr, Ba, Ce, La, Zr, Nb, Y, and V relative to the tholeiites and lower Zr/Nb and Sr/Nb ratios. Some of the alkali basalts are extensively differentiated. Below 340 m all the flows are tholeiitic, with compositions broadly similar to the few “fresh” subaerial shield‐building Mauna Kea tholeiites studied to date. High‐MgO lavas are unusually abundant, although there is a wide range (7–28%) in MgO content reflecting olivine control. FeO/MgO relationships are used to infer parental picritic magmas with about 15 wt % MgO. Lavas with more MgO than this have accumulated olivine. The Mauna Loa lavas have compositional trends that are controlled by olivine crystallization and accumulation. They compare closely with trends for historical (1843–1984) flows, tending toward the depleted end of the spectrum. They are, though, much more MgO‐rich (9–30%) than is typical for most historical and young (<30 ka) prehistoric lavas. The unusual abundance of high‐MgO and picritic lavas is attributed to the likelihood that only large‐volume, hot, mobile flows will reach Hilo Bay from the northeast rift zone. FeO/MgO relationships are used to infer parental picritic magmas with about 17 wt % MgO. Again, lavas with more MgO than this have accumulated olivine. Systematic changes in incompatible element ratios are used to argue that the magma supply rate has diminished over time. On the other hand, the relatively constant Zr/Nb and Sr/Nb ratios that compare closely with historical and young (<30 kyr) prehistoric flows are used to argue that the source components for these lavas in the Hawaiian plume have remained relatively uniform over the last 100 kyr.
Increased long‐range intermediate‐magnitude earthquake activity prior to strong earthquakes in California
Authors: Knopoff L. , Levshina T. , Keilis‐Borok V. I. , Mattoni C.
Published: February 2004

We study the space‐time relationships between strong earthquakes in California and the intermediate‐magnitude earthquakes that both precede and follow them. All 11 earthquakes in California with nominal magnitudes greater than or equal to 6.8 from 1941 to 1993 were preceded by an increase in the rate of occurrence of earthquakes having magnitudes greater than 5.1. Ten of the 11 earthquakes occurred when or shortly after the intermediate magnitude activity was greater than its 75th percentile. Three of these strong earthquakes are in a conventional space‐time foreshock‐aftershock relationship with others of the 11 strong events. The precursory activity is concentrated in regions having linear dimensions of the order of a few hundred kilometers; these dimensions are significantly larger than the estimated fracture lengths of the ensuing strong earthquakes. The correlations are ill defined for smaller earthquakes and are almost unidentifiable for earthquakes with magnitudes less than about 4.6. The precursors to the strong earthquakes appear over a time interval of the order of 5 to 10 years before the strong earthquake, although the onset was about 25 years before the San Francisco earthquake. In the case of the Loma Prieta and San Francisco earthquakes, the onset of increased activity appears to be relatively abrupt. The increased activity is either switched off abruptly to distances of the order of hundreds of kilometers shortly after the occurrence of a strong earthquake, or the strong events are themselves part of a precursory pattern of continuing high activity before a second strong earthquake that takes place soon thereafter, with subsequent extinction of activity after it. Thus the intermediate‐magnitude precursors do not directly influence the time and location of the subsequent strong event, but the strong event has a strong influence on the stress field in the vicinity of intermediate‐magnitude earthquakes to distances of the order of many times the scale size of the strong earthquake.
Terrestrial heat flow in the Paraná Basin, southern Brazil
Authors: Hurter Suzanne J. , Pollack Henry N.
Published: February 2004

We present 56 new heat flow values from the intracratonic Paraná Basin in southern Brazil. This large basin is filled with up to 5 km of Paleozoic and Mesozoic sedimentary rocks. In the Late Jurassic‐Early Cretaceous a great igneous event capped most of the basin surface with flood basalts up to 1700 m thick. Geothermal gradients computed from 79 deep exploration boreholes range from 20 K km−1 to 30 K km−1 with the lower gradients generally located in the central part of the basin. Thermal conductivities were determined on 247 core samples. The harmonic mean thermal conductivity of the section encountered by the boreholes decreases from 3.0 W m−1 K−1 at the eastern basin margin to 2.0 W m−1 K−1 in the basin center; this variation is related to the thickening of the basalt cap toward the basin center. Surface heat flow values for the 56 sites range from 40 mW m−2 to 75 mW m−2, with larger and more variable values (50–70 mW m−2) occurring along the eastern margin of the basin in the region without basalt cover. The heat flow in the central part of the basin (40–50 mW m−2) is less than that on the basin margin by about 15 mW m−2 and is more uniform. We discount advective effects as an explanation of the heat flow pattern because if a topographically driven flow system existed, it would diminish heat flow in the elevated recharge area along the basin margin and augment heat flow in the discharge area along the basin axis, opposite to what is observed. Wholly conductive models show that larger‐scale thermal conductivity contrasts produced by the flood basalts do not refract significant heat into the surrounding higher‐conductivity sedimentary section on the periphery of the basalts. Other model calculations show that the heat flow at the surface reflects the heat input from the basement with only minor, if any, redistribution within the basin. We conclude that the thermal data indicate a dominantly conductive thermal regime within the basin and that the observed heat flow pattern is not likely to result from intrabasinal causes. The observed pattern likely reflects the larger‐scale thermal structure of the lithosphere of this region, developed at the time the flood basalts were generated and extruded.
Geomagnetic field inclinations for the past 400 kyr from the 1‐km core of the Hawaii Scientific Drilling Project
Authors: Holt John W. , Kirschvink Joseph L. , Garnier Florence
Published: February 2004

A volcanic record of geomagnetic field inclination for the past ∼400 kyr at Hilo, Hawaii, has been obtained from the 941.5 m of core recovered by the Hawaii Scientific Drilling Project. The analysis of 195 lava flows reveals six instances of near‐zero inclination and two instances of fully negative inclination (reverse polarity) within an otherwise normal‐polarity core. In particular, flow unit 23 (∼178 m depth) records a horizontal inclination and may be associated with the Laschamp event; flow units 40 and 42 (∼260 m depth) record negative inclinations and are close in age to the Blake event; and flow unit 55 (∼320 m depth) records a negative inclination with a relative declination change of ∼75° with respect to the overlying flow and is probably the Jamaica/Biwa I/Pringle Falls event. The five instances of shallow inclination found below 400 m depth appear to have resulted from long‐term secular variation as they are part of inclination swings between ∼0° and ∼60° with a periodicity of ∼10–50 kyr. In contrast, the inclination shifts at ∼178 m and ∼320 m depths significantly deviate from long‐term trends, suggesting the existence of at least two independent processes producing time variations of the geomagnetic field. The secular variation has a mean of 30.9° (α95 = 2.27°), which is significantly shallower than the expected dipole mean of 36°. The dispersion (σ = 12.5°) agrees with global paleosecular variation data for 0–5 Ma and secular variation models.
A statistical analysis of Arctic pressure ridge morphology
Authors: Davis N. R. , Wadhams P.
Published: February 2004

Individual pressure ridges were identified on both sensors: the shape on the upward looking sonar was used to obtain draft and apparent slopes and width of each ridge, while the orientation of the ridge as revealed by the side scan sonar was used to convert these figures into absolute slopes and width. The data set comprises 729 ridges. It was found that in addition to ridge spacing, ridge drafts fit a lognormal distribution better than a negative exponential; that the mean draft of ridges diminishes with downstream distance in the Trans‐3Polar Drift Stream at about 0.625 m per degree, suggesting a faster melt rate for ridge bottoms than for the ice as a whole; and that ridge widths and slopes also fit lognormal distributions. An empirical relationship was found relating ridge slope to the ratio of half width to draft. The mean ridge slope varied with location but always lay in the range 21.7° to 26.5°.
Phase‐Doppler anemometer measurement of bubble concentrations in laboratory‐simulated breaking waves
Authors: Asher William E. , Farley Paul J.
Published: February 2004

Breaking waves and the bubble plumes they generate are thought to be an important air‐sea gas transfer mechanism. In order to compare bubble populations generated by laboratory‐simulated breaking waves and model bubble‐mediated airwater gas transfer in a whitecap simulation tank (WST), it was necessary to measure bubble sizes and water velocities. The phase‐Doppler anemometer (PDA) is well suited for this research because it provides simultaneous measurement of the size and velocity of a bubble. Bubble concentrations can be estimated from PDA data records with an accuracy of 15% (±1σ) for bubbles with radii in the range of 50 to 160 μm and 40% (±1σ) for larger bubbles. However, estimation of bubble concentrations requires knowledge of the instrument‐scattering cross section as a function of bubble radius. A procedure for calibrating the scattering cross section of the PDA was developed and tested. The PDA was then used to measured size‐segregated bubble concentrations in the WST as a function of depth, water temperature, total concentration of dissolved gas, and surfactant concentration. These measurements show that increases in dissolved gas concentration or decreases in water temperature increase bubble concentrations. Surfactants increased the concentration of small bubbles. Comparison of the WST bubble populations to measurements of oceanic bubbles showed that the two agree within the experimental uncertainty of the PDA.
Investigating processes of marine ice formation in a floating ice tongue by a high‐resolution isotopic study
Authors: Souchez R. , Tison J.‐L. , Lorrain R. , Fléhoc C. , Stiévenard M. , Jouzel J. , Maggi V.
Published: February 2004

A better knowledge of boundary conditions near a grounding line is critical for understanding the behavior of ice shelves and floating glaciers. We show here thatsignificant information can be gained from a high‐resolution isotopic and textural studyof marine ice accreted at the bottom of a floating glacier near its grounding line. Two different types of marine ice have been found. Type 1 is bubble‐and debris‐free icewith properties which, we believe, can be explained by intrusion of brackish water inopen basal fissures. Closing of the fissures by progression of a freezing front from thesides is precluded, and filling by frazil ice is favored. Type 2 is made of thin, clear iceand debris layers which are thought to have formed when a subglacial water‐filledsediment enters into contact with seawater and is subjected to freezing under a double‐diffusion process. The paper also stresses that in a δD‐δ18O diagram the alignment of marine ice samples on a mixing line does not necessarily imply a mixture of continental water and seawater in varying proportions.
Particulate and dissolved spectral absorption on the continental shelf of the southeastern United States
Authors: Nelson James R. , Guarda Sonia
Published: February 2004

Visible absorption spectra of particulate and dissolved materials were characterized on the continental shelf off the southeastern United States (the South Atlantic Bight), emphasizing cross‐shelf and seasonal variability. A coastal front separates turbid coastal waters from clearer midshelf waters. Spatial and seasonal patterns were evident in absorption coefficients for phytoplankton, detritus, and colored dissolved organic matter (CDOM); spectral shape parameters for CDOM and detritus; and phytoplankton chlorophyll‐specific absorption. The magnitude of CDOM absorption reflected seasonal differences in freshwater discharge and the salinity of the midshelf waters. In the spring of 1993 (high discharge), CDOM absorption at 443 nm was >10 times that of total particulate absorption between 12 and 50 km offshore (0.28–0.69 m−1 versus 0.027–0.062 m−1) and up to 10 times the CDOM absorption measured in the previous summer (low discharge). Phytoplankton chlorophyll‐specific absorption in the blue increased with distance from shore (from <0.03 m2 mg−1 in inner shelf waters to ∼0.1 m2 mg−1 at the most seaward stations in summer) and, for similar chlorophyll concentrations, was higher in summer than in the winter‐spring. These spatial and seasonal patterns in phytoplankton chlorophyll‐specific absorption can be attributed to a shift in phytoplankton species composition (from predominantly diatoms inshore to a cyanobacteria‐dominated assemblage midshelf in summer), pigment packaging, and higher carotenoid:chlorophyll with distance from shore.
Satellite‐derived sea surface temperatures: Current status
Authors: Barton Ian J.
Published: February 2004

The current status of techniques for deriving sea surface temperatures (SST) from infrared satellite data is reviewed. A short history of the subject is presented, along with a survey of the different techniques employed for obtaining SST from space measurements. The discussion concentrates on the advanced very high resolution radiometer (AVHRR) and similar instruments, as these are found to provide the most useful data for SST estimation. A comparison of different algorithms for deriving SST from the AVHRR instruments shows that there has been little improvement in derivation accuracy over the past decade. The importance of the “first‐guess” principle is introduced and shows that some of the more recent “advances” in this field do not address the residual source of error in SST derivation. New algorithms that include coefficients dependent on the variances of the satellite brightness temperature images are found to be difficult to apply to real satellite data. Other new algorithms that include measurements of total atmospheric water vapor column may give a slight improvement in derivation accuracy, but they do not account for anomalous vertical structure in the atmosphere. Directions for future research to improve the accuracy of satellite‐derived SST are suggested.
Optical serial sectioned Chlorophyllamicrostructure
Authors: Palowitch Andrew W. , Jaffe Jules S.
Published: February 2004

An underwater serial sectioning tomography technique is shown capable of determining three‐dimensional Chlorophylla(Chla) microstructure in phytoplankton distributions. The technique involves scanning a 457‐nm illumination plane through a range of distances parallel to the imaging plane of a digital CCD camera. Images of induced fluorescence at 685 nm in the sequentially illuminated planes are recorded. A theoretical model of the imaging process in the form of i = lnc+Ac+xshows that the image i is a function of a spatially varying c (Chl a) dependent absorption component A and a spatially invariant attenuation componentx. A numerical inverse method, incorporating an environmentally dependent calibration constant, is used to calculate Chlavalues from the image intensities for three‐dimensional presentation. A stable reconstruction, not affected by noise‐induced error propagation, is demonstrated. Laboratory testing of a prototype system has shown that Chlaconcentrations from 0.1 to 2.0 mg Chlam−3at a 0.1 mg Chlam−3resolution can be determined. The limiting factor in the imaging process is the rapid attenuation of the fluoresced 685‐nm light which determines the system optical depth ξ. With 0.1 mg Chlam−3as a minimum, ξ = 0.97. At this ξ andc(685)≤0.7 m−1, a 1 m3volume of 0.1 mg Chlam−3can be imaged with a spatial resolution of 1 cm3. At higher Chlalevels, larger values of ξ permit larger volumes to be imaged. The remote sensing aspect of this underwater tomography technique permits in situ Chlamicrostructure information to be obtained without disturbing the naturally occurring phytoplankton distributions, thus providing a new mechanism for microscale biological and physical oceanographic investigation.
Bidirectional reflectance of oceanic waters: A comparison of modeled and measured upward radiance fields
Authors: Morel André , Voss Kenneth J. , Gentili Bernard
Published: February 2004

The bidirectional reflectance of oceanic waters is conveniently described in a normalized way by forming the ratio of the up welling irradiance Eu to any up welling radiance Lu(θ′, φ). This ratio, Q[θ′, θ0, (φ0 − φ)], where θ′, φ are the nadir and azimuth angles for the upward radiance and θ0, φ0 are the zenith and azimuth angles of the Sun, has been determined from measurements at sea and computed via Monte Carlo simulations using the inherent optical properties measured in the field and appropriate boundary conditions (clear sky, no wind, varying Sun angle). Experimental and computed Q values are in excellent agreement. This successful comparison confirms the importance of the bidirectional character of ocean reflectance, already pointed out from a purely numerical approach without field validation, and corroborates the extended range of the Q variations. The later point is of importance when interpreting the marine signals detected by an ocean color satellite‐borne sensor. The validation is extended by considering the historical data for the radiance distributions in Lake Pend Oreille determined at various depths. The closure issue in ocean optics is examined by solving the direct problem of radiative transfer and through a model‐data comparison in terms of radiance field.