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Showing papers by "United States Geological Survey published in 1986"


Journal ArticleDOI
TL;DR: Results indicate that iron reduction can outcompete methanogenic food chains for sediment organic matter when amorphous ferric oxyhydroxides are available in anaerobic sediments, and the transfer of electrons from organic matter to ferric iron can be a major pathway for organic matter decomposition.
Abstract: The potential for ferric iron reduction with fermentable substrates, fermentation products, and complex organic matter as electron donors was investigated with sediments from freshwater and brackish water sites in the Potomac River Estuary. In enrichments with glucose and hematite, iron reduction was a minor pathway for electron flow, and fermentation products accumulated. The substitution of amorphous ferric oxyhydroxide for hematite in glucose enrichments increased iron reduction 50-fold because the fermentation products could also be metabolized with concomitant iron reduction. Acetate, hydrogen, propionate, butyrate, ethanol, methanol, and trimethylamine stimulated the reduction of amorphous ferric oxyhydroxide in enrichments inoculated with sediments but not in uninoculated or heat-killed controls. The addition of ferric iron inhibited methane production in sediments. The degree of inhibition of methane production by various forms of ferric iron was related to the effectiveness of these ferric compounds as electron acceptors for the metabolism of acetate. The addition of acetate or hydrogen relieved the inhibition of methane production by ferric iron. The decrease of electron equivalents proceeding to methane in sediments supplemented with amorphous ferric oxyhydroxides was compensated for by a corresponding increase of electron equivalents in ferrous iron. These results indicate that iron reduction can outcompete methanogenic food chains for sediment organic matter. Thus, when amorphous ferric oxyhydroxides are available in anaerobic sediments, the transfer of electrons from organic matter to ferric iron can be a major pathway for organic matter decomposition.

1,360 citations


Journal ArticleDOI
TL;DR: In this paper, the authors proposed a method to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks using gridded magnetic or gravity anomaly data.
Abstract: Cordell and Grauch (1982, 1985) discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks. The final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes. We attempt to automate their final step. Our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources. The method applies to both local surveys and to continent-wide compilations of magnetic and gravity data (e.g., Zietz, 1982; Simpson et al., 1983a; Kane et al., 1982).

810 citations


ReportDOI
01 Jan 1986
TL;DR: Lowland terrains as mentioned in this paper consist of all plains-forming units between the highland-lowland boundary scarp and the north edge of the map area, exclusive of materials of the western volcanic assemblage on the Tharsis swell.
Abstract: Although the origin and composition of many units are obscure or controversial, their interpretations are based on objective descriptions of morphologic characteristics visible on Viking photomosaics and images. LOWLAND TERRAIN MATERIALS Consist of all plains-forming units between the highland-lowland boundary scarp and the north edge of the map area, exclusive of materials of the western volcanic assemblage on the Tharsis swell. Northern plains assemblage Materials deposited in widespread sheets on northern plains. Within each formation, members mapped at places on basis of crater density; these contacts are approximately located. Assemblage postdates highland-lowland boundary scarp (Scott, 1979). ARCADIA FORMATION—Forms low-lying plains in Arcadia, Amazonis, and Acidalia Planitiae. Embays highland margins and partly buries outflow channels of Kasei, Shalbatana, Simud, Tiu, and Ares Valles. Members distinguished on basis of morphology, albedo, and crater density; common boundaries of older members mapped arbitrarily at places. Flows with lobate margins and small hills with summit craters visible in many places. High-resolution pictures show that sources of some flows are small cratered cones. Interpretation: Mostly lava flows and small volcanoes Aa5 Member 5—Relatively small areal extent. Dark, fresh-appearing flows; few superposed impact craters. Type area: lat 47° N., long 30° Aa4 Member 4—In Arcadia Planitia underlies member 5 and has similar appearance; one other occurrence in channel system of Chryse Planitia. Type area: lat 45° N., long 175° Aa3 Member 3—Forms smooth plains west of Olympus Mons aureoles; embays both the aureoles and fractured terra of Acheron Fossae. Flow fronts visible in places. Type area: lat 15° N., long 155° Aa2 Member 2—Underlies members 3, 4, and 5 in Arcadia Planitia. Includes many small (<10-km-diameter) structures resembling volcanoes and cinder cones. Curved concentric ridges visible on surfaces of flows. Type area: lat 45° N., long 155° Aa1 Member l—Widespread in Chryse and Amazonis Planitiae. Mare-type (wrinkle) ridges common. Type area: lat 30° N., long 40° MEDUSAE FOSSAE FORMATION—Consists of extensive, relatively flat sheets, generally smooth to grooved and gently undulating; deposits appear to vary from soft to indurated; albedo moderate. Occurs near equator in western part of map area. Total thickness may exceed 3 km Amu Upper member—Discontinuous but widespread deposits extend from south of Olympus Mons westward across Amazonis Sulci to map boundary. Smooth, flat to rolling, light-colored surfaces; sculptured into ridges and grooves in places (as in Medusae Fossae); broadly curved margins, locally serrated. Type area: lat 0° N., long 160°. Interpretation: Nonwelded ash-flow …

744 citations


Journal ArticleDOI
07 Nov 1986-Science
TL;DR: Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.
Abstract: Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

718 citations


Journal ArticleDOI
TL;DR: The availability of microbially reducible Fe(III) in surficial sediments demonstrates that microbial Fe( III) reduction can be important to organic matter decomposition and iron geochemistry, however, the overall extent of microbial Fe (III) reduction is governed by the inability of microorganisms to reduce most of the Fe(II) in the sediment.
Abstract: The distribution of Fe(III), its availability for microbial reduction, and factors controlling Fe(III) availability were investigated in sediments from a freshwater site in the Potomac River Estuary. Fe(III) reduction in sediments incubated under anaerobic conditions and depth profiles of oxalate-extractable Fe(III) indicated that Fe(III) reduction was limited to depths of 4 cm or less, with the most intense Fe(III) reduction in the top 1 cm. In incubations of the upper 4 cm of the sediments, Fe(III) reduction was as important as methane production as a pathway for anaerobic electron flow because of the high rates of Fe(III) reduction in the 0- to 0.5-cm interval. Most of the oxalate-extractable Fe(III) in the sediments was not reduced and persisted to a depth of at least 20 cm. The incomplete reduction was not the result of a lack of suitable electron donors. The oxalate-extractable Fe(III) that was preserved in the sediments was considered to be in a form other than amorphous Fe(III) oxyhydroxide, since synthetic amorphous Fe(III) oxyhydroxide, amorphous Fe(III) oxyhydroxide adsorbed onto clay, and amorphous Fe(III) oxyhydroxide saturated with adsorbed phosphate or fulvic acids were all readily reduced. Fe3O4 and the mixed Fe(III)-Fe(II) compound(s) that were produced during the reduction of amorphous Fe(III) oxyhydroxide in an enrichment culture were oxalate extractable but were not reduced, suggesting that mixed Fe(III)-Fe(II) compounds might account for the persistence of oxalate-extractable Fe(III) in the sediments. The availability of microbially reducible Fe(III) in surficial sediments demonstrates that microbial Fe(III) reduction can be important to organic matter decomposition and iron geochemistry. However, the overall extent of microbial Fe(III) reduction is governed by the inability of microorganisms to reduce most of the Fe(III) in the sediment.

674 citations


Journal ArticleDOI
TL;DR: In this article, a global stratigraphy of Mars was developed from a global geologic map series derived from Viking images; a new chronostratigraphic classification system which consists of lower, middle, and upper Noachian, Hesperian, and Amazonian systems is described.
Abstract: A global stratigraphy of Mars was developed from a global geologic map series derived from Viking images; the stratigraphy is composed of three maps. A new chronostratigraphic classification system which consists of lower, middle, and upper Noachian, Hesperian, and Amazonian systems is described. The crater-density boundaries of the chronostratigraphic units and the absolute ages of the Martian epochs aer estimated. The relative ages of major geologic units and featues are calculated and analyzed. The geologic history of Mars is summarized on the maps in terms of epochs.

584 citations


Journal ArticleDOI
07 Feb 1986-Science
TL;DR: The San Francisco Bay estuary has been rapidly modified by human activity; the introduction of exotic species has transformed the composition of its aquatic communities; reduction of freshwater inflow has changed the dynamics of its plant and animal communities; and wastes have contaminated its sediments and organisms.
Abstract: The San Francisco Bay estuary has been rapidly modified by human activity. Diking and filling of most of its wetlands have eliminated habitats for fish and waterfowl; the introduction of exotic species has transformed the composition of its aquatic communities; reduction of freshwater inflow by more than half has changed the dynamics of its plant and animal communities; and wastes have contaminated its sediments and organisms. Continued disposal of toxic wastes, the probable further reduction in freshwater inflow, and the possible synergy between the two provide the potential for further alteration of the estuary's water quality and biotic communities.

580 citations


Journal ArticleDOI
04 Jul 1986-Science
TL;DR: Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere, and Voyager images confirm the extremely low albedo of the ring particles.
Abstract: Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70 degrees to -27 degrees ), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20 degrees . Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 10(2) new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earth's moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Miranda's surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

408 citations


Journal ArticleDOI
TL;DR: In this article, the concentration ratios of Hf, Zr and REE between zircon, apatite, and liquid were determined for three igneous compositions: two andesites and a diorite.
Abstract: Concentration ratios of Hf, Zr, and REE between zircon, apatite, and liquid were determined for three igneous compositions: two andesites and a diorite. The concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient. Although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients, those for the apatite in the diorite may deviate from the partition coefficients. The HREE partition coefficients between zircon and liquid are very large (100 for Er to 500 for Lu), and the Hf partition coefficient is even larger. The REE partition coefficients between apatite and liquid are convex upward, and large (D=10–100), whereas the Hf and Zr partition coefficients are less than 1. The large differences between partition coefficients of Lu and Hf for zircon-liquid and for apatite-liquid are confirmed. These partition coefficients are useful for petrogenetic models involving zircon and apatite.

367 citations


Journal ArticleDOI
TL;DR: In this article, the skeleton structures of terrestrial cyclic terpenoids were analyzed in the solvent extractable material of fossil wood, ambers and brown coals and it was concluded from these structural distributions that aromatization is the main process for the transformation of terrestrial terpenoid during diagenesis.

323 citations


Journal ArticleDOI
17 Jan 1986-Science
TL;DR: High-resolution Viking orbiter images show evidence for quasi-viscous relaxation of topography, which suggests that ice at low latitudes has been lost via sublimation and diffusion through the regolith, thereby causing a net poleward transport of ice over martian history.
Abstract: High-resolution Viking orbiter images show evidenced for quasi-viscous relaxation of topography. The relaxation is believed to be due to creep deformation of ice in near-surface materials. The global distribution of the inferred ground ice shows a pronounced latitudinal dependence. The equatorial regions of Mars appear to be ice-poor, while the heavily cratered terrain poleward of + or - 30 deg latitude appears to be ice-rich. The style of creep poleward of + or - 30 deg varies with latitude, possibly due to variations in ice rheology with temperature. The distribution suggests that ice at low latitudes, which is not in equilibrium with the present atmosphere, has been lost via sublimation and diffusion through the regolith, thereby causing a net poleward transport of ice over martian history.

Journal ArticleDOI
TL;DR: In this paper, the compositional gap between andesite and rhyolite has been generated by fractional crystallization, and the inclusions of parent andesitic inclusions are used to record the early part of the differentiation history.
Abstract: At Medicine Lake Volcano, California, the compositional gap between andesite (57–62 wt.% SiO2) and rhyolite (73–74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite. Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H2O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions.

Journal ArticleDOI
TL;DR: In this paper, a generalized model of humic materials in soils and sediments, which is consistent with their observed properties, is presented, providing a means of understanding the interaction of hydrophobic pollutants with humic material.

Journal ArticleDOI
TL;DR: In this article, the frequency distribution of the ratio radius of curvature/channel width has a modal value in the range of 2 to 3; about one third of the 79 values is less than 2.

Journal ArticleDOI
01 Nov 1986-Icarus
TL;DR: In this paper, the authors estimate that Mars had at least 0.5 to 1 km of water, 10 to 20 bar of CO2, and 0.1 to 0.3 bar of N2 at the end of heavy bombardment.

Journal ArticleDOI
04 Apr 1986-Science
TL;DR: Speciation calculations for aluminum, in water samples taken from a drainage basin containing acid mine waters, demonstrate a distinct transition from conservative behavior for pH below 4.6 to nonconservative behavior above 4.9, corresponding to the pK for the first hydrolysis constant of the aqueous aluminum ion.
Abstract: Speciation calculations for aluminum, in water samples taken from a drainage basin containing acid mine waters, demonstrate a distinct transition from conservative behavior for pH below 4.6 to nonconservative behavior for pH above 4.9. This transition corresponds to the pK for the first hydrolysis constant of the aqueous aluminum ion and appears to be a consistent phenomenon independent of field location, ionic strength, and sulfate concentration. Nonconservative behavior is closely correlated with the equilibrium solubility of a microcrystalline gibbsite or amorphous aluminum hydroxide.

Book ChapterDOI
01 Jan 1986
TL;DR: In this article, a linear regression analysis of the mean random vitrinite reflectance (Rm in %) with the maximum burial temperature (Tmax in °C) was performed.
Abstract: Mean random vitrinite reflectance (Rm in %) shows a strong correlation (r2=0.7, n>600) with maximum burial temperature (Tmax in °C). These data are modelled by the linear regression equation: $$\ln (Rm) = 0.078 T_{\max } - 1.2$$ Tmax and Rm were compiled from over 35 systems, rich in humic organic matter to minimize the effect of chemical composition on Rm. The thermal maturation data span a range from early diagenesis to greenschist metamorphismover a Tmax interval from about 25 – 325° C and 0.2 – 4.0 % Rm. Burial history reconstruction indicates that the functional heating duration (elapsed time as temperature increases within 15° C of Tmax) of these systems ranges from 10,000 yr to more than 10 m.y. The strong correlation of Tmax with Rm, irrespective of functional heating duration and in diverse geologic systems, indicates that increasing time at Tmax has little influence on thermal maturation of sedimentary organic matter. Instead, uncertainty in correction of borehole temperature logs, Tmax determination, and Rm measurement explains much of the remaining variability not accounted for by the regression analysis. We did not attempt to correct the measured borehole temperature to equilibrium reservoir conditions because there is no consensus on which method to use, the necessary data is often unrecorded, and predictions made from our Tmax — Rm calibration are compared to uncorrected Tmax data. We found that Tmax is difficult to determine in sedimentary environments that have cooled because of the weak thermal imprint on the rocks in low temperature systems and poorly-known burial histories. Variability in Rm measurement appears mainly due to operator or laboratory bias, increasing bireflectance with rank, and variation in diagenetic history which causes reflectance suppression.

Journal ArticleDOI
TL;DR: The chemical and isotopic composition of rainfall and stream water was monitored during a storm in the Mattole River basin of northwestern California as mentioned in this paper, where 250 mm of rain fell during 6 days (∼80% within a 42-hour period) in late January, 1972, following 24 days of little or no precipitation.

Journal ArticleDOI
TL;DR: In this paper, a cubic spline interpolating function is used to solve the 3D electromagnetic scattering problem in the frequency domain using sine and cosine digital filters or with the decay spectrum.
Abstract: The three‐dimensional (3-D) electromagnetic scattering problem is first formulated in the frequency domain in terms of an electric field volume integral equation. Three‐dimensional responses are then Fourier transformed with sine and cosine digital filters or with the decay spectrum. The digital filter technique is applied to a sparsely sampled frequency sounding, which is replaced by a cubic spline interpolating function prior to convolution with the digital filters. Typically, 20 to 40 frequencies at five to eight points per decade are required for an accurate solution. A calculated transient is usually in error after it has decayed more than six orders in magnitude from early to late time. The decay spectrum usually requires ten frequencies for a satisfactory solution. However, the solution using the decay spectrum appears to be less accurate than the solution using the digital filters, particularly after early times. Checks on the 3-D solution include reciprocity and convergence checks in the frequenc...

Journal ArticleDOI
TL;DR: In this article, a correlation between the hydrogen index and carbon isotopic composition (σ 13C) of Cretaceous organic carbon (OC) was found to be a strong indicator of the marine source of organic carbon.

Journal ArticleDOI
TL;DR: The results indicate the methanogens and sulfate reducers compete for DMS when it is present at low concentrations; however, at high concentrations, DMS is a "noncompetitive" substrate for methanogenesis.
Abstract: Addition of dimethylsulfide (DMS), dimethyldisulfide (DMDS), or methane thiol (MSH) to a diversity of anoxic aquatic sediments (eg, fresh water, estuarine, alkaline/hypersaline) stimulated methane production The yield of methane recovered from DMS was often 52 to 63%, although high concentrations of DMS (as well as MSH and DMDS) inhibited methanogenesis in some types of sediments Production of methane from these reduced methylated sulfur compounds was blocked by 2-bromoethanesulfonic acid Sulfate did not influence the metabolism of millimolar levels of DMS, DMDS, or MSH added to sediments However, when DMS was added at ∼2-μM levels as [14C]DMS, metabolism by sediments resulted in a 14CH4/14CO2 ratio of only 006 Addition of molybdate increased the ratio to 18, while 2-bromoethanesulfonic acid decreased it to 0, but did not block 14CO2 production These results indicate the methanogens and sulfate reducers compete for DMS when it is present at low concentrations; however, at high concentrations, DMS is a “noncompetitive” substrate for methanogens Metabolism of DMS by sediments resulted in the appearance of MSH as a transient intermediate A pure culture of an obligately methylotrophic estuarine methanogen was isolated which was capable of growth on DMS Metabolism of DMS by the culture also resulted in the transient appearance of MSH, but the organism could grow on neither MSH nor DMDS The culture metabolized [14C]-DMS to yield a 14CH4/14CO2 ratio of ∼28 Reduced methylated sulfur compounds represent a new class of substrates for methanogens and may be potential precursors of methane in a variety of aquatic habitats

Journal ArticleDOI
TL;DR: The Green River is one of the principal tributaries in the Colorado River basin and drains 44,700 m 2 in Wyoming, Colorado, and Utah as mentioned in this paper, where flows of the Green River have been regulated by Flaming Gorge Reservoir.
Abstract: The Green River is one of the principal tributaries in the Colorado River basin and drains 44,700 m 2 in Wyoming, Colorado, and Utah. Since October 1962, flows of the Green River have been regulated by Flaming Gorge Reservoir, which is located 412 river miles upstream from its confluence with the Colorado River. Mean annual runoff has not been affected by the reservoir. The duration of the relatively large discharges that transport most of the annual sediment load, however, has decreased significantly. As a result, the mean annual sediment discharge has decreased by 54% to 3.21 × 10 6 tons from 6.92 × 10 6 tons at the Jensen gage located 105 river miles downstream from the reservoir and by 48% to 8.83 × 10 6 tons from 17.0 × 10 6 tons at the Green River, Utah, gage located 290 river miles downstream from the reservoir. Sediment supply to I he channel equals the annual transport within a relatively short distance, 68 river miles, downstream from the reservoir. Downstream from river mile 166, the supply of sediment from upstream plus tributary inflow exceeds the transport of sediment by ∼5.4 × 10 6 tons per year on an average. The quasi-equilibrium that appears to have existed prior to the reservoir no longer occurs along a majority of the Green River. In response to the reduced peak discharges, the bankfull channel width of the Green River has decreased by ∼10%. Adjustment of the channel to decreased peak flows and altered sediment loads is nowhere complete. At present, it appears that a century or more will be required for the Green River to adjust to the effects of Flaming Gorge Reservoir.

Journal ArticleDOI
01 Feb 1986-Geology
TL;DR: A geochemical explanation for the extensive dissolution observed along the carbonate coast of the Yucatan Peninsula, Mexico is provided in this article, where a mixture of fresh ground water with subterranean Caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonites.
Abstract: A geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the Yucatan Peninsula, Mexico. Mixing of fresh ground water with subterranean Caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite. Mixing-zone dissolution caused by ground-water discharge is a major geomorphic process in developing caves, coves, and crescent-shaped beaches along the Yucatan coast. Such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record.

Journal ArticleDOI
01 Jan 1986-Geology
TL;DR: In this article, small shear zones in granitic rocks of the Sierra Nevada, California, and near Roses (Rosas), northeast Spain, display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation.
Abstract: Small shear zones in granitic rocks of the Sierra Nevada, California, and near Roses (Rosas), northeast Spain, display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation. Many zones have sharp, nearly planar boundaries between their highly deformed interiors and the undeformed wall rock. The Roses shear zones narrow continuously to hairline fractures at their ends. Mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode. Dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation. Gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones. We suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes, thereby promoting chemical alteration and/or hydrolytic weakening.

Journal ArticleDOI
01 Nov 1986-Nature
TL;DR: The western interior of North America has the only known non-marine sections that contain the iridium-rich clay interpreted as the Cretaceous-Tertiary boundary.
Abstract: The western interior of North America has the only known non-marine sections that contain the iridium-rich clay interpreted as the Cretaceous–Tertiary (K–T) boundary1–7. Because vegetation and climate can be directly inferred from physiognomy of leaves8–15 and because leaf species typically represent low taxonomic categories, studies of leaf floras in these sections provide data on the effects of a terminal Cretaceous event on the land flora, vegetation and climate. A previous study based on detailed sampling of leaves and their dispersed cuticle16 in the Raton Basin provides a framework for interpretation of other leaf sequences over 20 degrees of latitude. We conclude that at the boundary there were: (1) high levels of extinction in the south and low levels in the north; (2) major ecological disruption followed by long-term vegetational changes that mimicked normal ecological succession; (3) a major increase in precipitation; and (4) a brief, low-temperature excursion, which supports models of an ‘impact winter’

Journal ArticleDOI
TL;DR: In this article, the authors present a new view of familiar themes, focusing on the changes implied by the way these feedbacks respond to perturbations, rather than the nature of the steady state maintained by complex networks of feedbacks.
Abstract: Readers of this book will generally fit into two groups. One group consists of geologists and geochemists, who have studied the global carbon cycle for many decades. These readers will find that the papers in this book present a new view of familiar themes. Whereas much previous work on the carbon cycle and other geochemical cycles has emphasized the nature of the steady state maintained by complex networks of feedbacks, recent attention has shifted to the changes implied by the way these feedbacks respond to perturbations.

Journal ArticleDOI
TL;DR: Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka as mentioned in this paper, which is dominated by the 6 km wide Cinque Denti caldera, which formed ca. 45 Ka during the Green Tuff eruption.
Abstract: Situated in a submerged continental rift, Pantelleria is a volcanic island with a subaerial eruptive history longer than 300 Ka. Its eruptive behavior, edifice morphologies, and complex, multiunit geologic history are representative of strongly peralkaline centers. It is dominated by the 6-km-wide Cinque Denti caldera, which formed ca. 45 Ka ago during eruption of the Green Tuff, a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls, surges, and pyroclastic flows. Soon after collapse, trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it. Progressive chemical evolution of the chamber between 45 and 18 Ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera. Beginning about 18 Ka ago, inflation of the chamber opened old ring fractures and new radial fractures, along which recently differentiated pantellerite constructed more than 25 pumice cones and shields. Continued uplift raised the northwest half of the intracaldera trachyte cone 275 m, creating the island's present summit, Montagna Grande, by trapdoor uplift. Pantellerite erupted along the trapdoor faults and their hingeline, forming numerous pumice cones and agglutinate sheets as well as five lava domes. Degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18-3-Ka episode led to entrainment of subjacent, crystal-rich, pantelleritic trachyte magma as crenulate inclusions. Progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0.82 km3 of magma erupted during the episode. The 45-Ka-old caldera is nested within the La Vecchia caldera, which is thought to have formed around 114 Ka ago. This older caldera was filled by three widespread welded units erupted 106, 94, and 79 Ka ago. Reactivation of the ring fracture ca. 67 Ka ago is indicated by venting of a large pantellerite centero and a chain of small shields along the ring fault. For each of the two nested calderas, the onset of postcaldera ring-fracture volcanism coincides with a low stand of sea level. Rates of chemical regeneration within the chamber are rapid, the 3% crystallization/Ka of the post-Green Tuff period being typical. Highly evolved pantellerites are rare, however, because intervals between major eruptions (averaging 13−6 Ka during the last 190 Ka) are short. Benmoreites and mugearites are entirely lacking. Fe-Ti-rich alkalic basalts have erupted peripherally along NW-trending lineaments parallel to the enclosing rift but not within the nested calderas, suggesting that felsic magma persists beneath them. The most recent basaltic eruption (in 1891) took place 4 km northwest of Pantelleria, manifesting the long-term northwestward migration of the volcanic focus. These strongly differentiated basalts reflect low-pressure fractional crystallization of partial melts of garnet peridotite that coalesce in small magma reservoirs replenished only infrequently in this continental rift environment.

Journal ArticleDOI
TL;DR: The authors analyzed the extent of isoleucine epimerization (aIle/Ile ratio) in protein preserved in molluscan fossils collected from raised marine deposits.
Abstract: The age and correlation of shorelines around the Mediterranean basin have been addressed by analyzing the extent of isoleucine epimerization (aIle/Ile ratio) in protein preserved in molluscan fossils collected from raised marine deposits. The taxodont genera Glycymeris and Arca were selected as the primary taxa for this study because of their simple shell structure, reproducible aIle/Ile ratios, and ubiquitous occurrence. Direct comparison of aIle/Ile ratios in associated mollusks allows correlation of disjunct marine deposits and relative dating of sequential marine units in nearby areas that have similar thermal histories. The thermal gradient across the Mediterranean basin is, however, sufficiently high that shells from isochronous shorelines have significantly higher ratios at warmer than at cooler localities. Absolute dating, primarily U-series dates on corals directly associated with molluscan samples, provides an independent calibration of the amino acid data and compensates for dissimilar thermal histories. AIle/Ile ratios in shells from 46 marine units cluster into 5 discrete groups (C, E, F, G, and K) that are related to positive sea-level events (interglacials/interstadials) associated with odd-numbered deep-sea isotopic stages. The most complete sequences are in southern Italy, where group C ratios in Glycymeris that average 0.30 are associated with the Neotyrrhenian, a post–last-interglacial (late stage 5), high–sea-level event. Group E ratios (0.38) are associated with classical Eutyrrhenian deposits from which four U-series coral dates (126 ± 7 ka) substantiate the correlation to isotope substage 5e. Group F ratios (0.50) are associated with U-series coral and mollusk dates between 200 and 300 ka; the deposits are tentatively correlated with stage 7. Deposits that have group G ratios (0.58) are correlated with stage 9. Shells from lower Pleistocene marine deposits that have ratios between 1.0 and 1.2 (group K) constrain the ages of the younger groups. An exponential decrease in the epimerization rate inhibits resolution of the older events. AIle/Ile ratios in last-interglacial deposits are similar in the northern and central region of the study area but increase sharply in southern Sicily, North Africa, and Crete, similar to the modern thermal gradient. A lacuna between group G and group K is similar to gaps identified in California and Alaska, suggesting generally lower interglacial sea levels between stage 11 and sometime before the Brunhes/Matuyama boundary.

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TL;DR: In this paper, near-source strong motion velocity records and teleseismic short-period P waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 Morgan Hill earthquake.
Abstract: Near-source strong motion velocity records and teleseismic short-period P waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 Morgan Hill earthquake. Both forward modeling and constrained, least-squares inversion techniques are used to interpret the strong motion velocity waveforms in the frequency range of approximately 0.2 to 2.0 Hz. These data support a nearly unilateral rupture to the southeast with a rupture propagation velocity of nine-tenths of the local S-wave velocity. The majority of the slip occurs over a fault length of 25 km and to a first approximation can be interpreted as two main source regions, each with an extent of about 5 km with their centers separated by about 12 km. However, each of the sources has detailed structure of its own, and a simple two-point-source model is not an accurate representation of the Morgan Hill earthquake. The second source occurs about 4.5 sec after the first and is approximately 3 times larger. The maximum dislocation on the fault plane is about 1 m. The total moment of the earthquake is estimated to be 2.1 × 10^(25) dyne-cm. The Morgan Hill earthquake offers convincing evidence for very inhomogeneous slip and stress distributions on shallow strike-slip faults.

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TL;DR: The isotopic Pb, Sr, Nd, and Hf compositions of nine Hawaiian volcanos were determined using the analytical procedures described by Tatsumoto and Unruh.