bluuebunny/crossref_metadata_2025 huggingface dataset:
A Methanotrophic Marine Molluscan (Bivalvia, Mytilidae) Symbiosis: Mussels Fueled by Gas
Authors: Childress James J. , Fisher C. R. , Brooks J. M. , Kennicutt M. C. , Bidigare R. , Anderson A. E.
Published: October 2006

An undescribed mussel (family Mytilidae), which lives in the vicinity of hydrocarbon seeps in the Gulf of Mexico, consumes methane (the principal component of natural gas) at a high rate. The methane consumption is limited to the gills of these animals and is apparently due to the abundant intracellular bacteria found there. This demonstrates a methane-based symbiosis between an animal and intracellular bacteria. Methane consumption is dependent on the availability of oxygen and is inhibited by acetylene. The consumption of methane by these mussels is associated with a dramatic increase in oxygen consumption and carbon dioxide production. As the methane consumption of the bivalve can exceed its carbon dioxide production, the symbiosis may be able to entirely satisfy its carbon needs from methane uptake. The very light (δ
13
C = -51 to -57 per mil) stable carbon isotope ratios found in this animal support methane (δ
13
C = -45 per mil at this site) as the primary carbon source for both the mussels and their symbionts.
From Stimulation to Undulation: A Neuronal Pathway for the Control of Swimming in the Leech
Authors: Brodfuehrer Peter D. , Friesen W. Otto
Published: October 2006

Initiation and performance of the swimming movement in the leech (
Hirudo medicinalis
) are controlled by neurons organized at at least four functional levels—sensory neurons, gating neurons, oscillator neurons, and motor neurons. A paired neuron, designated as Trl, in the subesophageal ganglion of the leech has now been shown to define a fifth level, interposed between sensory and gating neurons. Cell Trl is activated by pressure and nociceptive mechanosensory neurons, which mediate bodywall stimulus-evoked swimming activity in intact leeches. In the isolated leech nervous system, brief stimulation of cell Trl elicits sustained activation of the gating neurons and triggers the onset of swimming activity. The synaptic interactions between all five levels of control are direct. Discovery of the Trl cells thus completes the identification of a synaptic pathway by which mechanosensory stimulation leads to the swimming movements of the leech.
Community Diversity: Relative Roles of Local and Regional Processes
Authors: Ricklefs Robert E.
Published: October 2006

The species richness (diversity) of local plant and animal assemblages—biological communities—balances regional processes of species formation and geographic dispersal, which add species to communities, against local processes of predation, competitive exclusion, adaptation, and stochastic variation, which may promote local extinction. During the past three decades, ecologists have sought to explain differences in local diversity by the influence of the physical environment on local interactions among species, interactions that are generally believed to limit the number of coexisting species. But diversity of the biological community often fails to converge under similar physical conditions, and local diversity bears a demonstrable dependence upon regional diversity. These observations suggest that regional and historical processes, as well as unique events and circumstances, profoundly influence local community structure. Ecologists must broaden their concepts of community processes and incorporate data from systematics, biogeography, and paleontology into analyses of ecological patterns and tests of community theory.
Identification of a Juvenile Hormone-Like Compound in a Crustacean
Authors: Laufer H. , Borst D. , Baker F. C. , Reuter C. C. , Tsai L. W. , Schooley D. A. , Carrasco C. , Sinkus M.
Published: October 2006

Juvenile hormone (JH) has central roles in the regulation of insect development and reproduction but has not previously been identified in other arthropod classes. The hemolymph of a crustacean,
Libinia emarginata
(Leach), has now been analyzed for JH-like compounds. Samples contained 0.003 to 0.030 nanogram of JH III per milliliter and 10 to 50 nanograms of methyl farnesoate per milliliter; methyl farnesoate is a compound structurally related to JH III that has JH bioactivity. Several tissues were examined for synthesis and secretion of JH-like compounds. Of these tissues, only the mandibular organs produced and secreted JH III and methyl farnesoate. However, microchemical analysis revealed that this JH III was racemic, and thus likely an artifactual oxidation product of methyl farnesoate. Secretion of methyl farnesoate was related to reproduction in females, with the highest rates observed in
Libinia
near the end of the ovarian cycle when oocyte growth and vitellogenesis are greatest. These results indicate that JH-like compounds such as methyl farnesoate have regulatory roles in crustaceans.
Geologic Evolution of Northern Tibet: Results of an Expedition to Ulugh Muztagh
Authors: Molnar Peter , Burchfiel B. Clark , Ziyun Zhao , K'uangyi Liang , Shuji Wang , Minmin Huang
Published: October 2006

A reconnaissance expedition across the northern margin of the Tibetan plateau revealed evidence of a late Cenozoic northward progression of the locus of crustal shortening and, therefore, of a northward growth of the area encompassed by the plateau. Active reverse faults crop out at the foot of the Altyn Tagh, on the northern edge of the plateau, and at the bases of several ranges within the Altyn Tagh and Kunlun, where the elevations of the neighboring basins are less than 4000 meters. Farther south, where elevations are higher, there was no evidence of recent faulting, but late Cenozoic rock in the Ayak Kum Köl basin has been strongly folded. South of this basin, Ulugh Muztagh, apparently the highest mountain in the eastern Kunlun, is underlain by late Miocene, tourmaline-bearing and two-mica granite. These rocks suggest that thickening of continental crust had begun in this area by late Miocene time. Overlying quartz-sanidine welded tuffs of Pliocene age imply that uplift and erosion occurred between Miocene and Pliocene time, but with little subsequent erosion. In addition, we found an east-west trending belt of mafic and ultramafic rock that probably marks a suture of a crustal fragment with southern Asia in Triassic or more recent time.
Can Microscale Chemical Patches Persist in the Sea? Microelectrode Study of Marine Snow, Fecal Pellets
Authors: Alldredge Alice L. , Cohen Yehuda
Published: October 2006

Microelectrode studies demonstrate the existence of persistent oxygen and
p
H gradients around flocculent, macroscopic marine particles known as marine snow. Oxygen is partially, but continuously, depleted within and around marine snow in the dark and can be completely depleted within large fecal pellets. Boundary layers hundreds of micrometers thick are maintained despite advection of fluid past the particles. The existence of chemical microhabitats on the scale of millimeters around macroscopic particles in the pelagic zone may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.
Earthquake Hazards on the Cascadia Subduction Zone
Authors: Heaton Thomas H. , Hartzell Stephen H.
Published: October 2006

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (

M
w

) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (

M
w

8) or a giant earthquake (

M
w

9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of

M
w

less than 8.2 is discussed. Strong ground motions from even larger earthquakes (

M
w

up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis.
Evidence for Great Holocene Earthquakes Along the Outer Coast of Washington State
Authors: Atwater Brian F.
Published: October 2006

Intertidal mud has buried extensive, well-vegetated lowlands in westernmost Washington at least six times in the past 7000 years. Each burial was probably occasioned by rapid tectonic subsidence in the range of 0.5 to 2.0 meters. Anomalous sheets of sand atop at least three of the buried lowlands suggest that tsunamis resulted from the same events that caused the subsidence. These events may have been great earthquakes from the subduction zone between the Juan de Fuca and North America plates.
Tsunamis Generated by Eruptions from Mount St. Augustine Volcano, Alaska
Authors: Kienle Jürgen , Kowalik Zygmunt , Murty T. S.
Published: October 2006

During an eruption of the Alaskan volcano Mount St. Augustine in the spring of 1986, there was concern about the possibility that a tsunami might be generated by the collapse of a portion of the volcano into the shallow water of Cook Inlet. A similar edifice collapse of the volcano and ensuing sea wave occurred during an eruption in 1883. Other sea waves resulting in great loss of life and property have been generated by the eruption of coastal volcanos around the world. Although Mount St. Augustine remained intact during this eruptive cycle, a possible recurrence of the 1883 events spurred a numerical simulation of the 1883 sea wave. This simulation, which yielded a forecast of potential wave heights and travel times, was based on a method that could be applied generally to other coastal volcanos.
Precise Timing of the Last Interglacial Period from Mass Spectrometric Determination of Thorium-230 in Corals
Authors: Edwards R. Lawrence , Chen J. H. , Ku T.-L. , Wasserburg G. J.
Published: October 2006

The development of mass spectrometric techniques for determination of
230
Th abundance has made it possible to reduce analytical errors in
238
U-
234
U-
230
Th dating of corals even with very small samples. Samples of 6 × 10
8
atoms of
230
Th can be measured to an accuracy of ±3 percent (2σ) and 3 × 10
10
atoms of
230
Th can be measured to an accuracy of ±0.2 percent. The time range over which useful age data on corals can be obtained now ranges from about 50 to about 500,000 years. For young corals, this approach may be preferable to
14
C dating. The precision with which the age of a coral can now be determined should make it possible to critically test the Milankovitch hypothesis concerning Pleistocene climate fluctuations. Analyses of a number of corals that grew during the last interglacial period yield ages of 122,000 to 130,000 years. The ages coincide with, or slightly postdate, the summer solar insolation high at 65°N latitude which occurred 128,000 years ago. This supports the idea that changes in Pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth's orbit and rotation axis.
Acid Rain: China, United States, and a Remote Area
Authors: Galloway James N. , Dianwu Zhao , Jiling Xiong , Likens Gene E.
Published: October 2006

The composition of precipitation in China is highly influenced by fossil fuel combustion and agricultural and cultural practices. Compared to the eastern United States, precipitation in China generally has higher concentrations of sulfate, ammonium, and calcium. Wet deposition rates of sulfur in China are 7 to 130 times higher than those in a remote area in the Southern Hemisphere. In many areas of the world, significant ecological changes have occurred in ecosystems that have acid deposition rates substantially less than those currently existing in China.
A Major Direct GABAergic Pathway from Zona Incerta to Neocortex
Authors: Lin Chia-Sheng , Nicolelis Miguel A. L. , Schneider Jay S. , Chapin John K.
Published: October 2006

Retrograde fluorescent tracers were used to demonstrate a previously unknown but sizable direct γ-aminobutyric acid (GABA)-containing neuronal pathway from the zona incerta to the neocortex in rats. This incertocortical pathway was found to project bilaterally to the entire neocortex and exhibited a rough corticotopic organization. Many of the zona incerta neurons projecting to the parietal and occipital cortices could also be immunohistochemically stained with antibodies to glutamic acid decarboxylase and GABA. Few of these neurons were immunoreactive to tyrosine hydroxylase antibodies, which identify dopamine-containing neurons. Injections in the frontal and entorhinal cortices labeled many neurons near or within the dopaminergic A13 subdivision of the zona incerta. In addition, the incertocortical system was found to be significantly larger during early postnatal (2 to 3 weeks) development. The projection pattern of this newly discovered pathway resembles that of the monoaminergic and cholinergic systems, arising from the brainstem and forebrain, suggesting possible similarities of function.
Seasonal Mixing and Catastrophic Degassing in Tropical Lakes, Cameroon, West Africa
Authors: Kling George W.
Published: October 2006

Lethal gas releases from Lakes Nyos and Monoun in Cameroon seem to be more lacustrine than volcanic in origin. Both of these events occurred in August and were only 2 years apart. Data show that the period of deepest mixing and lake turnover also occurs during late summer in this region of tropical Africa. In addition, recent trends of decreases in both air temperatures and effective insolation relative to long-term means suggest that weakening of stratification, coupled with a predictable seasonal interval of reduced stability in August, may be responsible for the timing of these events.
Lateral Isotopic Discontinuity in the Lower Crust: An Example from Antarctica
Authors: Kalamarides Ruth I. , Berg Jonathan H. , Hank Robert A.
Published: October 2006

The lower continental crust is one of the least known variables in the crust-mantle evolutionary equation. In order to study the nature and compositional heterogeneity of the lower crust, more than 20 inclusions of lower crustal granulites in volcanic rocks from the McMurdo Sound region of Antarctica were analyzed for strontium and oxygen isotopes. These inclusions were erupted from volcanic centers covering an area of 12,000 square kilometers. Along with results from analyses of major and trace elements, the isotopic data reveal a profound discontinuity in the composition and probably the age of the lower crust that coincides with the boundary between the Transantarctic Mountains and the Ross Embayment. Although this topographic boundary between East and West Antarctica is largely a Cenozoic development, which apparently reflects a simple subvertical faulting relationship due to crustal rifting, the isotopic differences in the lower crust across the boundary suggest that the current faulting and rifting may coincide with an older crustal suture, the age of which is uncertain.
Three-Dimensional X-Ray Microtomography
Authors: Flannery Brian P. , Deckman Harry W. , Roberge Wayne G. , D'Amico Kevin L.
Published: October 2006

The new technique of x-ray microtomography nondestructively generates three-dimensional maps of the x-ray attenuation coefficient inside small samples with approximately 1 percent accuracy and with resolution approaching 1 micrometer. Spatially resolved elemental maps can be produced with synchrotron x-ray sources by scanning samples at energies just above and below characteristic atomic absorption edges. The system consists of a high-resolution imaging x-ray detector and high-speed algorithms for tomographic image reconstruction. The design and operation of the microtomography device are described, and tomographic images that illustrate its performance with both synchrotron and laboratory x-ray sources are presented.
A Simple Model for Neutrino Cooling of the Large Magellanic Cloud Supernova
Authors: Spergel D. N. , Piran T. , Loeb A. , Goodman J. , Bahcall J. N.
Published: October 2006

A simplified analytic model of a cooling hot neutron star, motivated by detailed computer calculations, describes well the neutrinos detected from the recent supernova in the Large Magellanic Cloud. The observations do not require explanations that invoke exotic physics or complicated astrophysics. The parameters in this simple model are not severely constrained: 6.1
-3.6
+3.5
× 10
52
ergs emitted in electron antineutrinos, a peak temperature of 4.2
-0.8
+1.2
megaelectron volts, a radius of 27
-15
+17
kilometers, and a cooling time of 4.5
-2.0
+1.7
seconds.
New Opportunities in Synchrotron X-ray Crystallography
Authors: Prewitt C. T. , Coppens P. , Phillips J. C. , Finger L. W.
Published: October 2006

Several high-intensity synchrotron x-ray sources have been constructed over the past few years in the United States, West Germany, Great Britain, Japan, France, Italy, and the Soviet Union. Crystallographers have begun to use these facilities for experiments that take advantage of the characteristics of synchrotron radiation, namely, a broad distribution of wavelengths, high intensity, low divergence, strong polarization, and a pulsed time structure. In addition to more familiar diffraction experiments on single crystals and powdered samples, new types of crystallographic studies, for example, energy-dispersive and surface diffraction studies, have progressed rapidly with more general accessibility of synchrotron sources. These high-intensity sources allow diffraction experiments to be performed on very small crystals or on large biological molecules, and permit weak magnetic scattering to be detected. Anomalous dispersion experiments can exploit the ability to vary the wavelength of the radiation, and the pulsed time structure of the beam makes possible fast time-resolved experiments. Because of the availability of synchrotron x-radiation, these and other kinds of experiments will be in the forefront of crystallographic research for the next several years.
In Situ X-ray Absorption Study of Surface Complexes: Selenium Oxyanions on α-FeOOH
Authors: Hayes Kim F. , Roe A. Lawrence , Brown Gordon E. , Hodgson Keith O. , Leckie James O. , Parks George A.
Published: October 2006

A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at an α-FeOOH(goethite)—water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.
Wind-Driven Ocean Currents and Ekman Transport
Authors: Price James F. , Weller Robert A. , Schudlich Rebecca R.
Published: October 2006

Oceanographers have long sought to verify the theoretical Ekman transport relation, which predicts that a steady wind stress acting together with the Coriolis force will produce a transport of water to the right of the wind. In situ measurements of wind and ocean currents provide a detailed view of this phenomenon. By separating the wind-driven current from the measured total current and by averaging over a long record, it is found that the observed transport is consistent with theoretical Ekman transport to within about 10 percent. In this case the wind-driven transport is strongly surface trapped, with 95 percent occurring in the upper 25 meters as a result of fair summer weather.
Antibody-Catalyzed Porphyrin Metallation
Authors: Cochran Andrea G. , Schultz Peter G.
Published: October 2006

An antibody elicited to a distorted
N
-methyl porphyrin catalyzed metal ion chelation by the planar porphyrin. At fixed Zn
2+
and Cu
2+
concentrations, the antibody-catalyzed reaction showed saturation kinetics with respect to the substrate mesoporphyrin IX (2) and was inhibited by the hapten,
N
-methylmesoporphyrin IX (1). The turnover number of 80 hour
-1
for antibody-catalyzed metallation of 2 with Zn
2+
compares with an estimated value of 800 hour
-1
for ferrochelatase. The antibody also catalyzed the insertion of Co
2+
and Mn
2+
into 2, but it did not catalyze the metallation of protoporphyrin IX (3) or deuteroporphyrin IX (4). The antibody has high affinity for several metalloporphyrins, suggesting an approach to developing antibody-heme catalysts for redox or electron transfer reactions.
Pliocene and Pleistocene Hominid-Bearing Sites from West of Lake Turkana, Kenya
Authors: Harris J. M. , Brown F. H. , Leakey M. G. , Walker A. C. , Leakey R. E.
Published: October 2006

Pliocene and Pleistocene fossil localities near the western shoreline of Lake Turkana, ranging in age between 1 million and 3.5 million years in age, have produced important new hominid specimens including most of a
Homo erectus
skeleton and a relatively complete early robust australopithecine cranium. The lacustrine, fluviatile, and terrestrial strata are designated the Nachukui Formation, which is subdivided into eight members. The distribution of sedimentary facies within the Nachukui Formation suggests that, as today, the Labur and Murua Rith ranges formed the western margin of the basin and were drained by eastward-flowing rivers that fed into the forerunner of the present lake or a major river system. There is also stratigraphic evidence for tectonic movement during the deposition of these sediments. Twenty-three of the tuffs observed in the succession occur also in the Koobi Fora Formation east of the lake and in the Shungura Formation of the lower Omo Valley and permit precise correlation among these three localities. Forty-seven fossiliferous sites from West Turkana have yielded more than 1000 specimens of 93 mammalian species. The mammalian fossils represent nine sequential assemblages that augment information about faunal and environmental change from elsewhere in the basin.
Aquatic Productivity and the Evolution of Diadromous Fish Migration
Authors: Gross Mart R. , Coleman Ronald M. , McDowall Robert M.
Published: October 2006

Diadromous migration, in which some fish species migrate from freshwater and feed in the ocean (anadromous species) and others migrate from the ocean and feed in freshwater (catadromous), has long been perplexing. However, when the distribution of diadromous species is examined with respect to global patterns in aquatic productivity, this apparent paradox is resolved. The contrasting directions of migration can largely be explained by the relative availability of food resources in ocean and freshwater habitats. Oceans are more productive than freshwaters in temperate latitudes, and anadromous species predominate. In contrast, catadromous species generally occur in tropical latitudes where freshwater productivity exceeds that of the ocean.
Formation of the Rocky Mountains, Western United States: A Continuum Computer Model
Authors: Bird Peter
Published: October 2006

One hypothesis for the formation of the Rocky Mountain structures in Late Cretaceous through Eocene time is that a plate of oceanic lithosphere was underthrust horizontally along the base of the North American lithosphere. The horizontal components of the motion of this plate are known from paleomagnetism, and the edges of the region of flat slab can be estimated from reconstructed patterns of volcanism. New techniques of finite-element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the North American plate. A model that has a realistic temperature-dependent rheology and a simple plane-layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present. Successful predictions of this model include (i) the location, amount, and direction of horizontal shortening that has been inferred from Laramide structures; (ii) massive transport of lower crust from southwest to northeast; (iii) the location and timing of the subsequent extension in metamorphic core complexes and the Rio Grande rift; and (iv) the total area eventually involved in Basin-and-Range style extension.

In a broad sense, this model has predicted the belt of Laramide structures, the transport of crust from the coastal region to the continental interior, the subsequent extension in metamorphic core complexes and the Rio Grande rift, and the geographic region of late Tertiary Basin-and-Range extension. Its principal defects are that (i) many events are predicted about 5 million to 10 million years too late and (ii) the wave of crustal thickening does not travel far enough to the east. Reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects.

The correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the Laramide orogeny is probably correct. The Rocky Mountain thrust and reverse faults formed in an environment of east-west to northeast-southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the North American crust. Nonuniform crustal thickening by simple-shear transport also caused relative uplifts; therefore, this model is consistent with both of the range-forming mechanisms that have been inferred (
1
). A new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional Basin-and-Range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away.
Coevolution of Sender and Receiver: Effect on Local Mate Preference in Cricket Frogs
Authors: Ryan Michael J.
Published: October 2006

Mate recognition in frogs requires congruence of call characters, such as dominant frequency, and properties of the auditory system, such as frequency sensitivity of inner ear organs. Two neighboring populations of cricket frogs (
Acris crepitans
) exhibit statistically significant differences in the dominant frequency of the advertisement call and the frequency to which the basilar papilla of the inner ear is most sensitive. Call frequency and frequency sensitivity are matched within but differ between populations. These characters usually are negatively correlated with body size, and thus their congruence and coevolution often is explained by pleiotropic effects of size. However, within this species call frequency and frequency sensitivity of the basilar papilla evolved independent of body size, yielding local mate preferences that could contribute to genetic differentiation among neighboring populations.
Reversal of Experimental Parkinsonism by Lesions of the Subthalamic Nucleus
Authors: Bergman Hagai , Wichmann Thomas , DeLong Mahlon R.
Published: October 2006

Although it is known that Parkinson's disease results from a loss of dopaminergic neurons in the substantia nigra, the resulting alterations in activity in the basal ganglia responsible for parkinsonian motor deficits are still poorly characterized. Recently, increased activity in the subthalamic nucleus has been implicated in the motor abnormalities. To test this hypothesis, the effects of lesions of the subthalamic nucleus were evaluated in monkeys rendered parkinsonian by treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The lesions reduced all of the major motor disturbances in the contralateral limbs, including akinesia, rigidity, and tremor. This result supports the postulated role of excessive activity in the subthalamic nucleus in Parkinson's disease.
A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas
Authors: Bourgeois Joanne , Hansen Thor A. , Wiberg Patricia L. , Kauffman Erle G.
Published: October 2006

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolide-water impact.
Forces Between Surfaces in Liquids
Authors: Israelachvili Jacob N. , McGuiggan Patricia M.
Published: October 2006

Recent developments in the direct measurements of forces between surfaces in liquids at the ångstrom resolution level are reviewed. The results reveal a rich variety of interactions and interaction potentials that depend on the nature of the surfaces and intervening liquids. These results also shed new insights into liquid structure adjacent to surfaces and the interactions occurring in complex systems, with implications in many different areas of chemical physics, biology, and technology. The origin of some important fundamental interactions, such as repulsive "hydration" forces and attractive "hydrophobic" forces, are still not understood and offer a challenge for experimental and theoretical work in this area.
Gibberellins: A Phytohormonal Basis for Heterosis in Maize
Authors: Rood Stewart B. , Buzzell Richard I. , Mander Lewis N. , Pearce David , Pharis Richard P.
Published: October 2006

Four commercially important maize parental inbreds and their 12 F
1
hybrids were studied to investigate the role of the phytohormone gibberellin (GA) in the regulation of heterosis (hybrid vigor). All hybrids grew faster than any inbred. In contrast, all inbreds showed a greater promotion of shoot growth after the exogenous application of GA
3
. Concentrations of endogenous GA
1
, the biological effector for shoot growth in maize, and GA
19
, a precursor of GA
1
, were measured in apical meristematic shoot cylinders for three of the inbreds and their hybrids by gas chromatography—mass spectrometry with selected ion monitoring; deuterated GAs were used as quantitative internal standards. In 34 of 36 comparisons, hybrids contained higher concentrations of endogenous GAs than their parental inbreds. Preferential growth acceleration of the inbreds by exogenous GA
3
indicates that a deficiency of endogenous GA limits the growth of the inbreds and is thus a cause of inbreeding depression. Conversely, the increased endogenous concentration of GA in the hybrids could provide a phytohormonal basis for heterosis for shoot growth.
[Cr
4
S(O
2
CCH
3
)
8
(H
2
O)
4
](BF
4
)
2
⋅H
2
O: Ferromagnetically Coupled Cr
4
S Cluster with Spin 6 Ground State
Authors: Bino A. , Johnston D. C. , Goshorn D. P. , Halbert T. R. , Stiefel E. I.
Published: October 2006

Data on the preparation, structure, and magnetism of the new complex [Cr
4
S(O
2
CCH
3
)
8
(H
2
O)
4
](BF
4
)
2
⋅H
2
O are presented. The metal-sulfur M
4
S core structure is similar to that found in M
4
S[S
2
As(CH
3
)
2
]
6
(where M is cobalt or zinc) compounds and in beryllium basic acetate. However, the six-coordinate metal ions in the new Cr
4
S system distinguish it from these structures. The tetranuclear chromium complex is the first example of a new structural type. Its magnetic spin
S
= 6 ground state, a striking example of intramolecular ferromagnetic coupling, was determined by variable-temperature magnetic susceptibility measurements. Long-range antiferromagnetic intercluster ordering was found below 170 millikelvin.
Neurotransmitters Increase Cyclic Nucleotides in Postganglionic Neurons: Immunocytochemical Demonstration
Authors: Kebabian John W. , Blood Floyd E. , Steiner Alton L. , Greengard Paul
Published: October 2006

Dopamine increases adenosine 3′,5′-monophosphate (cyclic AMP) but not guanosine 3′,5′-monophosphate (cyclic GMP) in slices of bovine sympathetic ganglion; this increase is localized to the postganglionic neurons. Conversely, acetylcholine increases cyclic GMP but not cyclic AMP in the ganglion; this increase also occurs within postganglionic neurons. Thus, different neurotransmitters can selectively alter cyclic nucleotide levels within the same neuronal population.
Psoriatic Fibroblasts Induce Hyperproliferation of Normal Keratinocytes in a Skin Equivalent Model in Vitro
Authors: Saiag P. , Coulomb B. , Lebreton C. , Bell E. , Dubertret L.
Published: October 2006

A skin equivalent model has been used to fabricate tissues with psoriatic and normal cells. Psoriatic fibroblasts can induce hyperproliferative activity in normal keratinocytes. The psoriatic epidermis from lesions continues to proliferate at high rates for at least 15 days in this model, and normal fibroblasts are unable to suppress this hyperproliferation. The primary defect in psoriatic skin may reside in the dermal fibroblast.
The Cellular Basis of Hearing: The Biophysics of Hair Cells
Authors: Hudspeth A. J.
Published: October 2006

A crucial event in the hearing process is the transduction of mechanical stimuli into electrical signals by hair cells, the sensory receptors of the internal ear. Stimulation results in the rapid opening of ionic channels in the mechanically sensitive organelles of these cells, their hair bundles. These transduction channels, which are nonselectively permeable, are directly excited by hair-bundle displacement. Hair cells are selectively responsive to particular frequencies of stimulation, both due to the mechanical properties of their hair bundles and because of an ensemble of ionic channels that constitute an electrical resonator.
Evidence for a Cultivar and a Chronology from Patterned Wetlands in Central Veracruz, Mexico
Authors: Siemens Alfred H. , Hebda Richard J. , Hernández Mario Navarrete , Piperno Dolores R. , Stein Julie K. , Zolá Báez Manuel G.
Published: October 2006

The patterning found in certain wetlands of lowland Mesoamerica has added an important element to the subsistence system that may be attributed to pre-Hispanic inhabitants of the region. The form of the remains, largely expressed in terms of surface vegetation, suggests agriculture on planting platforms, separated by canals. The physical and chemical aspects of the stratigraphy have clarified depositional environments but have not indicated agricultural horizons. Maize phytoliths at about 1 meter below the surface in two Central Veracruzan wetlands do confirm the practice of agriculture. Associated ceramics indicate wetlands agriculture was practiced by A.D. 500 and perhaps earlier.
Platelet Membrane Glycoprotein IIb/IIIa: Member of a Family of Arg-Gly-Asp—Specific Adhesion Receptors
Authors: Pytela Robert , Pierschbacher Michael D. , Ginsberg Mark H. , Plow Edward F. , Ruoslahti Erkki
Published: October 2006

Adhesive interactions of the platelet surface with plasma proteins such as fibrinogen and fibronectin play an important role in thrombosis and hemostasis. The binding of both of these proteins to platelets is inhibited by synthetic peptides containing the sequence Arg-Gly-Asp, which corresponds to the cell adhesion site in fibronectin and is also present in the α chain of fibrinogen. An affinity matrix made of an insolubilized heptapeptide containing the Arg-Gly-Asp sequence selectively binds the platelet membrane glycoprotein IIb/IIIa from detergent extracts of platelets. When incorporated into liposome membranes, the isolated protein confers to the liposomes the ability to bind to surfaces coated with fibrinogen, fibronectin, and vitronectin but not to surfaces coated with thrombospondin or albumin. This platelet receptor is related to the previously identified fibronectin and vitronectin receptors in that it recognizes an Arg-Gly-Asp sequence but differs from the other receptors in its wider specificity toward various adhesive proteins. These results establish the existence of a family of adhesion receptors that recognize the sequence Arg-Gly-Asp.
Caffeine-Induced Uncoupling of Mitosis from the Completion of DNA Replication in Mammalian Cells
Authors: Schlegel Robert , Pardee Arthur B.
Published: October 2006

Caffeine was shown to induce mitotic events in mammalian cells before DNA replication (S phase) was completed. Synchronized BHK cells that were arrested in early S phase underwent premature chromosome condensation, nuclear envelope breakdown, morphological "rounding up," and mitosis-specific phosphoprotein synthesis when they were exposed to caffeine. These mitotic responses occurred only after the cells had entered S phase and only while DNA synthesis was inhibited by more than 70 percent. Inhibitors of protein synthesis blocked these caffeine-induced events, while inhibitors of RNA synthesis had little effect. These results suggest that caffeine induces the translation or stabilizes the protein product (or products) of mitosis-related RNA that accumulates in S-phase cells when DNA replication is suppressed. The ability to chemically manipulate the onset of mitosis should be useful for studying the regulation of this event in mammalian cells.
Measurement of Single Channel Currents from Cardiac Gap Junctions
Authors: Veenstra Richard D. , DeHaan Robert L.
Published: October 2006

Cardiac gap junctions consist of arrays of integral membrane proteins joined across the intercellular cleft at points of cell-to-cell contact. These junctional proteins are thought to form pores through which ions can diffuse from cytosol to cytosol. By monitoring whole-cell currents in pairs of embryonic heart cells with two independent patch-clamp circuits, the properties of single gap junction channels have been investigated. These channels had a conductance of about 165 picosiemens and underwent spontaneous openings and closings that were independent of voltage. Channel activity and macroscopic junctional conductance were both decreased by the uncoupling agent 1-octanol.
Earthquake Hazard After a Mainshock in California
Authors: Reasenberg Paul A. , Jones Lucile M.
Published: October 2006

After a strong earthquake, the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures. A stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock. The probabilities depend strongly on the model parameters, which are estimated with Bayesian statistics from both the ongoing aftershock sequence and from a suite of historic California aftershock sequences. Probabilities for damaging aftershocks and greater mainshocks are typically well-constrained after the first day of the sequence, with accuracy increasing with time.
Cyclic AMP-Modulated Potassium Channels in Murine B Cells and Their Precursors
Authors: Choquet Daniel , Sarthou Pierre , Primi Daniele , Cazenave Pierre-André , Korn Henri
Published: October 2006

A voltage-dependent potassium current (the delayed rectifier) has been found in murine B cells and their precursors with the whole-cell patch-clamp technique. The type of channel involved in the generation of this current appears to be present throughout all stages of pre-B-cell differentiation, since it is detected in pre-B cell lines infected with Abelson murine leukemia virus; these cell lines represent various phases of B-cell development. Thus, the presence of this channel is not obviously correlated with B-cell differentiation. Although blocked by Co
2+
, the channel, or channels, does not appear to be activated by Ca
2+
entry. It is, however, inactivated by high intracellular Ca
2+
concentrations. In addition, elevation of intracellular adenosine 3′,5′-monophosphate induces at all potentials a rapid decrease in the peak potassium conductance and increased rates of activation and inactivation. Therefore, potassium channels can be physiologically modulated by second messengers in lymphocytes.
Polypeptide Sequences Essential for RNA Recognition by an Enzyme
Authors: Regan Lynne , Bowie James , Schimmel Paul
Published: October 2006

Many RNAs are complex, globular molecules formed from elements of secondary and tertiary structure analogous to those found in proteins. Little is known about recognition of RNAs by proteins. In the case of transfer RNAs (tRNAs), considerable evidence suggests that elements dispersed in both the one- and three-dimensional structure are important for recognition by aminoacyl tRNA synthetases. Fragments of alanine tRNA synthetase were created by in vitro manipulations of the cloned
alaS
gene and examined for their interaction with alanine-specific tRNA. Sequences essential for recognition were located near the middle of the polypeptide, juxtaposed to the carboxyl-terminal side of the domain for aminoacyl adenylate synthesis. The most essential part of the tRNA interaction strength and specificity was dependent on a sequence of fewer than 100 amino acids. Within this sequence, and in the context of the proper conformation, a segment of no more than 17 amino acids was responsible for 25% or more of the total synthetase-tRNA free energy of association. The results raise the possibility that an important part of specific RNA recognition by an aminoacyl tRNA synthetase involves a polypeptide segment that is short relative to the total size of the protein.
External Calcium Ions Are Required for Potassium Channel Gating in Squid Neurons
Authors: Armstrong Clay M. , Lopez-Barneo Jose
Published: October 2006

The effects of calcium removal on the voltage-dependent potassium channels of isolated squid neurons were studied with whole cell patch-clamp techniques. When the calcium ion concentration was lowered from 10 to 0 millimolar (that is, no added calcium), potassium channel activity, identified from its characteristic time course, disappeared within a few seconds and there was a parallel increase in resting membrane conductance and in the holding current. The close temporal correlation of the changes in the three parameters suggests that potassium channels lose their ability to close in the absence of calcium and simultaneously lose their selectivity. If potassium channels were blocked by barium ion before calcium ion was removed, the increases in membrane conductance and holding current were delayed or prevented. Thus calcium is an essential cofactor in the gating of potassium channels in squid neurons.
Auditory Pathways to the Frontal Cortex of the Mustache Bat,
Pteronotus parnellii
Authors: Kobler James B. , Isbey Susan F. , Casseday John H.
Published: October 2006

In primates, certain areas of the frontal cortex play a role in guiding movements toward visual or auditory objects in space. The projections from auditory centers to the frontal cortex of the bat
Pteronotus parnellii
were examined because echolocating bats utilize auditory cues to guide their movements in space. An area in the frontal cortex receives a direct projection from a division of the auditory thalamus, the suprageniculate nucleus, which in turn receives input from the anterolateral peri-olivary nucleus, an auditory center in the medulla. This pathway to the frontal cortex bypasses the main auditory centers in the midbrain and cortex and could involve as few as four neurons between the cochlea and the frontal cortex. The auditory cortex is also a major source of input to the frontal cortex. This area of the frontal cortex may link the auditory and motor systems by its projections to the superior colliculus.
Origin of Granulite Terranes and the Formation of the Lowermost Continental Crust
Authors: Bohlen S. R. , Mezger K.
Published: October 2006

Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.
Distribution and Detection of Positrons from an Orbiting Nuclear Reactor
Authors: Hones E. W. , Higbie P. R.
Published: October 2006

The Solar Maximum Mission (SMM) Gamma-Ray Spectrometer has on many occasions detected nuclear radiation produced by nuclear reactors carried on Soviet satellites. A unique feature of the observations is the measurement of bursts of 511—kiloelectron volt gamma rays that are thought to signal SMM encounters with positrons emanating from the Soviet satellites. A model of positron generation by an orbiting reactor has been developed that describes the resulting time-dependent distribution of positrons temporarily trapped in the geomagnetic field and estimates the response of the SMM spectrometer to passage through such distributions. The model successfully predicts onset times, durations, and intensities of the 511—kiloelectron volt gamma bursts, as we illustrate in a detailed analysis of one event, and thus confirms that these are due to positrons from the Soviet satellites. Reactor-generated positrons are potentially useful in magnetospheric research.
Geological and Geochemical Record of 3400-Million-Year-Old Terrestrial Meteorite Impacts
Authors: Lowe Donald R. , Byerly Gary R. , Asaro Frank , Kyte Frank J.
Published: October 2006

Beds of sand-sized spherules in the 3400-million-year-old Fig Tree Group, Barberton Greenstone belt, South Africa, formed by the fall of quenched liquid silicate droplets into a range of shallow-to deep-water depositional environments. The regional extent of the layers, their compositional complexity, and lack of included volcanic debris suggest that they are not products of volcanic activity. The layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions. Geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt, komatiite, and CI carbonaceous chondrite. The spherules are thought to be the products of large meteorite impacts on the Archean earth.
All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor
Authors: Diederich François , Rubin Yves , Knobler Carolyn B. , Whetten Robert L. , Schriver Kenneth E. , Houk Kendall N. , Li Yi
Published: October 2006

The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D

9
h

ground state geometry with alternating C-C (1.36 angstroms) and C≡C (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C
18
are described. The analysis of laser flash heating experiments on this precursor by time-of-flight mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C
18
as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor.