Corrected Late Triassic latitudes for continents adjacent to the North Atlantic

We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of approximately 0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today.     

Orbital influence on Earth's magnetic field: 100,000-year periodicity in inclination

A continuous record of the inclination and intensity of Earth's magnetic field, during the past 2.25 million years, was obtained from a marine sediment core of 42 meters in length. This record reveals the presence of 100,000-year periodicity in inclination and intensity, which suggests that the magnetic field is modulated by orbital eccentricity. The correlation between inclination and intensity shifted from antiphase to in-phase, corresponding to a magnetic polarity change from reversed to normal. To explain the observation, we propose a model in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not.     

Paleomagnetic record of a geomagnetic field reversal from late miocene mafic intrusions, southern nevada

Late Miocene (about 8.65 million years ago) mafic intrusions and lava flows along with remagnetized host rocks from Paiute Ridge, southern Nevada, provide a high-quality paleomagnetic record of a geomagnetic field reversal. These rocks yield thermoremanent magnetizations with declinations of 227 degrees to 310 degrees and inclinations of -7 degrees to 49 degrees , defining a reasonably continuous virtual geomagnetic pole path over west-central Pacific longitudes. Conductive cooling estimates for the intrusions suggest that this field transition, and mafic magmatism, lasted only a few hundred years. Because this record comes principally from intrusive rocks, rather than sediments or lavas, it is important in demonstrating the longitudinal confinement of the geomagnetic field during a reversal.     

Generation and migration of light hydrocarbons

Light hydrocarbons (containing from 1 to 14 carbon atoms) are formed from disseminated organic matter in sediments at the parts-per-billion level by biological and low-temperature (< 50 degrees C) chemical reactions and at the parts-permillion level by high-temperature (> 50 degrees C) cracking reactions. The cooler reactions produce mainly branched hydrocarbons, whereas the hotter reactions yield more straight chains. Hydrocarbon generation zones in the subsurface can be recognized on the basis of hydrocarbon distribution patterns. Hydrocarbons with tertiary carbon atoms form at lower temperatures than those with quaternary carbons. Methane and ethane migrate vertically through fine-grained shales by diffusion and solution, whereas many of the C(3+) hydrocarbons show little or no vertical migration. Concentrations of light hydrocarbons, including methane, in fine-grained source rocks decrease to low values in deep, high-temperature (>200 degrees C) sediments. This decrease may be one reason why no economic accumulation of gas has been found to date deeper than 8.2 kilometers (27,000 feet).     

Magnetic field of jupiter and its interaction with the solar wind

Jupiter's magnetic field and its interaction with the magnetized solar wind were observed with the Pioneer 10 vector helium magnetometer. The magnetic dipole is directed opposite to that of the earth with a moment of 4.0 gauss R(J)(3) (R(J), Jupiter radius), and an inclination of 15 degrees lying in a system III meridian of 230 degrees . The dipole is offset about 0.1 R(J) north of the equatorial plane and about 0.2 R(J) toward longitude 170 degrees . There is severe stretching of the planetary field parallel to the equator throughout the outer magnetosphere, accompanied by a systematic departure from meridian planes. The field configuration implies substantial plasma effects inside the magnetosphere, such as thermal pressure, centrifugal forces, and differential rotation. As at the earth, the outer boundary is thin, nor diffuse, and there is a detached bow shock.     

Paleomagnetic study of antarctic deep-sea cores

The magnetic inclinations and inten sities of about 650 samples from seven deepsea cores taken in the Antarctic were measured on a spinner magnetometer. This series of measurements provided a magnetic stratigraphy, based on zones of normally or reversally polar ized specimens for each core, which was then correlated with the magnetic stra tigraphy of Cox et al. (1). One core (V16-134) gave a continuous record of the paleomagnetic field back to about 3.5 million years. When selected samples were subject ed to alternatingfield demagnetization, most were found to have an unstable component that was removed by fields of 150 oersteds; all samples from two cores were partially demagnetized in a field of 150 oersteds. The average inclination in these two cores was then in good agreement with the average inclination of the ambient field for the latitude of the core site. It was also found that the intensities of the samples decreased at the points of reversal; this finding is to be expected if, as has been postulated by the dynamo theory, the intensity of the dipole field decreases to zero and builds again with opposite polarity. We believe that the magnetiza tion of the cores results from the pres ence of detrital magnetite, although other magnetic minerals also may be present. Four faunal zones (, X, , and ) have been recognized in these Antarctic cores on the basis of upward sequential disappearance of Radiolaria. The faunal boundaries and reversals consistently have the same relations to one another, indicating that they are both timedependent phenomena. Using previously determined times of reversal, one may date the following events in the cores: 1) Radiolarian faunal boundaries:-X, 2 million years; X-, 0.7 million years; -, 0.4 to 0.5 million years. These dates are in good agreement with ages previously extrapolated from radio metric dates. 2) Initiation of Antarctic diatom ooze deposition, approximately 2.0 mil-lion years ago. 3) First occurrence of ice- rafted detritus, approximately 2.5 million years ago. One can also calculate rates of sedi mentation, which vary in the cores studied from 1.1 to about 8.0 millimeters per 1000 years. Sedimentation rates for the Indian Ocean cores are higher than for the Bellingshausen Sea cores. The near coincidence of faunal changes and reversals in the cores suggests but does not prove a causal relation. We conclude from this study that paleomagnetic stratigraphy is a unique method for correlating and dating deep sea cores, and that future work with such cores may provide a complete or nearly complete record of the history of the earth's magnetic field beyond 4 million years.     

Spatial and temporal offsets between proxy records in a sediment drift

Chronologies for Late Quaternary marine sediment records are usually based on radiocarbon ages of planktonic foraminifera. Signals carried by other sedimentary components measured in parallel can provide complementary paleoclimate information. A key premise is that microfossils and other indicators within a given sediment horizon are of equal age. We show here that haptophyte-derived alkenones isolated from Bermuda Rise drift sediments are up to 7000 years older than coexisting planktonic foraminifera. This temporal offset, which is apparently due to lateral transport of alkenones on fine-grained particles from the Nova Scotian margin, markedly influences molecular estimates of sea surface temperatures. More broadly, the observation raises questions about both the temporal and the geographic fidelity of paleoenvironmental records encoded by readily transported components of sediments.     

Motions of the Earth's Core and Mantle, and Variations of the Main Geomagnetic Field

Theoretical work on the magnetohydrodynamics of the earth's liquid core indicates (a) that horizontal variations in the properties of the core-mantle interface that would escape detection by modern seismological methods might nevertheless produce measurable geomagnetic effects; (b) that the rate of drift, relative to the earth's surface, of nonaxisymmetric features of the main geomagnetic field might be much faster than the average zonal speed of hydrodynamic motion of core material relative to the surrounding mantle; and (c) why magnetic astronomical bodies usually rotate. Among the consequences of (a) and (b) are the possibilities that (i) the shortest interval of time that can be resolved in paleomagnetic studies of the geocentric axial dipole component of the earth's magnetic field might be very much longer than the value often assumed by many paleomagnetic workers, (ii) reversals in sign of the geomagnetic dipole might be expected to show some degree of correlation with processes due to motions in the mantle (for example, tectonic activity, polar wandering), and (iii) variations in the length of the day that have hitherto been tentatively attributed to core motions may be due to some other cause.     

A Statistical Model of the Fluctuations in the Geomagnetic Field from Paleosecular Variation to Reversal

The statistical characteristics of the local magnetic field of Earth during paleosecular variation, excursions, and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years. A model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a Maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations, in terms of both direction and intensity. This result suggests that the magnitude of secular variation vectors is independent of the magnitude of Earth's axial dipole moment and that the amplitude of secular variation is unchanged during reversals.     

不同倾角的秋刀鱼集鱼灯箱照度实验比较研究

秋刀鱼是西北太平洋北部温带水域的主要捕捞对象之一,集鱼灯灯光配置是秋刀鱼高效捕捞技术的重要内容。本文通过模拟灯箱光学特性实验,利用Matlab7.0软件建立秋刀鱼集鱼灯箱照度分布模型,对不同倾角的秋刀鱼集鱼灯箱的照度分布进行了研究。结果表明:当秋刀鱼集鱼灯倾角为30°、45°及60°时,集鱼灯箱的地面照度大小随地面距离的增加而减少,并且不同倾角时的集鱼灯箱地面照度分布之间存在显著差异,照度分布衰减程度也随灯箱倾角的增加而递减;不同倾角时的照度光场波动也存在差异,30°倾角时的集鱼灯灯光照度光场波动变化大于45°和60°倾角时的照度光场波动变化,而照度等位线密集度也随着倾角角度的增加而降低;秋刀鱼集鱼灯灯箱倾角为60°时,其照度分布和光场波动较稳定,诱集区域较大。     

Venus: topography revealed by radar data

Surface height variations over the entire equatorial region on Venus have been estimated from extended series of measurements of interplanetary radar echo delays. Most notable is a mountainous section of about 3-kilometer peak height located at a longitude of 100 degrees (International Astronomical Union coordinate system). The eastern edge has an average inclination of about 0.5 degrees, which is unusually steep for a large-scale slope on Venus. The resolution of the radar measurements along the surface of Venus varied between about 200 and 400 kilometers with a repeatability in altitude determination generally between 200 and 500 meters. The mean equatorial radius was found to be 6050.0+/-0.5 kilometers.     

Radiolarian evidence consistent with spreading of the pacific floor

In sediments west of the East Pacific Rise between the equator and 20 degrees N, Eocene microfossils are recorded no farther east than 134 degrees W; Oligocene, no farther east than 125 degrees W; while Miocene and Pliocene occurrences extend closer to the crest of the rise. This distribution may result from post-Eocene spreading of the sea floor here averaging about 8 centimeters per year.     

Fissure basalts and ocean-floor spreading on the East pacific rise

A basalt pavement outcrops almost continuously in a band along the crestal region of the East Pacific Rise from about 14 degrees S to 6 degrees S, that is, for more than 800 ikilometers; the outcrop may well extend beyond the above limits along the axis of the rise. The basalt band is generally between 40 and 60 kilometers wide and is replaced laterally by sediment. The lavas are fresh, "oceanic tholeiites" which were emplaced less than I million years ago by fissure eruptions. These findings, can be explained by the hypothesis of ocean-floor spreading; the basalts are the expression of material originating from the mantle and rising through fissures along the axis of the ridge. The absence of an axial rift valley on the East Pacific Rise may be explained by the fact that large volumes of lava are being outpoured along its crest.     

Paleotemperatures in the southwestern United States derived from noble gases in ground water

A paleotemperature record based on measurements of atmospheric noble gases dissolved in ground water of the Carrizo aquifer (Texas) shows that the annual mean temperature in the southwestern United States during the last glacial maximum was about 5 degrees C lower than the present-day value. In combination with evidence for fluctuations in mountain snow lines, this cooling indicates that the glacial lapse rate was approximately the same as it is today. In contrast, measurements on deep-sea sediments indicate that surface temperatures in the ocean basins adjacent to our study area decreased by only about 2 degrees C. This difference between continental and oceanic records poses questions concerning our current understanding of paleoclimate and climate-controlling processes.     

The structure of Mercury's magnetic field from MESSENGER's first flyby

During its first flyby of Mercury, the MESSENGER spacecraft measured the planet's near-equatorial magnetic field. The field strength is consistent to within an estimated uncertainty of 10% with that observed near the equator by Mariner 10. Centered dipole solutions yield a southward planetary moment of 230 to 290 nanotesla RM3 (where RM is Mercury's mean radius) tilted between 5 degrees and 12 degrees from the rotation axis. Multipole solutions yield non-dipolar contributions of 22% to 52% of the dipole field magnitude. Magnetopause and tail currents account for part of the high-order field, and plasma pressure effects may explain the remainder, so that a pure centered dipole cannot be ruled out.     

Implantation in interplanetary dust of rare-gas ions from solar flares

Measurements of excess Ar(36) + Ar(38) ( released mainly at 1200 degrees C) in magnetic concentrates of Pacific sediments and in a dense concentrate of Greenland dust agree within an order of magnitude with expected concentrations implanted by solar-flare ion streams of energy less than 10 Mev per atomic-mass unit. The agreement implies that more than 10 percent of each concentrate may be extraterrestrial, depending on size distribution and flare spectra. Rare-gas measurements on fine-grained dust can provide data on: solar-flare "paleo-ion" fluxes, energy spectra, and isotopic abundances; identification, mineralogy, and chemistry of interplanetary dust; influx rates to Earth and sedimentation rates of oceanic cores; and lunar-surface residence and mixing times.     

High geomagnetic intensity during the mid-Cretaceous from Thellier analyses of single plagioclase crystals

Recent numerical simulations have yielded the most efficient geodynamo, having the largest dipole intensity when reversal frequency is low. Reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled. We report data from 56 Thellier-Thellier experiments on plagioclase crystals separated from basalts of the Rajmahal Traps (113 to 116 million years old) of India that formed during the Cretaceous Normal Polarity Superchron. These data suggest a time-averaged paleomagnetic dipole moment of 12.5 +/- 1.4 x 10(22) amperes per square meter, three times greater than mean Cenozoic and Early Cretaceous-Late Jurassic dipole moments when geomagnetic reversals were frequent. This result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength.     

Geochemistry and thermolysis of flavonoids

The thermal and pH stability of selected flavonoids has been determined under simulated geologic conditions. Thermolytic rates and products for various regimes, as determined by gas chromatography-mass spectroscopy, indicate the potential usefulness of flavonoids as thermometric indicators in sediments. The parametric factors affecting flavonoid stability are used to geochemically characterize angiosperm "green leaves" (36 to 25 x 10(6) years old) from Succor Creek and indicate that these sediments have not experienced temperatures higher than 80 degrees C or extreme pH shifts (beyond the range 6.3 to 7.2) during postdepositional maturation.     

Extending the sub-sea-floor biosphere

Sub-sea-floor sediments may contain two-thirds of Earth's total prokaryotic biomass. However, this has its basis in data extrapolation from ~500-meter to 4-kilometer depths, whereas the deepest documented prokaryotes are from only 842 meters. Here, we provide evidence for low concentrations of living prokaryotic cells in the deepest (1626 meters below the sea floor), oldest (111 million years old), and potentially hottest (~100 degrees C) marine sediments investigated. These Newfoundland margin sediments also have DNA sequences related to thermophilic and/or hyperthermophilic Archaea. These form two unique clusters within Pyrococcus and Thermococcus genera, suggesting unknown, uncultured groups are present in deep, hot, marine sediments (~54 degrees to 100 degrees C). Sequences of anaerobic methane-oxidizing Archaea were also present, suggesting a deep biosphere partly supported by methane. These findings demonstrate that the sub-sea-floor biosphere extends to at least 1600 meters below the sea floor and probably deeper, given an upper temperature limit for prokaryotic life of at least 113 degrees C and increasing thermogenic energy supply with depth.