bluuebunny/crossref_metadata_2025 huggingface dataset:
The record of Late Pleistocene biogenic sedimentation in the eastern tropical Pacific Ocean
Authors: Lyle Mitchell , Murray David W. , Finney Bruce P. , Dymond Jack , Robbins James M. , Brooksforce Kathryn
Published: February 2008

We have generated ∼300 Kyr records of biogenic opal, calcite, and organic carbon (Corg) for three cores in the eastern and central equatorial Pacific Ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region. We find that Corg has been deposited in common pulses throughout the area, while opal has a much more local pattern of variation. Calcite varies regionally, but the record is shaped by superimposed dissolution and productivity processes. The most intense Corg peak occurs at 18 ka and can have greater than 2 times the Holocene Corg content. Other major Corg peaks occur 150 ka and perhaps at 280 ka. We have compared the Corg record in one of the cores, V19–28, to a model deepwater oxygen record developed from ∂13C data in the nearby V19–30 to test whether the Corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone. We found no coherence between the two records, implying that the Corg record is primarily a measure of productivity. By comparing the opal, calcite, and Corg records in V19–28, a core which is at or above the lysocline, we found that both increased calcite and opal deposition matches high Corg accumulation. We also found, however, that the calcite and opal records were uncorrelated, so that episodes of high opal deposition do not necessarily accumulate calcite rapidly. We hypothesize that at least two different plankton communities have been dominant in the waters above this site, one rich in opal‐secreting plankton and one more dominated by calcite producers. The opal‐rich plankton community was dominant during the intervals 10–15 ka and 35–60 ka.

Supplement data (Table 2) is available with entire articleon microfiche. Order from American Geophysical Union,2000 Florida Avenue, N.W., Washington, DC 20009.Document P88001; $2.50. Payment must accompany order.
Sedimentation at MANOP Site H (eastern equatorial Pacific) over the past 400,000 years: Climatically induced redox variations and their effects on transition metal cycling
Authors: Finney Bruce P. , Lyle Mitchell W. , Heath G. Ross
Published: February 2008

Gravity cores recovered from Manganese Nodule Project site H (6°33′N, 92°49′W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100‐kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity, redox conditions in sediments, and the cycling of transition metals.
Recent diatom record of McMurdo Sound, Antarctica: Implications for history of sea ice extent
Authors: Leventer Amy , Dunbar Robert B.
Published: February 2008

Analyses of diatom assemblages within surface sediments from McMurdo Sound, Antarctica, document the regional circulation patterns in this large embayment in the southwestern Ross Sea. Thalassiosira spp., indicative of water column primary productivity, is most common in eastern and portions of northwestern McMurdo Sound as a result of its advection into these regions from areas of open water primary productivity. Surface sediments from southwestern McMurdo Sound are composed mainly of Nitzschia curta, a small pennate form commonly found both as a member of the sea ice microbial community as well as a dominant component in ice edge blooms in the Ross Sea. The northward advection of oligotrophic water into the western Sound from beneath the McMurdo Ice Shelf results in the dominance of an autochthonous flora in the southwestern Sound. Sediments deposited during the past 500 years also have been dominated by Thalassiosira spp. and Nitzschia curta. Thalassiosira spp. concentrations reached a maximum between 1600 and 1875 A.D., corresponding to the time of the “Little Ice Age.” More persistent winds may have been responsible for more prevalent polynyas at that time, resulting in greater amounts of open water primary productivity and subsequent higher percentages of Thalassiosira spp. Increased areal extent and/or duration of coastal polynyas during the “Little Ice Age” suggests that within the southwestern Ross Sea, the production of High Salinity Shelf Water, and hence Antarctic Bottom Water, may have been greater at that time.
Late Quaternary oceanography of the mouth of the Gulf of California: The polycystine connection
Authors: Molina‐Cruz Adolfo
Published: February 2008

The mouth of the Gulf of California is a region where three principal water masses converge: the Equatorial Water, the water associated with the California Current, and the water from the gulf itself. These water masses exhibit a complex interaction which has been recorded by the polycystine radiolarian distribution patterns defined in the underlying sediments. The occurrence in the gulf of water associated with the California Current is indicated by the distribution of Cycladophora (?) davisiana (Ehrenberg), while the occurrence of the Equatorial Water is characterized by high abundance of Tetrapyle octacantha Müller. Where the California Current meets the Equatorial Water, a water mass boundary is formed, and this is depicted by the distribution of Druppatractus cf. pyriformis (Bailey). Micropaleontological analysis of six sedimentary gravity cores located along the mouth of the Gulf of California, correlated through δ18O stratigraphy, show that from approximately 33,000 to 15,500 years ago, the California Current frequently invaded the mouth of the gulf, displacing the water mass boundary system and the Equatorial Water southward. These oceanographic events preceded the global climatic changes indicated by the δ18O stratigraphy, occurring first in the surface and subsurface water in the mouth of the gulf and then in the deep waters.
Quaternary sediment patterns in the Weddell Sea: Relations and environmental conditions
Authors: Fütterer D. K. , Grobe H. , Grünig S.
Published: February 2008

Sediment patterns such as texture, composition, and facies from three selected areas of the Antarctic continental margin of the Weddell Sea are discussed in relation to environmental variations of the Quaternary hydrosphere and cryosphere. Advance and retreat of ice shelves as well as oscillations in sea ice coverage are reflected by particular sediment facies. The distribution of ice‐rafted detritus tracks the Antarctic Coastal Current, and the Weddell Sea Bottom Water contour current can be recognized by its distinctive winnowing and erosion pattern. Distribution and abundance of biogenic sediment components are mainly controlled by the duration of sea ice coverage reflecting the long‐term climatic evolution.
Geology and structure of the Astrid Ridge, Dronning Maud Land, Antarctica
Authors: Siddiquie H. N. , Rao D. Gopala , Ramana M. V.
Published: February 2008

Bathymetric (458 line km), Seismic reflection, higher energy sparker source (405 line km) and magnetic (452 line km) studies of the Astrid ridge, a prominent broad bathymetric feature in water depths of about 1520–2900 m on the continental margin of Dronning Maud Land, are carried out. The crest of the ridge in the area surveyed trends N‐S in the south but veers to NNW‐SSE at about 68° S Latitude and is found to change to NNE‐SSW at about 67° S Latitude. The western flanks of the ridge in the south are gently sloping while the eastern flanks are marked by NNE‐SSW trending scarps of about 80–280 m. The northern area is marked by a NW to NNW‐SE to SSE trending valley, about 5–15 km wide and 240–380 m deep with steep gradients. The ridge is mantled by layered sediments of a top transparent layer (50 to 0.4 s) underlain by a prominent reflector (50 ms to 100 ms) and the acoustic basement. The basement occur at 0.4 to 0.6 s depth. The sediments dip away from the crest of the ridge to the flanks. The seismic records further indicated that the NNE‐SSW trending scarps on the eastern flanks represent faults which extend only to the younger sediments. They are not reflected in the magnetics indicating that they are not deeper structures. The NW‐NNW to SE‐SSE trending narrow valleys with steep gradient walls occurring on the ridge crest in the study area are grabens where the basement and associated sediments have been faulted down. The graben in the north is marked by a series of magnetic anomalies of about 50 to 200 nT superposed over the peak of a broad high amplitude anomaly of a mafic material buildup and appears to be a deep crustal feature. The positive bathymetric feature of the ridge part in the south of the study area is not reflected in magnetic anomaly signatures, implying heterogeneity in composition. The magnetic basement is interpreted at depths to top of 1.8 km to 4.0 km. The graben appears to be a major tectonic feature of the area and marks a change in the trend of the ridge at about 68°S Latitude.
Late Cenozoic environments in the Barents Sea
Authors: Vorren Tore O. , Hald Morten , Lebesbye Erland
Published: February 2008

We examine the paleoceanography and paleogeography of the Barents Sea during the late Cenozoic glacial regime by seismostratigraphy and core analysis. During the smaller glaciations which probably dominated the first period of the glaciogenic regime (approximately 2.5–0.8 Ma?), glaciomarine processes with sediment influx from the adjacent mainland prevailed. The Barents Sea has been covered by grounded ice sheets several (five to 10) times during the late Cenozoic. We speculate that these glaciations occurred during the last 0.8 Ma. During these larger glaciations the margin of the ice sheet expanded to the shelf edge, and the continental shelf break and slope acted as a depocenter and prograded rapidly. Frequent sediment gravity flows accumulated on the lower continental slope. During interglacials, the Barents Sea was converted to a starved continental margin. Most of the Barents Sea was glaciated during the maximum of the last glaciation (19–16 ka). A two‐stepped deglaciation, 16 to 13 ka and 13 to 10 ka, occurred. Incipient Atlantic water reached the western areas about 13 ka and totally replaced the Arctic water by 10 ka. These paleoceanographic events were delayed in the eastern Barents Sea.
Surface area control of shallow cratonic to deep marine carbonate accumulation
Authors: Opdyke Bradley N. , Wilkinson Bruce H.
Published: February 2008

Sites of calcium‐magnesium carbonate accumulation have been shifting from shallow shelf to deep marine settings since the late Cretaceous. During this time, total area of flooded epicratonic shelf has decreased about 50%, and areas of shelf carbonate deposition have declined from 27 × 106km² to present areas of 1.4 × 106km² while the maximum extent of platform carbonate accumulation has decreased from about 50° to 28° north latitude. In terms of cratonic carbonate, this decrease in tropical shelf area has resulted in a sixfold reduction in shoal‐water limestone accumulation from a Cretaceous rate of 0.6 × km³/m.y. to a present value of about 0.1 × 106km³/m.y.. Over the same time period, the global carbonate compensation depth has deepened by about 1.5 km, attendant with a threefold areal increase of calcite ooze depositionn (40 to 130 × 106km²). This expansion has resulted in an eightfold increase in pelagic limestone accumulation from a Cretaceous rate of 0.05 × 106km³/m.y. to a present rate of about 0.4 × 106km³/m.y. Transfer of accumulation from shallow cratonic to deep marine depositional settings records the central role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to Phanerozoic global oceans.
Paleoceanography of the eastern equatorial Pacific during the Pliocene: A High‐Resolution Radiolarian Study
Authors: Hays P. E. , Pisias N. G. , Roelofs A. K.
Published: February 2008

Pliocene sediments from hydraulic piston cores of Deep Sea Drilling Project sites 572 and 573 in the eastern equatorial Pacific provide material for a high‐resolution stratigraphic and paleoceanographic study during a period of time from 2.4 to 3.7 Ma. Radiolarian stratigraphy of these two sites reveals two major faunal events. The older event involves the gradual disappearance of five species and the appearance of three equatorial surface‐dwelling radiolarian species and occurs at the Gauss/Gilbert paleomagnetic reversal boundary which is coincident with the time of the closing of the Isthmus of Panama around 3.5 Ma [Berggren and Hollister, 1974; Keigwin, 1978, 1982a, b]. The younger event involves a disappearance and appearance of two mainly Subarctic species and occurs at 2.9 Ma, prior to the onset of northern hemisphere glaciation as dated in North Atlantic sections. Analysis of quantitative radiolarian data for extant species divides the Pliocene fauna into two assemblages which are composed of radiolarian species that are abundant in the surface sediments of central and eastern equatorial regions. From the time series variations of these fauna, the variability of these factors has remained constant over the time interval at the eastern site but underwent a stepwise decrease at the western site. Paleotemperature transfer function techniques provided quantitative comparisons between Pliocene and Pleistocene paleooceanographic variability. At the more western site, a shift in paleotemperature estimates occurred. Using the modern day observation that the slope of the thermocline is related to the magnitude of westward wind stress, this change corresponds to an increase in wind stress. This event has been dated at about 2.8 Ma and may represent an important oceanic and atmospheric precursor to the major onset of northern hemisphere glaciation at 2.4 Ma. From the modern relationship between seasonal differences in temperatures at each site the Pliocene gradient in seasonally has been compared with the Pleistocene (and modern) gradients. Relative to the Pleistocene, seasonal differences in temperature were the same for the more eastern site but lower at the western site. The Pliocene gradient in seasonally was thus located farther east toward the South American continent than during the Pleistocene. No influence of the final closure of the Isthmus of Panama is seen in the extant radiolarian data for sites 572 and 573 during the time interval from 2.4 Ma to 3.7 Ma.
Diatoms of the Fortuna Basin, southeast Spain: Evidence for the intra‐Messinian inundation
Authors: Müller Daniel W. , Schrader Hans
Published: February 2008

The Fortuna Basin in southeast Spain, part of the Betic Strait, was one of the connecting pathways between the Atlantic Ocean and the Mediterranean Sea during the late Miocene. The basin contains a remarkable record of Messinian marine sediments. Two well‐laminated, marly, diatomaceous sections were examined: the 1‐m‐thick El Fenazar section (F section) and the 36‐m‐thick Rio Chicamo Diatomite Member (RCD section). Both sections are intercalated with gypsum horizons. The diatoms in the Rio Chicamo Diatomite suggest an Atlantic origin and strong coastal upwelling associated with high productivity. On the basis of event stratigraphy, the diatomites correlate to the intra‐Messinian inundation of the Mediterranean Sea during the Messinian salinity crisis. Four biostratigraphically useful species, Nitzchia jouseae, Thalassiosira oestrupii/T. praeoestrupii, Denticulopsis hustedtii, and Asterolampra acutiloba, occur in the west Pacific between ∼5.35 and 4.5 Ma (lower reversed Gilbert chron). This interpretation places the subsequent Upper Evaporite deposition into the lower reversed Gilbert chron. Our results indicate (1) the existence of a marine connection between the Atlantic Ocean and the Mediterranean Sea through the Betic Strait (southeast Spain) during the Messinian salinity crisis, (2) a possible earlier first appearance of Nitzchia jouseae in the Betic Strait compared to the Atlantic and Pacific oceans according to the stratigraphy of the Fortuna Basin, and (3) the probability that many of the “Tripolilike” and younger diatomites in the Mediterranean are local deposits and diachronous with each other.
Extended Cretaceous/Tertiary boundary extinctions and delayed population change in planktonic foraminifera from Brazos River, Texas
Authors: Keller Gerta
Published: February 2008

High‐resolution planktonic foraminiferal analysis of three Brazos River sections indicates a nearly continuous Cretaceous/Tertiary [K/T] boundary sedimentary record second only to the world's most complete record at El Kef, Tunisia. Species extinctions occur over an extended period of time and with two major extinction episodes. The first extinction episode with 46% of the species extinct occurs at and just below [10–15 cm] a short hiatus at the base of a sandy shell hash and clay‐sand unit which was interpreted by Bourgeois et al. [1988] to represent a tsunami bed generated by the K/T boundary bolide impact. The top of this tsunami bed is about 17–20 cm below the K/T boundary as defined by the first appearance of Tertiary planktonic foraminifera. The second extinction phase with 45% of the species extinct occurs 25 cm above the K/T boundary [Zone P0/P1a boundary]. Of the remaining seven surviving Cretaceous species, six gradually disappear during planktonic foraminiferal Subzones P1a and basal P1b. No species extinctions or major faunal assemblage changes are directly associated with the K/T boundary. Iridium distribution is ambiguous, with one peak in the upper part of the tsunami bed and a second peak at the micropaleontologically defined K/T boundary. Relative abundances of dominant species are stable through the Late Maastrichtian, and only minor abundance changes coincide with the first extinction episode or the K/T boundary. The first major faunal change in the dominant species group coincides with the second extinction episode and leads to decline and eventual extinction of this group in Subzone P1a. Species disappearing at the two extinction episodes [46% and 45%] constitute only a small percentage [8% and 5%] of the individuals of the total planktonic foraminiferal population. This suggests that weakened species with low numbers of individuals and sensitive to relatively minor environmental changes were primarily affected by these extinction episodes. Magnetostratigraphy indicates that the first extinction phase began about 310,000 years before the K/T boundary, and the second extinction phase occurred 50,000 years after the K/T boundary. This stepped pattern of species extinctions suggests a progressively stressed ecosystem in continental shelf settings which may be related to an observed sea level regression and global cooling. The hypothesis of a global catastrophic mass extinction at the K/T boundary caused by a large extraterrestrial impact is not supported by the Brazos River planktonic foraminiferal data.
Oceanic freshwater transport during the Last Glacial Maximum
Authors: Miller James R. , Russell Gary L.
Published: February 2008

Annual mean freshwater transports during the last ice age are calculated using data from a global atmospheric general circulation model. The atmospheric model is used to simulate the climate of 18,000 years ago using Climate: Long‐Range Investigation, Mapping, and Prediction data for the model's seasonally varying sea surface temperatures, ice coverage, and surface topography. The water transports are calculated for each ocean basin using the model's evaporation, precipitation, and runoff. For the present climate, the model's glacial ice sheets and the global water budget are near equilibrium. For the ice age the model does not maintain the glacial ice sheets at steady state but causes them to melt; therefore we reduced the runoff from melting ice to bring the global water budget into equilibrium. Recent studies have shown that salinity anomalies in the North Atlantic caused by changes in the freshwater transport could change the thermohaline circulation. For the present climate the evaporation exceeds the sum of precipitation and runoff in the Atlantic Ocean which leads to a higher mean salinity in the Atlantic than in the Pacific Ocean. The results suggest that this water budget reversed in the Atlantic during the last ice age. This conclusion, however, cannot be stated firmly because the model's global water budget was not in equilibrium for the ice age. The model's freshwater transports for the last ice age are compared with those for the present climate, and implications on changes in thermohaline circulation are discussed.
A salt oscillator in the glacial Atlantic? 1. The concept
Authors: Broecker Wallace S. , Bond Gerard , Klas Millie , Bonani Georges , Wolfli Willy
Published: February 2008

As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5°C air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50°N. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.
Reconstructing past particle fluxes in the tropical Atlantic Ocean
Authors: Curry W. B. , Lohmann G. P.
Published: February 2008

Using a research strategy analogous to modern sediment trap studies, sediment accumulation patterns on submarine rises can be interpreted in terms of past ocean chemistry and circulation. We have followed this research strategy to reconstruct the history of surface water productivity and deep‐water chemistry and circulation in the eastern equatorial Atlantic (Sierra Leone Rise) and western equatorial Atlantic (Ceara Rise) during the last glacial maximum (∼18,000 B.P.). On shallow sections of these rises, at depths with little carbonate particle degradation, we assume that the accumulation of skeletal carbonate approximates the carbonate production rate in surface water. During the last glacial maximum, the rate of carbonate productivity was lower in the eastern Atlantic than it is today, while the western Atlantic exhibited no glacial‐interglacial difference in carbonate productivity. Based on the difference in carbonate accumulation rates between shallow and deep cores on the rises, we observed greater dissolution in both basins during the last glaciation. The eastern Atlantic always had a lower rate of dissolution than the western Atlantic, despite having deep water with a lower δ13C during the last glacial maximum. Organic carbon accumulation in the eastern Atlantic increased with depth in the water column during the last glaciation, suggesting that there was a bathymetric decrease in dissolved oxygen concentration at that time. These observations are consistent with a glacial decrease in the production rate of northern source deep water during the last glaciation. At that time, the mixing zone between northern source and southern source deep water migrated to the north in the Atlantic, resulting in a greater proportion of corrosive, southern source deep water in the western Atlantic, and entering the eastern Atlantic through low‐latitude fracture zones. Today the ratio of northern and southern components entering the eastern Atlantic is about 4:1. Based on δ13C and carbonate accumulation, our best estimate of this ratio during the last glaciation is 1:1.
Younger Dryas‐Style Cooling Events at Glacial Terminations I‐VI at ODP Site 658: Associated benthic δ13C anomalies constrain meltwater hypothesis
Authors: Sarnthein Michael , Tiedemann Ralf
Published: February 2008

High‐resolution planktonic and epibenthic stable isotope records from Ocean Drilling Program site 658 off northwest Africa provide a basis for a detailed study of glacial terminations I‐VI during the last 650,000 years. The duration of the terminations was about one half to one quarter of an orbital precession cycle (5800–10,700 years), when its amplitude was high. At low amplitudes, the terminations lasted longer than half an obliquity cycle (29,000 years). Marked climatic rebounds similar to the Younger Dryas, each with a duration of 1000–2500 years, subdivided all six terminations into distinct steps A, B, and C. Important parts of the deglacial steps were as brief as 700–1000 years. The speed of climatic change suggests that special associations existed between orbital forcing and inherent instability of the ice sheets. In harmony with published models, the more rapid pulses of glacial meltwater incursions to the northern North Atlantic led to one or more brief short‐term shut‐downs of North Atlantic Deep Water formation. This process is reflected by pronounced benthic δ13C minima that precisely coincide, in most cases, with the end of the δ18O deglaciation steps and immediately terminate with succeeding Younger Dryas‐style cooling events. Thus we conclude that the rebounds resulted from a short‐term antecedent estuarine circulation regime in the North Atlantic, except for the Younger Dryas itself, which succeeded the Alleröd with its well ventilated Atlantic deepwater circulation and hence continues to be an enigma.
Contemporary electronic surveying
Authors: Laurila Simo H.
Published: February 2008

Electronic surveying—its feasibility, portability, and accuracy in geophysical mensurations—is analyzed. Geometries of various systems are presented, and circular, hyperbolic, and direct approaches are compared. Short derivations are made, and several phenomena of wave propagation are surveyed: shape of the ray path, velocity anomalies due to atmosphere and variable soil conductivity, reflections of long waves from the E layer and microwaves from the ground. The meaning of the use of various wavelengths in geophysical surveying is explained. An encyclopedic review is given of the different methods used in air to ground, ground to sea, and ground to ground surveying missions.
Q
Authors: Knopoff L.
Published: February 2008

Measurements of the specific attenuation factor 1/Q in homogeneous materials in the laboratory and in the field show overwhelmingly that 1/Q is substantially independent of frequency, whereas 1/Q varies as the first power of frequency in liquids. This conclusion, the result of observations over a wide range of frequencies in metals and nonmetals, in rocks in the laboratory and in the field, suggests that the mechanism for attenuation in solids is substantially different from that of liquids; a proposed nonlinear mechanism for attenuation is reviewed. The available data on attenuation of body waves, surface waves, and free oscillations are reviewed. An inversion method is described whereby the intrinsic Q in shear of the earth's mantle is computed from the surface wave and free oscillation data. The restrictions and assumptions in the calculation are (1) Q must be positive, (2) Q is independent of frequency, and (3) the mechanism of energy dissipation is through a complex modulus. The results show that, in shear, the upper mantle has a much higher attenuation than the lower mantle. Q for the upper mantle is estimated to be 110 from the surface to a depth of 650 km; for the lower mantle, below 650 km, it is much higher than this, but the exact value cannot be estimated with precision. There are hints of a fine structure for Q in the upper mantle, but present accuracy of the data and the assumptions used do not permit the literal use of this result. Partial melting in a low velocity layer at shallow depth is considered, and a small amount of partial melting is not inconsistent with the above result and the data.
Experimental cumulus dynamics
Authors: Simpson Joanne , Simpson Robert H. , Andrews Donald A. , Eaton Max A.
Published: February 2008

Cumulus dynamics is approached as an experimental science. Its development since 1947 is briefly reviewed and the current convection models described, in particular models evolved from laboratory studies, such as the ‘starting plume’.

A preliminary series of experiments on real tropical cumuli has been conducted as a method for testing and improving models and for assessing the range of applicability of the laboratory results. A silver iodide seeding technique was used in a joint U. S. Weather Bureau‐U. S. Navy experimental program called Project Stormfury. Eleven tropical cumulus clouds were probed by instrumented aircraft on four days in August 1963. Six were seeded by dropping pyrotechnic silver iodide generators into their tops at intervals of 100 meters or less; five were studied as ‘controls.’ All control clouds died without further growth. Of the treated clouds, one was seeded by misunderstanding when its top was below 10,000 feet; it collapsed. The remaining five treated clouds grew. Of these, three were seeded at low enough elevations so that natural glaciation was unlikely; these are called the ‘test’ clouds. All three test clouds were observed to ‘explode’ in two phases. The first phase was a vertical growth of 10,000–20,000 ft, occupying 10–12 minutes; the second was a horizontal expansion, with the resulting giant cloud persisting more than 30 minutes. Using laboratory and theoretical results, we construct a dynamic model of the first growth phase and test it against the aircraft measurements and photographic data. The model predicts tower ascent rates, excess temperatures, and water contents, with environment and cloud base conditions as input. It predicts that unmodified clouds could not have attained the observed heights. Furthermore, the postulated effects of seeding (fusion heating, expansion, and altered precipitation fallout) are incorporated into the model and are shown able to account for the excessive growth of the test clouds following treatment. A set of extensive control calculations shows that the main deductions are insensitive to the existing uncertainties in the input data and the modeling assumptions. Improved experiments are suggested to resolve some of the basic questions in cumulus dynamics, as well as seeding effects.
Transcontinental crustal and upper‐mantle structure
Authors: Pakiser L. C. , Zietz Isidore
Published: February 2008

Transcontinental seismic, aeromagnetic, and gravity measurements, together with geologic observations, suggest that the conterminous United States is divided by the Rocky Mountain system into two crustal and upper‐mantle superprovinces. In the eastern superprovince, the velocity of compressional waves in the upper‐mantle rocks is everywhere greater than 8 km/sec, the mean crustal velocity is generally greater than 6.4 km/sec, and the crust is generally thicker than 40 km. In the western superprovince, the velocity of compressional waves in the upper‐mantle rocks is everywhere less than 8 km/sec (except along the margin of the Pacific Ocean basin), the mean crustal velocity is generally less than 6.4 km/sec, and the crust is generally thinner than 40 km. Aeromagnetic data are characterized by anomalies of large amplitude in the eastern superprovince, indicating an abundance of magnetic minerals, whereas the magnetic field in the western superprovince is relatively featureless. The weakly magnetic crust of the western superprovince is relatively devoid of magnetic minerals at shallow depths and may be above the Curie temperature for magnetite (578°C) and therefore nonmagnetic at depth. Gravity measurements and considerations of isostasy indicate that crustal and upper‐mantle densities vary with velocity. These observations, together with the Cenozoic geologic record of diastrophism and volcanism in the western superprovince and relative Cenozoic inactivity in the eastern superprovince, suggest a mobile upper mantle in the west and a predominantly silicic crust that is now receiving mafic and probably also silicic material from the mantle, whereas the upper mantle in the east is relatively stable and the now predominantly mafic crust has been extensively intruded with mafic material from the mantle; additional mafic material has been added by extrusion of lava. The primitive continental crust that evolved from the mantle was probably silicic, and it has been made slowly more mafic by addition of mafic material from the mantle and removal of silicic material from the continental surface by erosion and stream transport.
Some geological implications of equilibrium between graphite and a C‐H‐O gas phase at high temperatures and pressures
Authors: French Bevan M.
Published: February 2008

The occurrence of graphite as a common accessory mineral in meteorites and in terrestrial metamorphic and igneous rocks gives particular importance to the study of equilibrium between graphite and a coexisting gas phase. By using a simplified model in which T, Pgas, and fO2 are independently specified for the system C‐H‐O, values of PCO2, PCO, PH2O, PH2 and PCH4 in a gas phase in equilibrium with graphite have been calculated for a wide range of geologically possible conditions by means of a high‐speed computer. The numerical results support the following general conclusions: (1) The assumption that Pgas = PH2O + PCO2 is significantly in error for many graphite‐bearing mineral assemblages. (2) Methane, CH4, may be a significant to dominant constituent of the gas phase in many possible geological environments involving moderate reduction; in particular, the occurrence of graphite with reduced minerals such as fayalite, wüstite, and iron is indicative of a methane‐rich gas phase. (3) Under metamorphic conditions, pure water is not stable with graphite, but graphite can coexist with a gas phase rich in CO2. (4) Original graphite in a sediment will stabilize increasingly reduced mineral assemblages during progressive thermal metamorphism. (5) The presence or absence of even small amounts of graphite can explain PO2 gradients observed over short distances or between adjacent layers in metamorphic rocks. (6) It is possible that the terrestrial atmosphere could have evolved by conversion of original methane to water and CO2 by reaction with graphite and other accessory minerals within the primordial earth at temperatures of 600° to 1000°C. Material requirements for such a conversion are not unreasonable, and the process itself is consistent with many proposed models for the origin of the earth.
Natural tritium
Authors: Nir A. , Kruger S. T. , Lingenfelter R. E. , Flamm E. J.
Published: February 2008

The subject of natural tritium is reviewed from the inception of the search for this isotope in 1932 until the present. Three sources of natural tritium are considered: production in the atmosphere by galactic cosmic rays, production in the atmosphere by solar flare accelerated particles, and accretion from the sun. A recalculation of the cosmic‐ray production rate utilizing experimental data for the last solar cycle yields a worldwide average of 0.20 ± .09 triton/cm² sec during solar minimum and 0.16 ± .09 triton/cm² sec during solar maximum. Production of tritium by interaction of solar flare accelerated particles with the atmosphere is found to be less than 3% of the production by galactic cosmic rays. Calculations of the tritium decay rate from material balance, utilizing measured tritium concentrations of rain and ocean water corrected for synthetic tritium, are consistent within the limits of error with the production rate by galactic cosmic rays. The best estimate of the pre‐bomb inventory corresponds to a decay rate of 0.5 ± .3 triton/cm² sec as compared with the estimated production rate of 0.19 ± .09 triton/cm² sec. In view of the large errors it is not possible to determine whether appreciable amounts of tritium are accreted from the sun.
A model for the evolution of strontium and lead isotopes in a dynamic Earth
Authors: Armstrong Richard Lee
Published: February 2008

Contrasting interpretations of existing models of Sr and Pb isotope evolution can be eliminated with a model in which crustal material is recycled through the mantle. In this model the earth's crust and upper mantle (above approximately 500 km depth) are in a steady‐state system, and the volumes and bulk compositions of ocean, continent, and mantle have been nearly constant for at least the last 2.5 b.y. and probably for most of the earth's history.

Sialic material is continuously eroded from continents into ocean basins and, as a consequence of this process, is isotopically homogenized. In continental‐margin orogenic belts and island arcs, the ocean basin, rise, and trench sediments are dragged into the mantle. Isotopic equilibration between sialic and simatic material takes place within the mantle, and the sialic material is returned to the continents or island arcs as juvenile‐appearing volcanics, thus completing the geochemical cycle. Most of the radioactive parent isotopes reside within the continental sial, whereas the mantle remains depleted and unable to sustain its observed isotope evolution.

With this model it is possible to explain Pb isotope evidence of widespread ancient continents and common Pb evolution in a system which appears to have a very uniform U/Pb and Th/U ratio, even though most of the U and Th are highly enriched in the heterogeneous sialic crust. At the same time the model provides an explanation for Sr isotope evidence of continual addition of material to continents. Sr isotope evolution is dominated by the reservoir of Sr in the mantle; in contrast, Pb isotope evolution is dominated by isotopic mixing during erosion and sedimentation.

The apparent differences in the evolutions of Sr and Pb isotopes are due to differing responses to various parts of the steady‐state cycle as a consequence of the differences in parent to daughter ratios in the sialic crust as compared with the upper mantle and in the degree of enrichment of parent and daughter products in the crust. Identical mathematical models may be used to describe the evolution of both isotope systems.
Laboratory experiments related to the solar wind and the magnetosphere
Authors: Schindler Karl
Published: February 2008

The increasing number of laboratory experiments related to the solar wind and the magnetosphere calls for a critical review of the conditions under which a laboratory experiment can provide information about space phenomena. Naturally, the starting point is a discussion of the problem of physical similarity. For a unified presentation of the several sets of scaling laws that have been used, it is convenient to use the similarity laws of the Vlasov theory as a reference system. Clearly, the biggest obstacle for a strict reproduction of the solar‐wind‐magnetosphere interaction in the laboratory is collisional interaction. In the solar wind at the earth's orbit, strict simulation is possible only for phenomena with a dimension of the order of a few hundred kilometers or smaller, such as the bow shock structure. A discussion of the existing terrella experiments shows that most of them obtain similarity by scaling the interplanetary field and the geomagnetic field differently. This determines the processes for which quantitative agreement can be expected, e.g., the shape of the frontal part of the cavity boundary and the approximate shape and position of the bow shock. Strict similarity can be achieved in simulating the structure of the bow shock wave. The over‐all ranges of the shock parameters of seven collision‐free shock experiments discussed cover substantial parts of the bow shock parameter ranges. A comparative discussion of the magnetic‐field profiles of the estimated shock width and of the appearance of oscillatory structures shows strong resemblances between space and laboratory observations. Several other experiments related to the solar wind and the magnetosphere are also listed.
Quantitative models of the magnetosphere
Authors: Roederer Juan G.
Published: February 2008

A clear understanding of the basic forms of motion of charged particles in the magnetic and electric fields of the outer magnetosphere is an essential ingredient to the study of the earth's radiation belts and plasma. To achieve this, one must work with a reliable quantitative model of the magnetospheric field. The main purpose of any such model is to provide a mathematical description of the field that is reasonably accurate within a given spatial and temporal domain and that can be used to analyze in a quantitative way any process influenced or governed by this field in the corresponding domain. In this review, magnetic field models of the magnetosphere are critically examined and their fit to in‐situ field measurements is discussed. Special attention is paid to the analysis of typical long‐term time variations of the field configuration and the matching changes in model parameters. Implications for adiabatic particle motion, such as shell geometry, regions of pseudo‐trapping, and adiabatic effects of slow time variations of the field, are analyzed.
Main mechanisms in the formation of the Earth's radiation belts
Authors: Tverskoy B. A.
Published: February 2008

In this paper we review present theories relating to the formation of the earth’s radiation belts and consider prospects for future development of these theories. The first part is devoted to the discussion of the possibility of particle injection into the zone of trapped radiation when the drift orbits of particles are perturbed during geomagnetic storms. In the second part we show that the experimental data on particle diffusion in the belts are generally in agreement with the concept of sudden impulses. The third is an analysis of the problem of proton and electron leakage due to ionization deceleration and cyclotron instability.
Body waves as normal and leaking modes, 3. Pseudo modes and partial derivatives on the (+ −) sheet
Authors: Cochran Michael D. , Woeber A. F. , De Bremaecker J.‐Cl.
Published: February 2008

In a multiple elastic wave guide, the phase velocity curves of the Π and Σ modes on the (+ −) sheet combine to form two families of almost continuous curves. The curves of the first family, called Π pseudo modes, are formed by the plateaus of the dispersion curves; they have a high group velocity, a low attenuation, and a low excitation; they depend solely on the P velocities. The curves of the second family, called Σ pseudo modes, are formed by the slopes of the dispersion curves and have the opposite characteristics. The Π pseudo modes make up the oscillatory part of the seismogram between P and S. The study is based on a simplified CIT 11 GB model, which consists of 6 layers, is 653 km thick, and contains a 54‐km low‐velocity P channel. The frequency range studied is from 0 to 0.146 Hz. The influence of the P channel is very restricted. At wavelengths smaller than ⅓ of its thickness, the layer below the channel (α = 8.19 km/sec) supports what is essentially a head wave.
Solar wind and solar energetic particles: Properties and interactions
Authors: Kavanagh Lawrence D. , Schardt Alois W. , Roelof Edmond C.
Published: February 2008

This paper reviews findings since 1966 about the properties of the interplanetary magnetic field; the solar wind plasma; solar wind interactions with the earth, the moon, Mars, and Venus; and the properties and propagation characteristics of energetic solar flare particles. For most purposes the solar wind can be characterized as a fluid that retains some of the properties of its source as it flows outward and that contains an anisotropic temperature distribution and embedded magnetic field. The magnetic field has a sector structure which slowly varies in a manner that resembles the evolution of magnetic features on the solar surface. As the solar wind encounters planetary bodies in its path, the nature of its interaction depends on the size and magnetic properties of the bodies. The solar wind creates a bow shock similar to that at earth as it interacts with Venus, and probably also with Mars. At Venus the plasma is apparently not diverted by a magnetic field, but by the planetary, ionosphere. At Mars the plasma is diverted either by a weak magnetic field or by an ionosphere. Further studies near earth have led to an explanation of the earth's bow shock in terms of hydromagnetic and ion waves. Energetic electrons (>400 kev) have been found in the neutral sheet of the geomagnetotail. The absence of thermal plasma in the earth's outer magnetosphere, beyond the plasmapause, has been explained in terms of a large‐scale convection field. The solar wind interaction with the moon is of an entirely different character. No bow shock is found, and the interplanetary magnetic field appears to convect through the moon. A single‐particle model of the interaction appears adequate to explain the observed phenomena, including the region behind the moon that is void of plasma but contains an enhanced magnetic field. Energetic electrons (20 kev to several Mev), protons (500 kev to 200 Mev), and α particles produced in solar flares have been studied in conjunction with solar X rays and radio bursts. It is now believed that these particles are not necessarily produced in an impulsive event during the flash phase of the flare; proton precursors have been found in some events, and it appears that energetic particles continue to leave the vicinity of the flare for hours or even days. These particles propagate preferentially along interplanetary magnetic field lines. Thus, their velocity distribution is generally anisotropic as long as the injection continues.
Diurnal variations in thermospheric hydrogen
Authors: Patterson T. N. L.
Published: February 2008

The escape of atomic hydrogen from the high regions of the earth's atmosphere plays a fundamental role in determining the altitude distribution of this gas. The rate of escape is well known to be a very sensitive function of temperature, with the result that, if hydrogen gas responded instantaneously to diurnal exospheric temperature variations, a day‐night asymmetry as high as 20 in the concentration of hydrogen might be expected. Studies during the last few years have shown, however, that lateral flow of hydrogen gas in the exosphere will cause a considerable reduction in the magnitude of this asymmetry, a ratio of the order of 2 being much more likely. In this paper the results of earlier work will be reviewed and updated by more recent studies.
Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest
Authors: Hudson R. D.
Published: February 2008

This paper is devoted to a critical review of photoabsorption cross sections for molecules of aeronomic and astrophysical interest at wavelengths less than 3000 A. A discussion of the relative merits of various experimental techniques is given along with possible systematic and random errors that may be associated with them. The problems in data analysis associated with finite spectral bandwidths are reviewed, with special emphasis on the interpretation of published absorption cross sections. This review does not contain a complete set of cross‐section‐versus‐wavelength values for each molecule; the prepared figures are used to compare the results of several determinations or to point out where difficulties of interpretation might arise. However, references to all papers believed to contain the more reliable data are given.