bluuebunny/crossref_metadata_2025 huggingface dataset:
Extent and bathymetry of North American Platform Seas in the Early Silurian
Authors: Johnson Markes E.
Published: February 2008

The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post‐Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed. The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene‐Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral‐stromatoporoid/stromatolite/paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coral‐stromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coral‐stromatoporoid cycles (East Iowa Basin). With shallow bathymetries of 0–30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0–60 m and 10–90 m the deeper‐water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.
The Cretaceous/Tertiary boundary carbon and oxygen isotope stratigraphy, diagenesis, and paleoceanography at Zumaya, Spain
Authors: Margolis Stanley V. , Mount Jeffrey F. , Doehne Eric , Showers William , Ward Peter
Published: February 2008

Carbon and oxygen isotope analysis of 240 m of Upper Cretaceous and Lower Tertiary limestones and marlstones from Zumaya, Spain, has revealed three distinct, negative carbon isotope excursions of up to 2 per mil, occurring both before and during the Cretaceous/Tertiary (K/T) transition. Although diagenesis has affected these rocks to varying degrees during burial and lithification, we believe that these excursions may reflect paleoceanographic changes. Sr/Ca, electron microscopic, and petrographic analysis have been used to assess the type and magnitude of diagenesis, allowing us to distinguish samples with significantly altered isotopic values. Carbon isotopes of rocks at Zumaya exhibit absolute values and ranges similar to semiconsolidated sediments of the same age. Oxygen isotopes of these rocks, however, exhibit a 1–2 per mil or more depletion, defined here as a diagenetic overprint, in comparison with values from semi‐consolidated sediments of the same age. Ammonite and inoceramid macrofossils disappear in this section 12.5 and 120 m below the K/T boundary, coincident with two distinct episodes of negative δ13C values. These may reflect localized paleoceanographic or ecological changes. The last and largest of the isotope excursions corresponds with the K/T boundary marl and mass extinctions of calcareous plankton. The K/T boundary carbon isotope excursion is one of three that took place during the last 1 million years of the Cretaceous, each one possibly being associated with biotic changes in the oceanic realm. The K/T boundary excursion coincides with iridium concentration and spherules of unidentified origin, perhaps indicating that paleoceanographic change coincided with either extraterrestrial impacts or volcanic events that amplified their effects and accelerated biotic changes during times of stressed oceanic ecosystems.
Stable isotope stratigraphy and paleoceanography during the Last Deglaciation on the continental shelf off Troms, northern Norway
Authors: Hald Morten , Vorren Tore O.
Published: February 2008

A stable oxygen and carbon isotope stratigraphy is established for a Late Weichselian/Holocene glaciomarine/marine sequence in Andfjorden and Malangsdjupet on the continental shelf off Troms, Northern Norway. The stratigraphy demonstrates that the global signals, Termination I B and possibly also I A (upper parts), are present and radiocarbon date to 10.3–9.7 kyr B.P. and >14–13.5 kyr B.P., respectively. A temperature increase of 5°–6°C and possibly a small salinity increase occurred during Term. I. A near‐glacial environment between 13 and 14 kyr B.P. was characterized by poorly ventilated bottom waters followed by a meltwater pulse at circa 13 kyr B.P. During the beginning intrusion of Atlantic Water between 13 and 10 kyr B.P., the bottom water was characterized by somewhat fluctuating temperatures and salinities. Temperatures close to those of the present were established around 9.7 kyr B.P. and seem to have been rather stable since.
Atlantic‐Mediterranean water exchange during the Late Neocene
Authors: Thunell Robert C. , Williams Douglas F. , Howell Michael
Published: February 2008

The nature of water exchange between the Mediterranean and the Atlantic is examined for three time periods during the late Neogene in order to assess the impact of flow patterns between these basins on bottom water conditions in the oceans. The large volume of carbonates and evaporites deposited in the Mediterranean during the Messinian salinity crisis appears to have caused the open ocean to become more undersaturated in calcium carbonate, resulting in increased dissolution of deep‐sea carbonates during the latest Miocene. During the earliest Pliocene, an estuarine‐type of flow existed between the Atlantic and Mediterranean. Waters entered the Mediterranean at depth and flowed out at the surface. Benthic foraminiferal stable isotopic evidence from the North Atlantic indicates that North Atlantic Deep Water (NADW) was produced throughout the early Pliocene and that the absence of saline Mediterranean outflow did not have a significant effect on bottom water production. Eustatic lowering of sea level during the last glacial maximum (18,000 years B. P.) reduced water exchange between the Atlantic and the Mediterranean. As a result, salinities in the Mediterranean increased substantially, but it seems unlikely that diminished NADW production during the last glacial episode can be attributed to a reduction in Mediterranean outflow.
Paleodepth distribution of Neocene deep‐sea hiatuses
Authors: Keller Gerta , Barron John A.
Published: February 2008

The depth of formation of Miocene to middle Pliocene deep‐sea hiatuses (NH1 to NH8)has been determined using 152 Deep Sea Drilling Project sites in the Atlantic, Pacific and Indian oceans. Megahiatuses representing maxima in deep‐sea erosion, occur at three water depths: below 3800 m, at intermediate depths between about 2000 and 3000 m, and in shallower waters above 1500 m paleodepth. Both the paleodepths and distribution of these hiatus maxima suggest that flow of Antarctic Bottom Water, Antarctic Intermediate Water, and North Atlantic Deep Water masses largely caused this erosion. Correlation of the hiatuses with oxygen isotope, carbonate and sea level records indicates that prior to 11 Ma, brief hiatuses including those resulting from nondeposition in high‐productivity equatorial regions largely correlate with global cooling episodes, high‐carbonate content, and lowstands of sea level. During the last 11 m.y., hiatuses also seem to correlate with cooling episodes, but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand. During the NH4, NH7, and NH8 intervals, it is possible that a rise in sea level corresponds with polar cooling, but there is uncertainty in correlation. Brief hiatuses during rising sea levels can be explained by basin‐shelf fractionation of carbonates. Hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of CO2and increased productivity during increased upwelling.

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Eocene equator‐to‐pole surface ocean temperatures: A significant climate problem?
Authors: Barron Eric J.
Published: February 2008

Eocene surface ocean paleotemperature data which illustrate a reduced equator‐to‐pole temperature gradient, cooler tropical sea surface temperatures, and a relatively small global temperature warming (˜2°C) present a significant climatic enigma if the reconstructions are correct. Such a temperature distribution can be achieved through greater poleward heat transport by the ocean. However, the mechanism for a greater oceanic role is uncertain. In addition, the results from climate models for conditions of a substantially greater poleward heat transport by the oceans indicate a weak atmospheric circulation and highly seasonal continental interior climates. The weak atmospheric circulation is consistent with the record of continent‐derived eolian sediment recovered in deep‐sea cores. However, the Eocene paleobotanical evidence from continental interiors indicates a very mild seasonality. The lack of consistency between model results, oceanic data, and terrestrial data defines the nature of the problem of Eocene climates. The solution must involve reinterpretation of at least one major source of information.
Paleoenvironmental implications of rock‐magnetic properties of Late Quaternary sediment cores from the eastern equatorial Atlantic
Authors: Bloemendal Jan , Lamb Bonnie , King John
Published: February 2008

Rapid and nondestructive rock‐magnetic measurements of late Quaternary sediment cores from the Senegal continental slope and the Sierra Leone Rise in the eastern equatorial Atlantic correlate strongly with the SPECMAP oxygen isotope master curve and carbonate content. The rock‐magnetic‐paleoclimatic relationship results from differences in the concentration, accumulation rate, grain size, and composition of magnetic minerals between glacials and interglacials. At both sites, magnetic mineral assemblages of glacial episodes are characterized by higher concentrations (and accumulation rates) of magnetic minerals, by coarser‐grained ferrimagnetic minerals, e.g., magnetite, and by a lower ratio of ferrimagnetic to antiferromagnetic, e.g., goethite, hematite, minerals. By serving as a sensitive tracer for various deep‐sea sedimentary components, rock‐magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic techniques.
Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian‐Turonian boundary interval: Implications for the origin and timing of oceanic anoxia
Authors: Bralower Timothy J.
Published: February 2008

A detailed nannofossil zonation has been derived for the Cenomanian‐Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons. This biostratigraphy, based on the investigation of numerous sections from Europe, North America, and Africa, allows accurate correlation of sections from shelf and deep‐sea environments. The increased resolution helps solve several pertinent paleoceanographic and sedimentologic problems in this time period. Facies patterns are variable in this interval for numerous reasons. A widespread hiatus occurred in the latest Cenomanian and earliest Turonian in much of northwest Europe and may have been an indirect result of peak marine transgression. In parts of the deep Tethys, dissolution of carbonate took place in undersaturated bottom waters or during early diagenesis. Only a few boundary sections, mostly within the Western Interior, United States, are characterized by continuous carbonate deposition. In many sections a ∥13C excursion (which has been associated with the burial of large amounts of marine organic matter) coincides with a facies change, lower ratios of coccolith carbonate to micrite, and poorer nannofossil preservation. This excursion therefore may be strongly overprinted by diagenesis. Numerous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior, and that the shift occurred distinctly later in this region than in northwest Germany. The data obtained do not invalidate the concept of a Cenomanian‐Turonian ocean‐wide anoxic event but show how local tectonic, climatic, and oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion which can be rationalized with overall sluggish middle Cretaceous circulation. Nannofossil assemblages in almost all sequences observed contain exceptionally high abundances of two dissolution‐resistant taxa in particular samples: Eprolithus floralis and Broinsonia sp.. However, the overall composition of assemblages does not change dramatically in the boundary interval, in contrast to the macrofossils and foraminifera, which suffered widespread extinctions.
Cadmium: Chemical tracer of deepwater paleoceanography
Authors: Boyle Edward A.
Published: February 2008

The oceanic distribution of cadmium resembles that of phosphorus. Because the cadmium content of foraminiferal shells is governed by the cadmium content of seawater, planktonic and benthic fossil shells can be used to infer nutrient distributions within ancient oceans. Empirical studies demonstrate that cadmium in benthic foraminiferal shells is related to the bottom water composition through a proportionality constant D ≃ 2.9. This constant is the same for each of the species studied: Cibicidoides wuellerstorfi, Cibicidoides kullenbergi, Nuttallides umbonifera, and Uvigerina.spp. Downcore cadmium data from high‐quality Pacific and Atlantic sediment cores suggest that the cadmium inventory of the ocean did not change significantly between the most recent glacial maximum and the present. Hence changes in the cadmium content of fossils at a site directly reflect changes in nutrient distributions due to altered oceanic circulation patterns. Studies of cadmium in Pleistocene sediments show that deep ocean circulation patterns were significantly different during the most recent glacial maximum. In the western North Atlantic Ocean, the nutrient content of waters from 2500 to 3500 m was twice as high during glacials as during interglacial periods, signifying an increase in the proportion of waters of Antarctic origin in this depth interval and a decrease (but not a cessation) in the flux of North Atlantic Deep Waters through this depth range. Below 3500 m, the nutrient increase was greater, indicating a higher proportion of Antarctic Bottom Water. Above 2500 m, the nutrient content was lower during glacial periods than it has been during interglacial times. The Cd content of high‐latitude North Atlantic and southern ocean surface waters does not show a significant glacial/interglacial change, which argues against theories for changes atmospheric carbon dioxide that require changes in high‐latitude nutrient concentrations.
Early history of the Arctic Ocean
Authors: Clark David L.
Published: February 2008

The Lomonosov Ridge divides the Arctic Ocean into two basins with different histories. The older Amerasian Basin consists of a ridge‐basin complex reflecting multiple tectonic cycles that are poorly understood. The younger Eurasian Basin includes the present Arctic Mid‐Ocean Ridge. Its origin is linked to Cenozoic spreading. The entire older sediment record of the Arctic Ocean is known from only four short piston cores, all recovered from the Alpha Ridge of the Amerasian Basin. The oldest sediment recovered is a single core of black mud consisting of abundant terrestrial plant material deposited in a marine environment. Dinoflagellates are late Campanian. The black mud resulted from rapid accumulation of organic matter rather than from any of the well‐known Cretaceous anoxic events. Two cores taken 100 to 200 km from the black mud consist of late Maastrichtian siliceous ooze of diatoms and silicoflagellates. This sediment has been interpreted to represent oceanic upwelling and high productivity in the Late Cretaceous Arctic Ocean. A Paleogene core consisting of similar biogenic silica has been recovered approximately 100 km from the Cretaceous cores. Silicoflagellates of this core are either Paleocene or middle to late Eocene. The sediment, like that of the Maastrichtian cores, has been interpreted to represent oceanic upwelling and high productivity. The older Arctic cores were recovered as a result of fortuitous sampling in areas of thin late Cenozoic sediment. All other Arctic cores consist of late Cenozoic sediment. The older sediment record is strong evidence of a Late Cretaceous to early Paleogene Arctic Ocean with no perennial ice and with high productivity of dinoflagellates or siliceous organisms in water that circulated with lower‐latitude oceans. Recent modeling predicts some of the same climatic and oceanographic events recognized in the oldest sediment of the Arctic Ocean. The sequence of events from a warm Arctic Ocean to an ice‐covered condition involved circulation changes, seasonality, and uneven spreading rates that increased North Atlantic‐Arctic ventilation.
Processes controlling the organic carbon content of open ocean sediments
Authors: Emerson S. , Hedges J. I.
Published: February 2008

Use of sedimentary organic carbon concentrations as a record of paleoceanographic conditions is complicated by an insufficient understanding of the mechanisms controlling present‐day variations in the organic matter content of surface open ocean sediments. This paper is a review of organic carbon distributions in marine sediments, the global marine balance of particulate and dissolved organic carbon and the processes controlling organic matter diagenesis. The discussion focuses on the last topic with the intention of bringing together mass balance and organic chemical evidence for mechanisms that control the preservation of organic matter in open ocean sediments.
Comparison between radiocarbon ages obtained on coexisting planktonic foraminifera
Authors: Broecker W. S. , Andree M. , Bonani G. , Wolfli W. , Klas M. , Mix A. , Oeschger H.
Published: February 2008

As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera. While ideally no differences should exist, we do find them. In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic‐planktonic age differences.
Miocene deepwater oceanography
Authors: Woodruff Fay , Savin Samuel M.
Published: February 2008

A global synthesis of Miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that Miocene thermohaline circulation evolved through three regimes corresponding approximately to early, middle, and late Miocene times. There is evidence for major qualitative differences between the circulation of the modern ocean and the Miocene ocean prior to 11 Ma. The 13C/12C ratios of the benthic foraminifera Cibicidoides are interpreted in terms of water mass aging, i.e., the progressive depletion of dissolved O2 and lowering of δ13C values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans. Both isotopic and faunal data indicate that the early Miocene regime, from 22 to 15 Ma, was the most different from today's. During that interval intermediate and deep waters of both the Atlantic and the Pacific oceans aged in a northward direction, and the intermediate waters of the Indian, the South Atlantic and the South Pacific oceans were consistently the youngest in the global ocean. We speculate that early Miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water, Tethyan Indian Saline Water, from the Tethys into the northern Indian Ocean. The isotopic and faunal data suggest that flow from the Tethyan region into the Indian Ocean diminished or terminated at about 14 Ma. Isotopic and faunal data give no evidence for North Atlantic Deep Water (NADW) formation prior to about 14.5 Ma (with the exception of a brief episode in the early Miocene). From 14.5 to 11 Ma NADW formation was weak, and circumpolar and Antarctic water flooded the deep South Atlantic and South Pacific as the Antarctic ice cap grew. From about 10 Ma to the end of the Miocene, thermohaline circulation resembled the modern circulation in many ways. In latest Miocene time (6 to 5 Ma) circulation patterns were very similar to today's except that NADW formation was greatly diminished. The distribution pattern of siliceous oozes in Miocene sediments is consistent with our proposed reconstruction of thermohaline circulation. Major changes which occurred in circulation during the middle Miocene were probably related to the closing of the Tethys and may have contributed to rapid middle Miocene growth of the Antarctic ice cap.

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Some thoughts about the radiocarbon budget for the glacial Atlantic
Authors: Broecker W. S.
Published: February 2008

The goal of this paper is to explore whether the low deep to surface radiocarbon age difference of 600 years reconstructed for the glacial Atlantic requires strong North Atlantic Deep Water production and its consequent large heat delivery to the atmosphere. An estuarine type circulation for the glacial Atlantic is explored as a possible alternative. While such a model can explain the low radiocarbon age difference, it cannot easily explain nutrient distribution reconstructed for glacial time.
Opening the carbon isotope “vital effect” black box 1. Seasonal temperatures in the euphotic zone
Authors: Spero H. J. , Williams D. F.
Published: February 2008

Recent laboratory experiments with Orbulina universa show that the carbon and oxygen isotope signals from this symbiont‐bearing planktonic foraminifera record distinct environmental parameters, irradiance and temperature. Since light levels are correlated with depth in the marine environment, the carbon isotopic composition of O. universa can be utilized to differentiate individuals from different depth habitats. By combining δ18O and δ13C data from individuals in an assemblage, estimates of seasonal temperature ranges are possible for the upper and lower euphotic zone of the ocean. Reconstruction of seasonal temperatures using individual Holocene O. universa from western equatorial Pacific Ocean boxcore ERDC‐83 yields ranges of 23.5° to 29.9°C and 20.1° to 24.5°C for the shallow and deep euphotic zone respectively. These values are in excellent agreement with recent temperature measurements from this region. Analyses of individual, late glacial (≈12,000–13,000 years B.P.) O. universa from this site suggest the seasonal upper euphotic zone temperature range was approximately 2.5°C lower than it is today, a function of cooler summers.
Atlantic Ocean thermohaline circulation of the last 150,000 years: Relationship to climate and atmospheric CO2
Authors: Oppo D. W. , Fairbanks R. G.
Published: February 2008

The high‐resolution δ18O and δ13C records of benthic foraminifera from a 150,000‐year long core from the Caribbean Sea indicate that there was generally high δ13C during glaciations and low δ13C during interglaciations. Due to its 1800‐m sill depth, the properties of deep water in the Caribbean Sea are similar to those of middepth tropical Atlantic water. During interglaciations, the water filling the deep Caribbean Sea is an admixture of low δ13C Upper Circumpolar Water (UCPW) and high δ13C Upper North Atlantic Deep Water (UNADW). By contrast, only high δ13C UNADW enters during glaciations. Deep ocean circulation changes can influence atmospheric CO2 levels (Broecker and Takahashi, 1985; Boyle, 1988a; Keir, 1988; Broecker and Peng, 1989). By comparing δ13C records of benthic foraminifera from cores lying in Southern Ocean Water, the Caribbean Sea, and at several other Atlantic Ocean sites, the thermohaline state of the Atlantic Ocean (how close it was to a full glacial or full interglacial configuration) is characterized. A continuum of circulation patterns between the glacial and interglacial extremes appears to have existed in the past. Subtracting the deep Pacific (∼mean ocean water) δ13C record from the Caribbean δ13C record yields a record which describes large changes in the Atlantic Ocean thermohaline circulation. The δ13C difference varies as the vertical nutrient distribution changes. This new proxy record bears a striking resemblance to the 150,000‐year‐long atmospheric CO2 record (Barnola et al., 1987). This favorable comparison between the new proxy record and the atmospheric CO2 record is consistent with Boyle's (1988a) model that vertical nutrient redistribution has driven large atmospheric CO2 changes in the past. Changes in the relative contribution of NADW and Pacific outflow water to the Southern Ocean are also consistent with Broecker and Peng's (1989) recent model for atmospheric CO2 changes.
Mid‐Cretaceous ocean circulation: Results from model sensitivity studies
Authors: Barron Eric J. , Peterson William H.
Published: February 2008

A series of ocean general circulation model (GCM) experiments for mid‐Cretaceous continental paleogeography with atmospheric forcing prescribed from atmospheric GCM experiments are utilized to investigate the nature of the mid‐Cretaceous ocean circulation. The experiments illustrate a Tethys system of clockwise currents and predominant eastward flow, in marked contrast with earlier reconstructions. This result is apparently robust for changes in bathymetry and in the degree of planetary warmth. Changes in sea level have the largest potential to produce alternative circulation characteristics. Two major current systems outside of Tethys dominate the model simulation. The site of bottom water formation is the North Pacific, however, in a sensitivity experiment for a warmer climate, the eastern Tethys became a site of deep water formation.
Thorium 230 profiling in deep‐sea sediments: High‐resolution records of flux and dissolution of carbonate in the equatorial Atlantic during the last 24,000 years
Authors: Francois Roger , Bacon Michael P. , Suman Daniel O.
Published: February 2008

Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point‐by‐point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported 230Th into deep‐sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess 230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of 230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid‐Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2 below the level of the Romanche Fracture Zone in the eastern basin.
Application of the Ce anomaly as a paleoredox indicator: The ground rules
Authors: German Christopher R. , Elderfield Henry
Published: February 2008

Much attention has been paid, in recent years, to the potential application of the Ce anomaly, measured in various marine phases, as a paleoceanographic indicator of widespread marine anoxia. In this paper we present and discuss results from recent studies of present‐day rare earth element (REE) distributions (and hence Ce anomaly distributions) in the marine environment which are particularly pertinent to paleoceanography. Subsequently, we review and discuss the validity of the recent literature in which Ce anomalies have been employed as paleoredox indicators.
Heat flow data analysis
Authors: Lee W. H. K.
Published: February 2008

Six hundred and eighty‐seven heat flow data from more than 780 individual determinations and estimations were compiled. Of these, 634 data were analyzed. The range of the present heat flow per unit area is approximately 0 to 8 µcal/cm2 sec. The world's arithmetic mean is 1.62, and the standard deviation of a datum is 1.21. The data do not seem to form any simple distribution. Since the sampling is poor and biased, a weighted mean technique is developed. On the basis of three assumptions, the world's average heat flow per unit area was found to be 1.5 ± 10 per cent at 95 per cent confidence. This is significantly higher than the value 1.2 suggested in 1954. There is no simple dependence of heat flow per unit area on latitude and longitude. A spherical harmonic analysis indicates that all the coefficients of the harmonics are at least an order of magnitude less than the mean of about 1.5. Heat flow and geological parameters, as well as difficulties in heat flow research and sources of heat, are briefly discussed. The purpose of this study is to summarize all the available data in a comprehensive list with extensive bibliographies and to set up computer programs for data analysis that can be used in the future.
Thermal effects of intrusions
Authors: Jaeger J. C.
Published: February 2008

The mathematical theory used in calculating temperatures of intrusions is reviewed, primarily from the point of view of finding temperatures in the country rock outside them. It is shown that the detailed behavior inside the intrusion, for example the mechanism of solidification and the possible effects of convection, becomes progressively less important as the distance from the contact increases, so that at distances of one‐quarter of the thickness or more the simple theory of Lovering is adequate. The theory for sheets, stocks, laccoliths, and some irregularly shaped bodies is given, together with the effect of dissimilar thermal conductivities. The effects of latent heat, convection, and the circumstances of intrusion are discussed. Applications to metamorphism, rock magnetism, and argon loss caused by heating by intrusions are reviewed.
Spread F and ionospheric F‐region irregularities
Authors: Herman John R.
Published: February 2008

The results of ionospheric observations by means of a variety of techniques are summarized to show that magnetic field‐aligned irregularities exist in the ionosphere. The irregularities appear in patches having horizontal dimensions up to 1000 km in the east‐west direction and several hundred kilometers in the north‐south direction. Size of the irregularities is typically about 1 km transverse to the magnetic field and elongated along the field line. Irregularities associated with spread F occur in the altitude range from 250 km up to the height of Fmax. Correlations between ionogram spread F and other ionospheric observations indicate that spread echoes are closely related to the field‐aligned irregularities. A critique of theories explaining why the irregularities cause spread echoes is given. Statistical studies of the geographic and temporal variations in spread‐F occurrence are summarized. The fundamental question of how irregularities causing spread F are produced in the ionosphere is discussed.
Longshore current velocity: A review of theory and data
Authors: Galvin Cyril J.
Published: February 2008

A proven prediction of longshore current velocity is not available, and reliable data on longshore currents are lacking over a significant range of possible flows. Theoretical studies have been based on over‐simplified models, and empirical predictions have been hampered by lack of data. The empirically modified, momentum‐flux theory now accepted as the best prediction is based on an untenable assumption and supported by inappropriate data. Regardless of their validity, however, all six of the testable equations agree fairly well with at least one of six sets of published data, and two agree with both of the better sets of data. These two equations may be used as empirical guides for velocity prediction in the absence of a proven theory. The best prospect for a generally valid velocity prediction appears to be an empirical correlation based on reliable data.
Geological and geophysical implications of paleomagnetic results from Africa
Authors: McElhinny M. W. , Briden J. C. , Jones D. L. , Brock A.
Published: February 2008

A critical review of all African paleomagnetic results has yielded 31 sets of data that are useful for discussions of the behavior of the paleomagnetic field and of polar wandering and continental drift. Most of the reliable data are new. They largely confirm the previously published polar‐wander path relative to Africa for Permian and younger time and add detail particularly about rates of polar shift relative to that continent. They permit a tentative extension of the polar‐wander path back to about 600 m.y. (beginning of Cambrian time). A complete revision of the previously published Precambrian polar‐wander curve is presented in two parts, covering the periods from 2700 m.y. to about 2100 m.y. and from 2000 m.y. to 1300 m.y., respectively. For these two periods it is possible to use paleomagnetism as a tool in geological correlation and as a means of dating. There is still no information on the interval from about 2100 m.y. to 1950 m.y. or for the interval between 1300 m.y. and 600 m.y. The Mesozoic data remain consistent with Du Toit's reconstruction of Gondwanaland for the Jurassic. The Permo‐Carboniferous data confirm the period of rapid drift within the Carboniferous, which has already been identified in Australia by Irving, and confirm the general correctness of the relative positions of Australia and Africa suggested by L. C. King. Comparison of African data with data for the Canadian shield for the period 2000 to 1200 m.y. enables us to suggest tentatively that the Canadian and Southern African cratons did not move relative to each other during that time. This is the first attempt at reconstruction of the relative positions of two continental regions from Precambrian paleomagnetic data. The oldest Precambrian data confirm the existence of a terrestrial magnetic field which occasionally reversed itself at least 2700 m.y. ago. The dispersion of directions in reliable paleomagnetic data is taken as a measure of paleosecular variation; only 3 of the 19 estimates are significantly different from the secular variation associated with the present geomagnetic field.
Low‐energy plasma in the Earth's magnetosphere
Authors: Gringauz K. I.
Published: February 2008

The low‐energy plasma fluxes in the earth's magnetosphere, with energies from thermal up to ∼50 kev, are an important factor in determining many of the essential properties of the magnetosphere. Currents produced by the motion of these charged particles play a key role in the development of magnetic storms and in the production of auroras and the geomagnetic tail. This paper will review the experimental observations of this plasma obtained over the last decade in different regions of the magnetosphere, considered in order of increasing distance from the earth. The similarity of results from measurements of the ion and electron components of the earth's thermal plasma envelope and the dependence of the altitude of the boundary of this envelope (the ‘plasmapause’) on geomagnetic activity are described. Within the radiation belts there exists a low‐energy plasma component that is much more variable than the energetic component; the variation of low‐energy plasma fluxes in the outer belt seems to be the main cause of geomagnetic storms. The radiation belts are always surrounded by low‐energy plasma fluxes; on the night side these fluxes become a part of the thick plasma layer within which is imbedded the neutral sheet of the geomagnetic tail. Experimental data on low‐energy plasma at geocentric distances >2 RE and geomagnetic latitudes >45° are very scarce; this region needs intensive investigation.
Magnetospheric convection
Authors: Axford W. I.
Published: February 2008

Magnetospheric convection, which implies the existence of large‐scale electric fields, appears to be the dominant process for the acceleration and injection of moderately energetic plasma into the magnetosphere. The aurora, the radiation belts, and the ‘thermal’ plasma beyond the plasmapause are directly controlled by the convection induced by the solar wind. The existence of the plasmapause and its variations with time can be understood in terms of convection, and, even within the plasmapause, the radiation belts and the ionospheric plasma are strongly affected by tidally induced convection and the more random convection associated with DP1 and DP2 geomagnetic activity. The basic features of convection are discussed briefly in this review. Direct and indirect observations that provide evidence for the existence of convection are described, and the expected nature of the solar‐wind‐induced convection in the outer magnetosphere is discussed at some length. A fairly complete bibliography concerning the theory of magnetospheric convection is provided in the references.
Electric acceleration of auroral particles
Authors: Chamberlain Joseph W.
Published: February 2008

Measurements of the distribution of auroral particles with energy and pitch angle have not given much of a clue to the precipitating mechanism, except to indicate that it occurs locally, in the magnetosphere. The available data, including observations of periodicities in auroral bombardment, hint that at least two different mechanisms are operative. With the aid of a simplified model for an accelerating electric field, the kind of dependences on angle and energy to be expected are illustrated. Quantitative discussion is restricted to cases where the electric potential is small compared with the energy dispersion of trapped particles, and the results are thus probably more applicable to instability mechanisms than to the electrostatic precipitation treated by Taylor and Hones. Simultaneous measures of the spectrum at different pitch angles above the atmosphere could be combined to test any given model of electric acceleration. In spite of the wealth of measurements, adequate data do not yet seem to be available.
Particles and fields: Significant achievements, 2
Authors: Schardt Alois W. , Opp Albert G.
Published: February 2008

Satellite observations of energetic particles and magnetic fields during the last years have contributed greatly toward understanding our environment. Studies of nonadiabatic processes that occur in the radiation belts have led to a qualitative understanding of several source and loss mechanisms responsible for the observed particle population. A plasma instability leading to spontaneous emission of VLF and ELF wave has been found that limits the maximum flux of protons and electrons that can be trapped in the geomagnetic field. It has been shown that diffusion of particles into the magnetosphere is accompanied by a gain in particle energy; this process probably accounts for most of the lower‐energy particles found in the Van Allen belts. A comprehensive description of the geomagnetic field has been completed. Corrections for the current systems at the magnetospheric boundary and in the magnetotail have been applied in a comprehensive model. Secular changes in the main field have been interpreted in terms of core motion in the earth. Initial measurements have been performed of the galactic cosmic‐ray gradient in the solar system between Venus and Mars. Theories for explaining this gradient in terms of observable properties of the solar wind are being perfected. The relative abundance and energy spectra of hydrogen and helium isotopes in cosmic radiation have been explained in terms of interstellar matter traversed by the cosmic‐ray particles.
Physical mechanisms of seismic‐wave attenuation
Authors: Jackson David D. , Anderson Don L.
Published: February 2008

The theoretical and experimental evidence concerning mechanisms likely to be responsible for the attenuation of seismic waves are reviewed. Intergranular thermoelastic relaxation, atomic diffusion, and dislocation mechanisms cannot be ruled out as significant causes of seismic attenuation, but the most effective mechanisms seem to be associated with partial melting, grain‐boundary relaxation, and a poorly understood mechanism called ‘high‐temperature, internal‐friction background’ which obeys an equation of the form

Q‐1 = (A/ƒ) exp (—H*/RT)

Here Q−1 is a dimensionless measure of anelasticity, f is the frequency, T is the absolute temperature, and A, H*, and A, H*,and RR are constants for a material of uniform composition and grain size. Many of the mechanisms considered here have a strongly frequency‐dependent Q−1. However, in a material as complex as a rock, there is unlikely to be a single discrete relaxation time or activation energy, or a single ‘typical’ grain size. The observed Q−1 is likely to result from a superposition of several mechanisms and involve a spectrum of parameters leading to weaker frequency dependence than is predicted from a single simple mechanism. The seismic data cannot at present resolve the question of whether or not Q−1 has an intrinsic frequency dependence. The extent and the limitations of the available seismic data are discussed. Experimental data on the attenuation of oxides are summarized, the measurements at high temperature and low frequency being emphasized.
Effect of crustal structure on rayleigh waves generated by atmospheric explosions
Authors: Harkrider David G. , Flinn Edward A.
Published: February 2008

Theoretical seismograms are calculated at a teleseismic distance from atmospheric sources over oceanic and continental earth models. Vertical surface displacements of the fundamental and first higher‐mode Rayleigh waves are obtained for each of the models. Source altitudes range from 0.3 to 4.88 km for a 1‐kiloton nuclear explosion in a stratified thermal atmosphere. At 20‐sec period, an explosion over the oceanic model exhibits amplitudes an order of magnitude greater than the equivalent amplitudes from an explosion of the same burst height and yield over any of the three continental structures. If the differences in anelastic attenuation over the paths are included, this effect will be reversed at large enough distances.
A geometrical picture of anisotropic elastic tensors
Authors: Backus George
Published: February 2008

A geometrical picture of fourth‐order, three‐dimensional elastic tensors in terms of Maxwell multipoles is developed and used to obtain the elastic tensors appropriate to various crystal symmetry groups. Simply examining the picture shows whether an elastic tensor, described by its 21 independent components relative to an ill‐chosen coordinate system, has an axis of symmetry of any order. The picture also facilitates obtaining the elastic tensor from the observed dependence of the three body‐wave phase velocities
on the direction
of the propagation vector κ. In particular, for q = 2, 4, and 6,
is a linear combination of the surface spherical harmonics of even orders up to and including q. Since
determine
uniquely, all the body‐wave phase‐velocity dependence on
can be summarized by the 6 coefficients of spherical harmonics in P(2), the 15 coefficients in P(4), and the 28 coefficients in P(6). For nearly isotropic media, the anisotropy in the P velocity determines 15 of the 21 elastic coefficients, whereas
determines the other 6 elastic coefficients. Our description of elastic tensors is generalized to all fourth‐order tensors in three dimensions and certain fourth‐order tensors in higher dimensions. The problem in higher dimensions produces simple examples of unitary representations of the rotation group ON+ with N ≥ 4 which contain no harmonic irreducible components.
Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes
Authors: Isacks Bryan , Molnar Peter
Published: February 2008

A region‐by‐region analysis of 204 reliable focal‐mechanism solutions for deep and intermediate‐depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake‐generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World‐Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre‐1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double‐couple or shear dislocation model of the source mechanism is adequate for all the data.