bluuebunny/crossref_metadata_2025 huggingface dataset:
Sediment accumulation in the central western Black Sea over the past 5100 years
Authors: Hay Bernward J.
Published: February 2008

The postdepositional particle flux (accumulation rate) over the last 5100 years was studied in the anoxic sediment units I and II from the central western Black Sea. Deposition of unit II (sapropel) in the deep Black Sea basin was divided further into two subunits. The beginning of the deposition of the sapropel in the Black Sea (5100–3100 years B.P., subunit IIb) was characterized by higher organic carbon accumulation rates due largely to higher primary production. Accumulation rates of lithogenic matter were at a minimum compared to younger units. During the subsequent 2100 years (3100–1000 years B.P., subunit IIa) organic carbon accumulation rates decreased considerably while accumulation rates of lithogenic matter increased to the present level. At 1000 years B.P. (unit I), total mass accumulation rates increased by a factor of 3. This increase was largely a result of an increase in carbonate accumulation rates by a factor of 11, which was caused by the rapid appearance of Emiliania huxleyi in the Black Sea. Higher carbonate accumulation rates in unit I are mainly the cause for the considerably lower organic carbon content compared to unit II, since the organic carbon accumulation rate in unit I did not decrease. General correspondence of accumulation rates of lithogenic matter in different cores through time suggests overregional long‐term (100‐year scale) climatic effects on the terrigenous particle supply to the Black Sea.
The role of magnetostratigraphy in the development of geological time scales
Authors: Hailwood E. A.
Published: February 2008

One of the major targets of geochronology is the “calibration” of chronostratigraphic boundaries (defined principally from biostratigraphic information) in terms of numerical ages derived from isotopic age data. Few isotopic age determinations correspond precisely with such chronostratigraphic boundaries. Furthermore the analytical (and other) uncertainties in isotopic age data are often greater than the duration of the age or zone whose boundaries need to be dated. Consequently, it is necessary to synthesize isotopic age data which are distributed both in space and time in order to obtain age estimates for the geological boundaries themselves. This synthesis requires the introduction of some linearizing operator on which to base the necessary interpolations or extrapolations. Commonly used linearizing operators involve assumptions of uniform rates of sedimentation and/or biological evolution. However, such assumptions are often highly questionable. An alternative and potentially more reliable linearizing operator utilizes the assumption of a uniform seafloor spreading rate at certain midocean ridges for certain periods in the (post mid‐Jurassic) geological past. The use of this operator for the Chronometric calibration of chronostratigraphic time scales is based upon the fact that successive polarity changes of the geomagnetic field are recorded simultaneously in the pattern of lineated magnetic anomalies formed by seafloor spreading and also in the paleomagnetic record of sedimentary sequences in which the chronostratigraphic boundaries can be defined. This approach provides a number of advantages over the use of more traditional linearizing operators, and these are reviewed in this paper. It has been used in several recent geological time scales. Its applicability depends upon a number of factors, including the reliability of the basic seafloor spreading polarity time scale and that of the paleomagnetic record in isotopically and/or biostratigraphically dated sequences. This paper reviews these topics, in particular the use of the constant spreading rate operator, and suggests that the full implications of “apparent spreading rate” changes which have been identified in previously published polarity time scales have not been fully addressed. Necessary remedial action is outlined, but the paper does not attempt to produce a further revision to the polarity time scale. A number of questions of terminology in magnetostratigraphy are discussed and the desirability of establishing a new rationalized or “systematized” magnetochron labeling scheme is reviewed.
Planktonic foraminifera: Differential dissolution and the Quaternary stable isotope Record in the west equatorial Pacific
Authors: Wu Guoping , Berger W. H.
Published: February 2008

Differential dissolution affects the isotopic composition of different species of planktonic foraminifera in different ways. In the two species studied here in cores from Ontong Java Plateau, the less resistant species, Globigerinoides sacculifer, is more readily affected at a shallower depth than the more resistant species, Pulleniatina obliquiloculata (2.9 versus 3.4 km), but shows a smaller and less predictable response to partial dissolution (0.2 to 0.3 per mil versus 0.6 to 0.7 per mil). Comparison of isotopic values from the last glacial period with those from the late Holocene indicates that the apparent dissolution effect is considerably reduced during the last glacial, presumably due to reduced dissolution intensity during glacial time. A change in the level of the lysocline of about 400 m is suggested. In the published isotope records from Pacific cores V28‐238 and V28‐239, the dissolution‐generated difference in δ18O (noted previously by Shackleton and Opdyke [1976]) is seen to describe a mid‐Brunhes dissolution maximum, between 300 and 500 thousand years ago. This mid‐Brunhes dissolution excursion is well known from the Pacific and the Indian Ocean.
Adriatic deep water formation during the Holocene: Implication for the reoxygenation of the deep eastern Mediterranean Sea
Authors: Fontugne Michel R. , Paterne Martine , Calvert Steve E. , Murat Anne , Guichard François , Arnold Maurice
Published: February 2008

Under modern conditions, the reoxygenation of eastern Mediterranean deep water is the result of the formation of dense bottom water in the southern Adriatic Sea. Oxygen isotopic records of planktonic foraminfera used as a paleosalinity tracer show that Po River and/or the alpine glaciers meltwater are not directly responsible of the inhibition of the deep water formation in the Adriatic, during sapropel (S1) formation at 9,000 years B.P.. Sedimentological evidence indicates that fully oxygenated conditions in the deep Adriatic occurred only by 1,650±100 years B.P., a long time after the total reoxygenation of the deep eastern Mediterranean. Organic carbon content and isotopic ratio indicate that persistence of a reduced sediment phase until 1,650 years B.P., does not result from increased primary productivity or large input of terrestrial organic matter.
Atlantic paleogene biserial heterohelicid foraminifera and oxygen minima
Authors: Boersma Anne , Silva Isabella Premoli
Published: February 2008

Through most of the Paleogene the planktonic foraminifera called biserial heterohelicids record the most depleted carbon isotope ratios among planktonic foraminiferal species. They are interpreted to occupy an oxygen minimum habitat. Stronger interplanktonic carbon isotope contrasts and more enriched near surface carbon isotope values correspond to higher abundances of biserial heterohelicids. This group appears to have preferred latitudes and times when the oxygen minimum was well developed. Biserial heterohelicid generic and specific diversity are directly related to the interplanktonic carbon isotope contrast. Minimal specific diversity characterizes the Oligocene when interplanktonic and surface to bottom carbon isotope contrasts were consistently low. Diversity and abundance decreases were coeval with increased oceanic ventilation.
Carbon isotope composition of tropical surface water during the past 22,000 years
Authors: Oppo Delia W. , Fairbanks Richard G.
Published: February 2008

Carbon isotope values of many species of planktonic foraminifera are size‐dependant. Values of δ13C of individual size fractions are therefore an integration of low δ13C values of calcite precipitated in the juvenile growth stage and high values precipitated in later stages. In general, the selection of a particular size fraction for downcore studies has been subjective. Analysis of different size fractions of planktonic foraminifera, however, yields different time series in downcore studies. By measuring the δ13C of many size fractions, the δ13C precipitated throughout the growth of the foraminifera can be calculated. Time series of the δ13C of calcite precipitated in the late growth stages of planktonic foraminifera from two nearby cores from the Caribbean Sea result in a better correlation than time series of constant size fractions. These δ13C records indicate similar δ13C values during the last glaciation and the late Holocene and a broad deglacial minimum approximately synchronous with a minimum in the δ13C records of benthic foraminifera from intermediate‐depth Atlantic cores. The deglacial minimum may be a reflection of the isotopic composition of the underlying intermediate water (which may be affected by circulation changes or changes in preformed δ13C values) and/or of the flux of water upwelled into the surface layer. The small (−0.2 to 0.0‰) glacial‐interglacial δ13C difference in tropical surface water is also believed to be affected by the chemistry of the underlying source water.
Climatic change and CaCO3 preservation: An 800,000 year bathymetric Reconstruction from the central equatorial Pacific Ocean
Authors: Farrell John W. , Prell Warren L.
Published: February 2008

Sixteen deep‐sea cores from the central equatorial Pacific are used to reconstruct a continuous 800,000‐year (800‐kyr) record of bathymetric variations in carbonate preservation as measured by calcium carbonate (CaCO3) content. The depth of the sedimentary lysocline has fluctuated markedly in conjunction with late Pleistocene climate cycles while the carbonate critical depth (CCrD), which is the water depth where the sediments contain 10% CaCO3, has remained relatively constant. As a result, the depth of the lysocline controls the bathymetric position and thickness of the CaCO3 transition zone, defined as the depth from the lysocline to the CCrD. Modern and interglacial‐aged sediments show poor CaCO3 preservation and thick CaCO3 transition zones. Glacial‐aged sediments show good preservation and deep, thin zones due to the deepening of the lysocline. Detailed comparison of the CaCO3 preservation and oxygen isotope records from the central equatorial Pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages. During the nine major glacial stages of the last 800 kyr, the lysocline deepened by at least 400 to 800 m. This deepening indicates an increase in the abyssal carbonate ion concentration ([CO3=]) and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate‐saturated water mass. The bottom of the transition zone has remained at a relatively constant depth during the Brunhes Chron, indicating a balance between CaCO3 sedimentation and dissolution in the deepest waters of the central equatorial Pacific.
Globorotalia truncatulinoides' Growth and chemistry as probes of the past thermocline: 1. Shell size
Authors: Lohmann G. P. , Schweitzer Peter N.
Published: February 2008

This work identifies features of the life history of the deep‐living planktonic foraminifer Globorotalia truncatulinoides that offer possibilities for reconstructing characteristics of the past ocean's main thermocline. Part 1 considers two aspects of variation in shell size: the predictability of growth‐related size changes, which we need to calculate changes in shell chemistry, and a model for environmentally forced changes in shell size frequency, which suggests how shell size may be a direct source of paleoceanographic information. Even though size variation among shells of G. truncatulinoides at the same stage of growth (same number of chambers) exceeds the size difference between successive growth stages, growth‐related size changes are predictable because shells tend to grow at the same rate. Variation in initial shell size is sufficient to account for the large size variation found among shells at the same stage of growth. Several lines of evidence suggest that G. truncatulinoides reproduces at ∼600 m in the water column, and it appears that vertical mixing to that depth is required to return juvenile shells to the surface. Anything that prevents this, such as shallowing of the thermocline, can interfere with G. truncatulinoides' life history in such a way that its shell size‐frequency distributions are altered. Among shells falling to the seafloor from a reproducing population are large numbers of both juvenile and adult sizes and, if reproduction is inhibited, the frequencies of those sizes are reduced and the relative frequency of intermediate‐sized shells increases. A paleoceanographically useful result of this is that the topography of the main thermocline tends to be reflected in the size‐frequency distribution of G. truncatulinoides' shells from the underlying sediments: In areas where the top of the main thermocline is shallower than ∼600 m, production of juveniles (125–177 µm) and reproductive adults (>425 µm) is reduced, leaving predominantly intermediate‐sized (212–355 µm) shells.
Simulated circulation of an idealized ocean for Pangaean time
Authors: Kutzbach J. E. , Guetter P. J. , Washington W. M.
Published: February 2008

An eight‐level dynamical ocean model, driven by prescribed surface forcing (wind stress, temperature, precipitation‐minus‐evaporation) from a previously published Pangaean time climate simulation, is used to study possible features of ocean circulation, temperature, and salinity for the early Mesozoic. The simulations are sensitivity experiments rather than estimates of the actual circulation because many simplifying assumptions are made concerning the prescribed surface forcing and the ocean basin's shape and depth. Simulated features include relatively warm water in high latitudes, warm deep water, a westward flowing equatorial current across Panthalassa and into the Tethys Sea, poleward and eastward flowing currents along the Tethys coasts, and relatively vigorous meridional and longitudinal vertical overturning cells. Cool, relatively fresh polar water combines with warm, saline Tethys water to produce the relatively warm deep water. There are few geological estimates of early Mesozoic (Pangaean time) ocean circulation available for comparison with the simulated circulation.
Energy balance constraints on 18O based paleo‐sea surface temperature estimates
Authors: Horrell Mark A.
Published: February 2008

Oxygen isotope based paleo‐sea surface temperature estimates from the tropics consistently indicate values well below that of the present (27°C), with some estimates from the Middle Tertiary reaching as low as 15°C. Using an energy balance model allows a test of several scenarios for producing and maintaining such cool tropical temperatures; of particular interest are (1) increased cloud cover, (2) increased zonal wind speed, and (3) increased poleward energy transport. The latter is the most likely process since enhanced poleward energy transport would lead to warmer poles, a point which agrees well with high latitude geological and paleontological evidence. However, none of the scenarios tested could provide the degree of equatorial cooling needed to be in agreement with paleotemperature estimates. In summary, published estimates of cool equatorial surface temperatures must be questioned since they cannot be supported by energy balance arguments or geological evidence.
Ocean General Circulation Model Sensitivity Experiment with an open Central American Isthmus
Authors: Maier‐Reimer Ernst , Mikolajewicz Uwe , Crowley Thomas
Published: February 2008

We have tested the sensitivity of an ocean general circulation model to changes in solid boundary conditions by opening the central American isthmus in order to simulate possible circulation patterns existing prior to about 3.0–4.0 Ma. The model was driven by present winds, atmospheric temperatures, and moisture fluxes, with the only difference between control and perturbed runs being the open isthmus. Significant changes result for the perturbed case, almost all of which can be traced to changes in surface salinity distribution. With an open isthmus, lower‐salinity waters from the Pacific dilute North Atlantic surface waters by >1.0‰. The lower surface salinities cause the thermohaline cell in the North Atlantic to collapse, with North Atlantic Deep Water (NADW) production reduced to near zero. This response greatly weakens poleward ocean heat transport in the North Atlantic. The “no‐NADW” result appears to be insensitive to initial conditions and represents a circulation mode different than a “low‐NADW” mode found in the same ocean model using present geographic forcing. There is some agreement between model predictions and geologic data, which indicate a significantly different deep‐water circulation pattern prior to ∼2.4 Ma. However, observations suggests that the time of major transition in circulation regimes occurs earlier than the final closure of the isthmus. This discrepancy may reflect the fact that near‐closure of the isthmus by 7–10 Ma could have been sufficient to alter the circulation. As ocean heat transport is a key source of warmth to the high latitudes of the North Atlantic, some compensating factor (CO2?) may have been required to restore warmth to that region prior to ∼3.0–4.0 Ma.
Climatic effects of reduced Arctic sea ice limits in the Giss II General Circulation Model
Authors: Raymo M. E. , Rind D. , Ruddiman W. F.
Published: February 2008

In this paper we present results of an atmospheric general circulation model (GCM) experiment in which Arctic sea ice limits were substantially reduced in all months. March sea ice limits were set equivalent to modern September limits, and all sea ice was removed in September. Sea ice coverage for other months varied between these two extremes. This climate sensitivity experiment makes predictions about mean northern hemisphere atmospheric conditions (including temperature, pressure, wind patterns, and precipitation) consistent with these boundary constraints. The major effects of reduced sea ice limits are observed in winter. They include large regional warming of the circum‐Arctic region, northward migration of the Icelandic low pressure system, and strengthening of the Azores high. Changes in net heating over the North Atlantic Ocean suggest that increases in sea surface temperatures and salinities in this region would also accompany reductions in Arctic sea ice limits. In the wind field, a weakening of the polar easterlies and an intensification of cyclonic circulation over the Norwegian‐Greenland Sea suggest that surface water exchange between the Atlantic and Arctic would increase when sea ice limits are reduced. However, zonally averaged changes in strength of the westerlies or upper level jet stream are minimal. The late Pliocene cooling of the North Atlantic Ocean and North American Arctic margin may have been linked in part to the development of perennial sea ice cover.
Variability in upwelling fields in the northwestern Indian Ocean 2. Data‐model comparison at 9000 years B.P.
Authors: Prell Warren L. , Marvil Rebecca E. , Luther Mark E.
Published: February 2008

Theoretical investigations, in the form of general circulation model (GCM) simulations and empirical studies of pollen, lake levels, and marine sediments suggest that the Indian summer monsoon was intensified 9000 years ago, at a time of increased summer radiation (about 8%) in the northern hemisphere. The southwesterly summer monsoon winds and their pattern of wind stress curl cause coastal upwelling along the coast of Arabia. Stronger summer winds, therefore, should intensify coastal upwelling. Incorporation of the broad‐scale GCM winds into a high‐resolution ocean model of the Arabian Sea produces distinct patterns of stronger upwelling at 9000 years B.P. To verify these patterns in the geologic record, we mapped upwelling assemblages of planktonic foraminifera and estimated sea surface temperature (SST) to define the magnitude and spatial distribution of monsoonal upwelling at 9000 years B.P. We found that the upwelling assemblage increased in abundance and the August SSTs were several degrees Celsius cooler off Arabia than present. However, in the central and eastern Arabian Sea the tropical assemblage increased and the SSTs were warmer than present values. The SST gradient across the Arabian Sea was greater (6°C) at 9000 years B.P. than it is today (4°C). These observations are consistent with the model results that show increased divergence, upwelling, and a thinner upper layer or shallower thermocline in the westernmost Arabian Sea and increased convergence, downwelling, and a thicker upper layer in the central and eastern Arabian Sea. In this experiment, the use of low resolution winds from a general circulation model (NCAR‐CCM) in a high‐resolution ocean model seems to be appropriate for simulating the long‐term averages observed in the marine sediments.
Late Quaternary history of the Agulhas Current
Authors: Winter Amos , Martin Keith
Published: February 2008

The Agulhas Current, located in the South West Indian Ocean, is one of the world's major western‐boundary currents. Vertical and lateral shifts in the Current's movements, especially during glacial intervals, would effect inter‐ and intra‐ocean heat transfers and thus affect the world's climate, particularly off southern Africa. Oxygen isotope ratios of two cores recovered from beneath the Agulhas Current record isotopic shifts probably induced by ice volume effects with little additional shift related to temperature changes, suggesting that temperature and mean position of the flow has been stable for at least 150,000 years. Changes in the percent abundances of coccolithophore species from the same cores suggest changes in paleoproductivity due to eddy formation, lowering of sea level or nutrient content from land sources. Absence of any cold water microfossils at our southern core location suggests that during the Last Glacial, the Agulhas Current probably transferred heat from low to high latitudes and probably reached as far south as 38°S before retroflecting close to its modern‐day position.
Transgressions, regressions and relative sea level changes on the Cretaceous shelf of Israel and adjacent countries. A critical evaluation of Cretaceous global sea level correlations
Authors: Lewy Zeev
Published: February 2008

Cretaceous transgressive regressive cycles over the northern margins of the Arabo‐Nubian craton in the region of present‐ day Israel and the adjacent countries were reconstructed by tracing the position of the ancient Tethyan shorelines. Many of these cycles suggest inverse relationships to the relative sea‐level changes deduced from litho‐ and biofacies analyses. Transgressions are frequently associated with shallowing, resulting in local exposure. The following rapid relative sea‐level rise is associated with regressive oscillations caused by prograding sedimentation. Generally, the relative sea‐level gradually rose throughout the Cretaceous, as reflected by the onlapping trend of the marine sediments. With the exception of one greater relative sea‐level drop in the Middle Turonian, probably tectonically induced, all other relative sea‐level variations were short stillstands and very minor falls. The dominant tectonic control on the relative sea‐level changes and transgressive‐ regressive cycles during the Late Coniacian‐Maastrichtian times, and on the Middle Turonian and early Late Coniacian shallowing events recognised in Israel render their previous attribution exclusively to eustasy erroneous. The opposing relationship between relative bathymetric changes and transgressive‐regressive tendencies, as well as the poor biostratigraphic control in many of the sea‐level curves compiled on the basis of these two criteria, raise additional doubts as to their presumed significance for global correlations. The present study supports Jeletzky's [1977, 1978] critical analysis, and his call for similar critical re‐evaluation of previous studies throughout the world, rather than the “emendation” of data to improve correlation to a given scheme.
Glacial to Holocene changes in carbonate and clay sedimentation in the equatorial Pacific Ocean estimated from thorium 230 profiles
Authors: Yang Yong‐Liang , Elderfield Henry , Ivanovich Miro
Published: February 2008

Uranium series nuclide concentrations have been measured on sediments from five box cores from an equatorial Pacific transect. 230Thexcess activities show discontinuities at the Holocene‐glacial boundary as dated by 14C. The glacial sedimentation rates determined by 230Th and 14C are 2.5‐3.0 cm/kyr. The Holocene rates from 230Th are much lower than those dated by 14C (1.9–2.3 cm/kyr) because of carbonate dissolution. 230Th sedimentation fluxes exceed water column supply by factors of 1.2–1.8 in the Holocene and 1.8–3.0 in the glacial sections. A number of models have been applied to calculate carbonate dissolution rates. The results show that carbonate dissolution rates in the Holocene (in g/cm²kyr) equal 1.5 × 10−3 exp (1.4D) where D is water depth in kilometers. A point‐by‐point estimation of sediment fluxes through time show that clay accumulation rates in the area have been near constant at 0.1–0.2 g/cm²kyr over the past 20 kyr whereas carbonate accumulation rates have decreased dramatically from 0.6–1.0 g/cm²kyr in the glacial sections of the cores to 0.2–0.6 g/cm²kyr in the Holocene. The errors caused by the uncertainties in the age of the termination of the last glacial period have been investigated and results show that a range of 11–14 kyr leads to an error upper limit of about 30% in the estimation of CaCO3 dissolution rates. The response time of CaCO3 and 230Thex concentrations in the mixed layer of sediments due to an impulse of change in CaCO3 dissolution rate has also been discussed, showing that the observed changes in carbonate dissolution may be explained in terms of a single or a continuous change, depending upon the thickness of the mixed layer.
Response of manganese in the ocean to the climatic cycles in the Quaternary
Authors: Mangini A. , Eisenhauer A. , Walter P.
Published: February 2008

Profiles of the 230Th concentration in Mn crusts from the central Pacific Ocean measured at extremely high depth resolution reveal that the growth rates of Mn crusts are influenced by climate. Based on a “constant flux model” the sections of maximum 230Th concentration correspond to periods of slow growth during glacial stages. Fast growth occurred during interglacial stages 1, 5, and 7, probably due to a larger supply of Mn oxides from the water column. High‐resolution profiles of 230Th and 10Be in sediment cores from high biological productivity areas display radioisotope maxima in the interglacial stages and minima during glacial periods, the ratio of the fluxes of 10Be/230Th being ≥4. The only exceptions are observed at 135 and 270 kyr B.P., where the ratio of the fluxes is as low as 0.1 to 0.3. We presume that this “230Th anomaly” reflects short periods of time at the end of glacials when precipitation of MnO2 occurred. This hypothesis is confirmed by peaks of Mn observed in sediment cores mainly at the transitions from glacial to interglacial stages. The standing crops of Mn in these layers suggest release of Mn2+ from the sediments during glacial stages and buildup of Mn in the water column to concentrations of up to 10 µmol/L.
Nutricline variation in the equatorial Atlantic coincident with the Younger Dryas
Authors: Molfino Barbara , McIntyre Andrew
Published: February 2008

Variations in nutricline depth during the last deglaciation are recorded by the phytoplankton species Florisphaera profunda. A stacked record of six deep‐sea cores shows that across the entire equatorial Atlantic there was a brief shallowing of the nutricline, centered on 11 ka, coincident with the Younger Dryas. This shallowing is linked with high‐latitude North Atlantic deglacial events. We invoke high albedo and cool sea surface temperatures in the northern latitudes during 11 ka which steepened an already higher‐than‐expected winter insolation (and hence temperature) gradient for a time of June perihelion. We postulate that the resultant increased pressure gradient caused an intensification of the northern hemisphere winter tropical easterlies which in turn caused equatorial divergence to occur in boreal winter.
Deglacial climatic oscillations in the Gulf of California
Authors: Keigwin L. D. , Jones G. A.
Published: February 2008

A high‐resolution, accelerator radiocarbon dated climate record of the interval 8,000–18,000 years B.P. from Deep Sea Drilling Project site 480 (Guaymas Basin, Gulf of California) shows geochemical and lithological oscillations of oceanographic and climatic significance during deglaciation. Nonlaminated sediments are associated with cooler climatic conditions during the late glacial (up to 13,000 years B.P.), and from 10,300 to 10,800 years B.P., equivalent to the Younger Dryas event of the North Atlantic region. We propose that the changes from laminated (varved) to nonlaminated sediments resulted from increased oxygen content in Pacific intermediate waters during the glacial and the Younger Dryas episodes, and that the forcing for the latter event was global in scope. Prominent events of low δ18O are recorded in benthic foraminifera from 8,000 to 10,000 and at 12,000 years B.P.; evidence for an earlier event between 13,500 and 15,000 years B.P. is weaker. Maximum δ18O is found to have occurred 10,500, 13,500, and 15,000 years ago (and beyond). Oxygen isotopic variability most likely reflects changing temperature and salinity characteristics of Pacific waters of intermediate depth during deglaciation or environmental changes within the Gulf of California region. Several lines of evidence suggest that during deglaciation the climate of the American southwest was marked by increased precipitation that could have lowered salinity in the Gulf of California. Recent modelling studies show that cooling of the Gulf of Mexico due to glacial meltwater injection, which is believed to have occurred at least twice during deglaciation, would have resulted in increased precipitation with respect to evaporation in the American southwest during summertime. The timing of deglacial events in the Gulf of Mexico and the Gulf of California supports such an atmospheric teleconnection.
Determination of the Earth's gravitational field
Authors: Kaula William M.
Published: February 2008

In all three principal methods of measuring the earth's gravitational field—gravimetry, satellite orbit perturbations, and astrogeodetic networks—both mathematical theory and observational techniques have been developed in recent years to more than sufficient accuracy to define and determine the irregularities of the field significant on a planetary scale. The common defect of all three methods is inadequate distribution of observations. At present, the different methods agree only for major features such as the Indian Ocean minimum, and the normalized spherical harmonic coefficients of the potential
are known to be of an order of magnitude of about ±1.3 × 10−6/(n ‐ 1) with an uncertainty of about ±0.6 × 10−6 for the tesseral terms. Gradual improvement is anticipated with additional observations and the application of better statistical techniques made possible by modern computers.
History of the lunar orbit
Authors: Goldreich Peter
Published: February 2008

A method of calculating the past states of the earth‐moon system is developed. The method is based on the existence of three distinct time scales for dynamical change. The short time scale is determined by the revolution periods of the sun and moon about the earth or, equivalently, by the year and current month. The intermediate time scale is set by the precessional motions of the lunar orbit plane and the earth's equator plane. The rate at which tidal friction alters the state of the earth‐moon system defines the long time scale. The equations of motion governing the earth‐moon system are successively averaged over the short and then the intermediate time scales. These averaged equations are then integrated back a short interval on the long time scale. The equations of motion appropriate to this new state of the earth‐moon system are then re‐averaged on the short and intermediate time scales, and once again the averaged eqations are stepped back on the tidal time scale. The first step in this procedure (i.e. averaging on the short time scale) is performed analytically, whereas the calculations on the intermediate and long time scales require the use of a large computer.

At present, the inclination of the lunar orbit plane to the ecliptic remains nearly constant during the precessional motion. On the other hand, if the moon's semimajor axis were ever less than 10R⨁, the inclination of the lunar orbit plane would have maintained a fixed value with respect to the earth's equator plane. The current investigation shows that this inclination could never have been less than 10° and, therefore, that the moon could never have moved on an equatorial orbit. This result contradicts theories that postulate fission of the earth to form the moon and also those which propose that the moon formed by accretion within 10R⨁
A thermodynamic study of homogeneous nucleation
Authors: Long Alexis B.
Published: February 2008

A thermodynamic study of homogeneous nucleation is presented. A thermodynamic equilibrium condition sufficiently general to be applied to any closed system is presented and then used to describe the thermodynamic equilibrium state of a liquid drop‐vapor system. The Einstein development of the theory of fluctuations is applied to homogeneous nucleation, a fluctuation phenomenon, and the thermodynamic meaning of homogeneous nucleation obtained. The activation energy for homogeneous nucleation is discussed and explicitly calculated. A peculiarity of the equilibrium state of the liquid drop‐vapor system indicating possible stability of this state is discussed. A thorough discussion of previous thermodynamic studies of homogeneous nucleation is presented in order to distinguish the correct and incorrect approaches to the problem.
The morphology, distribution, and origin of lunar sinuous rilles
Authors: Schubert Gerald , Lingenfelter Richard E. , Peale Stanton J.
Published: February 2008

The selenographic locations and measurements of such morphological characteristics as length, width, meander wavelength, and minimum average depth have been obtained for about 130 lunar sinuous rilles. We conclude that the morphology of the sinuous rilles requires that they be features of surface water erosion, which could occur in the present lunar environment, since a layer of ice can pressurize the water and maintain it in a liquid phase. Water is assumed to be trapped beneath a permafrost layer, which is shown to be stable on a geologic time scale. The concentration of the rilles around the circular mare basins and in craters with mare‐type floors implies a concentration of water and other volatiles along the margins of such circular basins and craters and associates the outgassing of the lunar interior with major impacts. The rille meander pattern is characterized by a median wavelength to width ratio >4, a value not inconsistent with ratios found for terrestrial alluvial rivers. The generally smaller ratio of meander wavelength to width for lunar sinuous rilles than for terrestrial rivers may indicate that the lunar rivers responsible for eroding the rilles transported a larger percentage of suspended‐load material than is carried by terrestrial rivers. We estimate that the amount of water ultimately available on the lunar surface is two orders of magnitude greater than that required to erode all the sinuous rilles. This estimate of the available water is in agreement with the measured water content of the Apollo 11 lunar samples.
Deformation of a homogeneous Earth model by finite dislocations
Authors: Ben‐Menahem Ari , Singh Sarva Jit , Solomon Fäiza
Published: February 2008

The deformation of a homogeneous, nongravitating, elastic sphere induced by a finite displacement dislocation inside the sphere is obtained in the form of rapidly converging series for arbitrary value of the Poisson ratio. Surface displacements, tilts, strains, and stresses are computed for (1) strike‐slip and (2) dip‐slip source models with various vertical extents. The numerical results are mapped on an epicentral tangential plane. It is estimated that a strike‐slip fault 600 km long extending from the free surface to a depth of 100 km and with a dislocation of 20 meters would yield a strain of the order of 10−9 at an equatorial station with respect to the source. The corresponding estimate for a dip‐slip source is 10−11. However, for deeper sources the difference in the estimated values of the strains decreases. Thus the seismic events such as the Alaskan earthquake of March 28, 1964, should produce strains large enough to be detected by modern extensometers and tiltmeters in the far field. Though the field for a strike‐slip source is approximately proportional to the vertical extent of the source, the field for a dip‐slip source varies as the square of the vertical extent. The problem of determining the number and position of nodal lines for a sphere is much more complicated than for a half‐space. For example, it is known that for a half‐space the vertical component of the displacement for strike‐slip has one more nodal line than the source in an infinite medium. On the other hand, for a sphere there may be two more nodal lines or none at all, depending on the vertical dimensions of the source. Similarly, though the azimuthal component of the displacement for strike‐slip has no nodal line when the source is in a half‐space, there is always one nodal line when the source is in a sphere. The finiteness of the source may introduce a strong asymmetry in the field if the pole is not taken at mid‐fault. Comparison of results for a sphere and for a half‐space reveals two important facts for strike‐slip. First, the absolute value of the ratio (sphere/half‐space) of the displacements lies between zero and ∼4; second, the sign of this ratio for the radial and the azimuthal components of the displacement is negative everywhere in the far field. The negative sign cannot be due to a different sign convention, because, as the point of observation approaches the epicenter, the ratio tends to the value +1. From the stress patterns it is concluded that on the basis of our model the stresses from one earthquake are not likely to trigger another one in the far field.
Plasma instabilities in the magnetosphere
Authors: Hasegawa Akira
Published: February 2008

An introductory review of theories of plasma instabilities in the magnetosphere is presented. Part A is a review of theories of plasma instabilities that are relevant to magnetospheric plasmas. Instabilities arising from velocity‐distribution anisotropies, such as a pitch‐angle anisotropy or the presence of beams, as well as instabilities from nonuniform distributions of plasmas and magnetic fields, are discussed. Particular emphasis is placed on the effect of a mixture of a cold plasma in a high‐β plasma. Part B is a summary of works related to actual plasma instabilities in the magnetosphere. In view of the observed plasma parameters, it is shown that the magnetosphere is rather stable against most macroscopic instabilities, and its dynamics are predominantly governed by microscopic instabilities.
Laurentide Ice Sheet: Estimated volumes during Late Wisconsin
Authors: Paterson W. S. B.
Published: February 2008

Estimates are made of the volumes of the Laurentide ice sheet and the Innuitian ice sheet (covering the Canadian Arctic Islands north of latitude 74°N) from 18,000 to 6000 B.P. Relevant parts of the existing theory of flow in an ice sheet are first reviewed. Two limiting models are considered: a steady‐state ice sheet, the dimensions of which do not change with time, and a stagnant ice sheet that thins at the same rate everywhere. Formulas relating ice thickness to radius and volume to area are given. The relation between area and volume of six existing ice sheets is examined to guide the choice of numerical parameters. The history of the Laurentide and Innuitian ice sheets is reviewed to help decide which model (or whether a combination of the two) is more appropriate at different times. Their volumes and thicknesses are then calculated from areas measured on a recent map of ‘speculative’ positions of the ice margins. Volume estimates for the Laurentide ice sheet are 26.5 × 106 km³ at maximum, 17.5 × 106 km³ at 11,800 B.P., and 6 × 106 km³ at 8500 B.P. The sea entered Hudson Bay shortly before 8000 B.P. and Foxe Basin a few hundred years thereafter. This divided the ice sheet into separate Keewatin, Labrador, and Foxe‐Baffin ice sheets, which had an estimated total volume of 106 km³ at 7500 B.P. The Keewatin and Labrador sectors had disappeared by about 6000 B.P. The volume of the Innuitian ice sheet at maximum is estimated at 106 km³. It had decreased to its present‐day value of 5 × 104 km³ by about 8000 B.P. Maximum errors in these figures are estimated at between. ±16% and ±20% at different times, plus an unknown amount for inaccuracies in the map of the ice margins. From maximum until about 12,000 B.P. the volume of the Laurentide ice sheet decreased much more rapidly than its thickness. If, at the ice sheet maximum, bedrock below its center in Hudson Bay was in isostatic equilibrium, between 150 and 390 meters of uplift must have occurred between 9000 and 8000 B.P. Melting of ice in the Laurentide and Innuitian ice sheets between 15,000 and 6000 B.P. can account for a rise in sea level of between 56 and 76 meters.
Kinetic models of the solar and polar winds
Authors: Lemaire J. , Scherer M.
Published: February 2008

In this paper the application of the kinetic theory to the collisionless regions of the polar and solar winds is discussed. A brief historical review is given to illustrate the evolution of the theoretical models proposed to explain the main phenomenon and observations. The parallelism between the development of the solar wind models and the evolution of the polar wind theory is stressed especially. The kinetic approaches were in both cases preceded by the hydrodynamic models, and their publication gave rise to animated controversies; later on, semikinetic and hydromagnetic approximations were introduced. A kinetic method, based on the quasi neutrality and the zero current condition in a stationary plasma with open magnetic field lines, is described. The applicability of this approach on the solar and polar winds is illustrated by comparison of the predicted results with the observations. The kinetic models are also compared with hydrodynamic ones. The validity of the criticism and remarks uttered during the Chamberlain‐Parker controversy (for the solar wind), and the dispute between Banks and Holzer on the one hand, and Dessler and Cloutier on the other (for the polar wind), are carefully analyzed. The main result of this study is that both approaches are in fact not contradictory but complementary. The classical hydrodynamic descriptions are only appropriate in the collision‐dominated region, whereas the kinetic theory can be applied only in the collision‐free domain.
Lunar seismicity, structure, and tectonics
Authors: Lammlein David R. , Latham Gary V. , Dorman James , Nakamura Yosio , Ewing Maurice
Published: February 2008

Natural seismic events have been detected by the long‐period seismometers at Apollo stations 16, 14, 15, and 12 at annual rates of 3300, 1700, 800, and 700, respectively, with peak activity at 13‐ to 14‐day intervals. Repetitive moonquakes from 41 hypocenters produce seismograms characteristic of each. About 90% of the long‐period signals are from these and other numerous, less active hypocenters, and meteoroid impact signals account for the remainder. At each hypocenter, moonquakes occur only within an active period of a few days during a characteristic phase of the monthly lunar tidal cycle. An episode of activity may contain up to four quakes from one hypocenter. Nearly equal numbers of hypocenters are active at opposite phases of the monthly cycle, accounting for the 14‐day peaks in total lunar seismic activity. A period of about 206 days in the seismic activity of several of the hypocenters is superimposed on a strong one‐to two‐year trend where the signal amplitudes decrease to the instrumental detection threshold. A 206‐day period with no secular decrease in amplitude is also observed in the total lunar seismic activity, suggesting that the total number of active hypocenters does not vary appreciably with time. Moonquake magnitudes range between 0.5 and 1.3 on the Richter scale with a total energy release estimated to be about 1011 ergs annually. With several possible exceptions, the moonquake foci located to date occur in two narrow belts on the near side of the moon. Both belts are 100–300 km wide, about 2000 km long, and 800–1000 km deep, and they lie along great‐circle arcs. Seismic data from a far‐side focus and a large far‐side meteoroid impact define the base of the lunar lithosphere at a depth of about 1000 km. In our present model the rigid lithosphere overlies an asthenosphere of reduced rigidity in which present‐day partial melting is probable. Tidal deformation presumably leads to critical stress concentrations at the base of the lithosphere, where moonquakes are found to occur. The striking tidal periodicities in the pattern of moonquake occurrence and energy release suggest that tidal energy is the dominant source of energy released as moonquakes. Thus, tidal energy is dissipated by moonquakes in the lithosphere and probably by inelastic processes in the asthenosphere. The low level of seismicity and the absence of shallow seismicity implies that the moon is neither expanding nor contracting at an appreciable rate. The secular accumulation of strain implied by the uniform polarities of moonquake signals may result from weak convection in the asthenosphere or from secular recession of the moon from the earth.
Origin of the plasma sheet
Authors: Hill T. W.
Published: February 2008

The plasma sheet is a region of hot plasma (∼107 °K) that surrounds the earth, filling the distant magnetosphere and separating the northern and southern lobes of the magnetospheric tail; its origin is one of the basic outstanding problems of magnetospheric physics. A critical review of the available observations indicates that the plasma sheet source must provide roughly 1025–1026 particles/s to balance known loss mechanisms operating in the quiet time magnetosphere and that an adiabatic or quasi‐adiabatic (ΔE ∝ E) acceleration mechanism must provide most of the observed particle energy. These constraints together with auroral helium ion measurements indicate that the plasma sheet particles must originate in the solar wind rather than in the terrestrial ionosphere. The available hypotheses for a solar wind source are examined, and it is concluded that a two‐step process of magnetopause diffusion plus electrostatic acceleration is the simplest source mechanism that is consistent with the available observations.