bluuebunny/crossref_metadata_2025 huggingface dataset:
Ba distribution in surface Southern Ocean sediments and export production estimates
Authors: Fagel Nathalie , Dehairs Frank , André Luc , Bareille Gilles , Monnin Christophe
Published: October 2002

We present excess Ba (Baxs) data (i.e., total Ba corrected for lithogenic Ba) for surface sediments from a north‐south transect between the Polar Front Zone and the northern Weddell Gyre in the Atlantic sector and between the Polar Front Zone and the Antarctic continent in the Indian sector. Focus is on two different processes that affect excess Ba accumulation in the sediments: sediment redistribution and excess Ba dissolution. The effect of these processes needs to be corrected for in order to convert accumulation rate into vertical rain rate, the flux component that can be linked to export production. In the Southern Ocean a major process affecting Ba accumulation rate is sediment focusing, which is corrected for using excess 230Th. This correction, however, may not always be straightforward because of boundary scavenging effects. A further major process affecting excess Ba accumulation is barite dissolution during exposure at the sediment‐water column interface. Export production estimates derived from excess 230Th and barite dissolution corrected Baxs accumulation rates (i.e., excess Ba vertical rain rates) are of the same magnitude but generally larger than export production estimates based on water column proxies (234Th‐deficit in the upper water column; particulate excess Ba enrichment in the mesopelagic water column). We believe export production values based on excess Ba vertical rain rate might be overestimated due to inaccurate assessment of the Baxs preservation rate. Barite dissolution has, in general, been taken into account by relating it to exposure time before burial depending on the rate of sediment accumulation. However, the observed decrease of excess Ba content with increasing water column depth (or increasing hydrostatic pressure) illustrates the dependence of barite preservation on degree of saturation in the deep water column in accordance with available thermodynamic data. Therefore correction for barite dissolution would not be appropriate by considering only exposure time of the barite to some uniformly undersaturated deep water but requires also that regional differences in degree of undersatuation be taken into account.
Postglacial marine diatom record of the Palmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) 1. Total diatom abundance
Authors: Sjunneskog Charlotte , Taylor Fiona
Published: October 2002

Late Pleistocene to Holocene climate variability in the Antarctic Peninsula is described from Ocean Drilling Program (ODP) Leg 178 Site 1098 (Palmer Deep) using total diatom abundance (valves per gram dry sediment) as a paleoproductivity proxy. Diatom frustules are absent in the diamicton at the base of the core, which was deposited prior to age calibrated circa 13.2 ka. Immediately above, the diamicton frustules are in low abundance as rare fragments and become increasingly more abundant and well preserved up core. A rapid and major increase in productivity after circa 12.8 ka is evident from distinctly laminated sediments that overlie the diamicton. The strongly laminated section was deposited during a “deglaciation phase” that followed the retreat of grounded ice after the Last Glacial Maximum. Diatom productivity decreased during a climatic cooling and/or change in meltwater supply, until circa 9.0 ka. Between 9.0 and 4.4 ka, productivity increased steadily again, with diatom abundance maxima reached during a mid‐Holocene “climatic optimum.” The optimum is interrupted by a brief cold event of ∼1000 years duration 6.0–5.0 ka. After 4.4 ka, productivity generally decreased, and the last 500 years have experienced the lowest diatom productivity of the postglacial period. The Palmer Deep sediment is compared to ice core and lake sediment records from other regions around the Antarctic Peninsula. These diverse data sets confirm that the Antarctic Peninsula has been a region of rapid and variable climate fluctuation since the Last Glacial Maximum.
Postglacial marine diatom record of the Palmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) 2. Diatom assemblages
Authors: Taylor Fiona , Sjunneskog Charlotte
Published: October 2002

The distribution of Late Pleistocene to Holocene diatom assemblages from Ocean Drilling Program (ODP) Leg 178 Site 1098 (Palmer Deep) is described and compared using multivariate statistical methods (cluster analysis and analysis of variance). Five assemblages are identified from Site 1098, which are correlated with the climatic zones identified previously in the Palmer Deep [Domack et al., 2001]. A Thalassiosira diatom assemblage characterizes the “deglaciation phase” (13.2–11.0 ka) and was deposited during glacial retreat at the end of the regional Last Glacial Maximum. A well‐preserved Rhizosolenia assemblage was deposited during a “climatic reversal” (11.0–9.0 ka) and indicates periods of seasonally open water with diatom blooms. An open water Fragilariopsis kerguelensis assemblage characterizes the mid‐Holocene climatic optimum (9.0–3.7 ka). In the Neoglacial (late Holocene, <3.7 ka), climatic cooling, increased sea ice cover, and increased wind are inferred from a Cocconeis assemblage. During the climatic optimum/Neoglacial transition (4.4–1.8 ka), laminae characterized by Corethron criophilum were preserved. It is uncertain whether the laminae represent short‐lived (interannual) bloom events with high settling rates or have been deposited over longer time periods.
Equatorial ocean circulation in an extremely warm climate
Authors: Moore T. C. , Rea D. K. , Lyle M. , Liberty L. M.
Published: October 2002

The warm climates of the early Paleogene and the associated diminished near‐surface winds should have resulted in a reduction in near‐surface ocean circulation. One check on this deduction is the delineation of biogenic sediments associated with an equatorial current system of the early Eocene Pacific. A latitudinal seismic reflection transect across the tropical Pacific along early Paleogene ocean crust reveals a basal high‐amplitude reflection package that we take to be the lower Eocene section. This unit varies in thickness by a factor of about two, with the thickest portion forming a low mound some 3°–4° north of the 56 Ma paleoequator. This mound may represent the position of a divergence generated in the frontal region between two currents flowing in opposite directions, and its position suggests that the wind‐driven equatorial circulation of the early Eocene was one without a pronounced equatorial divergence.
Paleotemperatures and ice volume of the past 27 Myr revisited with paired Mg/Ca and 18O/16O measurements on benthic foraminifera
Authors: Billups K. , Schrag D. P.
Published: October 2002

We explore the applicability of paired Mg/Ca and 18O/16O measurements on benthic foraminifera from Southern Ocean site 747 to paleoceanographic reconstructions on pre‐Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27–0 Ma) history of paleotemperatures and the evolution of the δ18O values of seawater (δ18Osw) at a temporal resolution of ∼100–200 kyr. Absolute paleotemperature estimates depend on assumptions of how Mg/Ca ratios of seawater have changed over the past 27 Myr, but relative changes that occur on geologically brief timescales are robust. Results indicate that at the Oligocene to Miocene boundary (23.8 Ma), temperatures lag the increase in global ice‐volume deduced from benthic foraminiferal δ18O values, but the smaller‐scale Miocene glaciations are accompanied by ocean cooling of ∼1°C. During the mid‐Miocene phase of Antarctic ice sheet growth (∼15–13 Ma), water temperatures cool by ∼3°C. Unlike the benthic foraminiferal δ18O values, which remain relatively constant thereafter, temperatures vary (by 3°C) and reach maxima at ∼12 and ∼8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an ∼4°C cooling at site 747. A comparison of our δ18Osw curve to the Haq et al. [1987] sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18O/16O ratios. At ∼12–11 Ma in particular, when benthic foraminiferal δ18O values do not support a further increase in ice volume, the δ18Osw curve comes to a maximum that corresponds to a major mid‐Miocene sea level regression. The agreement between the character of our Mg/Ca‐based δ18Osw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the δ18Osw on pre‐Pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater Mg/Ca ratios. The Mg/Ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes, which can lead to misinterpretations of the δ18O record.
Late Oligocene to early Miocene geochronology and paleoceanography from the subantarctic South Atlantic
Authors: Billups K. , Channell J. E. T. , Zachos J.
Published: October 2002

At Ocean Drilling Program (ODP) Site 1090 on the Agulhas Ridge (subantarctic South Atlantic) benthic foraminiferal stable isotope records span the late Oligocene through the early Miocene (25–16 Ma) at a temporal resolution of ∼10 kyr. In the same time interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing secure correlation of the isotope record to the GPTS. On the basis of the isotope‐magnetostratigraphic correlation we provide refined age calibration of established oxygen isotope events Mi1 through Mi2 as well as several other distinctive isotope events. Our data suggest that the δ18O maximum commonly associated with the Oligocene/Miocene (O/M) boundary falls within C6Cn.2r (23.86 Ma). The δ13C maximum coincides, within the temporal resolution of our record, with C6Cn.2n/r boundary and hence to the O/M boundary. Comparison of the stable isotope record from ODP Site 1090 to the orbitally tuned stable isotope record from ODP Site 929 across the O/M boundary shows that variability in the two records is very similar and can be correlated at and below the O/M boundary. Site 1090 stable isotope records also provide the first deep Southern Ocean end‐member for reconstructions of circulation patterns and late Oligocene to early Miocene climate change. Comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow. Oxygen isotope gradients between sites suggest that the deep Southern Ocean was cold in comparison to the North Atlantic, Indian, and the Pacific Oceans. Dominance of cold Southern Component Deep Water at Site 1090, at least until 17 Ma, suggests that relatively cold circumpolar climatic conditions prevailed during the late Oligocene and early Miocene. We believe that a relatively cold Southern Ocean reflects unrestricted circumpolar flow through the Drake Passage in agreement with bathymetric reconstructions.
On the 1470‐year pacing of Dansgaard‐Oeschger warm events
Authors: Schulz Michael
Published: October 2002

The oxygen isotope record from the Greenland Ice Sheet Project 2 (GISP2) ice core was reanalyzed in the frequency and time domains. The prominent 1470‐year spectral peak, which has been associated with the occurrence of Dansgaard‐Oeschger interstadial events, is solely caused by Dansgaard‐Oeschger events 5, 6, and 7. This result emphasizes the nonstationary character of the oxygen isotope time series. Nevertheless, a fundamental pacing period of ∼1470 years seems to control the timing of the onset of the Dansgaard‐Oeschger events. A trapezoidal time series model is introduced which provides a template for the pacing of the Dansgaard‐Oeschger events. Statistical analysis indicates only a ≤3% probability that the number of matches between observed and template‐derived onsets of Dansgaard‐Oeschger events between 13 and 46 kyr B.P. resulted by chance. During this interval the spacing of the Dansgaard‐Oeschger onsets varied by ±20% around the fundamental 1470‐year period and multiples thereof. The pacing seems unaffected by variations in the strength of North Atlantic Deep Water formation, suggesting that the thermohaline circulation was not the primary controlling factor of the pacing period.
On the causes for salinity variations in the Baltic Sea during the last 8500 years
Authors: Gustafsson Bo G. , Westman Per
Published: October 2002

The Baltic Sea became connected to the open sea about 8500 years B.P., and that marked the beginning of the still continuing Litorina Sea stage. There have since been significant long‐term variations of the salinity from a maximum of some 10–15‰ to the present of 7–8‰. The salinity variations have been known since the beginning of the twentieth century, and various mechanisms have been proposed to explain these. However, so far, no one has actually tried to quantify and thereby rank the relative importance of the different proposed mechanisms. In this paper we used a simple oceanographic model to quantify the impact of changes in cross‐sectional areas of the inlets to the Baltic. We found that such changes cannot explain the full variation of the salinity. We propose that the remaining salinity variations (0.5–5‰) are explained by changes in net freshwater input in the range 15–60%. The calculated variations in the freshwater supply are in large coherent with known variations of the northern European climate. We show that other possible factors as increased mixing, frontal displacements, higher deep water salinity, and larger depths in Kattegat and larger oscillating flows across the sills due to increased sea level variability are less likely to have made substantial contributions to the salinity variations in the Baltic Sea.
Response of the thermohaline circulation to cold climates
Authors: Wang Zhaomin , Mysak Lawrence A. , McManus Jerry F.
Published: October 2002

A coupled atmosphere‐ocean‐sea ice‐land surface‐ice sheet model of intermediate complexity, the so‐called McGill Paleoclimate Model, is employed to study the response of the thermohaline circulation (THC) to various global climate coolings, which are realized by increasing the present‐day planetary emissivity to various values. Generally, it is found that the response of the THC to global cooling is nonlinear: For a slightly cold climate the THC in the North Atlantic and the Pacific upwelling become intensified. For a very cold climate the THC in the North Atlantic may be weakened or even collapsed. The associated Pacific upwelling for a very cold climate also becomes weak when the THC is weakened, and intermediate deep water may form in the Pacific when the THC is collapsed. Some support for this nonlinear response is found in recent paleoceanographic data. The reduced atmospheric poleward moisture transport due to the global cooling is mainly responsible for the intensification of the THC in the North Atlantic for a slightly cold climate. For a very cold climate the global cooling may lead to a decrease of the meridional surface density gradient and an increase of the vertical density difference (lower layer density minus upper layer density) in the deep water formation region, which can weaken or shut down the THC. It is the temperature‐dependent part of the density differences that is mainly responsible for the weakening or shutting down of the THC. The potential influence of surface temperature changes must be taken into account for a full understanding of the role of the THC in the climate system.
Pacific sea surface temperature field reconstruction from coral δ18O data using reduced space objective analysis
Authors: Evans Michael N. , Kaplan Alexey , Cane Mark A.
Published: October 2002

A systematic methodology for the reconstruction of climate fields from sparse observational networks of proxy data, employing the technique of reduced space objective analysis, is applied to the reconstruction of gridded Pacific Ocean Basin sea surface temperature (SST) from coral stable isotope (δ18O) data for the period 1607–1990. In this approach we seek to reconstruct only the leading modes of large‐scale variability which are both observed in the modern climate and resolved in the proxy data. We find that the coral data verifiably resolve two spatial patterns of SST variability. The first and dominant pattern is that of the El Niño–Southern Oscillation (ENSO). A second pattern reflects uniform changes over most of the Pacific Basin. Calibration and verification tests for 1856–1990 show that root‐mean‐square variance is small (≤0.5°C RMS) and reconstruction errors are large (≤0.6°C RMS), limiting interpretation to the tropical region. Periods of enhanced ENSO activity similar to those observed in the past two decades are evident in the reconstruction for the early nineteenth century. The changing frequency of ENSO warm phase events appears to coincide with warming of the Pacific mean state inferred from similar reconstruction efforts using tree ring indicators over the last two centuries.
Stable isotope and minor element proxies for Eocene climate of Seymour Island, Antarctica
Authors: Dutton Andrea L. , Lohmann Kyger C , Zinsmeister William J.
Published: October 2002

Abundant and well‐preserved marine macrofossils on Seymour Island, Antarctica, provide a valuable resource to establish paleoenvironmental conditions at high southern latitudes during the warm Paleogene. Stable isotope, minor element, and87Sr/86Sr compositions have been measured for the aragonite bivalveCucullaeafrom the Eocene La Meseta Formation. The87Sr/86Sr stratigraphy suggests an early to late Eocene age for the La Meseta Formation. Sea surface temperature estimates based on δ18O values range from 14.5° to 15.5°C in the early and middle Eocene to 10.5°C in the late Eocene. High‐resolution sampling along growth structures provides estimates of seasonality in temperature, which decrease considerably up section. To address the relative contribution of temperature and ice volume or salinity to the δ18O record, Sr/Ca was measured and evaluated as a potential independent paleothermometer forCucullaea.
Direct comparison of marine and terrestrial climate variability during marine isotope stages 6 and 5: Results from Santa Barbara Basin ODP Hole 893A
Authors: Friddell Julie E. , Thunell Robert C. , Heusser Linda E.
Published: October 2002

Planktonic foraminiferal δ18O, pollen, and opal records from Santa Barbara Basin are used to evaluate climate changes in the northeast Pacific during marine isotope stages (MIS) 6 and 5. While the oak and foraminifera record the major glacial‐interglacial changes, opal is not correlated with these large‐scale transitions. Surface water warming in the basin was synchronous with terrestrial warming but earlier than ice sheet melting and increasing Northern Hemisphere insolation at Termination II. During and immediately after deglaciation, our marine proxies indicate the presence of an isothermal upper water column. Later in substage 5E, the water column became thermally stratified, surface productivity increased, and laminated sediments began accumulating. Sediment trap and hydrographic data suggest these changes were due to the onset of a strong seasonal cycle similar to that seen today in the basin. A series of rapid coolings within MIS 5 appears to be synchronous with North Atlantic ice‐rafting events and Greenland stadials.
Paired benthic foraminiferal Cd/Ca and Zn/Ca evidence for a greatly increased presence of Southern Ocean Water in the glacial North Atlantic
Authors: Marchitto Thomas M. , Oppo Delia W. , Curry William B.
Published: October 2002

Benthic foraminiferal δ13C and Cd/Ca studies suggest that deep Atlantic circulation during the Last Glacial Maximum was very different from today, with high‐nutrient (low δ13C, high Cd) deep Southern Ocean Water (SOW) penetrating far into the North Atlantic. However, if some glacial δ13C values are biased by productivity artifacts and/or air‐sea exchange processes, then the existing δ13C data may be consistent with the continual dominance of North Atlantic Deep Water (NADW). Cibicidoides wuellerstorfi Cd/Ca results presented here indicate that the glacial North Atlantic was strongly enriched in dissolved Cd below ∼2500 m depth. If NADW formation was still vigorous relative to SOW formation, these data could be explained by either increased preformed nutrient levels in the high‐latitude North Atlantic or by increased organic matter remineralization within lower NADW. High glacial Zn/Ca values in the same samples, however, are best explained by a substantially increased mixing with Zn‐rich SOW. The cause was most likely a partial replacement of NADW by less dense Glacial North Atlantic Intermediate Water. This reorganization also lowered deep North Atlantic CO32− concentrations by perhaps 10 to 15 μmol kg−1.
Circulation changes and nutrient concentrations in the late Quaternary Aegean Sea: A nonsteady state concept for sapropel formation
Authors: Casford J. S. L. , Rohling E. J. , Abu‐Zied R. , Cooke S. , Fontanier C. , Leng M. , Lykousis V.
Published: October 2002

The modern Aegean Sea is an important source of deep water for the eastern Mediterranean. Its contribution to deep water ventilation is known to fluctuate in response to climatic variation on a decadal timescale. This study uses marine micropalaeontological and stable isotope data to investigate longer‐term variability during the late glacial and Holocene, in particular that associated with the deposition of the early Holocene dysoxic/anoxic sapropel S1. Concentrating on the onset of sapropel‐forming conditions, we identify the start of “seasonal” stratification and highlight a lag in δ18O response of the planktonic foraminifer N. pachyderma to termination T1b as identified in the δ18O record of G. ruber. By use of a simple model we determine that this offset cannot be a function of bioturbation effects. The lag is of the order of 1 kyr and suggests that isolation of intermediate/deep water preceded the start of sapropel formation by up to 1.5 kyr. Using this discovery, we propose an explanation for the major unresolved problem in sapropel studies, namely, the source of nutrient supply required for export productivity to reach levels needed for sustained sapropel deposition. We suggest that nutrients had been accumulating in a stagnant basin for 1–1.5 kyr and that these accumulated resources were utilized during the deposition of S1. In addition, we provide a first quantitative estimate of the diffusive (1/e) mixing timescale for the eastern Mediterranean in its “stratified” sapropel mode, which is of the order of 450 years.
Millennial‐ to decadal‐scale paleoenvironmental change during the Holocene in the Palmer Deep, Antarctica, as recorded by particle size analysis
Authors: Warner Nathaniel R. , Domack Eugene W.
Published: October 2002

A record of Holocene paleoenvironmental variability from the Antarctic Peninsula has been produced using sediment cores from Site 1098 gathered on Ocean Drilling Project Leg 178. The results provide an accurate and continuous record of biotic and sedimentologic responses to Holocene environmental change. Holes 1098B and 1098C were used to produce the first ultrahigh‐resolution record of grain size variability for the past 13,000 years. We utilized samples every 2.5 cm (at 3–5 yr cm−1) which involved the analysis of 1466 samples from two holes at the site. Particle size was measured on uniform suspensions of dispersed particles using a laser diffraction method. The particles are mainly grains of silt (65–85%) and clay (15–35%) size. The analyses revealed that in most intervals, biotic particles in the form of diatom frustules (mostly spores) dominate the silt component, which is inversely reflected by magnetic susceptibility (MS). In intervals of the core that deviate from this relationship it was found that the MS signal correlated with medium to fine or coarse silt that was dominated by terrigenous rather than a biogenic (diatom) component. Spectral analyses of the variability in clay and medium‐fine silt content over the last ∼9000 calendar years reveal cycles of 1800, 400, 200, 100, 70, 60, and 50 years. The analyses revealed that particle size, specifically medium‐fine silt, is an excellent proxy for environmental change in the Palmer Deep region. Such changes may be forced by solar variability (the 200 and 400 year cycles), lunar tidal changes (the 1800 year cycles), and an as yet undetermined multidecadal forcing phenomena that operates in the SE Pacific sector of the Southern Ocean.
Late Eocene tropical sea surface temperatures: A perspective from Panama
Authors: Tripati Aradhna , Zachos James
Published: October 2002

We have reconstructed mean annual coastal temperatures and seasonality during the Eocene (Priabonian; 36.6–40 Ma) using oxygen isotope profiles of shallow marine aragonitic gastropod shells (Gatuncillo Formation, Panama). These data provide a unique opportunity to document low‐latitude coastal climates during a preglacial “greenhouse” period. Assuming shell oxygen isotope profiles primarily reflect changes in water temperature, our results indicate water temperatures in a middle to outer shelf setting (20–50 m depth) varied by 6°–8°C, and mean annual temperatures (MAT) were >26°C. These temperatures are in agreement with pollen‐based surface temperature reconstructions for the late Eocene and are consistent with other Eocene mollusk‐based tropical coastal temperature estimates but are at odds with the few Eocene foraminiferal‐based estimates of mean annual tropical sea surface temperatures (∼17°–22°C).
Flux‐forced simulations of the paleocirculation of the Mediterranean
Authors: Myers Paul G.
Published: October 2002

A series of experiments with an ocean general circulation model of the Mediterranean forced by artificial (but realistic) surface fluxes of heat and freshwater are performed. The model has a stable thermohaline circulation under the baseline fluxes. Small decreases in excess evaporation (8%) produce a linear weakening of the thermohaline circulation with less water formation and smaller strait transports. Levantine Intermediate Water production is shown to be very sensitive to additional freshwater input. The dominant circulation mode in “wetter” climates is one with Adriatic Intermediate Water formation and dispersal, leading to ventilation of intermediate depths of the basin. Although the deep layers are not ventilated, stratification is generally weak. As the excess evaporation decreases, stratification increases. A 60–80% decrease leads to conditions closest to those suggested for sapropel S1, with a west‐east salinity gradient of half of today.
Intra‐annual variation in the stable oxygen and carbon and trace element composition of sclerosponges
Authors: Swart P. K. , Thorrold S. , Rosenheim B. , Eisenhauer A. , Harrison C. G. A. , Grammer M. , Latkoczy C.
Published: October 2002

This paper presents data to support the presence of (1) intra‐annual signals in the chemical composition (δ18O and Sr/Ca) of the skeletons of sclerosponges from the Bahamas and (2) variable rates of skeletal accretion. These conclusions are based on data obtained by using a microsampling method for the stable oxygen and carbon isotopes in which material was extracted at a resolution of one sample every 34 μm and a laser microprobe which obtained trace element data every 20 μm (Sr, Mg, and Pb). An age model was established using a combination of changes in the concentration of Pb, the change in the δ13C of the skeleton of the sclerosponges, and U/Th isotopic measurements. These methods yield a mean growth rate of 220 μm/yr but suggest that the growth rate in this particular sclerosponge was not constant. The calculated growth rate is within error identical to that determined by U/Th methods. The variable growth rate was confirmed through spectral analysis of the δ18O and Sr/Ca data that showed peaks corresponding to the annual cycle in these parameters as well as peaks corresponding to growth rates of approximately 128, 212, 270, and 400 μm/yr. The presence of these additional frequencies suggests a growth rate between approximately 100 and 300 μm/yr. These conclusions were supported by modeling of oxygen isotopic data measured on a scleractinian coral as well as model isotope data generated on synthetic time series. These findings have important implications for the use of sclerosponges as proxies of paleoclimate because they emphasize the need for a precise yearly chronology in order that proxy data can be compared with climatic variables.
ADVANCED SPECTRAL METHODS FOR CLIMATIC TIME SERIES
Authors: Ghil M. , Allen M. R. , Dettinger M. D. , Ide K. , Kondrashov D. , Mann M. E. , Robertson A. W. , Saunders A. , Tian Y. , Varadi F. , Yiou P.
Published: October 2002

The analysis of univariate or multivariate time series provides crucial information to describe, understand, and predict climatic variability. The discovery and implementation of a number of novel methods for extracting useful information from time series has recently revitalized this classical field of study. Considerable progress has also been made in interpreting the information so obtained in terms of dynamical systems theory. In this review we describe the connections between time series analysis and nonlinear dynamics, discuss signal‐to‐noise enhancement, and present some of the novel methods for spectral analysis. The various steps, as well as the advantages and disadvantages of these methods, are illustrated by their application to an important climatic time series, the Southern Oscillation Index. This index captures major features of interannual climate variability and is used extensively in its prediction. Regional and global sea surface temperature data sets are used to illustrate multivariate spectral methods. Open questions and further prospects conclude the review.
SIGNIFICANCE OF MUD VOLCANISM
Authors: Kopf Achim J.
Published: October 2002

Mud volcanism and diapirism have puzzled geoscientists for ∼2 centuries. They have been described onshore and offshore in many places on Earth, and although they occur in various tectonic settings, the majority of the features known to date are located in compressional tectonic scenarios. This paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described. Mud volcanoes show variable geometry (up to tens of kilometers in diameter and several hundred meters in height) and a great diversity regarding the origin of the fluid and solid phases. Gas (predominantly methane), water, and mud may be mobilized at subbottom depth of only a few meters but, in places, can originate from several kilometers depth (with minor crustal or mantle input). The possible contribution of mud extrusion to global budgets, both from quiescent fluid emission and from the extrusive processes themselves, is important. In regions where mud volcanoes are abundant, such as the collision zones between Africa and Eurasia, fluid flux through mud extrusion exceeds the compaction‐driven pore fluid expulsion of the accretionary wedge. Also, quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and, hence, to greenhouse climate.
ENVIRONMENTAL CHARACTERIZATION OF GLOBAL SOURCES OF ATMOSPHERIC SOIL DUST IDENTIFIED WITH THE NIMBUS 7 TOTAL OZONE MAPPING SPECTROMETER (TOMS) ABSORBING AEROSOL PRODUCT
Authors: Prospero Joseph M. , Ginoux Paul , Torres Omar , Nicholson Sharon E. , Gill Thomas E.
Published: October 2002

We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics. The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad “dust belt” that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. There is remarkably little large‐scale dust activity outside this region. In particular, the Southern Hemisphere is devoid of major dust activity. Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200–250 mm. Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas. Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits. Many sources are associated with areas where human impacts are well documented, e.g., the Caspian and Aral Seas, Tigris‐Euphrates River Basin, southwestern North America, and the loess lands in China. Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity. Thus, on a global scale, dust mobilization appears to be dominated by natural sources. Dust activity is extremely sensitive to many environmental parameters. The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions. With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future. It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores.
Radar equations in the problem of radio wave backscattering during bistatic soundings
Authors: Berngardt O. I. , Potekhin A. P.
Published: October 2002

This paper outlines a method for obtaining the relation between the singly scattered signal and the Fourier spectrum of medium dielectric permittivity fluctuations, with regard for the fact that the scattering volume is determined by antenna patterns and is not small. On the basis of this equation we obtained the radar equation relating the scattered signal spectrum to the spatial spectrum of fluctuations. Also, a statistical radar equation is obtained that relates the mean statistical power of the scattered signal to the spectral density of the dielectric permittivity fluctuations without a classical approximation of the smallness of the irregularities' spatial correlation radius. The work deals with the bistatic sounding case, when the exact forward scattering and exact backward scattering are absent and sounding signal has sufficiently narrow spectral band for scattered volume to change slowly on ranges of Fresnel radius order. The statistical radar equations obtained differ from the classical ones in the presence of coherent structures with big correlation radii, and so the received signal spectrum can differ from the intrinsic spectrum of irregularities.
Evaporation duct assessment from meteorological buoys
Authors: Hitney Herbert V.
Published: October 2002

The evaporation duct over the sea is usually assessed using bulk meteorological measurements. This paper investigates the utility of meteorological buoys as a source for these bulk measurements and compares evaporation duct assessments using two buoys in southern California waters separated by 128 km. A simple radio propagation experiment at 2.4 GHz between one of the buoys and the coast on an 18.2 km path is described. Observed propagation loss from this experiment is compared to modeled loss based on the meteorological measurements at each buoy. The purpose of this paper is to investigate radio propagation effects using established and accepted methods already described in the literature. Accordingly, no discussion of atmospheric surface layer meteorology affecting radio propagation is given.
Electromagnetic field of a vertical electric dipole on a perfect conductor coated with a dielectric layer
Authors: Zhang Hong‐qi , Pan Wei‐yan
Published: October 2002

The simple asymptotic analytical expressions of the electromagnetic fields induced by a vertical electric dipole on a perfect conductor coated with a dielectric layer have been obtained. These expressions show that the induced fields consist of the direct wave, reflected wave, lateral wave and trapped surface wave. The propagation wave number of the trapped surface wave is between that of the wave number k0 in the air and the wave number k1 in the dielectric. The amplitude of the trapped surface wave attenuates as ρ−1/2 in the ρ direction near the surface and attenuates exponentially as e−γz in the z direction. The expressions for the direct wave, reflected wave and lateral wave in this paper coincide with those given by King and Sandler [1994]. When the thickness of the dielectric layer is not small, the lateral wave and the trapped surface wave are combined the total field to produce an interference pattern.
Surface integral equations for material layers modeled with tensor boundary conditions
Authors: Topsakal Erdem , Volakis John L. , Ross Daniel C.
Published: October 2002

A new class of tensor boundary conditions is introduced for modeling multilayered dielectric structures. Surface integral equations are then developed on the basis of these tensor conditions for a more efficient modeling of nonmetallic bulk structures. Numerical examples are presented to demonstrate the accuracy and applicability of these boundary conditions.
Reconstruction of nonmonotonic height profiles of electron density from vertical sounding ionograms by the regularization method
Authors: Beloff N. , Denisenko P. F. , Gough M. P.
Published: October 2002

The problem of reconstruction of daytime nonmonotonic height profiles of electron density N(h) from ionograms of ground‐based vertical sounding is considered. The regularization (REG) scheme has been proposed for determination of valley parameters. A comparison of the least squares method (LSM) with the regularization (REG) scheme was carried out using computer simulations. It is shown that LSM results are very sensitive to random errors in virtual heights. This fact leads to an unpredictably large deviation of the reconstructed N(h) profile from the real one. The application of REG methods makes the results more stable to random errors in measurements and allows us to decrease the deviation of the restored real height and valley depth from their model values by several times in comparison with LSM results.
First application of the radioholographic method to wave observations in the upper atmosphere
Authors: Pavelyev A. , Igarashi K. , Reigber C. , Hocke K. , Wickert J. , Beyerle G. , Matyugov S. , Kucherjavenkov A. , Pavelyev D. , Yakovlev O.
Published: October 2002

Wave phenomena in the upper atmosphere can be studied using the high‐precision Global Positioning System (GPS) radio navigational field. In this paper, basic principles, accuracy, and vertical resolution of the radioholographic technique for studies of ionospheric wave phenomena are presented for the general case when the orbits of the satellites are arbitrary. Results of testing of the radioholographic method are discussed using orbital station MIR and geostationary satellites (MIR/GEO) and GPS/Meteorology (GPS/MET) radio occultation data. The radioholographic method has high vertical (12–30 m) and angular (4–8 μrad) resolution, which has been validated by directly observing multibeam propagation in the atmosphere and revealing signals, reflected from the sea, in GPS/MET and MIR/GEO radio occultation data. We show that this method allows one to determine the vertical profile of the electron density and monitoring wave structures in the upper atmosphere. As an example of this approach, observations of the summer Antarctic mesosphere on 7 February 1997 are presented. We show, by combining phase and amplitude analysis, that a vertical resolution of 0.3–0.5 km reveals wavelike structures with spatial periods from 1–2 km to 8–10 km in the vertical electron density distribution in the D and E regions. Variations in the gradient of the electron density from ±5 × 103 to ±8 × 103 el/(cm3 km) at altitudes of 72–95 km were observed. The obtained results demonstrate the high‐technology level of the radioholography approach and open new perspectives for radio occultation experiments: measurements of the characteristics of the natural processes in the atmosphere, mesosphere, and ionosphere and observations of the state of the sea surface by measuring parameters of reflected signal simultaneously with radio occultation experiments.
Some issues related to the novel spectral acceleration method for the fast computation of radiation/scattering from one‐dimensional extremely large scale quasi‐planar structures
Authors: Torrungrueng Danai , Johnson Joel T. , Chou Hsi‐Tseng
Published: October 2002

The novel spectral acceleration (NSA) algorithm has been shown to produce an (Ntot) efficient iterative method of moments for the computation of radiation/scattering from both one‐dimensional (1‐D) and two‐dimensional large‐scale quasi‐planar structures, where Ntot is the total number of unknowns to be solved. This method accelerates the matrix‐vector multiplication in an iterative method of moments solution and divides contributions between points into “strong” (exact matrix elements) and “weak” (NSA algorithm) regions. The NSA method is based on a spectral representation of the electromagnetic Green's function and appropriate contour deformation, resulting in a fast multipole‐like formulation in which contributions from large numbers of points to a single point are evaluated simultaneously. In the standard NSA algorithm the NSA parameters are derived on the basis of the assumption that the outermost possible saddle point, ϕs,max, along the real axis in the complex angular domain is small. For given height variations of quasi‐planar structures, this assumption can be satisfied by adjusting the size of the strong region Ls. However, for quasi‐planar structures with large height variations, the adjusted size of the strong region is typically large, resulting in significant increases in computational time for the computation of the strong‐region contribution and degrading overall efficiency of the NSA algorithm. In addition, for the case of extremely large scale structures, studies based on the physical optics approximation and a flat surface assumption show that the given NSA parameters in the standard NSA algorithm may yield inaccurate results. In this paper, analytical formulas associated with the NSA parameters for an arbitrary value of ϕs,max are presented, resulting in more flexibility in selecting Ls to compromise between the computation of the contributions of the strong and weak regions. In addition, a “multilevel” algorithm, decomposing 1‐D extremely large scale quasi‐planar structures into more than one weak region and appropriately choosing the NSA parameters for each weak region, is incorporated into the original NSA method to improve its accuracy.
An investigation into the geometric optics approximation for indoor scenarios with a discussion on pseudolateral waves
Authors: Cotton Michael G. , Kuester Edward F. , Holloway Christopher L.
Published: October 2002

In this study we investigated the geometric optics (GO) approximation to the fields of an infinitesimal electric dipole above a half plane for geometries typical of wireless indoor communications. This inspection was motivated by efforts to establish a ray trace model to characterize indoor radio propagation channels. Eight canonical geometries were examined to isolate near‐surface and near‐field effects that are not accounted for in the GO approximation. Common building materials and physical dimensions (i.e., antenna separation and height) as small as 1 cm were investigated for frequencies up to 8 GHz. Theoretical fields were calculated via numerical evaluation of Sommerfeld integrals and compared to corresponding GO approximations. As expected, near‐field and near‐surface (e.g., surface wave) mechanisms which invalidate GO were observed. Close to the surface, an interesting interference pattern in the frequency domain was identified. Mathematical manipulation showed that this so‐called “pseudolateral wave” phenomenon was caused by adjacent fields in the two media propagating at different speeds. Next, we transformed the results to the time domain and used delay spread as a metric to quantify GO error. We also show that the pseudolateral wave manifests itself in the time domain as an additional pulse that arrives at a delay associated with the speed of a wave traveling in the lossy media.
Inferred electric field variability in the polarization jet from Millstone Hill E region coherent scatter observations
Authors: Erickson P. J. , Foster J. C. , Holt J. M.
Published: October 2002

We investigate the fine‐scale structure of the subauroral electric field in the vicinity of the polarization jet and subauroral ion drift (SAID) using coherent E region backscatter observed with the Millstone Hill 440 MHz UHF radar during a moderately disturbed period (Kp = 5) on 13 November 1998. We use a combination of data obtained at high time and spatial resolution and radar response modeling, incorporating magnetic aspect angle sensitivity and radar antenna beam shape. The modeled radar response function depends not only on the magnetic and radar pointing geometry but also on the altitude and extent of the irregularity layer. In addition to modeling of the radar system response, an optimal, regularized deconvolution technique is used to resolve fine‐scale spatial structure from the relatively long pulse length used. At the 440 MHz Millstone radar frequency, coherent backscattered power is a linear function of electric field amplitude, and we are able to relate changes in the range variation of deconvolved power to spatial/temporal structure in the electric field. We observe the polarization jet to have superimposed multiple instances of narrow (0.1°) intense SAID configuration electric fields having lifetimes as short as 1.5 min and gradients on their equatorward edge as large as 4 mV/m per kilometer. Simultaneous DMSP satellite observations of westward ion drift across our experimental field of view confirms our interpretation of the radar data in terms of a subauroral polarization jet structure spanning over 3° latitude moving poleward at 250–400 m/s velocity (cross‐L‐shell) past the radar beam with superimposed intense SAID‐like electric fields.
Design of leaky coaxial cables with periodic slots
Authors: Wang Jun Hong , Mei Kenneth K.
Published: October 2002

Based on the analysis of spatial harmonics of periodically slotted structures, the formulas for determining the slot period of the leaky coaxial cables according to the desired frequency band are given. Methods of suppressing the higher‐order harmonics for extending the frequency bandwidth are discussed. Diagrams for determining slot sizes and shapes according to the desired coupling loss are given.