bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ sulphate stimulation of mercury methylation in a boreal peatland: Toward a link between acid rain and methylmercury contamination in remote environments
Authors: Branfireun Brian A. , Roulet Nigel T. , Kelly Carol. A. , Rudd John W. M.
Published: September 2002

Recent studies have found that “pristine” peatlands have high peat and pore water methylmercury (MeHg) concentrations and that peatlands may act as large sources of MeHg to the downstream aquatic system, depending upon the degree of hydrologie connectivity and catchment physiography. Sulphate‐reducing bacteria have been implicated as principal methylators of inorganic mercury in many environments with previous research focused primarily on mercury methylation in aquatic sediments. Experiments in a poor fen in the Experimental Lakes Area, northwestern Ontario, Canada, demonstrated that the in situ addition of sulphate to peat and peat pore water resulted in a significant increase in pore water MeHg concentrations. As peatlands cover a large area of the Northern Hemisphere, this finding has potentially far ranging implications for the global mercury cycle, particularly in areas impacted by anthropogenically derived sulphate where the methylmercury fraction of total mercury species may be much larger than in nonimpacted environments.
The δ15N of nitrate in the southern ocean: Consumption of nitrate in surface waters
Authors: Sigman D. M. , Altabet M. A. , McCorkle D. C. , Francois R. , Fischer G.
Published: September 2002

We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton. The δ15N of nitrate increases by 1–2‰ from deep water into the Antarctic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarctic depth profiles yields values in the range of 5–6‰ in east Indian sector and 4–5‰ in the east Pacific sector. Surface transect data from the Pacific sector also yield values of 4–5‰. The major uncertainty in the profile‐based estimation of the isotope effect involves the δ15N of nitrate from the temperature minimum layer below the summertime Antarctic surface layer, which deviates significantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertically from the Subantarctic thermocline because of the distinctive relationships between the δ15N and concentration of nitrate in these two potential sources. Our Subantarctic samples, collected during the summer and fall, indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Pacific sector of the Subantarctic suggest an isotope effect of 4.5‰, indistinguishable from the Antarctic estimates in this sector.
Redfield revisited: 1. Regulation of nitrate, phosphate, and oxygen in the ocean
Authors: Lenton Timothy M. , Watson Andrew J.
Published: September 2002

The ratio of phosphate and nitrate concentrations in the deep ocean matches closely the Redfield ratio required by phytoplankton growing in the surface ocean. Furthermore, the oxygen available from dissolution in ocean water is, on average, just sufficient for the respiration of the resulting organic matter. We review various feedback mechanisms that have been proposed to account for these remarkable correspondences and construct a model to test their effectiveness. The model's initial steady state is cate responds to perturbation in 1000–2000 years and phosphate in 40,000‐60,000 years. However, recently increased estimates oflose to the Redfield ratios and stable against instantaneous changes in the sizes of the nitrate and phosphate reservoirs. When classic flux estimates are adopted, nitr the input and output fluxes of nitrate and phosphate suggest that they respond more rapidly to perturbation, nitrate in 500–1000 years and phosphate in 10,000–15,000 years. Nitrogen fixation tends to maintain nitrate close to Redfield ratio with phosphate, while denitrification tends to keep nitrate as the proximate limiting nutrient and tie it in Redfield ratio to dissolved oxygen. Under increases in phosphorus input to the ocean, the relative responsiveness of nitrogen fixation and denitrification determine whether nitrate remains close to Redfield ratio to phosphate or to oxygen. If nitrogen fixation is strongly limited (e.g., by lack of iron), increasing phosphorus input to the ocean can cause phosphate to deviate above Redfield ratio to nitrate. Hence nitrogen dynamics can control phosphate behavior and nitrate can potentially be the ultimate limiting nutrient over geologic periods of time. When nitrate and phosphate are coupled together by responsive nitrogen fixation, negative feedbacks on organic and calcium‐bound phosphorus burial stabilize their concentrations. If anoxia suppresses organic phosphorus burial, the resulting feedbacks on phosphate (positive) and oxygen (negative) improve regulation toward the Redfield ratios. Variants of the model are forced with a global record of phosphorus accumulation in biogenic sediments as a proxy for changes in phosphate input to the ocean over the past 40 Myr. Nitrate is generally regulated close to Redfield ratio to phosphate, despite large changes in phosphorus input. If nitrogen fixation is strongly limited, then there is one interval (∼15 Myr ago) when a very rapid increase in phosphate input forces phosphate above Redfield ratio to nitrate. Decreases in phosphorus input cause phosphate and nitrate to quickly deviate below Redfield ratio with oxygen, removing anoxia from the ocean, while increases in phosphorus input rapidly increase anoxia. Hence we conclude that there appears to be an element of chance in observing today's ocean “on the edge of anoxia” with nitrate, phosphate, and oxygen all close to the Redfield ratios.
Nitrous oxide in the deep waters of the world's oceans
Authors: Bange Hermann W. , Andreae Meinrat O.
Published: September 2002

We present a compilation and analysis Of N2O data from the deep‐water zone of the oceans below 2000 m. The N2O values show an increasing trend from low concentrations in the North Atlantic Ocean to high concentrations in the North Pacific Ocean, indicating an accumulation of N2O in deep waters with time. We conclude that the observed N2O accumulation is mainly caused by nitrification in the global deep‐water circulation system (i.e., the “conveyor belt”). Hydrothermal and sedimentary N2O fluxes are negligible. We estimate the annual N2O deep‐water production to be 0.3 ± 0.1 Tg. Despite the fact that the deep sea below 2000 m represents about 95% of the total ocean volume, it contributes only about 3–16% to the global open‐ocean N2O production. A rough estimate of the oceanic N2O budget suggests that the loss to the atmosphere is not balanced by the deep‐sea nitrification and pelagic denitrification. Therefore an additional source of 3.8 Tg N2O yr−1 attributed to nitrification in the upper water column (0–2000 m) might exist. With a simple model we estimated the effect of changes in the North Atlantic Deep Water (NADW) formation for deep‐water N2O. The upper water N2O budget is not significantly influenced by variations in the N2O deep‐water formation. However, the predicted decrease in the NADW formation rate in the near future might lead to an additional source of atmospheric N2O in the range of about 0.02‐0.4 Tg yr−1. This (anthropogenically induced) source is small, and it will be difficult to detect its signal against the natural variations in the annual growth rates of tropospheric N2O.
In situ evaluation of air‐sea gas exchange parameterizations using novel conservative and volatile tracers
Authors: Nightingale Philip D. , Malin Gill , Law Cliff S. , Watson Andrew J. , Liss Peter S. , Liddicoat Malcolm I. , Boutin Jacqueline , Upstill‐Goddard Robert C.
Published: September 2002

Measurements of air‐sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993. A conservative tracer, spores of the bacterium Bacillus globigii var. Niger, was used for the first time in an in situ air‐sea gas exchange experiment. This nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period. The first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings suggest that results from all dual tracer experiments are mutually consistent. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas ex‐ change relationship that shows a dependence on wind speed intermediate between those ofLiss and Merlivat [1986] and Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb‐derived radiocarbon. The degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates.
Changes in CH4 emission from rice fields from 1960 to 1990s: 1. Impacts of modern rice technology
Authors: van der Gon Hugo Denier
Published: September 2002

Four countries (Indonesia, Philippines, Thailand, and Nepal) were taken as an example to assess the impact of changes in rice cultivation on methane emissions from rice fields since the 1960s. The change of rice area by type of culture from 1960–1990s is estimated, and its relative contribution to national harvested rice area is calculated and multiplied with an emission factor, to derive the relative methane emission per unit rice land. Relative methane emission per ha rice land has increased since 1960 for all four countries, largely due to an increase in irrigated rice area and partly due to a decrease in upland rice area. Patterns of rice area changes and related emission changes differ considerably among countries. On the basis of the rice area increases between 1960 and the 1990s, significant increases in methane emissions from rice fields due to increases in total rice cultivated area are not to be expected in the future. The impact of modern rice variety adoption is assessed by relating methane emissions to rice production. The organic matter returned to the paddy soil is largely determined by rice biomass production which, given a certain yield, is different for traditional and modern rice varieties. By calculating total organic matter returned to rice paddy soils and assuming a constant fraction to be emitted as methane, rice production and methane emission can be related. The analysis indicates that (1) up to now, rice yield increases in countries with high modern rice variety adoption have not resulted in increased methane emissions per unit of harvested area and, (2) global annual emission from rice fields may be considerably lower than generally assumed. The introduction of modern rice varieties can be regarded as a historical methane emission mitigation strategy because higher rice yields resulted in lower or equal methane emissions.
Winter fluxes of greenhouse gases from snow‐covered agricultural soil:intra‐annual and interannual variations
Authors: van Bochove Eric , Jones H. Gerald , Bertrand Normand , Prévost Danielle
Published: September 2002

Despite the length of winter in cold temperate climates, few studies refer to greenhouse gas emissions from soils during the nongrowing season. In this study, N2O and CO2 fluxes from agricultural and forest soils in southeastern Quebec (Canada) were measured during winter and spring from 1994 to 1997, and the influences of climate, soil, and snow properties on the gaseous emissions were examined. N2O fluxes were far greater from the agricultural soil (2‐187 ng N2O m−2 s−1) than from the forest soil (< 3 ng N2O m−2 s−1), but CO2 fluxes were equivalent for both soil systems (2‐102 μ CO2 m−2 s−1). The higher N2O concentrations in the lower soil horizons could be explained by positive temperature gradients with depth and concomitant negative gas solubility gradients. However, the higher N2O concentrations could also be explained by variations in the expression of N2O reductase with depth, which can modify the N2/N2O ratios in relation to the availability of O2. Calculated N2O‐N fluxes showed that N losses by gaseous emissions from soils during winter and spring were comparable to, or exceeded, similar reported N losses during the growing season. The highest winter fluxes observed in 1997 were interpreted to be due to favorable meteorological conditions that prevailed for denitrification through high soil water content in summer and fall of 1996. Although interannual and nterseasonal variations of fluxes are important, this study shows that wintertime losses of N2O from agricultural soil can be up to 2 to 4 times greater than emissions measured during the growing season in similar agroecosystems.
Thermocline and Zonal Advective Feedbacks Within the Equatorial Ocean Recharge Oscillator Model for ENSO
Authors: Jin Fei‐Fei , An Soon‐Il
Published: September 2002

Through the dynamical coupling between ocean and atmosphere, the vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current constitute the so‐called the thermocline and zonal advective feedbacks that are essential for El Niño/Southern Oscillation (ENSO) coupled dynamics. In this paper, we demonstrate that these two feedbacks are dynamically linked because of the geostrophic balance between the upper ocean zonal current and the meridional gradient of the thermocline. Both feedbacks thus play important and similar roles in the growth and phase transition of ENSO. We further propose a new version of the conceptual recharge oscillator model for ENSO by including these two feedbacks. The new model retains the simplest possible form of a harmonic oscillator, yet presents a more complete description of slow physics for ENSO. Moreover, it also provides the reconciliation between the biased emphases in the ENSO theories oh the thermocline and zonal advective feedbacks.
Evidence for iron and sulfur enrichments in hydrothermal plumes at Axial Volcano following the January–February 1998 eruption
Authors: Feely Richard A. , Baker Edward T. , Lebon Geoff T. , Gendron James F. , Resing Joseph A. , Cowen James P.
Published: September 2002

In response to 12 days of seismic activity at Axial Volcano, Juan de Fuca Ridge in January–February 1998, the NSF/NOAA‐sponsored Axial Response Team conducted three response cruises in February, August, and August/September to map and sample hydrothermal plumes over the region. Vertical profiles of particulate Fe and S over the eruption site show high concentrations from about 1400 m to the bottom. Chemical and scanning electron microscope analysis of the February plume samples revealed anhydrite, Fe‐ferrihydrites, elemental sulfur, and angular glassy basalt shards up to 190 µm in the longest dimension. Many of these shards had halite coatings, which is consistent with subseafloor basalt seawater reactions at temperatures >450°C at 1500 m depth. In August and September cruises the basalt shards were no longer present in the hydrothermal plumes. Instead, the plumes were predominantly composed of Fe‐ferrihydrites, elemental sulfur, and sulfur filaments. A unique feature of this data set is the high concentrations of elemental sulfur in the lower 60 m of the water column. The sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time. Within the Axial caldera, approximately 10–20% of the total sulfur is present as sulfur filaments. These post‐eruption particle compositional changes have strong similarities to the results from the 1993 CoAxial eruption on the Juan de Fuca Ridge.
Anomalous helium and heat signatures associated with the 1998 Axial Volcano Event, Juan de Fuca Ridge
Authors: Lupton John , Baker Edward , Embley Robert , Greene Ron , Evans Leigh
Published: September 2002

Water‐column plumes sampled before and after the 1998 eruptive event on Axial Volcano, Juan de Fuca Ridge, showed a complex pattern in ³He and heat very different from that observed during previous magma injection events. Plumes detected in 1997 before the event had relatively high ³He/heat consistent with the volatile‐rich character of the known hydrothermal vents within the summit caldera. A response cruise in February 1998 immediately after the event failed to detect any event plumes, but instead found an intense steady‐state type plume with high‐temperature signals, moderate ³He enrichments, and low ³He/heat ratios. This plume was present over the caldera and extended a considerable distance off axis. In contrast, all of the plumes detected during follow‐up cruises in July–September 1998 had moderate temperature signals, high ³He concentrations, and elevated ³He/heat ratios. Comparison with previous events suggests that the low ³He/heat plume sampled in February was either the remnant of an event plume which had already migrated off axis, or an event plume‐like discharge which was smeared out by strong currents. It is likely that the high ³He/heat plumes observed in July–September were derived from the injected dike or from the new lava flow.
In situ observations of the onset of hydrothermal discharge during the 1998 Submarine Eruption of Axial Volcano, Juan de Fuca Ridge
Authors: Baker Edward T. , Fox Christopher G. , Cowen James P.
Published: September 2002

A volcanic eruption at the summit of Axial Volcano on January 25,1998, instantaneously created extensive and vigorous hydrothermal discharge. Moorings 2 km apart along the eruption fissure recorded temperature increases of ∼0.6°C up to 115 m above bottom within hours of initial seismic activity. Water temperatures at the mooring sites remained high for about 5 days, then declined steadily over the next 2 weeks. A response cruise 18 days after the eruption found hydrothermal temperature anomalies of ∼0.1°C over the eruption site, and a more intense and much thicker plume 20 km downstream of the eruption. We estimate the steady‐state heat flux required to produce this distal plume, evidence of discharge conditions perhaps 1–13 days after the eruption, as 60–230 GW. The Axial eruption thus produced the largest vent field heat flux yet measured, but these high levels lasted less than 3 weeks.
The landslide‐generated tsunami of November 3, 1994 in Skagway Harbor, Alaska: A case study
Authors: Rabinovich Alexander B. , Thomson Richard E. , Kulikov Evgueni A. , Bornhold Brian D. , Fine Isaac V.
Published: September 2002

We examine the origin and behavior of the catastrophic tsunami that impacted Skagway Harbor at the head of Taiya Inlet, Alaska, on November 3, 1994. Geomorphologic and tide gauge data, combined with numerical simulation of the event, reveal that the tsunami was generated by an underwater landslide formed during the collapse of a cruise‐ship dock undergoing construction. Use of a fine‐grid model for Skagway Harbor and a coarse‐grid model for Taiya Inlet enables us to explain many of the eyewitness accounts and to reproduce the dominant oscillations in the tide gauge record, including the persistent (∼1 h) 3‐min oscillation in Skagway Harbor. The occurrence of the landslide is linked to critical overloading of the slope materials at a time of extreme low tide.
Global warming may have slowed down the deep conveyor belt of a marginal sea of the northwestern Pacific: Japan Sea
Authors: Gamo Toshitaka
Published: September 2002

Weakening of the abyssal circulation (conveyor belt) in the Japan Sea during the 20th century is deduced from the decreasing trend of bottom dissolved oxygen (O2). This trend indicates a shortened O2 supply, caused by too inactive conveyor belt to compensate for biological O2 consumption at the bottom of the sea. Recent climatic changes during the winter season in the northern Japan Sea may play a significant role in prohibiting the formation of surface seawater dense enough to sink to the bottom. It is predicted that the Japan Sea bottom water will become anoxic within a few hundred years, as it was in the last glacial maximum.
Oceans or seas in the Martian northern lowlands: High resolution imaging tests of proposed coastlines
Authors: Malin Michael C. , Edgett Kenneth S.
Published: September 2002

Mars Global Surveyor Mars Orbiter Camera images that were targeted specifically to observe locations where published accounts argue for the presence of landforms created by the interaction of a large body of water with Martian topography fail to reveal any evidence to support the hypothesis that the northern lowlands were once the site of oceans or seas. Given the difficulty of identifying ancient coastlines on Earth from orbital and aerial photography in the absence of field work, this result does not preclude the possibility that Mars once had large standing bodies of water on its surface, but calls into question shorelines previously proposed.
A decadal spiciness mode in the tropics
Authors: Schneider Niklas
Published: September 2002

Analysis of a complex climate model suggests the existence of a mode of coupled ocean‐atmosphere variability in the tropical Pacific with the following dynamics. Anomalous strong Pacific trade winds accelerate the North Equatorial current and Countercurrent. At 10° to 15°N in the central Pacific thermocline this causes anomalous advection across mean gradients of temperature and salinity on isopycnal surfaces and generates cool/fresh spiciness anomalies of the order of 0.3°K. These are advected by the mean circulation via the western boundary region to the equatorial Pacific in approximately five years. At the equatorial outcrops of the isopycnals, the cool/fresh anomalies affect the surface heat budget and are hypothesized to initiate a relaxation of the trade winds that is reinforced by positive feedbacks with the slope of the thermocline and with air‐sea fluxes of heat and freshwater. At 5° to 10°S the decrease of the trades generates by anomalous advection cool/fresh spiciness anomalies that migrate to the equator in one to two years and provide a positive feedback. At 10° to 15°N, the decrease of the trades decelerates North Equatorial current and Countercurrent and causes warm/salty anomalies on isopycnals that arrive at the equator five years later and close the cycle.
The importance of flow in the Mozambique Channel to seasonality in the Greater Agulhas Current System
Authors: Biastoch A. , Reason C. J. C. , Lutjeharms J. R. E. , Boebel O.
Published: September 2002

The temporal variability of the greater Agulhas Current system has important climatological consequences. Some recent results have suggested that this variability contains a large seasonal component, due to changes in the circulation at latitudes poleward of Madagascar only. A model simulation shows that the contribution of Tropical Surface Water to Agulhas Current waters, via the Mozambique Channel, also has a distinct seasonal characteristic that is brought about by the seasonal wind stress over the tropical Indian Ocean. This simulated flow through the Channel contributes substantially to the seasonality of the Agulhas Current. This model result is shown to be not inconsistent with available hydrographic observations.
Implications of carbon isotope ratios of C27‐C33 alkanes and C37 alkenes for the sources of organic matter in the southern ocean surface sediments
Authors: Ohkouchi N. , Kawamura K. , Takemoto N. , Ikehara M. , Nakatsuka T.
Published: September 2002

Carbon isotopic ratios (δ13C) of C27‐C33n‐alkanes and C37 alkenes were measured in the surface sediments from Australian sector of Southern Ocean (47.6°–65.5°S). The δ13C of C31n‐alkane derived from higher plant wax shows little change along the latitude (−28.7±0.6 ‰), whereas δ13C of C37 alkenes derived from haptophyte algae decreases from −24.6±0.7 ‰ at 47.6°S to −32.1±1.2 ‰ at 63.9°S. Relatively heavy δ13C values of C31n‐alkane suggest significant contribution of C4 plant waxes or conifer resin whose δ13C of n‐alkanes are significantly heavier than those of C3 plants. The δ13C of total organic carbon exhibits a latitudinal pattern similar to that of C37 alkenes, suggesting that organic carbon in the sediments is primarily of marine origin.
Basin scale estimates of sea surface nitrate and new production from remotely sensed sea surface temperature and chlorophyll
Authors: Goes Joaquim I. , Saino Toshiro , Oaku Hiromi , Ishizaka Joji , Wong Chi Shing , Nojiri Yukihiro
Published: September 2002

The highly variable nature of T‐N relationships in oceanic waters has restricted nitrate (N) measurements from remotely sensed sea surface temperature (SST) to small time and space domains. Here we show that if changes in T‐N relationships resulting from phytoplankton (chlorophyll a) are taken into account, remote sensing can be exploited to provide high resolution maps of sea surface nitrate (SSN) that are valid over much larger scales than has been previously possible. We illustrate the potential of the method for monitoring basin scale, interannual variations in SSN in the north Pacific Ocean using co‐registered imagery of SST and chl a and demonstrate the usefulness of such data for estimating basin scale annual new production.
Evaluations of the saltwater‐groundwater interface from borehole temperature in a coastal region
Authors: Taniguchi Makoto
Published: September 2002

An interface between saltwater and groundwater was evaluated by monitoring the vertical groundwater temperature profiles. The temperature gradients were found to be constant below certain depths in boreholes, however the gradients decreased above the depths and the temperature profiles were convex in a coastal region. The depth to change in the vertical thermal gradient indicates a hydrological boundary, where upward groundwater flow exists above the boundary and no water flow exists below the boundary. Estimated depths to the interface between saltwater and fresh water using groundwater temperature agreed well with the depths calculated by traditional methods using groundwater potentials and with the results of the groundwater salinity. Results suggest that temperature may be an economical and reliable alternative to traditional methods.
Kelvin wave and Rossby wave interaction in the extratropical‐tropical Pacific
Authors: Wu Lixin , Liu Zhengyu , Hurlburt Harley E.
Published: September 2002

The adjustment of the equatorial Pacific thermocline to an initial extratropical thermocline anomaly is studied using analytical and high‐resolution (1/16°) primitive equation numerical models. It is found that the fast adjustment due to the coastal Kelvin wave and equatorial Kelvin and Rossby waves, and the slow adjustment due to the eastern boundary planetary wave play important roles in the redistribution of the initial anomalous mass, thereby determining the phase and magnitude of the equatorial thermocline transition induced by the initial extratropical thermocline transition. For a given extratropical anomaly, both analytical and numerical models show that the equatorial thermocline transition occurs about 2–5 years later, and the magnitude is about 5–10% of the initial anomaly. The mean circulation in the North Pacific intensifies the transition of the equatorial thermocline and reduces the phase‐lag of extratropical‐tropical thermocline transition.
Recent changes in Arctic Ocean sea ice motion associated with the North Atlantic Oscillation
Authors: Kwok Ronald
Published: September 2002

Examination of a new ice motion dataset of the Arctic Ocean over a recent eighteen year period (1978–1996) reveals patterns of variability that can be linked directly to the North Atlantic Oscillation. The intensity of the Icelandic Low, one of its centers of action, modulates the sea level pressure distribution over a broad region of the Arctic Ocean and the Greenland‐Iceland‐Norwegian and Barents Seas. Over the winters of 1988 through 1995, the Oscillation has remained in its positive phase contributing to coherent large‐scale changes in the intensity and character of ice transport in the Arctic Ocean. The significant changes include: the weakening of the Beaufort Gyre; the increase in ice export through the Fram Strait; the increase in ice import from the Barents/Kara Seas; the enhanced eastward transport of sea ice from the Laptev Sea; the weakening of the Transpolar Drift Stream; and, the reduction in ice extent in the Nordic Seas. All of these changes affect the regional and total sea ice mass balance of the Arctic Ocean.
Sensitivity of the Antarctic sea ice to the thermal conductivity of snow
Authors: Fichefet Thierry , Tartinville Benoît , Goosse Hugues
Published: September 2002

The sensitivity of a global, coarse‐resolution ice‐ocean model to a decrease in the thermal conductivity of the snow overlaying Antarctic sea ice (ksant) is investigated. This study was motivated by recent observations made in the eastern sector of the Southern Ocean and in the Bellingshausen, Amundsen, and Ross Seas, which suggest that the value of ksant usually used in large‐scale sea ice models is about a factor two too high. When ksant is reduced by half in our model, the average thickness of the Antarctic ice pack decreases by 10 cm (10%) and the geographical distributions of snow and ice thicknesses become more realistic. A strengthening of the Southern Ocean stratification and a concomitant weakening of the Antarctic Bottom Water meridional overturning are also noticed. This ultimately impacts on the sensible heat flux from the ocean to the ice and thus on the rate of thermodynamic ice growth.
Propagation velocities and dimensions of plasmoid structures in the near‐Earth magnetotail
Authors: Håland Stein , Søraas Finn , Ullaland Stein
Published: September 2002

Measurements from the High Energy Particle‐Low energy particle Detector experiment (HEP‐LD) and Magnetic Field experiment (MGF) onboard the GEOTAIL spacecraft are used to estimate characteristic flow velocities and scale sizes of plasmoid structures released in the near‐Earth magnetotail during substorm events. Bulk velocities are derived from the azimuthal distribution of energetic ions and typical scale sizes are thereafter calculated from the duration of the plasmoid signatures. The results show great variability from one event to another. Velocities range from 250–700 km/s and scale sizes were in the range 3–9 RE for the events studied. Despite the great variability, the velocities are somewhat lower than earlier observations from the distant tail.
Effective area for the northern polar cap magnetic activity index
Authors: Papitashvili V. O. , Rasmussen O.
Published: September 2002

In this letter, we report for the first time a study of an effective area where the polar cap (PC) magnetic activity index stably preserves a value. The 1998 data from three Greenlandic magnetic stations Qaanaaq (formerly Thule), Savissivik, and Kullorsuaq are utilized for derivation of 15‐min PC indices using normalization coefficients defined earlier for Thule. The results obtained show that Qaanaaq and Savissivik produce almost identical PCI through the year; however, the index from Kullorsuaq correlates poorly with the other two stations. Therefore, as defined only by the Thule coefficients, the effective area for PC index is estimated to be ∼15° in diameter centered at the north corrected geomagnetic pole.
A coupled‐map model for the magnetotail current sheet
Authors: Takalo J. , Timonen J. , Klimas A. J. , Valdivia J. A. , Vassiliadis D.
Published: September 2002

A magnetic field model of the magnetotail current sheet in the form of a coupled‐map lattice (CML) is presented. It is a continuously driven and based on the MHD diffusion equation. Solar wind vBs data (solar wind speed multiplied by the southward component of IMF) are used for driving the model, and it is shown to exhibit perturbations (avalanches) with power‐law scalings in their distributions of duration and size. Such distributions may indicate self‐organized critical (SOC) behavior. Furthermore, it is shown that the power spectra of the model outputs are of bicolor power‐law form with different slopes for high and low frequencies. The model parameters determine the frequency of the break points of the spectra, and the slopes of their low and high frequency regimes.
Four‐field model for dispersive field‐line resonances: Effects of coupling between shear‐Alfvén and slow modes
Authors: Bhattacharjee A. , Kletzing C. A. , Ma Z. W. , Ng C. S. , Otani N. F. , Wang X.
Published: September 2002

A new theoretical model is proposed for dispersive field‐line resonances in collisionless magnetospheric plasmas on the basis of reduced four‐field equations. The model improves upon the predictive capabilities of earlier two‐field models. In particular, due to the coupling of the shear‐Alfvén mode to the slow mode in the four‐field system, it is now possible to account for the observed low frequencies of field‐line resonances. Furthermore, parallel electric fields can be large without requiring the field‐aligned current density to be unrealistically large. Qualitative implications for recent FAST and ground‐based observations are discussed.
Estimates of atomic deuterium abundance and Lyman‐alpha airglow in the thermosphere of Jupiter
Authors: Parkinson C. D. , Griffioen E. , McConnell J. C. , Ben Jaffel L. , Vidal‐Madjar A. , Clarke J. T. , Gladstone G. R.
Published: September 2002

We have made calculations of the atomic D distribution in the thermosphere of Jupiter. The principal reactions determining the D abundance appear to be generation by reaction of H with vibrationally hot HD and loss by reaction of D with H2(υ=0,1) and CH3. The H and CH3 distributions have been calculated using a 1‐D photochemical‐diffusion model with the column H constrained using the Lyman‐α airglow. For H2 effective vibrational temperatures, Tυ, between 1 and 4 times kinetic we find D columns between 4×1011 and 2×1013 atoms cm−2. HD can be vibrationally excited due to VV energy transfer from H2(υ=1). Using a radiative transfer model with coupling of the H and D Lyman‐α lines we have calculated line profiles and total intensities across the Jovian disk and on the limb. For the above D columns and a H column ∼3.5×1017 cm−2, compatible with equatorial Lyman‐α airglow observations, the disk D intensity varies from 80 to 600 R for overhead Sun and viewing, whereas on the terminator the D maximum total intensity is ∼60 R at ∼860 km above the 1 bar level for the maximum D column.
Galileo's close‐up view of the Io sodium jet
Authors: Burger Matthew H. , Schneider Nicholas M. , Wilson Jody K.
Published: September 2002

The Galileo spacecraft has imaged a remarkable atmospheric escape process occurring in the atmosphere of Io, Jupiter's volcanic moon. Electrodynamic consequences of Io's motion through Jupiter's magnetosphere drive mega‐amp currents through Io's ionosphere; some of the sodium ions carrying this current are neutralized as they leave the atmosphere. The Galileo images show that the resulting fast sodium jet removes ∼5×1025 atoms sec−1 from Io's atmosphere. The spatial profile of the jet shows that the source region is much smaller than Io itself, perhaps confined to volcanically active regions, or to an ionosphere restricted to the denser atmosphere near Io's equator.
Enhanced solar wind ³He2+ associated with coronal mass ejections
Authors: Ho George C. , Hamilton Douglas C. , Gloeckler George , Bochsler Peter
Published: September 2002

Using data from the MASS high‐resolution solar wind spectrometer on the WIND spacecraft, six enhanced ³He2+/4He2+ periods were identified from January 1995 to May 1998. The ratios observed in these events are four to ten times higher than previously reported average solar wind values. All enhanced ³He2+ periods were associated with the passage of CME‐ejecta. The observed time periods were either within but towards the end of the CME‐associated magnetic clouds or trailing the magnetic cloud by about 24 hours. All the enhanced ³He2+ periods have a mixture of low (cold) iron charge states along with high (hot) oxygen charge states. The unusual charge state composition of these enhanced ³He2+ events suggests the plasma came from a relatively cold and dense solar region, or from a rapidly expanding plasma parcel in the corona. We suggest that the enhanced ³He2+ plasma originated from the prominence core embedded within the CME.
Global triangulation of intense lightning discharges
Authors: Füllekrug Martin , Constable Steven
Published: September 2002

A global network of three electromagnetic measurement instruments is used to simultaneously record time series of globally observable Extremely‐Low‐Frequency (ELF) magnetic field disturbances which propagate with little attenuation around the globe within the Earth‐ionosphere cavity. The triangulation of individual lightning flashes results in a picture of the temporal evolution of intense lightning discharge occurrences on the planetary scale during April 1998. The lightning flash charge moments are calculated with the short pulse approximation of the normal mode expansion. The majority of the triangulated lightning discharges exhibit charge moments with a potential to excite mesospheric sprites and ∼5–20% may account for air breakdown at sprite altitudes in ∼50–70 km height.
Stimulated thermal instability for ELF and VLF wave generation in the polar electrojet
Authors: Kuo S. P. , Lee M. C. , Kossey Paul , Groves Keith , Heckscher J.
Published: September 2002

Generation of ELF and VLF waves in the HF heating wave modulated polar electrojet is studied. Through the Ohmic heating by the amplitude‐modulated HF heating wave, the conductivity and thus the current of the electrojet is modulated to set up the ionospheric antenna current. However, it is shown that a stimulated thermal instability is also excited by the amplitude‐modulated HF heating wave. This instability introduces an electron temperature modulation more effectively than that by the passive Ohmic heating process and is expected to improve considerably the intrinsic efficiency of ELF and VLF wave generation by the amplitude‐modulated HF heating wave. Moreover, the generation efficiency and signal quality also depend on the HF wave modulation scheme. Thus, four amplitude‐modulation schemes are examined and compared.
Stability of the high‐latitude reconnection site for steady northward IMF
Authors: Fuselier S. A. , Petrinec S. M. , Trattner K. J.
Published: September 2002

The stability of the high‐latitude reconnection site under steady northward Interplanetary Magnetic Field (IMF) conditions was investigated using observations in the Earth's magnetospheric cusp. Using proton distributions with characteristic low‐energy cutoffs, the distance to the high‐latitude reconnection site was monitored during four cusp crossings that had relatively steady solar wind dynamic pressure and IMF clock angle. For these four events, the reconnection site location remained approximately fixed, indicating that the high‐latitude magnetic reconnection site remains stable for many minutes under steady IMF conditions. A possible explanation for this stability is that the magnetosheath flow remains sub‐Alfvenic even at high latitudes because of the presence of a plasma depletion layer.
Two‐species, 3D, MHD simulation of Europa's interaction with Jupiter's magnetosphere
Authors: Liu Yifan , Nagy Andrew F. , Kabin Konstantin , Combi Michael R. , DeZeeuw Darren L. , Gombosi Tamas I. , Powell Kenneth G.
Published: September 2002

The interaction of Europa with the Jovian magnetosphere has been studied by using a two species ideal magnetohydrodynamic (MHD) numerical model. This model considers the upstream plasma in the Jovian magnetosphere and the molecular oxygen ions in the ionosphere of Europa, separately. We present results from simulation studies, which take into account impact ionization, recombination, and the effect of a possible induced dipole magnetic field of Europa. The total mass loading of the magnetospheric flow and the ionization frequency used in the model are consistent with the estimates of Europa's ionosphere and atmosphere. The multi‐species MHD equations are solved by using a finite volume, high‐order, Godunov‐type method on an adaptively refined unstructured grid, which allows detailed modeling of the region near Europa's surface, while still resolving both the upstream region and the satellite's wake. We have paid special attention to the wake of Europa, in order to be able to make comparisons with the Galileo's E4 flyby observations, as well as other model calculations. The calculated escape flux of O2+ down the tail was found to be about 5.6 × 1025 s−1.
Equatorial waves and the 1997–98 El Niño
Authors: McPhaden Michael J. , Yu Xuri
Published: September 2002

New measurements and model results highlight the role of equatorial oceanic wave processes in affecting the evolution of the 1997–98 El Niño in ways not fully anticipated by delayed oscillator theory, a leading paradigm for the El Niño/Southern Oscillation (ENSO) cycle. The onset of the El Niño was linked to eastward propagating equatorial Kelvin waves forced by intraseasonal atmospheric oscillations originating over the Indian Ocean. The demise of the El Niño was related to a complex interplay of wind‐forced and western boundary‐forced waves that preconditioned the ocean to sudden sea surface temperature cooling initially centered between 125°W and 170°W along the equator. These large‐scale ocean wave processes, and the ocean‐atmosphere interactions that they mediate, have significant implications for understanding variability associated with El Niño‐related climate swings.
Buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones
Authors: Rowland Andrea , Davies J. Huw
Published: September 2002

The thickness of Earth's mechanical lithosphere is poorly defined. To investigate whether rheology controls the thickness of the overriding plate's mechanical lithosphere in subduction zones, the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity. It was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner, leading to unrealistically high heat flow. Since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere, we suggest that it is instead controlled by buoyancy. The source of buoyancy we assume is compositional, e.g. buoyant crust. Two end‐member models with crustal thickness of 10 and 70 km respectively were then undertaken, these had lower heat flow. This work supports the assumption of some earlier workers (e.g. Plank and Langmuir, 1988) who equated the mechanical lithosphere with the crust of the overriding plate.
First breaks—automatic phase pickings of P‐ and S‐onsets in seismic records
Authors: Fedorenko Yuri V. , Husebye Eystein S.
Published: September 2002

We have developed, validated and extensively tested a fast and versatile processing scheme for picking with confidence P‐ and S‐ arrival times in local seismic records. Out of 300 local events analyzed accurate P‐ and S‐ arrival times were automatically picked for 90% of these recordings. Essential ingredients in the processing scheme were i) prewhitening, ii) wavelet transform, iii) polarization filtering, iv) envelope transform and v) finally function fitting by a simulated annealing method for picking first breaks in approximately noise free envelope records.
Decelerating Nazca‐South America and Nazca‐Pacific Plate motions
Authors: Norabuena Edmundo O. , Dixon Timothy H. , Stein Seth , Harrison Christopher G. A.
Published: September 2002

Space geodetic estimates of the rate of Nazca‐South America convergence and Nazca‐Pacific spreading averaging over several years show that present day rates are significantly slower than the 3 million year average NUVEL‐1A model. The implied rates of deceleration are consistent with longer term trends extending back to at least 20 Ma, about the time of initiation of Andes growth, and may reflect consequences of ongoing subduction and construction of the Andes, e.g., increased friction and viscous drag on the subducted slab as the leading edge of South America thickens.
Statistical measures of crustal heterogeneity from reflection seismic data: The role of seismic bandwidth
Authors: Bean Christopher J. , Marsan David , Martini Francesca
Published: September 2002

In recent years there has been a growing realization that geological media vary over a broad scale range. As such heterogeneity cannot be described in a deterministic way, a parallel growth in stochastic analyses of geological/geophysical data has emerged. The stochastic description of these media is usually through some form of correlation function, of which the von Karman is the most widely employed. Using this form, media can be described in terms of a characteristic scale size (or correlation length), L and a coloured scaling regime, with scaling described by H, the Hurst exponent. Beyond the correlation length, the material follows a white noise spectrum where material average properties dominate, below the correlation length local heterogeneity dominates. Hence, the correlation length is a fundamental parameter in a range of geodynamical problems. In situ information about stochastic properties of deep crustal rocks can be obtained from the statistical analysis of reflection seismic data. Typical correlation distances within the crust are found to be several hundred metres. Here we show that correlation distances derived from reflection seismic data are strongly influenced by the spectral content of the source. In particular we conclude that there is no reliable evidence for hundred metre scale correlation lengths for crustal heterogeneity.
The role of flank failure in modifying the shallow plumbing system of volcanoes: An example from Somma‐Vesuvius, Italy
Authors: Ventura Guido , Vilardo Giuseppe , Bruno Pier Paolo
Published: September 2002

The joined interpretation of structural, geophysical, historical and DTM‐extracted data collected on the Somma‐Vesuvius volcano reveals the presence of a spoon‐shaped paleo‐depression, which is masked by the well known Vesuvius subsidence caldera. Such depression was caused by failure of the Somma southwestern flank. The flank failure was triggered by dike intrusion along a pre‐existing structural discontinuity. This event occurred between 20–25 and 18ka and marked a change in the composition of erupted products and style of activity, which shifted from an effusive and strombolian activity to a plinian‐type activity. The flank failure induced a reorientation of the gravity‐induced stress trajectory, which, below the depression, resulted parallel to the surface. This prevented the ascent of magma to the surface allowing the formation of a shallow reservoir in which more felsic magma evolved. When the magma pressure exceeded a critical value, plinian activity began.