bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the NOAA 16 AMSU‐A brightness temperatures observed over Libyan Desert
Authors: Mo Tsan
Published: October 2002

The brightness temperatures over the southeastern Libyan Desert (21°–23°N; 28°–29°E) from the NOAA‐16 Advanced Microwave Sounding Unit‐A (AMSU‐A) were investigated for the months of January and March 2001. Angular distributions of the data from each month show remarkably stable patterns. The stable pattern of angular distributions can be potentially useful for postlaunch calibration and validation of the AMSU‐A instrument, if a long‐term trend of the angular distributions can be established. Angular distributions from the three window channels (channels 1, 2, and 15 with frequencies centered at 23.8, 31.4, and 89 GHz, respectively) and one near‐surface sounding channel (channel 3 at 50.3 GHz) were simulated with a parametric model using the radiative transfer equation and a soil dielectric‐emissivity model that generates the required soil emissivity as a function of zenith angle. The simulated results agree well with the data over the Libyan Desert area. The best‐fit results indicate that radiation sensed by each channel comes from different thermal sampling depth and that these window channels could be used for temperature sounding in soils. The effect of spacecraft roll error on the angular distribution was also investigated. It was found that 1° roll error would produce a left‐right asymmetry of 2.6 K in the angular distribution.
In situ measurements of atmospheric methane at GAGE/AGAGE sites during 1985–2000 and resulting source inferences
Authors: Cunnold D. M. , Steele L. P. , Fraser P. J. , Simmonds P. G. , Prinn R. G. , Weiss R. F. , Porter L. W. , O'Doherty S. , Langenfelds R. L. , Krummel P. B. , Wang H. J. , Emmons L. , Tie X. X. , Dlugokencky E. J.
Published: October 2002

Continuous measurements of methane since 1986 at the Global Atmospherics Gases Experiment/Advanced Global Atmospherics Gases Experiment (GAGE/AGAGE) surface sites are described. The precisions range from approximately 10 ppb at Mace Head, Ireland, during GAGE to better than 2 ppb at Cape Grim, Tasmania, during AGAGE (i.e., since 1993). The measurements exhibit good agreement with coincident measurements of air samples from the same locations analyzed by Climate Monitoring and Diagnostics Laboratory (CMDL) except for differences of approximately 5 ppb before 1989 (GAGE lower) and about 4 ppb from 1991 to 1995 (GAGE higher). These results are obtained before applying a factor of 1.0119 to the GAGE/AGAGE values to place them on the Tohoku University scale. The measurements combined with a 12‐box atmospheric model and an assumed atmospheric lifetime of 9.1 years indicates net annual emissions (emissions minus soil sinks) of 545 Tg CH4 with a variability of only ±20 Tg from 1985 to 1997 but an increase in the emissions in 1998 of 37 ± 10 Tg. The effect of OH changes inferred by Prinn et al. [2001] is to increase the estimated methane emissions by approximately 20 Tg in the mid‐1980s and to reduce them by 20 Tg in 1997 and by more thereafter. Using a two‐dimensional (2‐D), 12‐box model with transport constrained by the GAGE/AGAGE chlorofluorocarbon measurements, we calculate that the proportion of the emissions coming from the Northern Hemisphere is between 73 and 81%, depending on the OH distribution used. However, this result includes an adjustment of 5% derived from a simulation of the 2‐D estimation procedure using the 3‐D MOZART model. This adjustment is needed because of the very different spatial emission distributions of the chlorofluorocarbons and methane which makes chlorofluorocarbons derived transport rates inaccurate for the 2‐D simulation of methane. The 2‐D model combined with the annual cycle in OH from Spivakovsky et al. [2000] provide an acceptable fit to the observed 12‐month cycles in methane. The trend in the amplitude of the annual cycle of methane at Cape Grim is used to infer a trend in OH in 30°–90°S of 0 ± 5% per decade from 1985 to 2000, in qualitative agreement with Prinn et al. [2001] for the Southern Hemisphere.
Climatic effects of an impact‐induced equatorial debris ring
Authors: Fawcett Peter J. , Boslough Mark B. E.
Published: October 2002

Several theoretical and laboratory studies suggest that some large impact events are capable of inserting material into space depending on mechanics of the impact. This material would quickly coalesce to form a temporary debris ring in orbit around the equator, which would cast its shadow on the winter hemisphere. The results of an atmospheric general circulation model (GCM) simulation where an orbiting equatorial debris ring is applied as a boundary condition to the model show how the longer‐term effects of a major impact could affect the climate system. The primary effect is a severe cooling in the tropics and the subtropics, especially under the seasonally migrating ring shadow. The globe cools and becomes drier, with the exception of monsoonal regions that become wetter. The Hadley cell is weakened resulting in drier tropics and weaker subtropical high‐pressure cells in the winter hemisphere. Because the tropics cool more than middle latitude regions, the equator‐to‐pole temperature gradient becomes shallower resulting in weaker tropospheric winds and less high‐latitude storminess. We suggest that the late Eocene impact(s) (35.5 Ma) could have generated a geologically temporary orbiting debris ring based on the global distribution of tektites associated with these events and patterns of climate change immediately above the iridium/microtektite layer. The Cretaceous‐Tertiary boundary event, while larger, did not produce a debris ring. We also suggest that an opaque debris ring could have acted as the trigger to at least one episode of global glaciation during the Neoproterozoic.
Climatological features of mesosphere and lower thermosphere stationary planetary waves within ±40° latitude
Authors: Forbes J. M. , Zhang X. , Ward W. , Talaat E. R.
Published: October 2002

Zonal and meridional wind measurements from the High Resolution Doppler Imager (HRDI) and Wind Imaging Interferometer (WINDII) instruments on the Upper Atmosphere Research Satellite (UARS) are used to construct monthly average stationary planetary wave (SPW) structures from 15 km to 110 km over the period December 1991 to September 1994. Because of viewing constraints of the instruments, intercomparisons between Northern Hemisphere (NH) and Southern Hemisphere (SH) structures and seasonal evolutions are mainly confined to ±40° latitude. The largest zonal wave number s = 1 amplitudes (10–25 m s−1) in the stratosphere and mesosphere at ±40° exist in the zonal wind fields and occur during September–January in the NH and July–October in the SH. SPW s = 2 structures are much less prevalent, especially above 80 km because of more restrictive filtering effects due to the mean winds. Above about 70 km, s = 1 wave amplitudes (∼5–10 m s−1) are more uniform, extending over all nonsummer months in both hemispheres up to 80–90 km, with similar amplitudes existing throughout the year at higher altitudes. Our interpretation is that the SPW above about 80–90 km during summer owe their existence to ducting from the winter hemisphere, in agreement with numerical simulations by Pogoreltsev and Sukhanova [1993]. The origin of SPW above about 80 km during winter at midlatitudes is not possible to determine unambiguously. Phase structures undergo a transition from vertical propagation with long vertical wavelengths (hundreds of kilometers) below ∼80 km to either evanescent behavior or an extreme shortening of vertical scale at higher altitudes. This change in phase behavior may be connected with negative values in refractive index Q for quasi‐geostrophic wave propagation, which occur over a narrow height region near 80 km during almost every month. This interruption of normal phase progression makes it impossible to distinguish the origin of the planetary wave oscillations from (1) those that “tunnel” through the Q < 0 region and (2) those that might be excited in situ by zonally asymmetric momentum deposition from gravity waves which have undergone filtering by the stratospheric planetary wave wind field. In addition, quantitative estimates of aliasing due to the diurnal tide indicate that it may be of sufficient magnitude to contaminate retrieved SPW structures between 80–90 km and 100–110 km, where local time coverages are incomplete. Comparisons are made between the observed SPW structure and those from the horizontal wind model [Hedin et al., 1993, 1996]. This model is found to provide a reasonable approximation to NH winter SPW s = 1 structures below 80 km, but severely underestimates the amplitudes of SPW in the SH.
Products of the OH radical‐ and O3‐initiated reactions of myrcene and ocimene
Authors: Reissell Anni , Aschmann Sara M. , Atkinson Roger , Arey Janet
Published: October 2002

We have investigated products formed from the gas‐phase reactions of OH radicals and O3 (at 296 ± 2 K and 740 Torr total pressure of purified air) with the biogenic organic compounds myrcene and ocimene, using gas chromatographic analyses. In addition to the formation of acetone the OH radical reaction with myrcene in the presence of NO resulted in the formation of 4‐vinyl‐4‐pentenal in 19 ± 4% yield, while the corresponding ocimene reaction showed no formation of the analogous product 4‐methyl‐3,5‐hexadienal (<2% yield). The O3 reactions with myrcene and ocimene in the presence of an OH radical scavenger formed acetone (21 ± 3%) and 4‐vinyl‐4‐pentenal (70 ± 13%) from myrene, and acetone (24 ± 4%) and 4‐methyl‐3,5‐hexadienal (33 ± 6%) from ocimene. Reaction pathways leading to these products are discussed.
Fatty acids in the marine atmosphere: Factors governing their concentrations and evaluation of organic films on sea‐salt particles
Authors: Mochida Michihiro , Kitamori Yasuyuki , Kawamura Kimitaka , Nojiri Yukihiro , Suzuki Keisuke
Published: October 2002

Fatty acids (C14–C32) in the marine boundary layer were measured in aerosols that were collected over the northern North Pacific from October 1996 to June 1997. Concentrations of lower molecular weight (C14–C19) saturated fatty acids (LFAs, 0.8–24 ng m−3) showed a positive correlation with sea‐salt concentrations, suggesting that LFAs are released from the ocean surface to the atmosphere with sea‐salt particles. The averaged ratios of [LFAs]air/[sea salt] in autumn, winter, spring and summer seasons were 1.8 (±1.3) × 10−4, 2.1 (±1.3) × 10−4, 3.7 (±2.9) × 10−4, and 4.6 (±1.8) × 10−4, respectively. The results indicate the seasonal variation of the sea‐to‐air flux of LFAs relative to that of sea salt with a maximum in spring to summer. The enhanced LFA flux was consistent with the satellite images of chlorophyll a over the northern North Pacific, which showed high biological productivity from spring to summer. On the basis of the ratios of [LFAs]air/[sea salt], relative humidity, and modeled size distribution of sea‐salt particles, the coverage of LFAs on sea‐salt particles was estimated to range from 0.3 to 14%. This study suggests that the coverage of fatty acids, together with other film‐forming materials, may have a significant effect on the physicochemical properties of aerosols, which may be affected by the high biological productivity in the high latitudinal ocean.
Estimate of top‐of‐atmosphere albedo for a molecular atmosphere over ocean using Clouds and the Earth's Radiant Energy System measurements
Authors: Kato Seiji , Loeb Norman G. , Rutledge C. Ken
Published: October 2002

The shortwave broadband albedo at the top of a molecular atmosphere over ocean between 40°N and 40°S is estimated using radiance measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument and the Visible Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission satellite. The albedo monotonically increases from 0.059 at a solar zenith angle of 10° to 0.107 at a solar zenith angle of 60°. The estimated uncertainty in the albedo is 3.5 × 10−3 caused by the uncertainty in CERES‐derived irradiances, uncertainty in VIRS‐derived aerosol optical thicknesses, variations in surface wind speed and variations in ozone and water vapor. The estimated uncertainty is similar in magnitude to the standard deviation of 0.003 that is derived from 72 areas which are divided by 20° latitude by 20° longitude grid boxes. The empirically estimated albedo is compared with the modeled albedo using a radiative transfer model combined with an ocean surface bidirectional reflectivity model. The modeled albedo with standard tropical atmosphere is 0.061 and 0.111 at the solar zenith angles of 10° and 60°, respectively. The empirically estimated albedo can be used to estimate the direct radiative effect of aerosols at the top of the atmosphere over oceans.
A pole to pole west Pacific atmospheric teleconnection during August
Authors: Hines Keith M. , Bromwich David H.
Published: October 2002

An observational analysis is presented that reveals an August teleconnection involving both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). The teleconnection includes three primary anomalies in the monthly average surface pressure field: (1) a high southern latitude component including Wilkes Land, Antarctica, the nearby Southern Ocean, and the Ross Sea, (2) a SH midlatitude component near Australia and New Zealand, and (3) a NH subtropical component over the extreme western Pacific Ocean. Surface pressure for the SH high‐latitude anomaly is negatively correlated to the other two primary anomalies. The SH midlatitude and NH subtropical component are positively correlated. Furthermore, the teleconnection is correlated to surface pressure near the Aleutian Islands. The teleconnection anomalies result from tropical convection on intraseasonal timescales. Monsoon rainfall over East Asia during August is correlated to the teleconnection. Furthermore, August precipitation at several other locations in the NH and SH is modulated by the teleconnection. Two different sets of numerical experiments with the National Center for Atmospheric Research (NCAR) Community Climate Model version 2 (CCM2) show a similar‐appearing teleconnection during late boreal summer. The modeled teleconnection, however, is forced from high southern latitudes.
Transatlantic transport of pollution and its effects on surface ozone in Europe and North America
Authors: Li Qinbin , Jacob Daniel J. , Bey Isabelle , Palmer Paul I. , Duncan Bryan N. , Field Brendan D. , Martin Randall V. , Fiore Arlene M. , Yantosca Robert M. , Parrish David D. , Simmonds Peter G. , Oltmans Samuel J.
Published: October 2002

We examine the transatlantic transport of anthropogenic ozone and its impact on surface ozone in Europe and North America by using a 5‐year (1993–1997) simulation with the GEOS‐CHEM global three‐dimensional model of tropospheric chemistry. Long‐term time series of ozone and CO at Mace Head (Ireland) and Sable Island (Canada) are used to evaluate transatlantic transport in the model. North American anthropogenic emissions contribute on average 5 ppbv to surface ozone at Mace Head, and up to 10–20 ppbv during transatlantic transport events, which are forerunners of broader events in Europe. These events are associated with low‐level westerly flow driven by an intense Icelandic low between Iceland and the British Isles. North American influence on ozone at Mace Head is strongly correlated with the North Atlantic Oscillation (NAO), implying that the NAO index can be used to forecast transatlantic transport of North American pollution to Europe. European anthropogenic emissions contribute on average less than 2 ppbv to surface ozone at Sable Island but up to 5–10 ppbv during transatlantic transport events. These events are associated with low‐level easterly flow established by anomalous low pressure at 45°N over the North Atlantic. North American anthropogenic emissions enhance surface ozone in continental Europe by 2–4 ppbv on average in summer and by 5–10 ppbv during transatlantic transport events; transport in the boundary layer and subsidence from the free troposphere are both important mechanisms. We find in the model that 20% of the violations of the European Council ozone standard (55 ppbv, 8‐hour average) in the summer of 1997 over Europe would not have occurred in the absence of anthropogenic emissions from North America. North American influence on surface ozone in Europe is particularly strong at the thresholds used for the European standards (55–65 ppbv).
Solar actinic radiation (280–420 nm) in the cloud‐free troposphere between ground and 12 km altitude: Measurements and model results
Authors: Hofzumahaus A. , Kraus A. , Kylling A. , Zerefos C. S.
Published: October 2002

Airborne measurements of the spectrally resolved actinic flux (280–420 nm) between the ground and 12 km altitude have been made using a new calibrated dual‐channel spectroradiometer. The measurements were made as part of the Photochemical Activity and Ultraviolet Radiation/Altitude Dependence of the Tropospheric Ozone Photolysis (PAUR/ATOP) measurement campaign in Greece during June 1996. Flights were made over the Agean Sea under cloudless conditions for various aerosol loads and solar zenith angles. The spectral actinic flux measurements are compared with radiative transfer model simulations based on the multistream discrete ordinate radiative transfer (DISORT) algorithm. All input to the radiative transfer model was provided by independent measurements performed simultaneously on the nearby island of Agios Efstratios. For altitudes between 3000–12,000 m the agreement between the measurements and the model simulations is within ±5% for wavelengths larger than 310 nm and within ±10% at 295–310 nm. For the lowest flight altitude, 108 m, the model underestimates the measured actinic flux systematically by about 12%. This may be partly explained by uncertainties in the aerosol optical properties and the surface albedo. Flights on days with small and large amounts of aerosols showed that under otherwise identical conditions the actinic flux increased by up to 10% when the aerosol amount was larger.
Light scattering by dust and anthropogenic aerosol at a remote site in the Negev desert, Israel
Authors: Andreae Tracey W. , Andreae Meinrat O. , Ichoku Charles , Maenhaut Willy , Cafmeyer Jan , Karnieli Arnon , Orlovsky Leah
Published: October 2002

We investigated aerosol optical properties, mass concentration, and chemical composition over a 2 year period at a remote site in the Negev desert, Israel (Sde Boker, 30° 51′N, 34° 47′E, 470 m above sea level). Light‐scattering measurements were made at three wavelengths (450, 550, and 700 nm), using an integrating nephelometer, and included the separate determination of the backscatter fraction. Aerosol coarse and fine fractions were collected with stacked filter units; mass concentrations were determined by weighing, and the chemical composition by proton‐induced X‐ray emission and instrumental neutron activation analysis. The total scattering coefficient at 550 nm showed a median of 66.7 Mm−1(mean value 75.2 Mm−1, standard deviation 41.7 Mm−1) typical of moderately polluted continental air masses. Values of 1000 Mm−1and higher were encountered during severe dust storm events. During the study period, 31 such dust events were detected. In addition to high scattering levels, they were characterized by a sharp drop in the Ångström coefficient (i.e., the spectral dispersion of the light scattering) to values near zero. Mass‐scattering efficiencies were obtained by a multivariate regression of the scattering coefficients on dust, sulfate, and residual components. An analysis of the contributions of these components to the total scattering observed showed that anthropogenic aerosol accounted for about 70% of scattering. The rest was dominated by the effect of the large dust events mentioned above and of small dust episodes typically occurring during midafternoon.
Temperature and humidity retrieval from simulated Infrared Atmospheric Sounding Interferometer (IASI) measurements
Authors: Lerner Jeffrey A. , Weisz Elisabeth , Kirchengast Gottfried
Published: October 2002

An efficient temperature and humidity retrieval algorithm for radiometric measurements at high spectral resolution is introduced and applied to climatological profiles. The algorithm is developed for analyzing Infrared Atmospheric Sounding Interferometer (IASI) data of the European weather satellite METOP‐1 (launch scheduled 2005) for climatological purposes but is also applicable for other purposes and to other similar data. The algorithm's core features are a channel selection methodology followed by a linearized optimal estimation. The key concept of the former is that a small subset (5–10%) of all available IASI channels (∼8000) is selected based on maximizing a suitable information content measure at each retrieval level. This enables efficiency and robustness of the retrieval algorithm and curtails the high redundancy in the measurements. In addition to profile and error covariance estimates optimal estimation furnishes various sensitivity functions of which we used “weighting functions” for quantifying the utility of measurement channels and “averaging kernel functions” for assessing the resolution of retrieved profiles. Results based on simulated IASI spectra computed from a set of standard climatological profiles and a realistic radiometric noise model demonstrate, for clear air, the capabilities of high spectral resolution measurements for improving temperature and humidity soundings compared to current operational sensors. In the troposphere (below ∼200 hPa), retrieved profiles exhibit temperature errors of <1 K and specific humidity errors of <10% at most heights, associated with a vertical resolution of ∼1.5–2 km. Promising performance was found in the upper troposphere (500–200 hPa), where about five independent reliable values of temperature and humidity are available indicating the high potential of the IASI sensor for monitoring climatic changes in upper tropospheric moisture. Tests on the sensitivity of retrieved profiles to the quality of a priori profiles showed weak sensitivity of temperature but significant sensitivity of humidity. The results provide a solid basis and clear guidance for improvements of the presented algorithm for reliable large‐scale application on cloud‐free spectra.
Magnetization of the Martian crust
Authors: Arkani‐Hamed Jafar
Published: October 2002

This paper presents the lateral variations of the vertically averaged magnetization model of the Martian crust determined from the degree 50 spherical harmonic model of the magnetic potential. The model potential is derived using the Mars Global Surveyor low‐altitude magnetic data acquired within elevations of 85–200 km. The potential model is first verified by comparing the three orthogonal components of the corresponding vector field with the observed high‐altitude (∼380 km) data. Using the paleopole positions calculated through modeling 10 isolated, small magnetic anomalies, the model potential is inverted to determine the vertically averaged magnetization of a nominal 50 km thick crust. The crust is highly magnetic, with a magnetization contrast (peak‐to‐trough) as high as 35 A/m. The lack of distinct magnetic signatures for the giant impact basins, Hellas, Argyre and Isidis, and the absence of a magnetic anomaly associated with the north‐south topographic dichotomy indicate that the upper parts of the crust are low magnetic and that the strong magnetic source bodies are in the lower parts.
Global mineral distributions on Mars
Authors: Bandfield Joshua L.
Published: October 2002

Determining the mineralogy of Mars is an essential part of revealing the conditions of the surface and subsurface. A deconvolution method was used to remove atmospheric components and determine surface mineralogy from Thermal Emission Spectrometer data at 1 pixel per degree (ppd). Minerals are grouped into categories on the basis of compositional and spectral similarity, and global concentration maps are produced. All binned pixels are fit well with RMS errors of ≤0.005 in emissivity. Higher RMS errors are attributed to short wavelength particle size effects on dust‐covered surfaces. Significant concentrations (>0.10) of plagioclase, high‐Ca pyroxene, sheet silicates/high‐Si glass, and hematite are detected and display distributions consistent with previous studies. Elevated concentrations of plagioclase and high‐Ca pyroxene are consistent with basaltic surfaces and are located in low‐albedo highlands regions north of ∼45°S. Significant concentrations of plagioclase and sheet silicates/high‐Si glass and low concentrations of high‐Ca pyroxenes are consistent with andesitic surfaces and are concentrated in both southern and northern high‐latitude, low‐albedo regions. Andesitic surfaces in the southern hemisphere have a lower spectral contrast than northern surfaces. An isolated surface located in Solis Planum is spectrally distinct but compositionally similar to other surfaces interpreted to be andesitic in composition. Concentrations of olivine below the detection limit correctly identify its presence in two of three locations. Potassium feldspar, low‐Ca pyroxene, basaltic glass, olivine, sulfate, carbonate, quartz, and amphibole are not detected with confidence at 1 ppd. The results presented here indicate a predominance of volcanic compositions within Martian dust‐free surfaces.
Observations of high‐altitude CO2 hot bands in Mars by the orbiting Thermal Emission Spectrometer
Authors: Maguire W. C. , Pearl J. C. , Smith M. D. , Conrath B. J. , Kutepov A. A. , Kaelberer M. S. , Winter E. , Christensen P. R.
Published: October 2002

We present limb spectral measurements of the Martian atmosphere for four seasonal periods produced from data acquired by the Mars Global Surveyor/Thermal Emission Spectrometer. Between the altitudes of approximately 50 and 90 km, nonlocal thermodynamic equilibrium (non‐LTE) emission from CO2 hot bands is observed at 10 μm. The centroids of these emissions vary in latitude and height with season, and their emission regions extend approximately 120° in latitude. The emission is considerably stronger in the southern summer/northern winter season than in the corresponding northern summer/southern winter season due to large solar flux variations resulting from the eccentricity of the Martian orbit. Modeling of this emission explains its observed altitude, solar zenith angle, and seasonal dependence.
On magnetic spectra of Earth and Mars
Authors: Voorhies C. V. , Sabaka T. J. , Purucker M.
Published: October 2002

The spectral method for distinguishing crustal from core‐source magnetic fields is reexamined, modified, and applied to both a comprehensive geomagnetic field model and an altitude normalized magnetic map of Mars. The observational spectra are fairly fitted by theoretical forms expected from certain elementary classes of magnetic sources. For Earth we find fields from a core of radius 3512 ± 64 km, in accord with the seismologic core radius of 3480 km, and a crust represented by a shell of random dipolar sources at radius 6367 ± 14 km, near the planetary mean radius of 6371.2 km. For Mars we find no sign of a core‐source field, only a field from a crust represented in the same way, but at radius 3344 ± 10 km, about 46 km below the planetary mean radius of 3389.5 km, and with sources about 9.6 ± 3.2 times stronger.
Venus: Analysis of the degree of impact crater deposit degradation and assessment of its use for dating geological units and features
Authors: Basilevsky Alexander T. , Head James W.
Published: October 2002

We use the degree of degradation of crater‐associated radar‐dark deposits on Venus to estimate the age of the crater and neighboring units. We analyzed craters ≥30 km in diameter superposed on regional plains (subpopulation 1; 138 craters) and on later units (subpopulation 2; 30 craters) and estimated percentages of craters with dark parabolas (DP), clear dark halo (CH), faint dark halo (FH), and no dark halo (NH). We constructed theoretical models of the evolution of these percentages with time for two possible interpretations of the upper boundary of the regional plains: (1) globally synchronous and (2) nonsynchronous (diachronous). They show that in the synchronous case the dark deposit lifetimes are proportional to the corresponding percentages observed for subpopulation 1: TDP = 0.15T, TCH = 0.3T, and TFH = 0.3T, where T is the mean global surface age of Venus. In diachronous cases the percentages may or may not be proportional to the appropriate lifetimes, depending on the range of the regional plains. If the time range of plains emplacement is not larger than ±0.5T, the use of TDP = 0.15T and TCH = 0.3T is appropriate. We consider the time ∼0.5T ago as the lower time boundary for the age of the now‐observed CH craters and the time 0.1–0.15T ago as the lower boundary for the age of DP craters. We propose that the Atlian Period of the geologic history of Venus be subdivided into Upper and Lower Epochs, with the boundary between them at ∼0.5T ago, and the lower boundary of the Aurelian Period be placed at 0.1–0.15T ago. We apply this approach to assess ages of activity of three volcanic‐tectonic structures on Venus: Beta Regio‐Devana Chasma (shows evidence of activity younger than 0.5T ago), Mylitta Fluctus (also younger than 0.5T), and Atla Regio (shows evidence of activity younger than 0.1–0.15T ago).
Geology and origin of Europa's “Mitten” feature (Murias Chaos)
Authors: Figueredo P. H. , Chuang F. C. , Rathbun J. , Kirk R. L. , Greeley R.
Published: October 2002

The “Mitten” (provisionally named Murias Chaos by the International Astronomical Union) is a region of elevated chaos‐like terrain in the leading hemisphere of Europa. Its origin had been explained under the currently debated theories of melting through a thin lithosphere or convection within a thick one. Galileo observations reveal several characteristics that suggest that the Mitten is distinct from typical chaos terrain and point to a different formational process. Photoclinometric elevation estimates suggest that the Mitten is slightly elevated with respect to the surrounding terrain; geologic relations indicate that it must have raised significantly from the plains in its past, resembling disrupted domes on Europa's trailing hemisphere. Moreover, the Mitten material appears to have extruded onto the plains and flowed for tens of kilometers. The area subsequently subsided as a result of isostatic adjustment, viscous relaxation, and/or plains loading. Using plate flexure models, we estimated the elastic lithosphere in the area to be several kilometers thick. We propose that the Mitten originated by the ascent and extrusion of a large thermal diapir. Thermal‐mechanical modeling shows that a Mitten‐sized plume would remain sufficiently warm and buoyant to pierce through the crust and flow unconfined on the surface. Such a diapir probably had an initial radius between 5 and 8 km and an initial depth of 20–40 km, consistent with a thick‐lithosphere model. In this scenario the Mitten appears to represent the surface expression of the rare ascent of a large diapir, in contrast to lenticulae and chaos terrain, which may form by isolated and clustered small diapirs, respectively.
A first look at dust lifting and dust storms near the south pole of Mars with a mesoscale model
Authors: Toigo Anthony D. , Richardson Mark I. , Wilson R. John , Wang Huiqun , Ingersoll Andrew P.
Published: October 2002

Surface wind stresses and dust lifting in the south polar region of Mars are examined with a three‐dimensional numerical model. The focus of this study is the middle to late southern spring period when cap‐edge dust lifting events are observed. Mesoscale model simulations of high southern latitudes are conducted at three dates within this season (Ls = 225°, 255°, and 310°). Assuming that dust injection is related to the saltation of sand‐sized grains or aggregates, the Mars MM5 mesoscale model predicts surface wind stresses of sufficient strength to initiate movement of sand‐sized particles (∼100 μm), and hence dust lifting, during all three periods. The availability of dust and/or sand‐sized particles is not addressed within this study. Instead, the degree to which the existence of sufficiently strong winds limit dust injection is examined. By eliminating forcing elements from the model, the important dynamical modes generating high wind stresses are isolated. The direct cap‐edge thermal contrast (and topographic slopes in some locations) provides the primary drive for high surface wind stresses at the cap edge, while sublimation flow is not found to be particularly important, at these three dates. Simulations in which dust is injected into the lowest model layer when wind stresses exceed a threshold show similar patterns of atmospheric dust to those seen in recent observations. Comparison between these simulations and those without active dust injection shows no signs of consistent positive or negative feedback due to dust clouds on the surface wind stress fields during the late spring season examined here.
North polar stratigraphy and the paleo‐erg of Mars
Authors: Byrne Shane , Murray Bruce C.
Published: October 2002

An accurate self‐consistent way of coregistering the imaging and topographic data sets of the Mars Global Surveyor mission was developed and used to begin a stratigraphic analysis of the northern polar region. A distinct change in the layering style exists at a definite stratigraphic horizon near the base of the north polar layered deposits. Occurrences of the contact between two distinct layered units can be mapped hundreds of kilometers apart at nearly the same Mars Orbiter Laser Altimeter (MOLA) elevation. The lower layered unit has a consistent association with sand dunes, leading to the conclusion that it is an eroding sand‐rich deposit that predates most of the overlying north polar layered deposits, which exhibits the expected features of a dust‐ice mixture. These results suggest that an areally extensive erg was in existence before the present ice cap and that the present circumpolar erg is likely composed of material reworked from this older deposit. The volume of this lower unit is estimated to be on the order of 105 km3. The presence of this deposit implies that there existed a period in Mars' history when there was no icy polar cap. A dramatic climatic change leading to the deposition of the upper layered (icy) unit in the present‐day polar layered deposits represents a major event in Mars' history. However, owing to uncertainties in the mechanics of layered deposits formation, such an event cannot be dated at this time.
Thermal and crustal evolution of Mars
Authors: Hauck Steven A. , Phillips Roger J.
Published: October 2002

We present a coupled thermal‐magmatic model for the evolution of Mars' mantle and crust that may be consistent with estimates of the average crustal thickness and crustal growth rate. By coupling a simple parameterized model of mantle convection to a batch‐melting model for peridotite, we can investigate potential conditions and evolutionary paths of the crust and mantle in a coupled thermal‐magmatic system. On the basis of recent geophysical and geochemical studies, we constrain our models to have average crustal thicknesses between 50 and 100 km that were mostly formed by 4 Ga. Our nominal model is an attempt to satisfy these constraints with a relatively simple set of conditions. Key elements of this model are the inclusion of the energetics of melting, a wet (weak) mantle rheology, self‐consistent fractionation of heat‐producing elements to the crust, and a near‐chondritic abundance of those elements. The latent heat of melting mantle material is a small (percent level) contributor to the total planetary energy budget over 4.5 Gyr but is crucial for constraining the thermal and magmatic history of Mars. Our nominal model predicts an average crustal thickness of ∼62 km that was 73% emplaced by 4 Ga. However, if Mars had a primary crust enriched in heat‐producing elements, consistent with SNC meteorite geochemistry, then our models predict a considerably diminished amount of post 4 Ga crustal emplacement relative to the nominal model. The importance of a wet mantle in satisfying the basic constraints of Mars' thermal and crustal evolution suggests (independently from traditional geomorphology or meteorite geochemistry arguments) that early Mars had a wet environment. Extraction of water from the mantle of a one‐plate planet such as Mars is found to be extremely inefficient, such that 90–95% of all water present in the mantle after the initial degassing event should still reside there currently. Yet extraction of even 5% of a modestly wet mantle (∼36 ppm water) would result in a significant amount (6.4 m equivalent global layer) of water available to influence the early surface and climate evolution of the planet.
Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid‐Cretaceous
Authors: Leckie R. Mark , Bralower Timothy J. , Cashman Richard
Published: October 2002

Mid‐Cretaceous (Barremian‐Turonian) plankton preserved in deep‐sea marl, organic‐rich shale, and pelagic carbonate hold an important record of how the marine biosphere responded to short‐ and long‐term changes in the ocean‐climate system. Oceanic anoxic events (OAEs) were short‐lived episodes of organic carbon burial that are distinguished by their widespread distribution as discrete beds of black shale and/or pronounced carbon isotopic excursions. OAE1a in the early Aptian (∼120.5 Ma) and OAE2 at the Cenomanian/Turonian boundary (∼93.5 Ma) were global in their distribution and associated with heightened marine productivity. OAE1b spans the Aptian/Albian boundary (∼113–109 Ma) and represents a protracted interval of dysoxia with multiple discrete black shales across parts of Tethys (including Mexico), while OAE1d developed across eastern and western Tethys and in other locales during the latest Albian (∼99.5 Ma). Mineralized plankton experienced accelerated rates of speciation and extinction at or near the major Cretaceous OAEs, and strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism. Elevated levels of trace metals in OAE1a and OAE2 strata suggest that marine productivity may have been facilitated by increased availability of dissolved iron. The association of plankton turnover and carbon isotopic excursions with each of the major OAEs, despite the variable geographic distribution of black shale accumulation, points to widespread changes in the ocean‐climate system. Ocean crust production and hydrothermal activity increased in the late Aptian. Faster spreading rates [and/or increased ridge length] drove a long‐term (Albian–early Turonian) rise in sea level and CO2‐induced global warming. Changes in ocean circulation, water column stratification, and nutrient partitioning lead to a reorganization of plankton community structure and widespread carbonate (chalk) deposition during the Late Cretaceous. We conclude that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.
Forcing of the deep ocean circulation in simulations of the Last Glacial Maximum
Authors: Schmittner A. , Meissner K. J. , Eby M. , Weaver A. J.
Published: October 2002

From the interpretation of different proxy data it is widely believed that the North Atlantic thermohaline circulation during the maximum of the last ice age ∼21,000 years ago was considerably weaker than today. Recent equilibrium simulations with a coupled ocean‐atmosphere‐sea ice model successfully simulated a reduction in North Atlantic Deep Water (NADW) formation consistent with reconstructions. Here we examine the influence of different air‐sea fluxes on simulated changes in the deep ocean circulation between the Last Glacial Maximum and present day. We find that changes in the oceanic surface freshwater fluxes are the dominant forcing mechanism for the reduced Atlantic overturning. Diminished export of freshwater out of the Atlantic drainage basin through the atmosphere decreases surface salinities in the North Atlantic, leading to less NADW formation in the colder climate. Changes in heat fluxes, which lead to increased sea surface densities in the North Atlantic and therefore to an enhanced overturning, are of secondary importance. Wind stress variations seem to play a negligible role. The degree to which the Atlantic freshwater export and hence the NADW formation are reduced depends on the formulation of the atmospheric hydrological cycle and on the strength of the overturning in the present‐day simulation. Simulated changes in sea surface properties for a large variety of overturning strengths are compared with different reconstruction data sets. The results depend strongly on the data set used. Sea surface temperature reconstructions from Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) and earlier salinity reconstructions based on planktonic foraminifera are most consistent with a significant reduction of the circulation, while recent reconstructions using dinocyst assemblages allow no unequivocal conclusion.
Bias in the paleoceanographic time series: Tests with a numerical model of U, Corg, and Al burial
Authors: Donnadieu Y. , Lecroart P. , Anschutz P. , Bertrand P.
Published: October 2002

We have constructed a nonsteady state numerical model of organic carbon, aluminum, and uranium burial to examine the dynamic response of sediments to variations on thousand years scale. The three components are defined by distinct behaviors in the sedimentary system. The aluminum is a conservative component, the organic carbon is a nonconservative component degraded in superficial sediment, and the uranium is a component affected by redox conditions. By introducing a simultaneous variation of incoming fluxes of Corg and Al, geochemical profiles of Al, Corg, and U clearly show separate distributions after burial. Each component record the synchronous variation of the incoming fluxes, but the position in the sedimentary pile of the recording differs from a component to another. We show that a minimum of U could be linked to a rapid ascent of the oxic front during an increase in Corg flux.
Arctic ice export events and their potential impact on global climate during the late Pleistocene
Authors: Darby Dennis A. , Bischof Jens F. , Spielhagen Robert F. , Marshall Steven A. , Herman Stephen W.
Published: October 2002

Ice sheets in the North American Arctic and, to a lesser extent, those in northern Eurasia calved large quantities of icebergs that drifted through Fram Strait into the Greenland Sea several times during the late Pleistocene. These icebergs deposited Fe oxide grains (45–250 μm) and coarse lithic clasts >250 μm matched to specific circum‐Arctic sources. Four massive Arctic iceberg export events are identified from the Laurentide and the Innuitian ice sheets, between 14 and 34 ka (calendar years) in a sediment core from Fram Strait. These relatively short duration (<1–4 kyr) events contain 3–5 times the background levels of Fe oxide grains. They began suddenly, as indicated by a steep rise in the number of grains matched to an ice sheet source, suggesting rapid purges of ice through Fram Strait, due perhaps to collapse of ice sheets. The larger events from the northwestern Laurentide ice sheet are preceded by events from the Innuitian ice sheet. Despite the chronological uncertainties, the Arctic export events appear to occur prior to Heinrich events.
Constructing a stacked benthic δ18O record
Authors: Karner Daniel B. , Levine Jonathan , Medeiros Brian P. , Muller Richard A.
Published: October 2002

Composite stacks were constructed by superimposing 6 to 13 benthic foraminiferal δ18O records covering the period 0–850 ka. An initial timescale for each core was established using radioisotopic age control points and assuming constant sedimentation rates between these points. The average of these records is our 13‐core “untuned” stack. Next, we matched the 41 kyr component of each record individually to variations in Earth's obliquity. Four of the 13 records produced timescales that were inconsistent with one or more of the known radioisotopic ages. The nine remaining cores were averaged to create a “minimally tuned” stack. Six of the minimally tuned cores were assembled into a “tropical” stack. For each stack we estimated the uncertainty envelope from the standard deviation of the constituents. Spectral analysis of the three stacks indicates that benthic δ18O is dominated by a 100 kyr oscillation that has a narrow spectral peak. The contribution of precession to the total variance is small when compared to prior results from planktic stacks.
Tropical Pacific climate at the mid‐Holocene and the Last Glacial Maximum simulated by a coupled ocean‐atmosphere general circulation model
Authors: Kitoh Akio , Murakami Shigenori
Published: October 2002

Simulations for the mid‐Holocene (6000 years before present: 6 ka) and the Last Glacial Maximum (LGM: 21 ka) have been performed by a global ocean‐atmosphere coupled general circulation model (GCM). After the initial spin‐up periods, both runs were integrated for about 200 years. For 6 ka the model shows an enhanced seasonal variation in surface temperature and a northward shift of the African and the Indian summer monsoon rain area. Overall circulation features in the tropics correspond to a strong Walker circulation state with negative sea surface temperature (SST) and precipitation anomalies in the central Pacific and positive precipitation anomalies over the Indian and Australian monsoon regions. It is noted that there is about a 0.35°C cooling of the global mean SST. In contrast to the 6 ka result, the simulated tropical climate anomaly at 21 ka corresponds to a weak Walker circulation state. The simulated LGM SST decrease is about 2°C over the tropical western Pacific. It is larger (about 5°C) over the Caribbean Sea. This SST anomaly is in broad agreement with the observed proxy data. Interestingly, the model simulates a warmer than present SST over the subtropical Pacific. This may be related to a weaker subtropical anticyclone due to weakened monsoon circulation at the LGM.
Stacked 2.6‐Ma grain size record from the Chinese loess based on five sections and correlation with the deep‐sea δ18O record
Authors: Ding Z. L. , Derbyshire E. , Yang S. L. , Yu Z. W. , Xiong S. F. , Liu T. S.
Published: October 2002

This study aims to establish a stacked climate record of the Quaternary period from the Chinese loess sequence and to address the forcing mechanisms for the regional climate history of the Loess Plateau by correlating the stacked record with a composite δ18O record in deep‐sea sediments. A total of 18,352 samples were obtained from five loess sections, located at Baoji, Lingtai, Jingchuan, Puxian, and Pingliang in the southern and middle Loess Plateau. These yielded high‐resolution grain size records. Between‐section correlation of these shows that although small depositional hiatuses are present in places within a single section, most parts of the sections display near‐continuous dust deposition throughout the Quaternary. The grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the Earth’s orbit. The grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series, termed the “Chiloparts” record. This resolves most of the orbital timescale paleoclimate events buried in the loess‐soil sequences of the southern and middle Loess Plateau and can be used as a regional archive of the Pleistocene climate history in the Loess Plateau. Comparison of the “Chiloparts” record with a composite marine δ18O record shows that for the past 1.8 Ma, the loess‐paleosol record can be correlated almost cycle by cycle with the marine record. Several discrepancies in the climatic events between the two records have also been identified, implying that regional forcing mechanisms may have played a part in the climatic evolution of the Chinese Loess Plateau.