bluuebunny/crossref_metadata_2025 huggingface dataset:
Long‐range transport of SOx and dust in East Asia during the PEM B Experiment
Authors: Xiao Hui , Carmichael Gregory R. , Durchenwald James , Thornton Donald , Bandy Alan
Published: February 2004

The transport of SO2 and sulfate in East Asia (including eastern China, Korea, and Japan) during the period of March 1 through March 14, 1994, is studied using a three‐dimensional regional‐scale atmospheric chemistry model. This period corresponds to that in which the Pacific Exploratory Mission in the Western Pacific Ocean (PEM‐West B) was being conducted around Japan. During this period, characterized by the passage of cold fronts and relatively dry conditions, the anthropogenic sulfur emitted from the source regions in East Asia is transported out into the central Pacific Ocean. The sulfur transport is largely limited to the lower 4 km of the atmosphere, with the maximum flux occurring in the 30° to 40°N latitude band containing the bulk of the anthropogenic emissions. The interactions between the sulfur cycle and mineral aerosol are also included in the analysis. It is found that the chemical conversion of SO2 to sulfate in the presence of mineral aerosol may be a significant process during this time period, and may contribute from 20% to 40% of the total sulfate production. Sulfur dioxide arising from volcanic sources in Japan is also discussed.
A comparison of sap flow and eddy fluxes of water vapor from a boreal deciduous forest
Authors: Hogg Edward H. , Black T. Andrew , den Hartog Gerry , Neumann Harold H. , Zimmermann Reiner , Hurdle Patrick A. , Blanken Peter D. , Nesic Zoran , Yang Paul C. , Staebler Ralf M. , McDonald Kyle C. , Oren Ram
Published: February 2004

Water flux to the atmosphere was measured from a mature stand of aspen (Populus tremuloides Michx.) in Saskatchewan, Canada, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Diurnal and seasonal changes in transpiration were monitored using two sap flow techniques and were compared against the difference between eddy correlation measurements of water vapor flux made above and below the aspen canopy. The three methods showed similar diurnal and seasonal trends in water flux, although sap flow lagged the eddy correlation measurements by about 1 hour diurnally due to changes in water storage within the trees. During the growing season, all methods showed a linear increase in midday transpiration with above‐canopy vapor pressure deficit (VPD) up to ∼1 kPa, beyond which transpiration was relatively constant (VPD 1–2.5 kPa). A similar relationship was obtained when total daily transpiration was plotted against mean daytime VPD. The results are consistent with other observations that stomatal conductance of the aspen canopy decreases at high VPD. The complementary benefits of simultaneous monitoring of canopy transpiration by both eddy correlation and sap flow measurements are discussed.
Measurements of the NOy‐N2O correlation in the lower stratosphere: Latitudinal and seasonal changes and model comparisons
Authors: Keim E. R. , Loewenstein M. , Podolske J. R. , Fahey D. W. , Gao R. S. , Woodbridge E. L. , Wamsley R. C. , Donnelly S. G. , Del Negro L. A. , Nevison C. D. , Solomon S. , Rosenlof K. H. , Scott C. J. , Ko M. K. W. , Weisenstein D. , Chan K. R.
Published: February 2004

The tracer species nitrous oxide, N2O, and the reactive nitrogen reservoir, NOy, were measured in situ using instrumentation carried aboard the NASA ER‐2 high altitude aircraft as part of the NASA Airborne Southern Hemisphere Ozone Expedition/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) and Stratospheric Tracers of Atmospheric Transport (STRAT) missions. Measurements were made throughout the latitude range of 70°S to 60°N over the time period of March to October 1994 and October 1995 to January 1996, which includes the period when the Antarctic polar vortex is most intense. The correlation plots of NOy with N2O reveal compact, near‐linear curves throughout data obtained in the lower stratosphere (50 mbar to 200 mbar). The average slope of the correlation, ΔNOy/ΔN2O, in the southern hemisphere (SH) exhibited a much larger seasonal variation during this time period than was observed in the northern hemisphere (NH). Between March and October in the potential temperature range of 400 K to 525 K, the correlation slope in the SH midlatitudes increased by 28%. A smaller but still positive increase in the correlation slope was observed for higher‐latitude data obtained within or near the edge of the SH polar vortex. At NH midlatitudes the correlation slope did not significantly change between March and October, while between October and January the slope increased by +7%. The larger SH midlatitude increase is consistent with ongoing descent throughout the winter and spring and also suggests that denitrification, the irreversible loss of HNO3 through sedimentation of cloud particles, is not a significant term (<10–15%) in the budget of NOy at SH midlatitudes during the wintertime. A secular increase in the correlation slope is ruled out by comparison with SH data obtained during the 1987 Airborne Antarctic Ozone Expedition (AAOE) aircraft campaign. These results suggest that a seasonal cycle exists in the correlation slope for both hemispheres, with the SH correlation slope returning to the April value during the SH spring and summer. Changes in stratospheric circulation also probably play a role in both the SH and the NH correlation slope seasonal cycles. Comparisons with two‐dimensional model results suggest that the slope decreases when the denitrified Antarctic vortex is diluted into midlatitudes upon vortex breakup in the spring and that through the descent of stratospheric air, the slope recovers during the following fall/winter period.
On the use of HF as a reference for the comparison of stratospheric observations and models
Authors: Chipperfield M. P. , Burton M. , Bell W. , Walsh C. Paton , Blumenstock T. , Coffey M. T. , Hannigan J. W. , Mankin W. G. , Galle B. , Mellqvist J. , Mahieu E. , Zander R. , Notholt J. , Sen B. , Toon G. C.
Published: February 2004

Hydrogen fluoride (HF) is often used as a simple reference for other column observations of chemically active stratospheric species. However, seasonal and shorter timescale variations in column HF make its use as a reference more complicated. In this paper we characterize the expected magnitude of these variations in HF, and variations of ratio quantities involving HF, using a two‐dimensional (2‐D) photochemical model and two versions of a three‐dimensional (3‐D) transport model. The 2‐D model predicts that the column ratios HNO3/HF and HCl/HF increase from midlatitudes to the tropics, although this is very sensitive to HCl and HNO3 abundances in the tropical upper troposphere. Seasonal variations in vertical motion modifys the predicted ratios; for example, wintertime descent at high latitudes decreases HCl/HF. The ratio HNO3/HF at high latitudes is strongly modified by seasonal variations in the chemical partitioning of the odd nitrogen (NOy) species. We compare these model predictions with ground‐based Fourier transform infrared spectroscopy (FTIR) observations of HF along with HCl, ClONO2 and HNO3 obtained at eight northern hemisphere sites between October 1994 and July 1995. We investigate quantitatively how HF can be used as a tracer to follow the evolution of observations at a single station and to intercompare results from different stations or with photochemical models. The magnitude of the 3‐D model HF column agrees well with the observations, except on some occasions at high latitudes, giving indirect support for the important role of COF2 in the stratospheric inorganic fluorine budget. The observed day‐to‐day variability in the column ratios HCl/HF and HNO3/HF is much larger at high latitudes. This variability is reproduced in the 3‐D models and is due to horizontal motion. Short timescale vertical displacement of the species profiles is estimated to have a small effect on the column ratios. In particular, we analyze the usefulness of the observed column ratio (ClONO2 + HCl)/HF as an indicator for chlorine activation. Current measurement uncertainties limit the degree of activation which can be unambiguously detected using this observed quantity, but we can determine that chlorine‐activated air was observed above Aberdeen (58°N) on 6 days in late January 1995.
Operational remote sensing of tropospheric aerosol over land from EOS moderate resolution imaging spectroradiometer
Authors: Kaufman Y. J. , Tanré D. , Remer L. A. , Vermote E. F. , Chu A. , Holben B. N.
Published: February 2004

Daily distribution of the aerosol optical thickness and columnar mass concentration will be derived over the continents, from the EOS moderate resolution imaging spectroradiometer (MODIS) using dark land targets. Dark land covers are mainly vegetated areas and dark soils observed in the red and blue channels; therefore the method will be limited to the moist parts of the continents (excluding water and ice cover). After the launch of MODIS the distribution of elevated aerosol concentrations, for example, biomass burning in the tropics or urban industrial aerosol in the midlatitudes, will be continuously monitored. The algorithm takes advantage of the MODIS wide spectral range and high spatial resolution and the strong spectral dependence of the aerosol opacity for most aerosol types that result in low optical thickness in the mid‐IR (2.1 and 3.8 μm). The main steps of the algorithm are (1) identification of dark pixels in the mid‐IR; (2) estimation of their reflectance at 0.47 and 0.66 μm; and (3) derivation of the optical thickness and mass concentration of the accumulation mode from the detected radiance. To differentiate between dust and aerosol dominated by accumulation mode particles, for example, smoke or sulfates, ratios of the aerosol path radiance at 0.47 and 0.66 μm are used. New dynamic aerosol models for biomass burning aerosol, dust and aerosol from industrial/urban origin, are used to determine the aerosol optical properties used in the algorithm. The error in the retrieved aerosol optical thicknesses, τais expected to be Δτa= 0.05±0.2τa. Daily values are stored on a resolution of 10×10 pixels (1 km nadir resolution). Weighted and gridded 8‐day and monthly composites of the optical thickness, the aerosol mass concentration and spectral radiative forcing are generated for selected scattering angles to increase the accuracy. The daily aerosol information over land and oceans [Tanré et al., this issue], combined with continuous aerosol remote sensing from the ground, will be used to study aerosol climatology, to monitor the sources and sinks of specific aerosol types, and to study the interaction of aerosol with water vapor and clouds and their radiative forcing of climate. The aerosol information will also be used for atmospheric corrections of remotely sensed surface reflectance. In this paper, examples of applications and validations are provided.
A semidiscrete model for the scattering of light by vegetation
Authors: Gobron Nadine , Pinty Bernard , Verstraete Michel M. , Govaerts Yves
Published: February 2004

An advanced bidirectional reflectance factor model is developed to account for the architectural effects exhibited by homogeneous vegetation canopies for the first orders of light scattering. The characterization of the canopy allows the simulation of the relevant scattering processes as a function of the number, size, and orientation of the leaves, as well as the total height of the canopy. A turbid medium approach is used to represent the contribution to the total reflectance due to the light scattering at orders higher than 1. This model therefore incorporates two previously separate approaches to the problem of describing light scattering in plant canopies and enhances existing models relying on parameterized formulae to account for the hot spot effect in the extinction coefficient. Simulation results using this model compare quite favorably with those produced with a Monte Carlo ray‐tracing model for a variety of vegetation cases. The semidiscrete model is also inverted against a well‐documented data set of bidirectional reflectance factors taken over a soybean canopy. It is shown that the inversion of the model against a small subset of these measurements leads to reasonable values for the retrieved canopy parameters. These values are used in a direct mode to simulate the bidirectional reflectance factors for solar and viewing conditions significantly different from those available in the subset of soybean data and compared with the full set of actual measurements.
Calibrating the ice core paleothermometer using seasonality
Authors: van Ommen Tas D. , Morgan Vin
Published: February 2004

High‐resolution oxygen isotope measurements on the Dome Summit South (DSS) ice core from Law Dome, Antarctica, provide a seasonal profile that is sufficiently stable and undistorted by biases in accumulation to permit comparison with measured temperature seasonality. This comparison yields an isotope‐temperature relation with a gradient (dδ/dT) of 0.44±0.02‰/°C and provides a new method for exploring the isotope‐temperature relationship at high‐accumulation sites. If applied to the observed isotope record from the DSS core, which extends through the last glacial and beyond, this calibration suggests that at its coldest the last glaciation was ∼13°C colder than present at this site (after correcting for elevation change). This finding compares with a temperature difference of ∼8°C computed by using the local spatially derived calibration.
Lithospheric rheology and flexure at Artemis Chasma, Venus
Authors: Brown C. David , Grimm Robert E.
Published: February 2004

Artemis Chasma is an arcuate, 2000‐km‐diameter zone of convergence and lithospheric underthrusting on Venus. Inelastic flexure modeling of the topography at Artemis, combined with observations of trench tectonics, allow us to document local temperature gradients below 4 K km−1 and an immense compressive in‐plane force at the trench. Lithospheric rheology on Venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments; in particular, the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting. Elastic plate bending models provide an adequate estimate of the bending moment at Artemis, but they fail to constrain—or even recognize—the in‐plane force. The inelastic analysis implies an in‐plane force of the order of −1 × 1014 N m−1; a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north. The low heat flux at Artemis, which is a comparatively young structure, is compatible with the notion that Venus has experienced a prolonged period of cooling in the last several hundred million years. The inference of such exceptionally low thermal gradients embraces three end‐member possibilities: (1) the surface age is >600 Ma, (2) the lithospheric thermal age is greater than the surface age, and/or (3) the upper mantle temperature is anomalously low (∼1550 K).
Annual (perihelion‐aphelion) cycles in the photochemical behavior of the global Mars atmosphere
Authors: Clancy R. T. , Nair H.
Published: February 2004

Ground‐based temperature and water vapor profiling of the atmosphere of Mars point to large annual variations in the water vapor saturation altitude, or hygropause, of the entire low‐to middle‐latitude atmosphere, as forced by the elliptical Mars orbit [Clancy et al., 1996]. We examine the effects of such an annual variation in the hygropause altitude (from <10 km to >30 km) on the photochemistry of the Mars atmosphere. The one‐dimensional, diffusive transport photochemical model of Nair et al. [1994] is run in a diurnally averaged mode for time‐dependent calculations of the annual behavior of Mars photochemistry at low to middle latitudes. The model incorporates a specified annual variation of the water vapor profile, based on the microwave observations of Mars water vapor and temperature profiles versus season (solar longitude, Ls). Due to their long photochemical lifetimes, Mars CO and O2 are expected and found to be in significant non‐equilibrium with the annually varying water vapor (and hence HOx) densities, and to present nearly constant abundances representative of the annual average water vapor profile. In contrast, Mars O3 has a photochemical lifetime of hours, and exhibits very large annual variations in response to the annual variation in the hygropause altitude. The global‐scale abundance of O3 at altitudes of 20–40 km is predicted to vary from >109 cm−3 around Mars aphelion (northern spring/summer, Ls = 71°) to ∼108 cm−3 around Mars perihelion (southern spring/summer, Ls = 251°). These model ozone variations are discussed in the context of the disparate Mars 5 (1974 [Krasnopolsky and Parshev, 1979]) and Phobos (1989 [Blamont and Chassefière, 1993]) measurements of low‐latitude ozone densities at 35–50 km altitude, as well as Hubble space telescope observations of enhanced low‐latitude ozone during the 1995 aphelion of Mars [Clancy et al., this issue].
Mapping Precambrian structures in the Sahara Desert with SIR‐C/X‐SAR radar: The Neoproterozoic Keraf Suture, NE Sudan
Authors: Abdelsalam Mohamed G. , Stern Robert J.
Published: February 2004

A major N‐trending Neoproterozoic suture between composite arc terranes of the Arabian‐Nubian Shield in the east and older crust of the Nile Craton to the west is inferred to trend N‐S close to the Nile in northern Sudan. We used shuttle imaging radar (SIR) C/X synthetic aperture radar (SAR) imagery to find and map these structures in the poorly known Keraf Suture which are not apparent on visible or near IR imagery due to extensive sand cover. L band (23 cm wavelength) radar images best resolve geologic structure; the other frequencies of the SIR‐C/X‐SAR system (X and C bands) permit qualitative evaluation of the effects of surface versus subsurface backscattering. Interpretation of L band images supported by field work indicates that the Keraf Suture is ∼50 km wide and >550 km long, making it the longest basement structure recognized to date in NE Africa. The northern part of the Suture comprises ophiolitic rocks which were thrust westward over tightly folded sediments of the Nile Craton. The southern Keraf Suture is dominated by N‐ and NNW‐trending, left‐lateral strike‐slip faults that affect previously deformed passive margin sediments. Associated with these faults are NE‐trending transpressional folds and a possible transtensional basin. These structures are interpreted to be due to NW‐SE oblique collision between the Arabian‐Nubian Shield and the Nile Craton, as east and west Gondwana collided in the last 150 m.y. of Neoproterozoic time.
Ejecta entrainment by impact‐generated ring vortices: Theory and experiments
Authors: Barnouin‐Jha O. S. , Schultz P. H.
Published: February 2004

Laboratory experiments indicate that an advancing ejecta curtain displaces the atmosphere and creates strong winds that entrain large amounts of fine‐grained ejecta. A theoretical model describing the interaction between an atmosphere and the outward moving ejecta curtain allows one to estimate the magnitude and velocity of the winds in the induced ring vortex in order to establish the importance of such winds at planetary scales. Model estimates for the initial magnitude of the flow within the basal vortex match experimental results within observational uncertainty and reveal that the flow in the ring vortex is turbulent. Successful comparisons of the vortex generation model with experiments allow preliminary applications to be made at much broader scales. Curtain generated winds by a 30‐km‐diameter crater should entrain ejecta particle diameters smaller than 5 mm on Mars, 30 cm on Earth, and 8 m on Venus.
Erosional and depositional patterns associated with the 1993 Missouri River floods inferred from SIR‐C and TOPSAR radar data
Authors: Izenberg N. R. , Arvidson R. E. , Brackett R. A. , Saatchi S. S. , Osburn G. R. , Dohrenwend J.
Published: February 2004

The Missouri River floods of 1993 caused significant and widespread damage to the floodplains between Kansas City and St. Louis. Immediately downstream of levee breaks, flood waters scoured the bottoms. As the floodwaters continued, they spread laterally and deposited massive amounts of sand as crevasse splays on top of agricultural fields. We explore the use of radar interferometry and backscatter data for quantitative estimation of scour and deposition for Jameson Island/Arrow Rock Bottoms and Lisbon Bottoms, two bottoms that were heavily damaged during the floods and subsequently abandoned. Shuttle imaging radar C (SIR‐C) L band (24 cm) HH (horizontally transmitted and horizontally received) radar backscatter data acquired in October 1994 were used together with a distorted Born approximation canopy scattering model to determine that the abundance of natural leafy forbs controlled the magnitude of backscatter for former agricultural fields. Forb areal density was found to be inversely correlated with thickness of sand deposited during the floods, presumably because thick sands prevented roots from reaching nutrient rich, moist bottoms soils. Using the inverse relationship, a lower bound for the mass of sand added was found to be 6.3 million metric tons over the 17 km2 study area. Digital elevation data from topographic synthetic aperture radar (TOPSAR) C band (5.6 cm) interferometric observations acquired in August 1994 were compared to a series of elevation profiles collected on the ground. Vertical errors in TOPSAR were estimated to range from 1 to 2 m, providing enough accuracy to generate an estimate of total mass (4.7 million metric tons) removed during erosion of levees and scour of the bottoms terrains. Net accretion of material to the study areas is consistent with the geologic record of major floods where sediment‐laden floodwaters crested over natural levees, initially scoured into the bottoms, and then deposited sands as crevasse splays as the flows spread out and slowed by frictional dissipation. The addition of artificial levees to the Missouri River system has undoubtedly enhanced flood damage, although quantitative estimation of the degree of enhancement will require additional work.
Vertical distribution of Titan's atmospheric neutral constituents
Authors: Lara L. M. , Lellouch E. , López‐Moreno J. J. , Rodrigo R.
Published: February 2004

The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N2, of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5–2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100–300 km that is 5–20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH4 or (preferably) C2H6. The observed CO2 profile can be modeled by assuming an external source of water of ∼6 × 106 cm−2 s−1. For a nominal CO mixing ratio of 5 × 10−5, the chemical loss of CO exceeds its production by ∼15%, and equilibrium is achieved for CO = 1 × 10−5.
Near‐surface geologic units exposed along Ares Vallis and in adjacent areas: A potential source of sediment at the Mars Pathfinder landing site
Authors: Treiman Allan H.
Published: February 2004

A sequence of layers, bright and dark, is exposed on the walls of canyons, impact craters and mesas throughout the Ares Vallis region, Chryse Planitia, and Xanthe Terra, Mars. Four layers can be seen: two pairs of alternating dark and bright albedo. The upper dark layer forms the top surface of many walls and mesas. The upper dark‐bright pair was stripped as a unit from many streamlined mesas and from the walls of Ares Valles, leaving a bench at the top of the lower dark layer, ∼250 m below the highland surface on streamlined islands and on the walls of Ares Vallis itself. Along Ares Vallis, the scarp between the highlands surface and this bench is commonly angular in plan view (not smoothly curving), suggesting that erosion of the upper dark‐bright pair of layers controlled by planes of weakness, like fractures or joints. These near‐surface layers in the Ares Vallis area have similar thicknesses, colors, and resistances to erosion to layers exposed near the tops of walls in Valles Marineris [Treiman et al., 1995] and may represent the same pedogenic hardpan units. From this correlation, and from analogies with hardpans on Earth, the light‐color layers may be cemented by calcite or gypsum. The dark layers are likely cemented by an iron‐bearing mineral. Mars Pathfinder instruments should permit recognition and useful analyses of hardpan fragments, provided that clean uncoated surfaces are accessible. Even in hardpan‐cemented materials, it should be possible to determine the broad types of lithologies in the Martian highlands. However, detailed geochemical modeling of highland rocks and soils may be compromised by the presence of hardpan cement minerals.
A three‐dimensional azimuthally symmetric model atmosphere for Io: 2. Plasma effect on the surface
Authors: Wong Mau C. , Johnson Robert E.
Published: February 2004

Because SO2 gas in Io's atmosphere is transported poleward where it condenses, the absence of bright polar caps on Io is intriguing. In this paper, we show that the energetic plasma ions measured by the low energy charged particles (LECP) instrument on Voyager carry most of the plasma energy and they deposit sufficient energy in Io's surface to alter the molecular composition and hence the “color” of the surface. We use an extrapolation of the LECP energy flux measured by Voyager, together with the calculated surface condensation rate, to obtain the energy deposited per molecule condensed on the surface in the region above 50° latitude. Because vapor deposited SO2 is spectrally flat above ∼0.4 μm, a small concentration of absorbers can efficiently darken the surface. Based on the laboratory data of Moore [1984], a dose ∼0.5–5 eV per molecule at 88 K produces significant color change in vapor‐deposited SO2. At the poles the energy deposited is ∼10 eV per molecule condensed and at 50° latitude it is ∼1 eV per molecule condensed so that these regions will not appear as a bright frost. In contrast, the fractional deposition rate in this region of photodissociation products is small, and the sputter removal rate of SO2 is slower than the chemical alteration rate.
The Mars Pathfinder Mission
Authors: Golombek Matthew P.
Published: February 2004

Mars Pathfinder, one of the first Discovery‐class missions (quick, low‐cost projects with focused science objectives), will land a single spacecraft with a microrover and several instruments on the surface of Mars in 1997. Pathfinder will be the first mission to use a rover, carrying a chemical analysis instrument, to characterize the rocks and soils in a landing area over hundreds of square meters on Mars, which will provide a calibration point or “ground truth” for orbital remote sensing observations. In addition to the rover, which also performs a number of technology experiments, Pathfinder carries three science instruments: a stereoscopic imager with spectral filters on an extendable mast, an alpha proton X ray spectrometer, and an atmospheric structure instrument/meteorology package. The instruments, the rover technology experiments, and the telemetry system will allow investigations of the surface morphology and geology at submeter to a hundred meters scale, the petrology and geochemistry of rocks and soils, the magnetic properties of dust, soil mechanics and properties, a variety of atmospheric investigations, and the rotational and orbital dynamics of Mars. Landing downstream from the mouth of a giant catastrophic outflow channel, Ares Vallis at 19.5°N, 32.8°W, offers the potential of identifying and analyzing a wide variety of crustal materials, from the ancient heavily cratered terrain, intermediate‐aged ridged plains, and reworked channel deposits, thus allowing first‐order scientific investigations of the early differentiation and evolution of the crust, the development of weathering products, and the early environments and conditions on Mars.
Selection of the Mars Pathfinder landing site
Authors: Golombek M. P. , Cook R. A. , Moore H. J. , Parker T. J.
Published: February 2004

The Mars Pathfinder spacecraft will land on a depositional fan near the mouth of the catastrophic outflow channel, Ares Vallis (19.5°N, 32.8°W). This site offers the prospect of analyzing a variety of rock types from the ancient cratered highlands, intermediate‐age ridged plains, and reworked channel deposits. Analyses of these rocks by Pathfinder instruments will enable first‐order scientific questions to be addressed, such as differentiation of the crust, the development of weathering products, and the nature of the early environment, as well as their subsequent evolution on Mars. Constraints imposed by (1) spacecraft and rover designs (which are robust), (2) entry, descent, and landing, (3) scientific potential at various sites, and (4) safety were important considerations in site selection. Engineering constraints require a 70 km by 200 km smooth, flat (low slopes) area located between 10° and 20°N that is below 0 km elevation, with average radar reflectivity, little dust, and moderate rock abundance. Three regions on Mars are between 10° and 20°N and below 0 km elevation: Chryse, Amazonis, and Isidis‐Elysium. Science considerations favor sites at the mouths of outflow channels (grab bag sites offer an assay of rock types on Mars), highland sites (early crustal differentiation and climate), and sites covered with dark (unoxidized) material. Sites are considered safe if they are clearly below 0 km elevation, appear acceptably free of hazards in high‐resolution (<50 m/pixel) Viking orbiter images and have acceptable reflectivity and roughness at radar wavelengths, thermal inertia, rock abundance, red to violet ratio, and albedo. Recent 3.5‐cm wavelength radar observations were used to verify elevation, reflectivity, and roughness within the landing ellipses. Three sites meet all of these criteria: Ares Vallis, Tritonis Lacus, and Isidis. Although Isidis appears to be safer than Tritonis and Ares, the greater scientific potential at Ares Vallis resulted in its selection. Comparisons of the Grand Coulee (channel) and the depositional Ephrata Fan of the Channeled Scabland in eastern Washington, with Ares Vallis and its depositional fan also suggest the Ares Vallis landing site is safe and scientifically interesting.
The atmosphere structure and meteorology instrument on the Mars Pathfinder lander
Authors: Seiff Alvin , Tillman James E. , Murphy James R. , Schofield John T. , Crisp David , Barnes Jeffrey R. , LaBaw Clayton , Mahoney Colin , Mihalov John D. , Wilson Gregory R. , Haberle Robert
Published: February 2004

An instrument on the Pathfinder lander has been designed to measure the structure of Mars' atmosphere during spacecraft entry and descent from ∼150 km altitude to the surface, and to measure meteorological parameters after landing for the landed duration of the mission. This is specified to be nominally 30 Mars days but potentially up to 1 Earth year. Landed sensors will measure surface level pressure; temperatures at 0.25, 0.5, and 1.0 m above the surface; and wind speed and direction at a height of 1.1 m. These sensors are mounted on a slender mast about 1.63 m from the center of the Lander base petal so as to avoid flow disturbance and thermal contamination insofar as possible. Wind sensing is most sensitive at velocities <20 m s−1 but can resolve speeds up to 50 m s−1, with directional accuracy ∼10°. A key point of interest in atmosphere structure is comparison with profiles obtained by Viking 20 years ago, to evaluate changes predicted to occur with solar activity and with dust loading of the atmosphere. The proximity of the landing site to that of Viking Lander 1 will permit comparison with and extension of the earlier lander data, which were taken over a period of more than 3 Mars years. The improved sensitivity of the lander instruments will also permit investigation of many new phenomena.
Measurement of H2O adsorption under Mars‐like conditions: Effects of adsorbent heterogeneity
Authors: Zent A. P. , Quinn R. C.
Published: February 2004

New measurements of the adsorption of H2O onto terrestrial materials, under Mars‐like conditions, disagree with predictions made on the basis of the most frequently used adsorption isotherms. We report here on additional measurements, which confirm that previous estimates of H2O adsorptive coverage of Martian surface minerals were too high. This discrepancy is a result of extrapolating an empirical expression for adsorption that can be shown to contain unrealistic assumptions. New isotherms, developed to describe adsorption on heterogeneous surfaces, are well suited to describing adsorption at Mars‐like conditions. The new isotherms predict that the adsorptive capacity of the regolith is lower than previously thought, if normalized for available surface area. The adsorptive behavior of the regolith materials, to first order, can be predicted on the basis of their specific surface area, without regard to composition. For a given specific surface area of a few tens of square meters per gram, we show that vapor diffusion is more rapid, and that predawn, near‐surface ground ice must occur more commonly than previously thought. We find that the presence of a few percent of smectites in the upper few centimeters of the regolith can dominate the adsorption cycle.
Determination of the chemical composition of Martian soil and rocks: The alpha proton X ray spectrometer
Authors: Rieder R. , Wänke H. , Economou T. , Turkevich A.
Published: February 2004

The alpha proton X ray spectrometer (APXS) for the Mars Pathfinder mission is designed to provide a complete and detailed analysis of chemical elements in Martian soil and rocks near the landing site. The APXS instrument is carried on the Pathfinder Microrover, which will provide transportation to places of interest on the Martian surface. It consists of a complex sensor head, mounted on a simple but sophisticated APXS deployment mechanism (ADM) outside the warm electronics box (WEB) of the Microrover, and the instrument electronics, mounted inside the WEB. The ADM permits the instrument sensor head to be placed against soil and rock samples in arbitrary positions, ranging from horizontal to vertical, in order to perform in situ analysis. The possibility to transport the APXS to an arbitrary location, preselected on Earth, and to perform in situ analysis there, constitutes one of the most exciting aspects of the Pathfinder mission. The principle of the APXS technique is based on three interactions of alpha particles from a radioisotope source with matter: simple Rutherford backscattering, production of protons from (α, p) reactions on light elements, and generation of characteristic X rays upon recombination of atomic shell vacancies created by a bombardment. Measurement of the intensities and energy distributions of these three components yields information on the abundance of chemical elements in the sample. In terms of sensitivity and selectivity, data are partly redundant and partly complementary: alpha backscattering is superior for light elements (C, O), while proton emission is mainly sensitive to Na, Mg, Al, Si, S, and X ray emission is more sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H and He) present at concentration levels above typically a fraction of 1%.
Distribution of otoliths in surficial sediments of the U.S. Atlantic Continental Shelf and slope and potential for reconstructing Holocene fish stocks
Authors: Elder Kathryn L. , Jones Glenn A. , Bolz George
Published: February 2004

We examined more than 1100 surface sediment samples from the Atlantic continental margin of the United States to determine the feasibility of using fossil fish otoliths as diagnostic tools in reconstructing paleoenvironments and latitudinal distribution of fish stocks during the Holocene. Although 63% of the 1107 samples collected were from shelf areas (<140 m), the total number of shelf‐derived otoliths represents only 0.3% of the entire sampled assemblage. The majority of otoliths occurred on the continental slope (400‐2000 m), with a maximum concentration in sediments at 500 to 600‐m water depth. Otoliths of the most commonly occurring species, Ceratoscopelus maderensis, exhibit a marked distributional boundary just south of Cape Hatteras, North Carolina (33°N), which mimics the distribution of their living counterparts. North of this boundary, C. maderensis constitutes greater than 70% of the preserved otolith assemblage, whereas more southerly regions contain no otoliths of this species. Although C. maderensis typically migrates diurnally over a depth of 300‐600 m, otoliths taken from live‐captured C. maderensis exhibit Δ14C values comparable to that of the dissolved inorganic carbon (DIC) of surface seawater in the study area. Accelerator mass spectrometry radiocarbon analyses of cooccurring otoliths and planktonic foraminifera from a sediment core collected south of Martha’s Vineyard (40°15′N 70°51′W, 265 m) demonstrate temporal concordance throughout the Holocene. Otoliths appear to be viable, underutilized paleoceanographic tools. Specimens are found in sufficient abundance to permit temporal reconstructions of the distribution of C. maderensis and potentially several other icthyospecies along the U.S. Atlantic continental margin.
Messinian deep‐water turbidites and glacioeustatic sea‐level changes in the North Atlantic: Linkage to the Mediterranean salinity crisis
Authors: Zhang Jijun , Scott David B.
Published: February 2004

Our benthic foraminiferal data clearly indicate eight layers of deep‐water turbidites during the Messinian (MTL 1‐8) and one in the early Pliocene (PTL 1) in Ocean Drilling Program Leg 105, Site 646B. These deep‐water tuibidite deposits are characterized by highly concentrated agglutinated marsh benthic foraminifera (e.g., Trochammina cf. squamata, Ammotium sp. A, Miliammina fusca), rounded quartz, polished thick‐walled benthic foraminifera, wood fragments, plant seeds, plant fruit, and highly concentrated mica and are interbedded with sediments containing deep‐water benthic faunas. We suggest these turbidites deposited during sea‐level low stands (∼80–100 m below sea level), and their ages are tentatively correlated to 6.59, 6.22, 6.01, 5.89, 5.75, 5.7, 5.65, 5.60, and 5.55 Ma, respectively, based on the Messinian oxygen isotope enrichments at Site 552A of Deep Sea Drilling Project Leg 81. The turbidites layers during the late Messinian, coeval with frequent climate changes suggested by six oxygen enrichment excursions of Site 552A, may have been in part linked to the late Messinian evaporite deposits in the Mediterranean Basin. The most profound climate changes at 5.75 and 5.55 Ma may have been related to the Lower and Upper Evaporites in the Mediterranean Basin.

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS .AGU.ORG, (LOGIN toAGU's FTP account using ANONYMOUS as the username and GUESTas the password. Go to the right directory by typing CD APEND. TypeLS to see what files are available. Type GET and the name of the file toget it. Finally, type EXIT to leave the system. (Paper 96PA00572,Messinian deep‐water turbidites and glacioeustatic sea‐level changes inthe North Atlantic: Linkage to the Mediterranean Salinity Crisis, JijunZhang and David B. Scott). Diskette may be ordered from AmericanGeophysical Union, 2000 Florida Avenue, N.W., Washington, D.C.20009; $15.00. Payment must accompany order.
Geochemical consequences of increased Late Cenozoic weathering rates and the global CO2 balance since 100 Ma
Authors: Compton John S. , Mallinson David J.
Published: February 2004

Large imbalances in the relative net CO2 flux over the last 100 m.y. are obtained from independently derived estimates of CO2 uptake by weathering and organic carbon burial and of CO2 outgassing assumed proportional to the mean mid‐ocean ridge (MOR) spreading and mantle plume production (MPP) rates. Increasing the river flux of Ca, Mg, P, and C by 50% since 30 Ma to parallel the proposed increase in the Sr river flux needed to explain the Sr isotopic evolution of seawater [Richter et al., 1992] has a large impact on the global carbon cycle by reducing the excess net CO2 imbalance in the late Cenozoic. CO2 uptake is calculated by assuming that 19% of the Ca river flux and 60% of the Mg river flux are derived from weathering silicates and that Mg is removed in the oceans by 1:1 molar exchange reactions with calcsilicates. An increase in the Mg river flux combined with an overall decrease in the MOR spreading rate predicts as much as a factor of 2 increase in the seawater Mg concentration since 100 Ma. The net organic carbon burial flux (burial minus weathering) largely reflects changes in the bulk carbonate δ13C record and, to a lesser extent, variations in the net carbonate burial flux and mean δ13C value of the carbonate weathering flux. Organic carbon burial efficiency is markedly less than that of P from the middle middle Miocene to the present because of early diagenetic transfer of organic P to francolite and oxidation of organic carbon during reworking of margin sediments by sea level fluctuations. The large CO2 imbalance of the Late Cretaceous requires significantly less carbonate subduction or greater weathering rates. A CO2 balance can be achieved in the Miocene by decreasing the proposed increase in river fluxes by up to half or by variably increasing the amount of carbonate subducted. Variations in shallow water carbonate burial, and hence the global carbonate record, are currently too poorly known to differentiate between the above possibilities. The decrease in the excess CO2 flux generally parallels the trend to cooler climate inferred from the δ18O record, but periods of large CO2 imbalance generally precede δ18O shifts by several million years.
Evaluation of the Plio‐Pleistocene astronomical timescale
Authors: Lourens L. J. , Antonarakou A. , Hilgen F. J. , Van Hoof A. A. M. , Vergnaud‐Grazzini C. , Zachariasse W. J.
Published: February 2004

An astronomically calibrated timescale has recently been established [Hilgen, 1991a, b] for the Pliocene and earliest Pleistocene based on the correlation of dominantly precession controlled sedimentary cycles (sapropels and carbonate cycles) in Mediterranean marine sequences to the precession time series of the astronomical solution of Berger and Loutre [1991] (hereinafter referred to as Ber90). Here we evaluate the accuracy of this timescale by (1) comparing the sedimentary cycle patterns with 65°N summer insolation time series of different astronomical solutions and (2) a cross‐spectral comparison between the obliquity‐related components in the 65°N summer insolation curves and high‐resolution paleoclimatic records derived from the same sections used to construct the timescale. Our results show that the carbonate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previously proposed. Application of the astronomical solution of Laskar [1990] (hereinafter referred to as La90) with present‐day values for the dynamical ellipticity of the Earth and tidal dissipation by the Sun and Moon results in the best fit with the geological record, indicating that this solution is the most accurate from a geological point of view. Application of Ber90, or La90 solutions with dynamical ellipticity values smaller or larger than the present‐day value, results in a less obvious fit with the geological record. This implies that the change in the planetary shape of the Earth associated with ice loading and unloading near the poles during the last 5.3 million years was too small to drive the precession into resonance with the perturbation term, s6−g6+g5, of Jupiter and Saturn. Our new timescale results in a slight but significant modification of all ages of the sedimentary cycles, bioevents, reversal boundaries, chronostratigraphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the astronomical timescales of ODP site 846 [Shackleton et al., 1995a, b] and ODP site 659 [Tiedemann et al., 1994] indicates that all obliquity‐related glacial cycles prior to ∼4.7 Ma in ODP sites 659 and 846 should be correlated with one obliquity cycle older than previously proposed.
Can increased poleward oceanic heat flux explain the warm Cretaceous climate?
Authors: Schmidt Gavin A. , Mysak Lawrence A.
Published: February 2004

The poleward transport of heat in the mid‐Cretaceous (100 Ma) is examined using an idealized coupled ocean‐atmosphere model. The oceanic component consists of two zonally averaged basins representing the proto‐Pacific and proto‐Indian oceans and models the dynamics of the meridional thermohaline circulation. The atmospheric component is a simple energy and moisture balance model which includes the diffusive meridional transport of sensible heat and moisture. The ocean model is spun up with a variety of plausible Cretaceous surface temperature and salinity profiles, and a consistent atmosphere is objectively derived based on the resultant sea surface temperature and the surface heat and freshwater fluxes. The coupled model does not exhibit climate drift. Multiple equilibria of the coupled model are found that break the initial symmetry of the ocean circulation; several of these equilibria have one‐cell (northern or southern sinking) thermohaline circulation patterns. Two main classes of circulation are found: circulations where the densest water is relatively cool and is formed at the polar latitudes and circulations where the densest water is warm, but quite saline, and the strongest sinking occurs at the tropics. In all cases, significant amounts of warm, saline bottom water are formed in the proto‐Indian basin which modify the deepwater characteristics in the larger (proto‐Pacific) basin. Temperatures in the deep ocean are warm, 10°–17°C, in agreement with benthic foraminiferal oxygen isotope data. The poleward transport of heat in the modeled Cretaceous oceans is larger than in some comparable models of the present day thermohaline circulation and significantly larger than estimates of similar processes in the present‐day ocean. It is consistently larger in the polar sinking cases when compared with that seen in the tropical sinking cases, but this represents an increase of only 10%. The largest increase over present‐day model transports is in the atmospheric latent heat transport, where an increased hydrological cycle (especially in the tropical sinking cases) contributes up to an extra 1 PW of poleward heat transport. Better constraints on the oceanic deepwater circulation during this period are necessary before the meridional circulation can be unambiguously described.