bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the Martian seasonal water cycle
Authors: Houben Howard , Haberle Robert M. , Young Richard E. , Zent Aaron P.
Published: February 2004

Ever since the observations of Percival Lowell, the annual cycle of Martian water has been a fascinating topic in planetary exploration. Observations by the Viking Orbiter, supplemented by Earth‐based microwave and infrared observations, have given us a reasonable picture of this cycle. We are now also able to model the cycle using our Mars Climate Model, a simplified atmospheric general circulation model designed specifically for this purpose. We find that a thin adsorbing layer of the Martian regolith plays a fundamental role in the water cycle, limiting the lower atmospheric relative humidity and preventing the formation of widespread ice deposits at low latitudes. We are thus able to estimate a large‐scale average value of the specific soil surface area of this regolith. Water which evaporates from the permanent north polar ice cap during summer is returned by a process of repeated evaporation and precipitation on the retreating seasonal cap the following spring, so that the global inventory of water outside the polar caps ranges within narrow limits. (There is a small net annual deposition of water ice at the south polar cap which is always at dry ice temperatures.) If ice on the residual south polar cap is exposed during the summer, it rapidly sublimes, generating vapor amounts similar to those observed in northern summer. Recovery to normal dry conditions in the southern atmosphere occurs very rapidly in the next year. Such an event could explain the otherwise anomalous Earth‐based pre‐Viking observations of a wet southern summer. If southern ice deposits at lower latitudes are exposed, the vapor can be transferred irreversibly through the strong Hadley cell to the north polar cap. We therefore speculate that the asymmetry of Mars' current orbit is responsible for the asymmetry of the present water distribution (with extensive permanent water ice deposits located only in the colder, aphelion summer, northern hemisphere).
Remote X‐ray spectrometry for NEAR and future missions: Modeling and analyzing X‐ray production from source to surface
Authors: Clark P. E. , Trombka J. I.
Published: February 2004

Remote X‐ray spectrometry will play a key role in the geochemical exploration of solar system bodies, provided the methodology for data analysis efficiently detects and removes solar source and flight trajectory‐induced geometric variations. In this paper, we discuss the nature of such variations expected for missions to an asteroid, the Moon, and Mercury. An effective means of removing the effects of solar variability from surface measurements, as indicated by the agreement between theoretical models presented here and Apollo X‐ray observations, is also discussed. We calculate X‐ray spectra anticipated for these targets using probable surface compositions, solar outputs, and flight trajectories. Generally, the spectra show three distinctive regions where line intensities are clearly correlated with surface abundances: a high‐energy Fe region, a moderate‐energy Ca region, and a low‐energy region which contains Mg, Al, and Si lines. In addition, we calculate anticipated integration times required for acceptable levels of certainty and estimate spatial resolutions achievable for those integration times for elements Mg, Al, Si, S, Ca, Ti, and Fe. Required integration times are lower (on the order of minutes or even seconds) and achievable spatial resolutions improved (on the order of kilometers) for the lower energy lines and for periods of higher solar activity. Using the Near Earth Asteroid Rendezvous (NEAR) mission to asteroid 433 Eros as an example, we describe a recommended approach for analysis of X‐ray measurements based on our findings. Most importantly, we clearly demonstrate that major scientific goals for future exploration of asteroids, Mercury, and the Moon can be met by obtaining remote orbital X‐ray measurements of these bodies.
Meteorological predictions for the Mars Pathfinder lander
Authors: Haberle Robert M. , Barnes Jeffrey R. , Murphy James R. , Joshi Manoj M. , Schaeffer James
Published: February 2004

The NASA Ames Mars general circulation and boundary layer models are used as guides to forecast the meteorological environment of the Pathfinder lander site. Based on these models we predict that for a Viking‐like atmospheric dust loading, significant vertical wave structure will be seen in the entry temperature profile above 50 km. Temperatures in this region will oscillate by up to 30 K about a mean value of ∼145 K. At the surface during the primary mission, winds are expected to rotate clockwise during the day at 2–10 m/s, while air temperatures will range from overnight lows in the mid 180s K to daytime highs near 250 K. If the atmosphere is dust free at arrival, as appears possible from recent Earth‐based observations, then we expect a cooler upper atmosphere (130–140 K) with less wave structure during entry, weaker surface winds, and a slight increase in near surface air temperatures. Regardless of dust loading, we predict that the daily averaged surface pressure will reach its annual minimum about 15–20 sols after landing. During the extended mission, the basic character of Pathfinder's meteorology will change from that of a summertime quasi‐regular regime to that of a wintertime highly variable regime. This transition is predicted to occur by sol 60. The strongest winds (30 m/s) and lowest daily‐averaged temperatures (∼200 K) will be recorded during early winter.
Thermal infrared emission spectroscopy of anhydrous carbonates
Authors: Lane Melissa D. , Christensen Philip R.
Published: February 2004

The objective of this study is to present the thermal emission spectra of various anhydrous, calcite‐ and dolomite‐series carbonate minerals to illustrate the effect of the structural cation (Ca2+, Mg2+, Fe2+, Mn2+, Zn2+) on shifting the positions of the carbonate absorption bands. All of the carbonate mineral emission spectra included in this study exhibit three absorption features related to three specific vibrational modes of the carbonate anion (CO32−). These anion vibrations include the out‐of‐plane bend, the asymmetric stretch, and the in‐plane bend (i.e., the v2, v3, and v4 modes, respectively). The positions of the absorption‐band emissivity minima are unique for each carbonate chemistry and are thus diagnostic of mineralogy. The average v2, v3 and v4 wavenumber positions for the various carbonate minerals are as follows: calcite (CaCO3) 883, 1523, and 712 cm−1; magnesite (MgCO3) 901, 1572, and 748 cm−1; siderite (FeCO3) 876, 1523, and 736 cm−1; rhodochrosite (MnCO3) 877, 1535, and 726 cm−1; smithsonite (ZnCO3) 882, 1509, and 742 cm−1; dolomite (CaMg(CO3)2) 894, 1547, and 728 cm−1; and kutnahorite (CaMn(CO3)2) 882, 1526, and 716 cm−1. Carbonates as a general mineral class crystallize in a variety of geological environments; however, each specific carbonate mineralogy typically is limited to a narrow range of geologic settings in which it forms. Thermal infrared emission data to be received from the Mars Global Surveyor thermal emission spectrometer will contain the spectral signature of carbonates if they are present above the detectibility limit on the surface of Mars. This study presents the spectral information necessary to recognize carbonate as a mineral class as well as identify the specific type of carbonate from thermal emissivity data. Knowledge of distinct carbonate mineralogy will be useful for interpreting the environmental conditions that were present on Mars during the carbonate formation. The result of this study is that the major carbonate mineral species (calcite, dolomite, magnesite, siderite, and smithsonite) can be identified from thermal infrared emission data, provided moderate (10 cm−1) spectral sampling. Because of the similarity of absorption band positions between kutnahorite and calcite, high (2 cm−1) spectral sampling is required to distinguish kutnahorite. Moderate spectral sampling data are also sufficient to determine the amount of Mg and Fe in Mg‐Fe solid solution minerals to within 3–5% of the cation abundance.
The neutron signature of Mercury's volatile polar deposits
Authors: Feldman W. C. , Barraclough B. L. , Hansen C. J. , Sprague A. L.
Published: February 2004

Neutron leakage flux spectra are simulated to explore whether the nature of volatile deposits residing within the permanently shaded craters near the poles of Mercury can be identified using state‐of‐the‐art space neutron detection techniques deployed aboard a flyby mission to Mercury. The most sensitive method for discriminating between H2O (ice)‐rich and S‐rich deposits involves measurement of the flux of epithermal neutrons. We conclude that a single flyby over crater Chao Meng‐Fu at an altitude of 200 km is sufficient to detect a circular deposit rich in elemental sulfur at the 2.6σ statistical significance level, and that a similar deposit rich in H2O can be identified at the 7.8σ level. Whereas identification of a S‐rich deposit is not unique but compelling because of auxiliary physical arguments, that of an H2O deposit is unique.
Rate coefficients for reactions of ethynyl radical (C2H) with HCN and CH3CN: Implications for the formation of complex nitriles on Titan
Authors: Hoobler Ray J. , Leone Stephen R.
Published: February 2004

Rate coefficients for the reactions of C2H+HCN→products and C2H+CH3CN→products have been measured over the temperature range 262–360 K. These experiments represent an ongoing effort to accurately measure reaction rate coefficients of the ethynyl radical, C2H, relevant to planetary atmospheres such as those of Jupiter and Saturn and its satellite Titan. Laser photolysis of C2H2 is used to produce C2H, and transient infrared laser absorption is employed to measure the decay of C2H to obtain the subsequent reaction rates in a transverse flow cell. Rate constants for the reaction C2H+HCN→products are found to increase significantly with increasing temperature and are measured to be (3.9–6.2)×10−13 cm3 molecules−1 s−1 over the temperature range of 297–360 K. The rate constants for the reaction C2H+CH3CN→products are also found to increase substantially with increasing temperature and are measured to be (1.0–2.1)×10−12 cm3 molecules−1 s−1 over the temperature range of 262–360 K. For the reaction C2H+HCN→products, ab initio calculations of transition state structures are used to infer that the major products form via an addition/elimination pathway. The measured rate constants for the reaction of C2H+HCN→products are significantly smaller than values currently employed in photochemical models of Titan, which will affect the HC3N distribution.
Volcanic degassing of argon and helium and the history of crustal production on Venus
Authors: Namiki Noriyuki , Solomon Sean C.
Published: February 2004

We develop a new methodology linking 40Ar and 4He degassing to crustal production on Venus in order to examine two different scenarios for the history of magmatism on that planet, both consistent with recent analyses of the cratering record of the Venus surface. The first scenario includes episodic global resurfacing events and modest levels of magmatism between such events, while the second scenario invokes different rates of steady magmatism before and after a given transition time. Our degassing models include distinct mantle, crustal, and atmospheric reservoirs. Diffusive transfer of 40Ar and 4He from the crust to the atmosphere is also taken into account. The 40Ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history. In contrast, helium escapes from the planetary atmosphere within a characteristic residence time of 200 Myr to 1.8 Gyr, so the present atmospheric 4He abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second. Unfortunately, large uncertainties in mineral partition coefficients and in the Ar and He mixing ratios in the lower atmosphere of Venus presently prevent the use of our degassing models to distinguish among crustal formation history models. We therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history. As an important step in the development of the Ar degassing model, the K budget in the bulk silicate fraction of Venus is re‐examined on the basis of Venera and Vega γ ray measurements of K, U, and Th concentrations in surface materials. For U and Th concentrations in the bulk silicate planet of 18–29 ppb and 64–94 ppb, respectively, the degree of mantle melting that formed the surface materials at the Venera 9 and 10 and Vega 1 and 2 landing sites is calculated to lie between 0.02 and 0.16. The degree of melting calculated for materials at each landing site is combined with measurements of surface K concentration to estimate the K concentration in the bulk silicate portion of the planet at between 100 and 300 ppm. These results suggest that Venus and Earth have similar heat production.
New insights into Coronae evolution: Mapping on Venus
Authors: Copp Duncan L. , Guest John E. , Stofan Ellen R.
Published: February 2004

Coronae are geologically and geophysically important features on Venus, because they are thought to contribute to planetary heat loss. It is thus necessary to understand their role in space and time in the evolution of Venus. Detailed mapping of five coronae in Guinevere and Sedna Planitia illustrates that although previous models invoked initial uplift, not all coronae can be explained in such a simple way. We show that the formation of corona annuli can be multistaged and that the position of the annulus does not always coincide with the main topographic ridges and troughs that outline the feature. The magnitude and timing of volcanism are not necessarily the same at each corona, and coronae can have long and complex histories, in contrast with the stratigraphic relations suggested by other workers. We demonstrate that coronae do not all have the same relative ages with respect to adjacent units. These results suggest that corona formation was (is) not confined to a single time period in the history of Venus and that detailed mapping is a reliable method of establishing the relative timing of corona formation.
Ice flow and rock glaciers on Mars
Authors: Colaprete Anthony , Jakosky Bruce M.
Published: February 2004

Several geologic features suggest the presence of rock glaciers on the surface of Mars. These features include lobate debris aprons, concentric crater fill and lineated valley fill. The lateral extent of these rock glaciers can range from 5 km to over 20 km. A simple time‐marching model is developed and used here to demonstrate the ability of ice and ice‐rock mixtures to flow under Martian conditions. For temperatures lower than about 220 K, even pure ice becomes too rigid to flow and a glacier 5 km long could not have formed. Results from this model indicate temperatures 20 to 40 K higher than present average midlatitude temperatures (210 K), an ice content of no less than 80%, and a net accumulation rate of at least 1 cm year−1 are required to produce rock glaciers of the size seen on Mars.
The influence of temperature on the spectra of the A‐asteroids and implications for their silicate chemistry
Authors: Lucey Paul G. , Keil Klaus , Whitely Robert
Published: February 2004

Previously obtained high‐quality near‐infrared spectra of asteroids revealed a class of objects, the A‐asteroids, with reflectance spectra similar to the mineral olivine. We compare the asteroid spectra to a compositional series of powdered olivines varying from Fe‐rich to Fe‐poor. With continua removed and absorption band strengths normalized, the A‐asteroid spectra are most similar, but not identical, to the spectrum of a very low‐Fe olivine suggesting the asteroidal olivine to be Fe‐poor. The spectra of the laboratory samples differ systematically from those of the A‐asteroids. The spectra of the A‐asteroids are much more similar to the spectra of low‐Fe olivines obtained at low temperatures reasonable for the heliocentric distance of the main asteroid belt. We conclude that the spectral differences between ambient temperature laboratory measurements of olivines and the spectra of A‐asteroids are mainly due to temperature‐dependent spectral effects. This is the first detection of these effects in the spectra of asteroids. We further conclude that the A‐asteroid silicate fraction is olivine with a composition less than Fa10±5. The low iron content of these asteroid silicates indicates the objects' origin in reduced parent bodies with compositions unlike the ordinary chondrites. The combination of monomineralic olivine silicate fraction and very low iron olivines is solely represented in the meteorite collection by the main group of pallasite meteorites. The silicate fraction of other olivine‐rich meteorites is too Fe‐rich to be represented on the A‐asteroids. This work strengthens the link between the pallasite meteorites and the A‐type asteroids.
Latitudinal control of astronomical forcing parameters on the high‐resolution clay Mineral distribution in the 45°–60° N range in the North Atlantic Ocean during the past 300,000 years
Authors: Bout‐Roumazeilles Viviane , Debrabant Pierre , Labeyrie Laurent , Chamley Hervé , Cortijo Elsa
Published: February 2004

The clay mineralogy of four 5.5‐ to 13.5‐m‐long cores sampled between 45° and 60°N in the North Atlantic Ocean has been investigated at high latitudes within a well‐constrained chronostratigraphic scale. Cross‐correlation spectral analyses have been performed on both clay mineral and δ18O planktonic records. Detrital clay minerals display strong signals which are coherent with the δ18O record, within the three main Milankovitch frequency bands (eccentricity, obliquity, and precession). The climatic control on clay mineral sedimentation largely depends on the latitudinal location of the sediment cores. The 100,000‐year signal occurs as a uniformly acting factor, whereas the 41,000‐year signal dominates clay sedimentation at high latitudes and the 23,000‐year signal dominates at midlatitudes. We suggest that the latitudinal variations of the orbital forcing on the detrital clay mineral distribution in the North Atlantic Ocean not only result from climatic control of the intensity of physical and chemical weathering, but also from latitudinal control on the detrital clay supply linked to influences of the high‐latitude wind‐driven and midlatitude ocean‐driven transportation processes, respectively.
Strontium/calcium ratios in modern porites corals From the Great Barrier Reef as a proxy for sea surface temperature: Calibration of the thermometer and monitoring of ENSO
Authors: Alibert Chantal , McCulloch Malcolm T.
Published: February 2004

High‐precision Sr/Ca ratios are reported for Porites corals from Davies Reef, at ∼19°S on the midshelf of the Great Barrier Reef, Australia. Skeletal growth parameters were also determined from coral slabs using X ray photographs and γ‐densitometry measurements. A calibration of the Sr/Ca thermometer has been obtained, using in situ measured sea surface temperatures, which gives Sr/Ca × 10³ = 10.48 (±0.01) − 0.0615 (±0.0004) × T, whereT is in °C. This calibration applies to different species of Porites and a large range of annual extension and calcification rates, provided sampling follows a major growth axis. Biases of 1°–2°C have been observed in the low‐density margins where growth is hampered by interfering fans of corallites. The analytical and the long‐term reproducibility of Sr/Ca‐derived temperatures are both better than ±0.3°C (2σ). Coral growth parameters such as the extension rate or density generally appear to respond to environmental changes such as rainfall but do not affect the Sr/Ca partitioning between coralline aragonite and seawater. No correlation has been found between calcification rate and Sr/Ca variations, consistent with Sr/Ca being mainly controlled by temperature. In the Great Barrier Reef, the interannual variability in SST is primarily related to the El Niño‐Southern Oscillation phenomenon and is typically marked by a cold anomaly during the winter preceding the mature warm phase. Instrumental records show that the surface waters of the Coral Sea around 19°S have experienced a significant cooling associated with the El Niños of 1965, 1972, and 1982‐1983 which the Porites corals record faithfully. The coral‐derived temperatures for the period from 1965 to 1993 show an increase of 1.3°C, which has occurred mainly since 1979. This warming is larger in magnitude but consistent with the trend for the southern hemisphere land and marine temperature records, which shows an increase of around 0.4°C.
Tropical Pacific Ocean thermocline depth reconstructions for the Last Glacial Maximum
Authors: Andreasen D. J. , Ravelo A. C.
Published: February 2004

We evaluate the relationship between ten surface ocean (0–300 m) hydrographic parameters and the spatial distribution of factor‐analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24°N–24°S) for core tops <3800 m. The spatial distribution of the first three faunal factor loadings (88% of the variance) are most highly correlated to subsurface variability (mixed layer depth, thermocline depth) and resistant species percent (RSP). However, RSP is not related to dissolution but is related to thermocline depth. Factor I (mixed layer species G. glutinata, G. ruber) and factor III (G. ruber) can be distinguished from each other by low abundances of G. glutinata in factor III. Both assemblages spatially comprise the deep mixed layer region of the western tropical and equatorial Pacific Ocean, but are associated with distinct water mass properties. A combination of Factor I and III loadings shows a higher correlation to thermocline depth (R² = 0.70). Factor II loadings (dominated by thermocline dwelling species N. dutertrei) are most significantly correlated with the thermocline depth (R² = 0.73). Most factors show only marginally significant correlation to sea surface temperatures (SSTs), indicating that SST is not the primary forcing factor on the planktonic foraminiferal species distributions in the tropical Pacific. A new transfer function was calculated to predict tropical Pacific thermocline depth from planktonic foraminifera abundances using the Imbrie‐Kipp Method (IKM) (standard deviation of residuals ±22 m (1σ)). An additional ±5‐m error is attributed to low species counts in the core top database. The modern analog technique (MAT) was also used to predict thermocline depth (standard deviation of residuals ±21 m). While last glacial maximum (LGM) thermocline depth changes by IKM and MAT were generally within error, estimated changes were geographically uniform, suggesting an oceanographic response to climate forcing. We estimate that the thermocline depth of the LGM was shallower than present by ∼20 m south of 8°S, possibly due to a shift in the South Pacific anticyclone to the northeast. Both the IKM and MAT estimate a steeper east‐west thermocline slope along the equator, suggesting that zonal wind stress (Walker circulation) was intensified during the LGM. Collectively, the thermocline estimates for the LGM suggest an equatorward compression of the climate zones in both hemispheres.
Trade wind forcing of upwelling, seasonally, and Heinrich events as a response to sub‐Milankovitch climate variability
Authors: Little Mark G. , Schneider Ralph R. , Kroon Dick , Price Brian , Summerhayes Colin P. , Segl Monika
Published: February 2004

Planktonic foraminifera recovered from two cores in the northern Benguela upwelling system reveal a history of rapid events with a variability at sub‐Milankovitch frequencies during the last 140 kyr. The “cold‐water” planktonic foraminifer, left coiling Neogloboquadrina pachyderma (Ehrenberg), shows rapid fluctuations in relative abundance, indicating changes in upwelling intensity. The periods of high abundance in left coiling N. pachyderma are referred to as “PS events” (pachyderma sinistral) and indicate increased intensity and zonality of the South Atlantic trade winds controlling the Benguela upwelling system. The good correlation between PS events, the North Atlantic Heinrich events, and the Dansgaard‐Oeschger cycles from the Greenland Ice Sheet Program (GISP2) ice core suggests large‐scale global oceanographic or climatic teleconnections between the South and North Atlantic via the trade wind system. The radiocarbon constrained timing of PS events younger than 45 kyr indicates that the South Atlantic leads the North Atlantic's response to trade wind changes, particularly during isotope stages 4–2 when the Earth was dominated by large ice sheets. At times of increased trade wind strength, tropical and subtropical waters are forced across the equator enhancing the pool of warm water to be transferred to the high latitudes of the North Atlantic via the Gulf Stream and North Atlantic Drift, increasing the pull of the thermohaline convective conveyor. The increased supply of warm water to the polar regions of the northern hemisphere increases the ice‐ocean moisture gradient and accelerates ice sheet growth, leading to eventual instability and collapse.
Magnitude of the CaCO3 dissolution events marking the onset of times of glaciation
Authors: Broecker Wallace S. , Sanyal Abhijit
Published: February 2004

The availability of CaCO3 and ³He content results for core TT 13‐72 from 4.3 km depth in the equatorial Pacific [Marcantonio et al., 1996] allows the magnitude of the excess (i.e., over ambient) CaCO3 dissolution at the onset of marine glacial stages 10, 8, and 6 to be estimated. These three events are remarkably similar; during each an integrated loss of about 28 g CaCO3 per cm² occurred. While the magnitude of this loss is consistent with that expected from the interglacial to glacial pH shifts reconstructed based on boron isotope measurements on benthic foraminifera [Sanyal et al., 1995], measurements at a number of other locations and water depths will be required before this approach can be used to evaluate the global toll of these dissolution events.
Regional differences in Quaternary subantarctic nutrient cycling: Link to intermediate and deep water ventilation
Authors: Ninnemann Ulysses S. , Charles Christopher D.
Published: February 2004

Several fundamental issues regarding carbon cycling in the glacial oceans rest on the development of reliable descriptions of high southern latitude surface waters. Here we compare new Subantarctic planktonic foraminiferal δ13C records with previously published records to demonstrate two distinct regional patterns over glacial cycles: (1) a low‐amplitude signal (∼0.7‰), previously observed in the Indian (primarily in Globigerina bulloides), that also dominates the Pacific, and (2) a higher‐amplitude signal, previously observed in Neogloboquadrina Pachyderma, that is confined to the Atlantic and western Indian sectors. The new observations from the Southeast Pacific, a primary region of Antarctic Intermediate Water (AAIW) formation, strengthen the suggestion that intermediate water acted as a conduit for transferring δ13C variability to low latitudes, because the timing and amplitude of the Indo‐Pacific low‐amplitude δ13C changes are similar to those observed in planktonic records from the tropical Pacific and Atlantic. A new benthic foraminiferal δ13C record from intermediate depths in the South Atlantic is also similar to the Southeast Pacific surface water records, further demonstrating that this link between high‐ and low‐latitude surface waters might be maintained. The widespread Indo‐Pacific Subantarctic surface water signal is obscured in records from the Atlantic sector by the large glacial‐interglacial δ13C signal (>1.0‰) that is most likely the result of nutrient changes related to variable North Atlantic Deep Water (NADW) production. We find that the large change found uniformly in Atlantic N. pachyderma δ13C records is not matched in a new G. bulloides δ13C record. The confined regional extent of the high‐amplitude signal and the discrepancy between the two species suggest that most of the excess nutrients in the glacial Atlantic (inferred from δ13C) may be removed seasonally by increased production in the Subantarctic.
Modeled estimates of global reef habitat and carbonate production since the Last Glacial Maximum
Authors: Kleypas J. A.
Published: February 2004

Estimated changes in reef area and CaCO3 production since the last glacial maximum (LGM) are presented for the first time, based on a model (ReefHab) which uses measured environmental data to predict global distribution of reef habitat. Suitable reef habitat is defined by temperature, salinity, nutrients, and the depth‐attenuated level of photosynthetically available radiation (PAR). CaCO3 production is calculated as a function of PAR. When minimum PAR levels were chosen to restrict reef growth to 30 m depth and less, modern reef area totaled 584–746 × 10³ km². Global carbonate production, which takes into account topographic relief as a control on carbonate accumulation, was 1.00 Gt yr−1. These values are close to the most widely accepted estimates of reef area and carbonate production and demonstrate that basic environmental data can be used to define reef habitat and calcification. To simulate reef habitat changes since the LGM, the model was run at 1‐kyr intervals, using appropriate sea level and temperature values. These runs show that at the LGM, reef area was restricted to 20% of that today and carbonate production to 27%, due primarily to a reduction in available space at the lower sea level and secondarily to lower sea surface temperatures. Nonetheless, these values suggest that reef growth prior to shelf flooding was more extensive than previously thought. A crude estimate of reef‐released CO2 to the atmosphere since the LGM is of the same order of magnitude as the atmospheric CO2 change recorded in the Vostok ice core, which emphasizes the role of neritic carbonates within the global carbon cycle. This model currently addresses only the major physical and chemical controls on reef carbonate production, but it provides a template for estimating shallow tropical carbonate production both in the present and in the past. As such, the model highlights several long‐standing issues regarding reef carbonates, particularly in terms of better defining the roles of light, temperature, aragonite saturation state, and topography on reef calcification.
Productivity and paleoceanographic implications on the Tore Seamount (Iberian Margin) during the last 225 kyr: Foraminiferal evidence
Authors: Lebreiro S. M. , Moreno J. C. , Abrantes F. F. , Pflaumann U.
Published: February 2004

Foraminifera counts and climatic assemblages from the Tore Seamount are used to approach the glacial and interglacial changes in temperature and productivity on the Iberian Margin over the last 225 kyr. Chronostratigraphy is based on Globigerinoides ruber and Globigerina bulloides oxygen isotopes and supported by foraminifera and carbonate stadial fluctuations. Foraminifera indicate cooling from late interglacial stage 5 to the beginning of Termination I (TI). Neogloboquadnna pachyderma‐s reflects cold conditions during glacial stages 4‐2. In contrast, glacial stage 6 is dominated by warmer N. pachyderma‐d and dutertrei and a restricted arctic assemblage. Past sea surface temperatures confirm the general cooling, reaching 4.3°C (SIMMAX.28) during stage 2. Multiple productivity proxies such as organic carbon, productivity‐related foraminifera, and δ13C constrain the changes observed. A productivity increase occurs after interglacial stage 5, enhanced from late glacial stage 3 to TI Present‐day satellite‐detected phytoplankton plumes off Portugal would have accounted in the past glacial stages for the general productivity increase over the Tore. On top of this, welldefined peaks of organic carbon and productivity‐related foraminifera correspond with Heinrich events 1‐4.
Glacial‐Holocene calcium carbonate dissolution at the central equatorial Pacific seafloor
Authors: Stephens Mark P. , Kadko David C.
Published: February 2004

Holocene sediments in the central equatorial Pacific have lower CaCO3 burial rates relative to sediments deposited during the last glacial period. This change could be related to shifts in the deep‐sea carbonate system and ultimately to the glacial‐interglacial atmospheric CO2 cycle. In this study the glacial‐Holocene sedimentation in the central equatorial Pacific is investigated using a numerical model applied to CaCO3, carbonate 14C, and excess 230Th. We also employ excess 231Pa to 230Th ratios as proxies of past particle fluxes. Although the CaCO3 burial rates decreased by 0.7–2.0 g cm−2 kyr−1, the 231Pa to 230Th ratios indicate that particle and CaCO3 fluxes have either remained nearly constant or increased since the last glacial. This suggests that the reduction in CaCO3 burial rates resulted from an intensification of CaCO3 dissolution. A small to moderate decrease in bottom water carbonate concentration, rather than an increase in sedimentary respiratory‐CO2 production, is the most plausible mechanism for driving the sedimentation changes.
Last Glacial Maximum paleochemistry and deepwater circulation in the Southern Ocean: Evidence from foraminiferal cadmium
Authors: Rosenthal Yair , Boyle Edward A. , Labeyrie Laurent
Published: February 2004

South Atlantic benthic foraminiferal Cd/Ca shows no glacial‐interglacial variation, suggesting that the glacial contribution of North Atlantic Deep Water to the Southern Ocean was not much different than at present. In contrast, Cd/Ca in southeast Indian Ridge cores show lower glacial bottom water Cd, comparable to levels in intermediate depths of the North Atlantic and significantly lower than in the deep South Atlantic. Low glacial Cd/Ca was also recorded in planktonic foraminifera, suggesting a substantial decrease in the nutrient concentration of Subantarctic surface water during the glacial maximum which most likely was caused by increased biological productivity. The Cd data are inconsistent with low glacial benthic foraminiferal δ13C which suggest higher nutrient concentration. We propose that the low Cd/Ca in the Southeast Indian Ridge records reflects a local source of nutrient‐depleted deepwater, formed during the last glacial maximum by open‐ocean convection near the Antarctic Polar Front, downstream of the Kerguelene Plateau. If this source was limited to the southeast Indian basin then its impact on the overall chemistry of glacial Circumpolar Deepwater was rather small. However, if during glaciations open‐ocean convection became the dominant mode of bottom water formation, it might have had a greater impact on CPDW chemistry.
Does growth rate affect ketone unsaturation and intracellular carbon isotopic variability in Emiliania huxleyi?
Authors: Popp Brian N. , Kenig Fabien , Wakeham Stuart G. , Laws Edward A. , Bidigare Robert R.
Published: February 2004

We present the first results of U37K′ and εalkenone from Emiliania huxleyi grown in chemostat cultures at constant temperature but at variable growth rates. A small change in U37K′ with growth rate was found for a noncalcifying strain of E. huxleyi, whereas results from a calcifying strain showed no systematic variation between U37K′ and growth rate. We conclude that nutrient‐limited growth rate effects will not produce serious errors in paleotemperature determinations using U37K′ or U37K′. However, U37K′ values for the two strains differed significantly and apparent temperatures calculated using unsaturation ratios obtained from chemostat experiments are very different from those derived from batch culture experiments. Results of isotopic analyses of alkenones and bulk cells indicate no effect of growth rate on εalkenone (εalkenone=4.2±0.2‰). Constancy of εalkenone values is a requirement for environmental and paleoenvironmental interpretations. These results indicate that isotopic analyses of alkenones are ideal markers for paleo‐[CO2(aq)] determinations.
Response of surface water masses and circulation to Late Quaternary climate change east of New Zealand
Authors: Weaver Philip P. E. , Carter Lionel , Neil Helen L.
Published: February 2004

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present‐day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4° of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°–2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.
Early Pliocene deep water circulation in the western equatorial Atlantic: Implications for high‐latitude climate change
Authors: Billups Katharina , Ravelo A. C. , Zachos J. C.
Published: February 2004

High‐resolution (∼3–4 kyr) stable isotope stratigraphies from sites drilled along a depth transect on Ceara Rise (Ocean Drilling Program Leg 154, Sites 925 and 929) are used to reconstruct the deep water circulation response to the long‐ and short‐term climate changes of the early Pliocene (3.2–4.7 Ma). Over the long term, benthic foraminiferal carbon isotope records show that the vertical δ13C gradient in this region was similar to that of the late Holocene, implying a steady flux of Northern Component Deep Water (NCDW) into the deep Atlantic during most of the early Pliocene. The vertical benthic foraminiferal oxygen isotope gradient is reversed with respect to that of the late Holocene. On the basis of density constraints imposed by seawater stability along this depth transect we attribute the reversed gradient to warmer and more saline NCDW (5°C and 35.1). On orbital timescales we find that the phase relationship between δ18O and δ13C values at the deeper Site 929 differs from the late Pliocene/Pleistocene, while that at the shallower Site 925 was essentially the same.
Hydrologic issues in arid, unsaturated systems and implications for contaminant transport
Authors: Scanlon Bridget R. , Tyler Scott W. , Wierenga Peter J.
Published: February 2004

Analysis of unsaturated flow and transport in arid regions is important, not only in water resource evaluation but in contaminant transport as well, particularly in siting waste disposal facilities and in remediating contaminated sites. The water fluxes under consideration have a magnitude close to the errors inherent in measuring or in calculating these water fluxes, which makes it difficult to resolve basic issues such as direction and rate of water movement and controls on unsaturated flow. The purpose of this paper is to review these issues on the basis of unsaturated zone studies in arid settings. Because individual techniques for estimating water fluxes in the unsaturated zone have limitations, a variety of physical measurements and environmental tracers should be used to provide multiple, independent lines of evidence to quantify flow and transport in arid regions. The direction and rate of water flow are affected not only by hydraulic head gradients but also by temperature and air pressure gradients. The similarity of water fluxes in a variety of settings in the southwestern United States indicates that vegetative cover may be one of the primary controls on the magnitude of water flow in the unsaturated zone; however, our understanding of the role of plants is limited. Most unsaturated flow in arid systems is focused beneath topographic depressions, and diffuse flow is limited. Thick unsaturated sections and low water fluxes typical of many arid regions result in preservation of paleoclimatic variations in water flux and suggest that deep vadose zones may be out of equilibrium with current climate. Whereas water movement along preferred pathways is common in humid sites, field studies that demonstrate preferential flow are restricted mostly to fractured rocks and root zones in arid regions. Results of field studies of preferential flow in humid sites, generally restricted to the upper 1–2 m because of shallow water tables, cannot be applied readily to thick vadose zones in arid regions.
Monitoring ice sheet behavior from space
Authors: Bindschadler Robert
Published: February 2004

Satellite remote sensing has revolutionized ice sheet research. A variety of instruments sensitive to different parts of the electromagnetic spectrum take what the human eye detects as a flat, white desert and provide data sets rich in scientific information. Image‐based maps of ice sheets are becoming commonplace and have become an integral component of field work. More than a pretty picture, the digital character of the satellite data from these instruments has become fundamental to the production of elevation, motion, accumulation, and reflectance data sets. Visible imagery shows the scientist a wealth of features that offer clues to the history and current behavior of the ice sheet. Radar and microwave imagery provide information from beneath the surface and have been used to estimate snow accumulation rates. Interferometry principles have recently been applied to measure surface topography and ice motion with unparalleled precision. Nonimaging instruments also keep a watchful eye, monitoring the ice sheet for indications of growth or shrinkage. Further expansion of the uses of satellite data is anticipated in the future.
Flow channeling in heterogeneous fractured rocks
Authors: Tsang Chin‐Fu , Neretnieks Ivars
Published: February 2004

Experimental observations and theoretical studies over the last 10 years or so have demonstrated that flow channeling or preferred flow paths is a common phenomenon in fractured rocks. The reason it has come to the forefront of scientific investigation is the recent interest in predicting solute transport in geological media as part of safety assessment of geologic isolation of nuclear or toxic wastes. Solute transport is much more sensitive to medium heterogeneity than is temperature or pressure. In this paper, experimental observations of tracer transport over distances ranging from centimeters to hundreds of meters are reviewed, and theoretical efforts to explain or model these observations are summarized. Processes that may explain some of the experimental observations without the use of flow‐channeling models are discussed. The paper concludes with a discussion of the implications of flow channeling on the practical problems related to contaminant transport in geologic systems.
Improved radio tomography of the ionosphere using EUV/optical measurements from satellites
Authors: Bernhardt P. A. , Dymond K. F. , Picone J. M. , Cotton D. M. , Chakrabarti S. , Cook T. A. , Vickers J. S.
Published: February 2004

Computerized tomography of the ionosphere employs radio beacons on satellites and ground‐based receivers that measure the integrated electron densities along line‐of‐sight propagation paths. The primary limitation to satellite‐based ionospheric radio tomography is the lack of near‐horizontal ray paths. This restricts the accuracy for the reconstruction of vertical profiles in the F region. Horizontal integration paths may be obtained using the natural extreme ultraviolet emissions in the ionosphere. The emissions at 91.1 nm are the result of radiative recombination of O+ ions and electrons and at 83.4 nm are produced by photoionization of atomic oxygen and subsequent scatter by the atomic oxygen ion. Combining data from radio and EUV measurements yields greatly improved ionospheric density reconstructions. This concept will be tested using the TERRIERS satellite scheduled for launch in early 1998.
Forecasting global lightning for atmospheric noise prediction
Authors: Warber Chris R. , Prasad Balram
Published: February 2004

In this paper we discuss an investigation into the feasibility of using current and forecasted weather data to forecast lightning occurrence. These lightning occurrence forecasts are intended to be used to improve the accuracy of near‐term long wave communication systems coverage predictions. Various weather information sources were reviewed to determine which data and parameters could be used to predict lightning and which techniques were best suited for selected forecast periods. Review of atmospheric and lightning physics resulted in several methods for establishing relationships between weather data parameters and lightning flash rates for selected forecasting periods. We review these methods and show results of tests which were conducted to compare the forecasted lightning occurrence with measured data. These tests were limited to the continental United States where empirical data on lightning occurrence from the National Lightning Detection Network are available. The results of the techniques developed for several example days are shown, as is the impact that using the techniques might have on the coverage prediction for an example long wave transmitter.
High‐latitude upgrade to the Wideband ionospheric scintillation model
Authors: Secan J. A. , Bussey R. M. , Fremouw E. J. , Basu S.
Published: February 2004

The high‐latitude sections of the Wideband ionospheric scintillation model (WBMOD) have been upgraded extensively, based on analysis of scintillation data from the Defense Nuclear Agency Wideband, HiLat, and Polar BEAR satellite‐beacon experiments. Data collected at Sondre Stromfjord, Greenland; Tromso, Norway; Fort Churchill, Canada; and Bellevue, Washington (United States) over a 4‐year period were analyzed, and the results of these analyses were used to construct a completely new model for the behavior of the height‐integrated irregularity‐strength parameter (CkL) at high latitudes. The new high‐latitude CkL model includes variations with sunspot number, geomagnetic activity (Kp), latitude, local time, longitude, and season. The new WBMOD CkL models (equatorial and high‐latitude) have been implemented in a more versatile code, denoted SCINTMOD, which has the capability to generate a wide range of user‐controlled maps of scintillation effects over large spatial areas. Examples of the types of graphical output that SCINTMOD can generate are presented.
GPS phase fluctuations in the equatorial region during sunspot minimum
Authors: Aarons J. , Mendillo M. , Yantosca R.
Published: February 2004

GPS multisite and multisatellite observations of phase fluctuations are used to characterize irregularity development in the equatorial region. We use 30‐s samples of total electron content obtained from phase differences between 1.2 and 1.6‐GHz signals as gathered by the International GPS Service for Geodynamics (IGS). These observations from a number of ground receivers allowed for simultaneous studies over a range of latitudes and longitudes in the region near and distant from the magnetic equator. Several storm periods were analyzed in the time period from November 3,1993, to October 2,1995. By examining phase fluctuations at latitudes near the magnetic equator, in the anomaly region, and poleward of the anomaly region, it was found that very high altitudes were reached by plumes of irregularities during seven magnetic storms in solar minimum years. These also occurred during periods of magnetic quiet. The data set allowed the determination of effective altitudes of equatorial irregularities. During low solar flux years, there was a plethora of different altitude regimes from thin layers to altitudes over 2000 km.