Incipient hydrothermal alteration of basalts and the origin of martian soil

The martian soil is a fine-grained regolith that is chemically basaltic in character with evidence for both gains and losses of volatile and mobile elements compared to martian basalt compositions. These chemical fractionations provide clues to geochemical processes on the surface of Mars. Geochemical processes contributing to the soil proposed in the past include the chemical and mechanical breakdown of rocks under surface conditions, the addition of volcanic aerosols containing S and Cl compounds, and the alteration of basaltic glass to palagonite. Our studies of terrestrial analogs suggest that hydrothermal alteration processes involving impact craters and volcanism could also contribute to the major element trends observed in martian soil. Data from Viking, Pathfinder, and the current MER missions consistently show that relative to basaltic martian meteorite compositions, the major element compositions of the soils are (1) depleted in the fluid-mobile element calcium, (2) generally similar or somewhat enriched in iron oxide and magnesium but MgO depleted compared to Gusev rocks, (3) locally variable in potassium, (4) possibly poorer in aluminum, and (6) very enriched in chlorine and sulfur. The major element trends, aside from the Cl and S enrichment, could be explained by the formation or addition of palagonite according to McSween and Keil (2000), but the missing CaO remains a problem. The chlorine and sulfur are probably derived from other processes such as volcanic aerosols and hydrothermal fluids. McSween and Keil (2000) also argued that hydrothermal alteration of basalts produce alteration trends that are inconsistent with the Mars soil, but this study concludes otherwise. We have used quantitative mass balance mixing models to investigate possible models involving mixtures of basaltic compositions with different types of alteration materials, including palagonite. We show that the Mars soil composition can be matched with a combination of unweathered basaltic martian meteorites with basaltic FeO-rich, CaO-poor alteration products. Palagonite is a possible, but not a necessary component of successful model mixtures. The hydrothermal alteration materials that form successful model mixtures are formed in low temperature, low water/rock ratio environments, and they can reproduce the required geochemical trends because they are poorer in CaO but not in FeO compared to their respective protoliths. These results argue that material altered by hydrothermal processes could be a plausible component of the soil, and that removal of CaO from the soil into some undiscovered reservoir after its formation is not required. The current soil on Mars, therefore, did not have to undergo an episode of in situ aqueous alteration but could represent a sink for materials that experienced aqueous processes in a different setting before erosion to form the soil. The soil can also represent a sink for mobile elements (e.g., S, Cl, and Br) derived from other sources such as volcanic aerosols and hydrothermal fluids.     

Potassium-argon ages and strontium isotope ratio measurements from volcanic rocks in northeastern Nigeria

The Cenozoic volcanic activity in northeastern Nigeria began with the intrusion into the Benue trough of a trachyte-phonolite suite of plugs 22-11 m.y. ago. Later activity, which was more widespread and dominantly basaltic in character, began some 7 m.y. ago and has continued until very recent times. It resulted in basaltic plugs and lava plateaux within the Benue trough, and cinder cones and lavas on the Jos Plateau.The initial ⁸⁷Sr/⁸⁶Sr ratios of nine of the fifteen analyzed basic and alkalic rocks lie in the range 0.7025–0.7032, and the highest ratio measured is 0.7129.The main group of trachytes and phonolites are considered to be fractional melts derived from the upper mantle, modified in small part by potassium feldspar crystal fractionation. Two Sr-rich phonolite plugs may have a separate origin from the main group of trachytes and phonolites.The Cenozoic volcanic activity in northeastern Nigeria is probably related to the nearby Cameroun volcanic line. The concentration of plugs within the Benue trough may reflect internal adjustments along old lines of weakness within the African lithosphere plate, in response to synchronous changes affecting the plate's external dimensions and internal structure, such as the growth of the Red Sea and the Gulf of Aden, and the volcanism of the east African rift valley.     

Cloud fraction at the ARM SGP site

The Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site has a rich history of actively sensed cloud observations. Fourteen years (1997–2010) of observations from the Millimeter Cloud Radar (MMCR), Micropulse Lidar (MPL), and Belfort/Vaisala Ceilometers are used to understand how instrument selection and sampling impacts estimates of Cloud Fraction (CF) at this location. Although all instruments should be used in combination for the best estimates of CF, instrument downtime limits available samples and increases observational errors, demanding that users make sacrifices when calculating CF at longer intervals relevant to climate studies. Selection of MMCR or MMCR + MPL cloud masks changes very little in the overall understanding of total CF. Addition of the MPL increases the 14-year average CF by 9 %, mainly through an increase in optically thin high clouds year-round, and mid-level clouds during the summer months. Splitting the period into two equal 7-year periods reveals negligible change in MMCR + MPL CF. For the MMCR, however, CF deceases by 6.1 %. This sudden change in CF occurs around the time the radar was upgraded, suggesting that this decrease is tied to hardware sensitivity or scanning strategy changes. Users must be cognizant of this and other issues when calculating CF from the variety of observations available at the ARM SGP site.     

Exploring the geophysical and socio-economic determinants of land cover changes in Eastern Mau forest reserve and Lake Nakuru drainage basin,Kenya

Understanding the linkages between the biogeophysical and socio-economic processes that operate at different spatial and temporal scales is important for land cover change mitigation. This study analysed several factors that explained the forest-shrubland conversions, grassland conversions and cropland expansions in Lake Nakuru drainage basin and Eastern Mau forest reserve in Kenya from 1985 to 2011. Logistic regression models were developed using a combination of remote sensing-based land cover data, and geographical information systems-based geophysical and socio-economic data (i.e., temperature, rainfall, elevation, slope, aspect, topographic wetness, curvature, soil pH, soil cation exchange capacity (CEC), population density and distance to road, river and town). The results were varied; for example, in the period 1985–2000, forest-shrubland conversions were linked to distance to road), distance to town, soil pH, soil CEC, rainfall, topographic wetness, curvature and aspect. The same factors, in addition to slope and distance to river, also determined the likelihood of forest-shrubland conversions in the period 2000–2011. Overall, significance of the determining factors varied depending on time and nature of land cover change. For example, topographical factors influenced grassland conversions in the period 1985–2000, while soil-related factors did not. But in the period 2000–2011, the converse was true. Therefore, policies for restoration, conservation and sustainable management of critical ecosystems (e.g., forests) should be spatially targeted and time-specific. These results broaden our knowledge of land cover dynamics in this locality, and provide a base for effective environmental policy formulation, planning and management.     

A depositional model for offshore deposits of the lower Blue Gate Member,Mancos Shale,Uinta Basin,Utah, USA

Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.     

Composition,Abundance, and Life History of Mysids (Crustacea: Mysida) in the Coastal Lagoons of MD,USA

The composition, abundance, biomass, and life history of mysid species were investigated and described for the first time in the Maryland Coastal Bays (38° N, 75° W), Mid-Western Atlantic, using data collected from 2010 to 2013. Three species of mysids were collected, with Neomysis americana being the most abundant species (maximum mean abundance 6.7 ± 6.4 numbers (nos.) m^?2 in July 2013 and biomass 2.78 ± 2.76-mg dry weight (DW) m^?2 in July 2012). Americamysis bahia was the second most abundant species (maximum mean abundance: 0.7 ± 0.4 nos. m^?2 and biomass: 0.23 ± 0.14 mg DW m^?2 in March 2012). Metamysidopsis swifti made up 0.02 to 2 % of mysids and were found in samples collected mainly from southern Chincoteague Bay close to that Bay’s inlet in the fall of 2012. The two most abundant mysid species reproduced continuously from March to July (Neomysis) and May to October (Americamysis). N. americana had larger body and brood sizes than A. bahia. Mysids were relatively low in abundance in late summer, a period of relatively high biomass of fish predators, than during other seasons, suggesting that intense predation might be controlling their abundance. The increase in mysid abundance in the fall following their disappearance in late summer without evidence of reproductive activities suggests species migration from coastal waters into the Maryland Coastal Bays. This annual mysid subsidy perhaps helps to sustain their populations within the bays.     

Comparison of Lower-Tropospheric Temperature Climatologies and Trends at Low and High Elevation Radiosonde Sites

Observations of rapid retreat of tropical mountain glaciers over the past two decades seem superficially at odds with observations of little or no warming of the tropical lower troposphere during this period. To better understand the nature of temperature and atmospheric freezing level variability in mountain regions, on seasonal to multidecadal time scales, this paper examines long-term surface and upper-air temperature observations from a global network of 26 pairs of radiosonde stations. Temperature data from high and low elevation stations are compared at four levels: the surface, the elevation of the mountain station surface, 1 km above the mountain station, and 2 km above the mountain station. Climatological temperature differences between mountain and low elevation sites show diurnal and seasonal structure, as well as latitudinal and elevational differences. Atmospheric freezing-level heights tend to decrease with increasing latitude, although maximum heights are found well north of the equator, over the Tibetan Plateau. Correlations of interannual anomalies of temperature between paired high and low elevation sites are relatively high at 1 or 2 km above the mountain station. But at the elevation of the station, or at the two surface elevations, correlations are lower, indicating decoupling of the boundary layer air from the free troposphere.Trends in temperature and freezing-level height are generally upward, both during 1979–2000 and during longer periods extending back to the late 1950s. However, some negative trends were found at extratropical locations. In many cases, statistically significant differences were found in trends at paired high and low elevation stations, with tropical pairs revealing more warming (and greater increases in freezing-level height) at mountain stations than at low elevations. This result is consistent with both the observed retreat of tropical glaciers and the minimal change in tropics-wide tropospheric temperatures over the past two decades.Overall, the analysis suggests that, on diurnal, seasonal, interannual, and multidecadal time scales, temperature variations at mountain locations differ significantly from those at relatively nearby (a few hundred kilometers) low elevation stations. These differences are greatest at the two surface levels, but can persist up to 2 km above the mountain site. Therefore, to determine the nature of climate variability at high elevation sites requires local observations, since large-scale patterns derived from low elevation observations may not be representative of the mountain regions. Conversely, temperature change in mountain regions should not be viewed as necessarily representative of global surface or tropospheric trends.     

Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex,West Texas

Late Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.