Observations of martian gullies and constraints on potential formation mechanisms

The discovery of presumably geologically recent gully features on Mars (Malin and Edgett, 2000, Science 288, 2330-2335) has spawned a wide variety of proposed theories of their origin including hypotheses of the type of erosive material. To test the validity of gully formation mechanisms, data from the Mars Global Surveyor spacecraft has been analyzed to uncover trends in the dimensional and physical properties of the gullies and their surrounding terrain. We located 106 Mars Orbiter Camera (MOC) images that contain clear evidence of gully landforms, distributed in the southern mid and high latitudes, and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. We find that the number of gully systems normalized to the number of MOC images steadily declines as one moves poleward of 30° S, reaches a minimum value between 60°-63° S, and then again rises poleward of 63° S. All gully alcove heads occur within the upper one-third of the slope encompassing the gully and the alcove bases occur within the upper two-thirds of the slope. Also, the gully alcove heads occur typically within the first 200 meters of the overlying ridge with the exception of gullies equatorward of 40° S where some alcove heads reach a maximum depth of 1000 meters. While gullies exhibit complex slope orientation trends, gullies are found on all slope orientations at all the latitudes studied. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 79% of the gully alcove bases lie at depths where subsurface temperatures are greater than 273 K and 21% of the alcove bases lie within the solid water regime. Most of the gully alcoves lie outside the temperature-pressure phase stability of liquid CO₂. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies.     

Non-equilibrium interphase heat and mass transfer during two-phase flow in porous media—Theoretical considerations and modeling

Mass and heat transfer occurring across phase-interfaces in multi-phase flow in porous media are mostly approximated using equilibrium relationships or empirical kinetic models. However, when the characteristic time of flow is smaller than that of mass or heat transfer, non-equilibrium situations may arise. Commonly, empirical approaches are used in such cases. There are only few works in the literature that use physically-based models for these transfer terms. In fact, one would expect physical approaches to modeling kinetic interphase mass and heat transfer to contain the interfacial area between the phases as a variable. Recently, a two-phase flow and solute transport model was developed that included interfacial area as a state variable [36]. In that model, interphase mass transfer was modeled as a kinetic process.     

Spatial pattern of localized disturbance along a Southeastern salt marsh tidal creek

Geomorphology may be an important predictor of vegetation pattern in systems where suceptibility to disturbance is unevenly distributed across the landscape. Salt marsh communities exhibit spatial pattern in vegetation at a variety of spatial scales. In coastal Georgia, the low marsh is a virtual monoculture ofSpartina alterniflora interspersed with patches of species that are more typical of the high marsh. These localized disturbances are most likely created by wrack mats, mats of dead vegetation which can compact and smother underlying vegetation creating bare patches for colonization by high marsh species. We investigated the spatial pattern of disturbed patches along a 2 km section of Dean Creek, a tidal creek at the southwestern end of Sapelo Island, Georgia, U.S. We used a discriminant model to explore the relationship between tidal creek morphology (e.g., the presence of drainage channels and creek bends) and the spatial distribution of disturbed patches. The model predicted vegetation pattern along the creek with relatively high accuracy (>70%). Areas where water movement is slowed or multidirectional (e.g., along creek bends and near drainage channels) were most susceptible to disturbance. Our findings suggest an important functional linkage between geomorphology and vegetation pattern in salt marsh communities.     

Does I = PAT Work in Local Places?

An aspect of global change currently not well understood is how processes operating on spatial scales finer than those used in recent global circulation models (GCMs) contribute to changes in atmospheric composition and the subsequent changes in climate. We use the ‘IPAT’ formulation as a framework to test relationships among social driving forces and user group greenhouse gas (GHG) emissions in northwestern North Carolina. Using regression, correlation, and bivariate mapping to examine relationships between a suite of socioeconomic variables and GHG emissions for the residential, commercial/industrial, and agricultural end‐user categories, we find that various measures of population and affluence serve equally well as explanatory variables.     

AMS 14C age constraints on geoglyphs in the lower colorado river region,arizona and california

Giant ground figures are widespread in the lower Colorado River area of southwestern North America, yet their chronology has remained unconstrained by numerical ages. Thirteen AMS ¹⁴C measurements reported here indicate that geoglyphs were made from before ˜A. D. 1200 to before ˜900 B. C. We account for potential contamination from prior organics in weathering rinds. All other potential errors point to ¹⁴C dates being minimum-limiting ages for the manufacturing of geoglyphs. Although these ages indicate considerable chronological complexity among geoglyphs, our data are consistent with the linguistic hypothesis that the Yuman people in the desert of southeastern California migrated from Baja California—rather than from the north. These results must, however, be placed under the cloud of uncertainty that hangs over the entire field of AMS dating of rock art: the untested assumption surrounding contemporeneity of organics in a surface context. © 1995 John Wiley & Sons, Inc.     

Flexural deformation controls on Late Quaternary sediment dispersal in the Garo-Rajmahal Gap,NW Bengal Basin

Subsurface deformation is a driver for river path selection when deformation rates become comparable to the autogenic mobility rate of rivers. Here we combine geomorphology, soil and sediment facies analyses, and geophysical data of the Late Quaternary sediments of the central Garo-Rajmahal Gap in Northwest Bengal to link subsurface deformation with surface processes. We show variable sedimentation characteristics, from slow rates (<0.8 mm/year) in the Tista megafan at the foot of the Himalaya to nondeposition at the exposed surface of the Barind Tract to the south, enabling the development of mature soils. Combined subsidence in the Tista fan and uplift of the Barind Tract are consistent with a N-S flexural response of the Indian plate to loading of the Himalaya Mountains given a low value of elastic thickness (15–25 km). Provenance analysis based on bulk strontium concentration suggests a dispersal of sediment consistent with this flexural deformation—in particular the abandonment of the Barind Tract by a Pleistocene Brahmaputra River and the current extents of the Tista megafan lobes. Overall, these results highlight the control by deeply rooted deformation patterns on the routing of sediment by large rivers in foreland settings.     

Inorganic arsenic speciation in the waters of the Penzé estuary (NW France): Seasonal variations and fluxes to the coastal area

Over this one-year study, the variations of inorganic As species were examined monthly along the salinity gradient of the Penzé estuary (NW France) in relation with different biogeochemical parameters. In most cases, dissolved As exhibited a non-conservative behaviour which resulted from the competition between two major processes. In the upstream section of the estuary, a strong input of both total inorganic As and As(III) occurred. Then, the removal of the same species, under precipitation of iron oxides/oxyhydroxides, was observed in the low-salinity range (S < 10). Using our experimental data, the fluxes of the various As species were estimated for the first time in estuarine waters. Inputs from the river were mainly constituted of particulate As (∼70%). Conversely, dissolved species were predominant in the net fluxes (∼65%) and As(III) accounted for ∼15% of the dissolved net flux.     

The impact of scour processes on a smothered reef system in the Irish Sea

The Irish Sea, like many marine areas, is threatened by anthropogenic activities. In particular the Pisces Reef system, a series of smothered rocky reefs are subject to fishing pressures as a result of their position within a Nephrops norvegicus fishery. In an area of sediment deposition and retention the reefs modify the environment by increasing the energy of near-bottom currents which results in localised scouring. This is the first study to attempt to characterise and investigate the ecological functioning of the Pisces Reef system. A multidisciplinary approach was essential for accurate investigation of the area. To facilitate more effective management of the benthic habitats of the Reef system, this study integrates acoustic, seismic, grab sampling and video ground-truthing methods for benthic habitat discrimination. Orientation of the scour hollows also suggest that seabed features could be used to infer dominant flow regimes such as the Irish Sea Gyre. The data revealed significant geology–benthos relationships. A unique biotope was described for the reef habitat and it was demonstrated that scouring may influence community composition through disturbance mechanisms. This study provides preliminary information required for management of a unique habitat within a uniform region.     

Investigating the effect of ballasting by CaCO3 in Emiliania huxleyi, II: Decomposition of particulate organic matter

The quantitative relationship between organic carbon and mineral contents of particles sinking below 1800 m in the ocean indicates that organisms with mineral shells such as coccolithophores are of special importance for transporting carbon into the deep sea. Several hypotheses about the mechanism behind this relationship between minerals and organic matter have been raised, such as mineral protection of organic matter or enhanced sinking rates through ballast addition. We examined organic matter decomposition of calcifying and non-calcifying Emiliania huxleyi cultures in an experiment that allowed aggregation and settling in rotating tanks. Biogenic components such as particulate carbon, particulate nitrogen, particulate volume, pigments, transparent exopolymer particles (TEP), and particulate amino acids in suspended particles and aggregates were followed over a period of 30 d. The overall pattern of decrease in organic matter, the amount of recalcitrant organic matter left after 30 d, and the compositional changes within particulate organic matter indicated that cells without a shell are more subject to loss than calcified cells. It is suggested that biogenic calcite helps in the preservation of particulate organic matter (POM) by offering structural support for organic molecules. Over the course of the experiment, half the particulate organic carbon in both calcifying and non-calcifying cultures was partitioned into aggregates and remained so until the end of the experiment. The partial protection of particulate organic matter from solubilization by biominerals and by aggregation that was observed in our experiment may help explain the robustness of the relationship between organic and mineral matter fluxes in the deep ocean.     

Surficial and Deep Earth Material Prediction from Geochemical Compositions

Prediction of true classes of surficial and deep earth materials using multivariate spatial data is a common challenge for geoscience modelers. Most geological processes leave a footprint that can be explored by geochemical data analysis. These footprints are normally complex statistical and spatial patterns buried deep in the high-dimensional compositional space. This paper proposes a spatial predictive model for classification of surficial and deep earth materials derived from the geochemical composition of surface regolith. The model is based on a combination of geostatistical simulation and machine learning approaches. A random forest predictive model is trained, and features are ranked based on their contribution to the predictive model. To generate potential and uncertainty maps, compositional data are simulated at unsampled locations via a chain of transformations (isometric log-ratio transformation followed by the flow anamorphosis) and geostatistical simulation. The simulated results are subsequently back-transformed to the original compositional space. The trained predictive model is used to estimate the probability of classes for simulated compositions. The proposed approach is illustrated through two case studies. In the first case study, the major crustal blocks of the Australian continent are predicted from the surface regolith geochemistry of the National Geochemical Survey of Australia project. The aim of the second case study is to discover the superficial deposits (peat) from the regional-scale soil geochemical data of the Tellus Project. The accuracy of the results in these two case studies confirms the usefulness of the proposed method for geological class prediction and geological process discovery.