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Numerical simulations and geologic studies suggest that high thermal anomalies beneath, within, and above thermally insulating layers of buried hydrated salts and gas hydrates could have triggered and sustained hydrologic processes on Mars, producing or modifying chaotic terrains, debris flows, gullies, and ice-creep features. These simulations and geologic examples suggest that thick hydrate deposits may sustain such geothermal anomalies, shallow groundwater tables, and hydrogeologic activity for eons. The proposed mechanism may operate and be self-reinforcing even in today's cold Martian climate without elevated heat flux. Keywords: Mars, heat flow, hydrous sulfates, clathrate hydrates, Aram Chaos. doi: 10.1130/G23783A.1

Geological mapping based on topographic analysis of Mars Orbiter Laser Altimeter (MOLA) data, together with photointerpretation of Mars Orbiter Camera (MOC) images and thermodynamic and heat-flow considerations, frame a new hypothesis for the formation of Martian outflow channels through catastrophic dewatering of evaporite deposits. MOLA transects across Valles Marineris show that the valley is located at the crest of a 3-km-high topographic bulge on the flank of the much larger Tharsis Rise. Interpretation of MOC images showing layered deposits within Valles Marineris as unconformably underlying Hesperian-age lava flows means that these thick deposits, thought to contain hydrous sulfates, were heated by an increased geothermal gradient due to development of Tharis. Increased temperatures adequate to dehydrate hydrous evaporites would trigger significant volumetric expansion and catastrophically release tremendous amounts of over-pressured water. Keywords: Mars, outflow channels, floods, evaporites.

Physical weathering of rock (spalling), removal of adhered rock varnish, and diffusive loss of noble gases are accelerated by heating in range fires, a dramatic and widespread geomorphic process that must be considered when samples are collected for determining surface exposure ages or for measuring in situ production rates of cosmogenic isotopes. Exposure ages and production rates will be minima if the effects of spalling and/or the effects of accelerated diffusive loss of gaseous isotopes such as He and Ne are not considered. To minize the effect of fire-induced diffusive loss, samples for noble-gas analysis should be collected 5-10 cm below the rock surface or from outcrops that are unlikely to have been heated by fire. In addition, by determining the concentration of in situ produced cosmogenic isotopes of different half-lives, the magnitude of spalling loss (site-specific erosion rate) can be constrained for each sample.

To compare methods of rock-varnish chemical analysis and cation-ratio determination, we prepared three glass standards, synthetic analogues of rock varnish. Analysis of these standards demonstrates that material of varnishlike composition can be accurately and precisely analyzed by means of quantitative, energy-dispersive electron microscopy (SEM-EDS). However, a blind interlaboratory comparison and reexamination of published data show that most published cation ratios of varnish are probably inaccurate because the technique used most frequently to determine rock-varnish composition, proton-induced X-ray emission (PIXE), did not accurately measure the concentration of Ti in the presence of Ba, a ubiquitous component of rock varnish. This inaccuracy suggests that the premise of cation-ratio dating and dates generated by this method should be reevaluated.

New data and statistical analyses indicate that the precision of the mean rock-varnish cation ratio determined for a geomorphic surface and the uncertainty of the age calculated from that ratio are controlled by the area of varnish included in each chemical analysis, the precision with which each cation ratio is determined, and the number of independent chemical analyses. This study has implications for the interpretation of published cation-ratio ages and the future use of cation-ratio dating.

The Chinese Loess Plateau (CLP) is a large, spatially well defined and persistent zone of loess accumulation developed near the fluctuating northwest margin of the East Asian monsoon. Many studies have analyzed its loess sediments to provide insights into paleoclimatic conditions. Although spatial and temporal variations in the grain sizes of CLP sediments are fundamental to this effort, controversy over the origin of the dominant coarse quartz silt has limited interpretations. Reexamination of the spatial pattern of grain-size distribution across the CLP and a field-scale experiment conducted in the Gobi Desert revealed a genetic association between the coarse silt fraction of the loess and primary production of coarse silt through eolian abrasion of sand in the proximal Mu-Us, Tengger, and Badain Jaran sandy deserts. Our results demonstrate the effectiveness of eolian abrasion of quartz sand in primary coarse silt production in Central Asia and identify this process as the most consistent with the well-recognized systematic northwest-southeast depositional pattern of the CLP. We suggest that only abraded coarse quartz grains transported short distances by long-term persistent eolian activity can build up thick loess sequences to form a massive and spatially well defined loess plateau. These results decouple the production and transport of coarse silt and finer silt and clay particles, which have a more distant and wider provenance, changing the constraints on previous paleoclimatic reconstructions. [ABSTRACT FROM AUTHOR]