The inland region of the McMurdo Dry Valleys (MDV) of Antarctica represents a subzero, hyper-arid endmember for physical weathering, and is Earth’s closest terrestrial analog to the Martian surface. In order to document the style and rate of rock breakdown in this region, I conducted field-based, experimental, and numerical modeling studies of supraglacial debris (Mullins till) on Mullins Glacier. These investigations were designed to (1) quantify the rate and processes of physical breakdown of surface rocks on Mullins till, particularly of Ferrar Dolerite, (2) determine the efficacy of thermal stress weathering as an agent in rock erosion, and (3) examine the role of physical weathering in altering the sublimation of buried glacial ice. Results from morphometric field surveys characterizing changes in rock shape, strength, and small-scale surface features, coupled with an iterative cosmogenic nuclide-based age model for Mullins Glacier, show that total erosion rates for clasts of Ferrar Dolerite on Mullins till range from 1.1 to 15 cm Myr-1. In situ field measurements of rock surface temperatures and local ambient conditions recorded at 15-second intervals, combined with a numerical finite element model elucidating changes in internal rock temperatures and associated strain, show that thermal stress weathering is sufficient to induce spalling by propagating existing microcracks of ≥1.1 cm that typically occur at the base of thin, mm-scale alteration rinds. The implication is that thermal stress weathering, previously undocumented in this region, may account for >80% of the total estimated erosion of Ferrar Dolerite. Furthermore, the spalled fragments (up to 5% of Mullins till) provide a negative feedback that slows the rate of subsurface ice sublimation and internal vapor diffusion. Experimental analyses in a controlled environmental chamber set with Mullins till and driven by local meteorological conditions measured in the field yields an average effective diffusivity of 4.5 x 10-6 m2 s-1 for Mullins till and annual rates of buried ice loss of <0.068 mm (assuming Fickian diffusion); these values are consistent with theoretical estimates, demonstrate the importance of physical weathering in modifying supraglacial deposits, and support arguments for persistent cold-desert conditions in the MDV for the last several million years.

The aim of this study is to characterise the structure of refrozen cracks, and to deduce the details of their formation. Surveys and experiments are conducted on straight-sided, linear, refrozen cracks of width 80 mm to 340 mm in land-fast first-year sea ice in McMurdo Sound, Antarctica. Refreezing of cracks is simulated analytically, and with a numerical fluid dynamics model of brine movement in the porous sea ice and in the ocean. Systematic arch-shaped patterns of inclusions, upstream-growing crystals, and two-dimensional variations in salinity are identified in completely and partially refrozen, natural cracks, and in artificial cracks. Using a two-dimensional thermistor array, a relationship between the development of the sea ice structure and the temperature records is found, which identifies the transition from the porous, skeletal layer to consolidated ice in artificial cracks. A two-dimensional analytical model is developed that predicts the measured thickness of consolidated ice in refreezing cracks for this study and for the studies of others. From a heat balance within the refreezing cracks, it is concluded that some of the experiments were conducted in the presence of a negative ocean heat flux. A two-dimensional thermistor array beneath the ice-water interface of a refreezing crack provides evidence for sporadic, cold temperature, advective events at night. A two-dimensional, numerical fluid dynamics model based on the finite volume method is developed to simulate desalination and fluid flow in refreezing cracks. This requires a permeability-porosity relationship for sea ice, which is deduced from data of other groups, combined with the numerical model. To make comparisons among data sets, an analytical approximation is derived for the relationship between connected pore space and total pore space of a random porous medium, based on a Monte Carlo model that is adapted to the crystal structure of sea ice. The permeability-porosity relationship derived in this study is in good agreement with permeability functions published recently. The refreezing of cracks simulated with the numerical fluid dynamics model is consistent with experiments and with the analytical model. In addition, the numerical model simulates the high porosity, arch-shaped freezing front and inclusion structure. Supercooling of the liquid is found to cause excessive heat loss in the simulation. Since a large oceanic heat flux was not observed in the experimental heat balance of refreezing slots, it is suggested that this indicates platelet ice formation or frazil ice formation at the vertical crack interface in Antarctic experiments.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 16, 2010). Includes bibliographical references. Also issued in print.

The aim of this research was to demonstrate the relationship between variations in summertime surface mass balance of the McMurdo Ice Shelf and atmospheric processes. The approach encompassed a broad range of techniques. An existing energy balance mass balance model was adapted to deal with debris-covered ice surfaces and modified to produce distributed output. Point based surface energy and mass balance for two key surfaces of the ice shelf were linked to different synoptic types that were identified using a manual synoptic classification. The distributed model was initialised with distributed parameters derived from satellite remote sensing and forced with data from a regional climate model. Patterns of summertime surface mass balance produced by the distributed model were assessed against stake measurements and with respect to atmospheric processes. During the summers of 2003-2004 and 2004-2005 an automatic weather station (AWS) was operated on bare and debris-covered ice surfaces of the McMurdo Ice shelf, Antarctica. Surface mass balance was calculated using the energy balance model driven by the data from the AWS and additional data from permanent climate stations. Net mass balance for the measurement period was reproduced reasonably well when validated against directly measured turbulent fluxes, stake measurements, and continuously measured surface height at the AWS. For the bare ice surface net radiation provided the major energy input for ablation, whereas sensible heat flux was a second heat source. Ablation was by both melt (70%) and sublimation (30%). At the debris-covered ice site investigated, it is inferred that the debris cover is sufficient to insulate the underlying ice from ablation. Synoptic weather situations were analysed based on AVHRR composite images and surface pressure charts. Three distinct synoptic situations were found to occur during the summers, these were defined as Type A, low pressure system residing in the Ross Sea Embayment; Type B, anticyclonic conditions across region; and Type C, a trough of low pressure extending into the Ross Sea Embayment. A dependence of surface energy fluxes and mass balance on synoptic situation was identified for the bare ice surface. The distributed model was found to produce spatial patterns of mass balance which compared well with stake measurements. Mass balance patterns show that the McMurdo Ice Shelf was generally ablating in the west, and accumulating in the east during summer. Areas of enhanced ablation were found which were likely to be caused by the surface conditions and topographic effects on the wind field. The mean summertime surface mass balance across the entire ice shelf for the 2003-2004 and 2004-2005 summers were –2.5 mm w.e. and –6.7 mm w.e. respectively. The differences between the two summers are inferred to be a result of more frequent type A conditions occurring during the summer of 2004-2005.

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Microbial community composition plays a vital role in soil biogeochemical cycling. Information that explains the biogeography of microorganisms is consequently necessary for predicting the timing and magnitude of important ecosystem services mediated by soil biota, such as decomposition and nutrient cycling. Theory developed to explain patterns in plant and animal distributions such as the prevalent relationship between ecosystem productivity and diversity may be successfully extended to microbial systems and accelerate an emerging ecological understanding of the "unseen majority." These considerations suggest a need to define the important mechanisms which affect microbial biogeography as well as the sensitivity of community structure/function to changing climatic or environmental conditions. To this end, my dissertation covers three data chapters in which I have 1) examined patterns in bacterial biogeography using gradients of environmental severity and productivity to identify changes in community diversity (e.g. taxonomic richness) and structure (e.g. similarity); 2) detected potential bacterial ecotypes associated with distinct soil habitats such as those of high alkalinity or electrical conductivity and; 3) measured environmental controls over the function (e.g. primary production, exoenzyme activity) of soil organisms in an environment of severe environmental limitations. Sampling was performed in the polar desert of Antarctica's McMurdo Dry Valleys, a model ecosystem which hosts microbially-dominated soil foodwebs and displays heterogeneously distributed soil properties across the landscape. Results for Chapter 2 indicate differential effects of resource availability and geochemical severity on bacterial communities, with a significant productivity-diversity relationship that plateaus near the highest observed concentrations of the limiting resource organic carbon (0.30mg C/g soil). Geochemical severity (e.g. pH, electrical conductivity) primarily affected bacterial community similarity and successfully explained the divergent structure of a subset of samples. 16S rRNA amplicon pyrosequencing further revealed in Chapter 3 the identity of specific phyla that preferentially exist within certain habitats (i.e. Acidobacteria in alkaline soils, Nitrospira in mesic soils) suggesting the presence of niche specialists and spatial heterogeneity of taxa-specific functions (i.e. nitrite oxidation). Additionally, environmental parameters had different explanatory power towards predicting bacterial richness at varying taxonomic scales, from 57% of phylum-level richness with pH to 91% of order- and genus-level richness with moisture. Finally, Chapter 4 details a simultaneous sampling of soil communities and their associated ecosystem functions (primary productivity, enzymatic decomposition) and indicates that the overall organic substrate diversity may be greater in mesic soils where bacterial diversity is also highest, thus a potentially unforeseen driver of community dynamics. I also quantified annual rates of soil production which range between 0.7 - 18.1g C/m2/yr from the more arid to productive soils, respectively. In conclusion, the extension of biogeographical theory for macroorganisms has proven successful and both environmental severity and resource availability have obvious (although different) effects on the diversity and composition of soil microbial communities. / Ph. D.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 20, 2010). Includes bibliographical references. Also issued in print.

Thesis (Ph. D.)--University of Canterbury, 2009. / Typescript (photocopy). Includes bibliographical references (leaves 177-204). Also available via the World Wide Web.