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  • Thumbnail for Geophysical evidence for englacial brine associated with Blood Falls, McMurdo Dry Valleys, Antarctica
    Geophysical evidence for englacial brine associated with Blood Falls, McMurdo Dry Valleys, Antarctica by Badgeley, Jessica

    Ground-penetrating radar (GPR) images of Taylor Glacier, in the McMurdo Dry Valleys, Antarctica, reveal an englacial drainage system near this polar glacier’s terminus at Blood Falls. Blood Falls is the surface manifestation of episodic releases of subglacial, iron-rich, hypersaline, microorganism-bearing brine. Locating englacial brine near Blood Falls would allow us to extract unoxidized brine in order to better understand the geochemistry and microorganisms as a proxy for life on other planets. In the current study, we collected a grid of GPR transects immediately upglacier from Blood Falls to locate the path by which brine surfaces and to inform future drilling operations in search of subsurface brine. Additionally, this study explores the extent of the subglacial and englacial brine reservoirs and seeks to refine hypotheses about the mechanisms driving the brine to the surface. In each of the GPR profiles, we found an englacial scattering zone located above a break in the basal-ice reflection. Downwarping of the basal-ice reflection on either side of the break and the break itself indicate that the scattering zone has slowed down the electromagnetic waves and prevented their further propagation into the glacier. We interpret this scattering zone as evidence of water-saturated and/or salty ice. A three-dimensional plot of the scattering zones visible on the profiles reveals a linear trend upglacier from Blood Falls nearly paralleling previously active brine-releasing cracks. Our evidence suggests that the zone is a recently or currently active englacial brine reservoir. In 2014, a team drilled near the area and successfully extracted pressurized brine ~16 m deep and upstream from Blood Falls at -7.1°C within surrounding ice of ~-17 °C. This brine temperature is consistent with the theoretical basal temperature of -7.8 °C that Hubbard et al. (2004) modeled near their hypothesized brine source 3-6 km upglacier from the terminus using geothermal heat flux and friction caused by ice deformation. Further study of the GPR data has allowed us to better understand the extent and movement of subglacial brine to the surface. Our cross-terminus traverse GPR transect shows that the subglacial brine reservoir may, in some form, extend all the way to the terminus and allow continuous brine release into Lake Bonney. As the subglacial brine surfaces, our data and analyses confirm hypotheses that it follows favorable pressure gradients up through surface cracks that penetrate the brine reservoir.

  • Thumbnail for Composition and rheology of overthickened lava flow units on Banks Peninsula, New Zealand
    Composition and rheology of overthickened lava flow units on Banks Peninsula, New Zealand by Kroner, Ryan

    Exceptional coastal exposures of Miocene lava sequences on Banks Peninsula, New Zealand provide an opportunity to understand the emplacement processes of over-thickened lava flows. Field observations of three geochemically variable (nephelinite, hawaiite, and mugearite) over-thickened flows reveal 2-4 m-thick lavas inland that transition into ~50 m cliff-forming compound units toward the ocean. These cliff sections are the result of rapid emplacement of multiple flows that enable cooling as a single unit. Possible eruption rates and reconstruction of various emplacement mechanisms can determine controls on the over-thickening of the aforementioned lava examples. Flow compositions modeled against temperatures and viscosities indicate that viscosities and compositions did not exert a significant influence on the over-thickened morphologies. There is no correlation between geochemistry (especially wt. %SiO2) and the morphology of the overthickened flows, and the close relationship of viscosity to geochemistry suggests that viscosity is not a major factor in the emplacement of these overthickened flows. Computational analysis of viscosities, coupled with measurements of flow dimensions and crystal content, reveal that high effusion rates may have been a critical factor in forming these overthickened flows, with rates of between 400 and 2000 m3 s-1 being likely. Production of more precise ranges of possible rates is hindered by uncertainties associated with taking dimensional measurements of flows as old, weathered, and poorly exposed as these. Additionally, a particularly large margin of error for flow temperatures prevents the drawing of any reasonably precise conclusions with regard to effusion rates. This example illustrates the problems inherent in attempting to quantify such precise numerical parameters for ancient lava flow units. It is very likely that paleo-topography was a very influential factor on these flows’ unique morphologies. Further work should focus on significantly narrowing the margins of error for flow dimensions (thickness and width) and flow temperatures.