The morphology of supraglacial lake ogives

Supraglacial lakes on grounded regions of the Greenland and Antarctic ice sheets sometimes produce ‘lake ogives’ or banded structures that sweep downstream from the lakes. UsingSupraglacial lakes on grounded regions of the Greenland and Antarctic ice sheets sometimes produce ‘lake ogives’ or banded structures that sweep downstream from the lakes. Using a variety of remote-sensing data, we demonstrate that lake ogives originate from supraglacial lakes that form each year in the same bedrock-fixed location near the equilibrium-line altitude. As the ice flows underneath one of these lakes, an ‘image’ of the lake is imprinted on the ice surface both by summer- season ablation and by superimposed ice (lake ice) formation. Ogives associated with a lake are sequenced in time, with the downstream ogives being the oldest, and with spatial separation equal to the local annual ice displacement. In addition, lake ogives can have decimeter- to meter-scale topographic relief, much like wave ogives that form below icefalls on alpine glaciers. Our observations highlight the fact that lake ogives, and other related surface features, are a consequence of hydrological processes in a bedrock-fixed reference frame. These features should arise naturally from physically based thermodynamic models of supraglacial water transport, and thus they may serve as fiducial features that help to test the performance of such models.Research conducted at the University of Chicago was supported by several US National Science Foundation (NSF) grants, including ARC-0907834, ANT-0944248 and ANT-0944193. We thank Dorian S. Abbot for helpful discussions and review of earlier manuscripts. This work began as a result of NSF-supported summer research internships awarded in 2010 to Pablo S. Wooley (Bowdoin College) and Julia E. Vidonish (University of Chicago). We thank S.G. Warren for informative discussions about the brightening of lake bottom surfaces. We also thank Roman J. Motyka for helpful discussions and the use of SPOT5 products. SPOT data products used in this study were provided by the SPOT5 stereoscopic survey of Polar Ice: Reference Images and Topographies (SPIRIT) during the fourth International Polar Year (2007–09). We acknowledge W.T. Colgan for helpful criticism of the ideas presented in this paper, and review of earlier versions of the manuscript. We acknowledge the use of data and/or data products from CReSIS generated with support from NSF grant ANT- 0424589 and NASA grant NNX10AT68G. Ed Waddington, Derrick Lampkin and and two anonymous referees provided comments that significantly improved the manuscript. We dedicate this manuscript to the memory of Keith Echelmeyer, who first described lake ogives and considerably enriched the science of glaciology throughout his life.Ye

Assessing Preferential Flow by Simultaneously Injecting Nanoparticle and Chemical Tracers

The exact manner in which preferential (e.g., much faster than average) flow occurs in the subsurface through small fractures or permeable connected pathways of other kinds is important to many processes but is difficult to determine, because most chemical tracers diffuse quickly enough from small flow channels that they appear to move more uniformly through the rock than they actually do. We show how preferential flow can be assessed by injecting 2 to 5 nm carbon particles (C-Dots) and an inert KBr chemical tracer at different flow rates into a permeable core channel that is surrounded by a less permeable matrix in laboratory apparatus of three different designs. When the KBr tracer has a long enough transit through the system to diffuse into the matrix, but the C-Dot tracer does not, the C-Dot tracer arrives first and the KBr tracer later, and the separation measures the degree of preferential flow. Tracer sequestration in the matrix can be estimated with a Peclet number, and this is useful for experiment design. A model is used to determine the best fitting core and matrix dispersion parameters and refine estimates of the core and matrix porosities. Almost the same parameter values explain all experiments. The methods demonstrated in the laboratory can be applied to field tests. If nanoparticles can be designed that do not stick while flowing through the subsurface, the methods presented here could be used to determine the degree of fracture control in natural environments, and this capability would have very wide ranging value and applicability.King Abdullah University of Science and Technology (KAUST). Grant Number: KUS-C1-018-0