Correcting Ice Sheet Mass Balance for Refreezing of Infiltrated Surface Melt: The Forgotten Grand Problem
by W. Tad Pfeffer
Recent images showing vastly expanded regions of surface melt on the Greenland Ice Sheet are testimony to the climatic anomalies now occurring in the Arctic with ever greater frequency and duration. What these images tell about the overall mass balance of the Greenland Ice Sheet, on the other hand, is another question, not directly addressed by determination of surface melt rates. The significance of that increased melt depends greatly upon whether the water thus produced runs off directly to the ocean via near-surface hydrologic pathways, or drains to the base of the ice sheet where it may influence ice sheet sliding, or percolates some short distance vertically to be retained, possibly as saturated firn or as refrozen “infiltration ice.” At present the observational evidence needed to investigate this problem is sparse (but not altogether absent), and relevant theory is in development, but incomplete. Significantly, there is still no observational method for directly measuring the outflow of water from the Greenland Ice Sheet, and all evaluations of the surface mass balance of the ice sheet depend to some extent on modeled runoff, based on unconfirmed assumptions about the mobility and transport of water once it has been generated by surface melt.
The fate of surface-generated melt on the Greenland Ice Sheet has been the focus of studies going back to the 1950s and 1960s when research motivated by Cold War military objectives was conducted on internal firn structure, and trafficability of the ice sheet surface. The earliest comprehensive global studies on future sea level rise began in the late 1970s, leading to research specifically on Greenland’s overall mass balance. At that time, retention of infiltrating melt water by refreezing was identified as a crucial unknown to be resolved if the mass balance of polar glaciers, and the Greenland Ice Sheet in particular, was to be modeled based on projections of future temperature. Subsequent developments in surface energy balance measurements and modeling have yielded a robust capacity to model the generation of surface melt, but to date the near-surface transport of that melt has at best been handled by one-dimensional models of infiltration and refreezing. Observations show, however, that melt water infiltration in subfreezing, heterogeneous, stratified firn is highly non-linear and typically involves the development of vertical channels, or “flow fingers,” where water flows downward through firn of higher relative saturation and hydraulic conductivity. The differences in transport characteristics between one-dimensional, homogenous and three-dimensional, heterogeneous infiltration can be dramatic.
I will review the fundamental physics of the problem of melt water infiltration in cold, permeable snow, trace its history as a research issue in the context of forecasting future sea level rise, discuss some interesting and very new developments, and make an initial evaluation of the effect of this unsolved problem on sea level projections.
Bio for W. Tad Pfeffer
W. Tad Pfeffer is a glaciologist, geophysicist, and photographer at the University of Colorado at Boulder. He is a Fellow of the University’s Institute of Arctic and Alpine Research and Professor in the Department of Civil, Environmental, and Architectural Engineering. Pfeffer’s research is focused on glacier mechanics and dynamics, and particularly on dynamics of oceanending glaciers and glacier contributions to sea level. He has done field research for more than 30 years in glacier regions from Alaska to Antarctica to the summit of Mt. Kilimanjaro. Pfeffer also leads the long-term study of Columbia Glacier, on Alaska’s South Central Coast, one of the world’s most extensively studied and most rapidly changing glaciers. He has served as an advisor to the United Nations Environmental Program (UNEP), the Arctic Monitoring and Assessment Program (AMAP), and is a Lead Author for Chapter 13 (Sea Level Change) in the IPCC Fifth Assessment/Working Group I. In addition to his scientific work, Pfeffer’s photography has appeared in many publications in the US and Europe. He is the author of The Opening of a New Landscape: Columbia Glacier at Mid-Retreat, published by the American Geophysical Union in 2007.
Evaluating the History of Vadose-Water Flow Through Yucca Mountain, Nevada, Using Secondary Hydrogenic Minerals – A Case for Slow and Steady
by James B. Paces, Ph.D.: USGS, Geoscience and Environmental Change Science Center, Denver, CO
Yucca Mountain, the erstwhile repository site for the Nation’s high-level radioactive waste, represents one of the most extensively studied patches of real estate on the planet. Although the waste-disposal program was suspended in 2010, a large body of science provided unprecedented insights into the geologic, hydrologic, and climate histories and processes, and how these features contribute to a natural barrier for isolating hazardous wastes over geological time periods. One of the most critical mechanisms affecting repository performance is the flow of water through the 500- to 700-m-thick vadose zone, which has the potential to interact with waste packages and transport radionuclides to the accessible environment. In addition to understanding present-day conditions, reliable performance evaluations require knowledge of hydrologic responses to climate changes expected over a functional life span of more than 100,000 years for a repository.
Secondary hydrogenic minerals formed in this environment, mostly calcite and opal, contain physical, chemical, and isotopic information that can be used to decipher the history of past flow at various time scales. In addition, the mineral deposits allow the investigation of the mechanisms and processes of fracture flow, the source of the solutions, and the physical conditions under which the minerals formed. Information derived from these deposits represents an important means of evaluating hydrologic flow and transport models that are based largely on computer simulations of rock properties rather than direct observations.
The presentation will describe the general nature of the problem of understanding vadose flow through time as well as the evolution of how the mineral record was deciphered by scientists working on the Yucca Mountain Project at the USGS. Isotopic compositions of secondary calcite and opal confirm a meteoric source of infiltration and downward percolation that has remained more-or-less constant despite large variations in surface-water availability caused by fluctuations in Pleistocene climate. Those records reflect a substantial degree of hydrological stability which is likely to be maintained for hundreds of thousands of years or longer.
Bio for Jim Paces
Jim Paces is a research geologist at the USGS who specializes in radiogenic isotope and geochronological studies. He received a Ph.D. degree from Michigan Technological University in 1988 that was focused on understanding the petrogenesis of flood basalts associated with the Proterozoic Midcontinent Rift. After completing a USGS-NRC postdoc working on lower crustal nodules from northern Michigan and mafic rocks of the Duluth Complex, he was hired by the Yucca Mountain Project Branch where he contributed to studies of paleoseismology, geomorphology, paleohydrology, and paleoclimate. After the Branch was disbanded in 2010, his focus has broadened to include areas outside of southern Nevada and his current work includes geochemical and isotope studies of speleothems, peat-rich wetland deposits, archeological materials, calcic soils, ground -water discharge deposits, and hydrologic flow systems.