Lake ice thickness and density measurements, McMurdo Dry Valleys, Antarctica (1989-2025, ongoing)

As part of the McMurdo Dry Valleys Long Term Ecological Research (LTER) project, ice thickness and density are measured annually for several lakes located throughout this region of Antarctica. Ice thickness is measured from the bottom of the ice cover to the piezometric water level and to the top of the ice cover. Because most limnological sampling holes are drilled in low-lying areas, it was thought that ice thickness measurements were biased. Therefore, random ice thickness measurements were performed in a 1000 m2 sampling grid on each lake in past years. Statistical analyses by Dr. Priscu have shown that measurements from three ice holes are representative of the ice thickness measured by this grid system.

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A 30 cm brass rod, with nylon string attached to one end, was fastened to a 10m tape measure and lowered through a newly drilled ice hole. Once through the ice, the tape measure was slowly pulled upward allowing the brass rod to catch on the bottom of the ice cover. The distances from the bottom of the ice cover to the piezometric water level (z-water), and to the top of the ice cover (z-ice) were then recorded. The difference between these two measurements (z-difference) or "freebore" provides some insight into the topographical features of the ice cover as well as density of the ice. Ice thickness measurements are typically performed on each hole drilled through the ice cover. Because most limnological sampling holes are drilled in low-lying areas, ice thickness measurements have been biased. Therefore, random ice thickness measurements were performed in a 1000 m2 sampling grid on each lake. At Lakes Fryxell, Hoare, East and West Bonney the sampling grids are located along a 100 m baseline due south of the blue instrument boxes. Prior to sampling, two sets of random numbers between 0 and 100 were generated as south and east coordinates in this 100x100m grid. For example, if the sampling coordinates were (56S, 72E), the hole would be located 56 m due south along the baseline, then similarly, 72 m due east of that baseline. A new method for measuring ice thickness was employed in the 2004-2005 season using the Aqua-Vu underwater viewing system, which allows us to see the exact point where the bottom of the ice cover is located. The Aqua-Vu underwater camera is lowered through a newly drilled (and usually at least partially melted) ice hole, and the camera lowered until the bottom of the ice cover is viewed through the above-water viewing system. The distance from the bottom of the ice cover to the piezometric water level (z-water) is marked on the viewing system cord and measured with a tape measure once the viewing system is brought out of the ice hole. The distance from the piezometric water level to the top of the ice cover (z-difference) is measured using a tape measure. If more than one replicate measurement is taken for z-water or z-difference, the replicates are averaged. Z-difference, or 'freebore,' provides some insight into the topographical features of the ice cover as well as density of the ice. The addition of z-water and z-difference gives the total ice thickness (z-ice). Ice thickness measurements are typically performed on each hole drilled through the ice cover. Because most limnological sampling holes are drilled in low-lying areas, it was thought that ice thickness measurements were biased. Therefore, random ice thickness measurements were performed in a 1000 m2 sampling grid on each lake in past years. However, statistical analyses by Dr. Priscu have shown that measurements from three ice holes are representative of the ice thickness measured by this grid system. Starting during the 2004-2005 season, ice thickness measurements were done on the two holes drilled for limnological sampling, plus one additional hole. In addition, notes were taken to describe the topography of the ice surrounding the ice hole (see comments section of data file). Generally ice thickness measurements are done on the two holes drilled for limnological sampling (1 hole inside the polarhaven, designated as "Inside Hole," and 1 hole outside) plus one additional outside hole. If there is an opportunity to take measurements in other holes that have been drilled, measurements are taken. Holes that are maintained open throughout the season are measured for ice thickness throughout the season. Note that ablation occurs around the sampling polarhaven, so take note of this when reviewing the Z-difference values for the "inside" holes throughout the season. Notes are taken to describe the topography of the ice surrounding the ice hole and are detailed in the comments section of the data file.