thickness

Ice Thickness for Taylor Valley Lakes, Antarctica

Abstract: 

Ice thickness was 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 m^2 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.

Associated Personnel: 

208
762
209
801

Dataset ID: 

67

Short name: 

icethickness

Methods: 

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.

Data sources: 

LIMNO_ICE_THICKNESS

Maintenance: 

The 1995/96 data from this table was submitted to INSTAAR by Ray Kepner at the Desert Research Institute. It was included in several e-mail messages sent in August 1997. Denise Steigerwald, the data manager used the information in these messages to create a file called thckness.dat.
       
 The 1996/97 and 1997/98 data was submitted to INSTAAR by John Priscu's team at Montana State University. The former is a Microsoft Excel version 6.0 file (icethk.xls), and the latter is an ascii text file. 
       
  Upon arrival at INSTAAR, the data manager  fine-tuned the files to allow them to be merged for use on the MCM LTER web site. Adjustments included the following: 
       
  Lake Descriptions" file. added to the "Limno Holes, Dive Holes, Lake Level Measurement Locations" file. 
       
 The icethk.xls file did not contain variables for latitude and longitude, but records for each "location" represented in this dataset were added to the "Limno Holes, Dive Holes, Lake Level Measurement Locations" file (leaving the latitude and longitude fields blank). The links made possible by doing this will allow users to call up further information about a given lake (thru the link with the "McMurdo Dry Valley Lake Descriptions" file), and/or about a given sampling location on the lake (thru the link with the "Limno Holes, Dive Holes, Lake Level Measurement Locations" file). 
       
 icethk.xls file had  variables for z-water, z-ice, and z-diff. After consulting with Craig Wolf (at Montana State University), the ice thickness variable was renamed to z-ice. project (S025) which is still taking place. John Priscu requested to have these records removed from the MCM LTER web site in an August, 1998 e-mail message. Any location codes containing a "C" followed by a number  (eg. C01) were removed from this file.
       
Each of these adjustments allowed the all of the ice thickness measurements to be represented in one data file on the MCM LTER web site. The file was imported into Microsoft Access on INSTAAR's Unix system, and can currently be found there. The file was then exported in ascii, comma delimited text and MS-DOS text (table layout) to present on the MCM LTER web site. Both of these files are linked to this web page above. 
       
Information for the metadata was obtained from the icethick.metadata.txt and Metaicethick9697.rtf files. The".rtf" file was called up using Microsoft Word version 6.0, and a simple text editor was used for the icethick.metadata.txt file. Text from these files was used to create this page in html format.
      
In 2006, Chris Gardner added Latitude and Longitude columns to the Ice Thickness table.  He also deleted and reloaded seasons 2001-2002 through 2003-2004 as some errors were found.  Lat/Longs were added to these dates as well. Metadata was also made conformant to the EML format, and served through the MCM webpages using an XSLT transformation.
 
In 2014, metadata was enhanced using the Drupal Ecological Information Management System (Inigo San Gil)
 
In 2016, enhancements to metadata and data updates were introduced using DEIMS (Inigo)

Additional information: 

Problems with the Kovacs drill prevented the limnological sampling team from attaining more than one "unbiased" measurement at Lake Fryxell during the 1996-97 season.
 
Notes about the 2004/05 season
 
Because sampling at Lake Vanda was done as a day trip, the Aqua-Vu system was not brought to this sampling location, and the ice thickness measurement was done using  the method employed in previous years.  A 30 cm brass rod, with nylon cord attached to one end, was fastened to a 10 m tape measure and lowered through the 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 recorded.
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