GISP2 Electrical Conductivity Measurements

REFERENCES: 

Taylor, K.C., R.B. Alley, G.W. Lamorey, and P.A. Mayewski. 1997. Electrical
measurements on the Greenland Ice Sheet Project 2 core. Journal of
Geophysical Research 102:26511-26517.

Taylor, K.C., P.A. Mayewski, M.S. Twickler, and S.I. Whitlow. 1996. Biomass
burning recorded in the GISP2 ice core: A record from eastern Canada? The
Holocene 6(1):1-6.

Chylek, P., B. Johnson, P.A. Damiano, K.C. Taylor, and P. Clement. 1995.
Biomass burning record and black carbon in the GISP2 ice core. Geophysical
Research Letters 22:89-92.

Taylor, K.C., G.W. Lamorey, G.A. Doyle, R.B. Alley, P.M. Grootes, P.A.
Mayewski, J.W.C. White, and L.K. Barlow. 1993. The 'flickering switch' of
late Pleistocene climate change. Nature 361:432-436.

Taylor, K.C., R.B. Alley, R.J. Fiacco, P.M. Grootes, G.W. Lamorey, P.A.
Mayewski, and M.J. Spencer. 1992. Ice core dating and chemistry by direct
current electrical conductivity. Journal of Glaciology 38:325-332.

DATA DESCRIPTION: 

The electrical conductivity measurement (ECM) measures the direct current
between two electrodes with a potential difference of 2,100 volts. The
electrodes have a surface area of 1 x 2 mm and are 1 cm apart. The
electrodes are made of bronze.  The current flowing through the electrodes
is digitally recorded every millimeter. In an aqueous solution, all the
ions can participate in the conduction of an electrical charge. In ice the
movement of the ions is restricted by the ice lattice which reduces the
ability of the ions to conduct a direct current. The direct current is
conducted by the movement of protons associated with the H+ of strong
acids; hence, the ECM current is considered to be a measure of the acidity
of the ice. The ECM has a spatial resolution of <1 centimeter (cm), yet is
rapid enough that it is practical to measure it continuously along the
entire core. These attributes make the ECM ideally suited for
investigation of short duration phenomena which influence the acid/base
balance of the ice. 

Two-meter sections of the core were passed under a horizontal bandsaw to
remove a 5 cm wide slab along the axis of the core. A rail mounted
microtome knife or milling machine was used to shave the saw cut surface
to remove surface contamination and surface irregularities. A computer
controlled the motion of electrodes along the axis of the core and
recorded the ECM current for each millimeter of travel. The operator used
a hand switch to identify locations where the electrodes encountered
fractures in the core. These data were removed from the data set. A
similar system was used on the GRIP core. 

Instrument related artifacts adversely influence some aspects of the GISP2
ECM record. The ECM current is influenced by the temperature of the ice
when the measurement was made. A correction based on the surface
temperature of the ice was made to determine what the ECM current would be
at a temperature of -20_C. When an ice core was undergoing a temperature
transition, the surface temperature did not represent the temperature of
the volume being measured.  There may be sections two to 50 m in length
that have a several percent variation from the correct ECM current because
of the temperature uncertainty. We now hold the ice at a constant
temperature for 24 hours prior to measurement to avoid this complication.
An error of several percent over hundreds of meters may have occurred due
to variations in the surface conditions of the electrodes.  Between 1100 m
and 1400 m, the core quality was low and there were numerous small pieces
of ice per 2 m segment of core. There was also an intermittent problem
with the electrical grounding of the system. Both of these factors reduced
the data quality in this section. Better methods of maintaining consistent
electrode surface conditions have been instituted. The preamplifier was
changed at a depth of 2250 m, which appears to have altered the response
of the ECM instrumentation at levels below 0.1 microamps. This results in
an artifact which incorrectly suggests that stadial periods have a
slightly lower ECM current above 2250 m than below 2250 m. The design of
the preamplifier has been stabilized to prevent this from recurring. 

The data is saved in files, each of which contains 50 m of data. The first
letter of the file indicates the core which the data is from. The 4 digits
which follow is the depth at the top of the file. 

The extension of .dat indicates that it is an ecm file versus depth (i.e.
The file d0200.dat is the ecm data for the GISP2 D core (which is the main
deep core) from a depth of 200 m to 250 m.) These files have two columns.
The first column is the depth, the second column is the ECM current in
microamps. 

The extension of .a indicates that it is an ecm file versus age (i.e. the
file d0200.a is the ecm data for the GISP2 D core from a depth of 200 m to
250 m. These files have two columns.  The first column is age (years B.P.
1950 on the Meese/Sowers timescale), the second column is the ECM current
in microamps. 

The ECM file ecmage.dat represents approximately 0.5 meter averages of the
data in the b****.dat and d****.dat files.