Accumulation Estimates from Ice Cores

J.R. McConnell, R.C. Bales, B. Snider, B. Matson

Dept. of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721

joe@hwr.arizona.edu, roger@hwr.arizona.edu,

betty@hwr.arizona.edu, matson@hwr.arizona.edu

Grant: NAG5-5031

Objectives

The primary objective of our work is to use multi-species chemical analyses of 20-m to 150-m deep ice cores to develop an estimate of annual accumulation at various sites on the Greenland ice sheet. Additional objectives include 1) the development of high-resolution, spatially distributed ice-core chemical data for use in modeling historical changes in atmospheric chemistry from pre-industrial time through the present and 2) the development and validation of methods for estimating shifts in sub-annual accumulation patterns based on ice-core chemistry. The main atmospheric chemistry question that we are addressing is the use of ice-core concentrations of H₂O₂ and HCHO as proxy records of atmospheric oxidation capacity.

Work Done in 1997

Field Work. Shallow firn cores were collected at eight sites (Figure 1) around the perimeter of the Greenland ice sheet at approximately the 2500 m elevation contour. The 4-inch cores were collected by 2 and sometimes 3 person teams using the ``sidewinder" coring device developed by Jay Kyne at PICO. At those shallow coring sites that were co-located with automatic weather stations (AWS), more than one core was collected in order to investigate issues of short-scale spatial variability in snow accumulation (Table 1). In addition, snow pits were dug and sampled at three of the four AWS sites.

Collection of the cores at the AWS sites (N.DYE_3, South_Dome, Tunu_S, and NASA_E) involved a minimum of 2 nights at each site. Because of weather delays and logistical costs, we switched to a ``commuter" collection mode for the four west coast sites where a single, 20-m core was collected, weighed and bagged in approximately 4 hours while the aircraft waited on the ground. The help of the flight crew at these sites (especially the crews from Flugfelag Nordurlands) was much appreciated. This mode of operation allowed us to carry only emergency survival gear in addition to the coring equipment and meant that the aircraft went in and came out fully loaded. If core collection had involved camping on the ice at these west coast sites where no AWS was located, the aircraft would have generally gone in full, and come out empty on deployment and then gone in empty and come out full on re-deployment.

Laboratory Analyses. The firn and ice-core chemistry is measured using a continuous melter and flow-through chemical analysis system. Resolution in depth (and so time) in the melter system is determined by the volume of ice melted compared to the volume of the flow-through analytical system. In our original melter design, a 1.0 cm diameter cylinder from the inner part of a 2.5 X 2.5 cm, longitudinal cut of the core was used in the chemical analyses. At low accumulation sites, however, this did not provide sufficient vertical resolution so we have recently modified the melter to increase the vertical resolution by doubling the volume of ice melted. We now use a 2.0 cm diameter inner core from a 3.5 X 3.5 cm longitudinal cut. We currently analyze for calcium (Ca⁺⁺), ammonium (NH₄), nitrate (HNO₃), hydrogen peroxide (H₂O₂), and formaldehyde (HCHO) and make two measurements of liquid conductivity. To further improve vertical resolution, we have switched to conductivity instruments with very small (7 ml) flow cells.

Results

Shallow Core Analyses. A subset of the shallow cores collected during 1997 recently arrived and about 55 m have been analyzed. An example of the melter results is given in Figure 2 for 10 m at the 7247 site. Note the very clear annual cycles in hydrogen peroxide and less well defined annual cycles in ammonium, calcium, liquid conductivity, and nitrate. Snow depths were converted to water equivalent using an average depth-density curve for the four west coast sites. Preliminary water equivalent accumulation values from the west coast sites are plotted on Figure 1.

Deeper Core Analyses. We have continued to analyze the deeper cores collected at the GITS and Tunu sites in 1996. Interpretation of the results is underway. Results from the NASA-U and Humbolt cores collected in 1995 are reported in Anklin et~al. [1997] and Bales [1997].

Plans for 1998

We would like to collect additional shallow (20-30 m) cores in order to better understand snow accumulation patterns in in the high accumulation areas along the west coast and in the south of Greenland. In addition to deriving accumulation estimates from new and existing core data, modeling will focus on understanding spatial variability in snow accumulation at sites were multiple cores have been collected and where AWS information is available. We also plan to use the results of recent air-snow transfer function modeling for H₂O₂, in combination with snow ventilation modeling from M. Albert and atmospheric photochemical modeling at NASA/Goddard, to estimate changes in sub-annual accumulation timing from the ice core chemical records.

Data availability

· Current: annual accumulation estimates for NASA-U (3 cores: 350, 30, 29 yr) and Humboldt Glacier (5 cores: 852, 66, 71, 68, 71 yr) cores.

· Spring 1998: annual accumulation estimates for GITS and Tunu cores (120-m and 20-m cores), and 8 sites listed in Table 1 (20-m cores).

Table 1. Shallow core locations, attributes, and samples collected.

Site	Latitude (N)	Longitude (W)	Elevation (m)	No. Cores (Depth (m)))	Snow Pit	AWS
N_DYE_3	66° 0.033¢	44° 30.083¢	2460	2 (19.06, 17.27)	Yes	Yes
South_Dome	63° 8.933¢	44° 49.0¢	2858	2 (24.54, 15.30)	Yes	Yes
Tunu_S	69° 49.017¢	34° 59.9¢	3083	3 (20.57, 10.43, 10.41)	Yes	Not Yet
NASA_E	75° 0.0¢	29° 59.98¢	2634	2 (20.19, 10.84)	No	Yes
7247.5	71° 55.552¢	47° 29.212¢	2277	1 (20.06)	No	No
7551	75° 0.0¢	50° 59.97¢	2265	1 (21.19)	No	No
7653	76° 0.0¢	53° 0.0¢	??	2 (14.91, 4.97)	No	No
7147	71° 4.15¢	47° 14.557¢	2134	1 (19.85)	No	No

References

Anklin, M., R. C. Bales, E. Mosley-Thompson, and K. Steffen, Annual accumulation at two sites in northwest Greenland during recent centuries, Journal of Geophysical Research, submitted, 1997.

Bales, R. C., Comparison of accumulation trends in recent centuries in northwest Greenland, Eos, Transactions, American Geophysical Union, Submitted to Fall Meeting, 1997.