KARL J. KREUTZ, PAUL A. MAYEWSKI, MARK S. TWICKLER, and SALLIE I. WHITLOW, Climate Change Research Center, Institute for the Study of Earth, Oceans, and Space and Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824
L. DAVID MEEKER, Climate Change Research Center, Institute for the Study of Earth, Oceans, and Space and Department of Mathematics, University of New Hampshire, Durham, New Hampshire 03824.
Deep ice cores collected from the interior of the west antarctic ice sheet and the interice stream ridges along the Siple Coast potentially contain long time-series records of Southern Hemisphere environmental change. One such location is Siple Dome, an approximately 120-kilometer (km) x 250-km ice dome located between ice streams C and D (figure 1). Because of promising results from reconnaissance glaciochemical (Mayewski, Twickler, and Whitlow 1995) and geophysical (Raymond et al. 1995) research, current U.S. deep ice-coring efforts are focused in the area. Drilling at Siple Dome is advantageous for several reasons, including the site's relatively simple geometry and internal layering (Raymond et al. 1995) and its sensitivity to changes in South Pacific lower atmospheric circulation (Kreutz and Mayewski in press). Changes in the strength of these circulation conditions over the last millennium have been documented using glaciochemical measurements from a 150-meter (m) ice core collected at Siple Dome in 1994 (Kreutz et al. 1997). As part of the U.S. WAISCORES program, the approximately 1,000-m ice core recovered from Siple Dome will extend such well-dated, multiparameter, high-resolution environmental reconstructions back about 100,000 years and be used to investigate several issues, including
In preparation for the recovery, analysis, and interpretation of the Siple Dome deep core, a thorough understanding of the modern glaciochemical spatial variability in the area is essential. Spatial studies were begun during the 1994-1995 season, when five snowpits were collected on a 10-km x 10-km surveyed grid centered on the Siple Dome summit (Mayewski et al. 1995). Sampling during the 1996-1997 season expanded the glaciochemical spatial investigation completed in 1994-1995 and, in addition, collected clean surface snow and firn samples from the deep-core site. In addition to snowpits covering approximately 4-10 years of deposition, shallow (approximately 100-m) ice cores collected on the same spatial grid will allow investigation of modern and longer term changes in the spatial patterns of chemical deposition, source regions, moisture flux, and the relationship between glaciochemical and other measurements (e.g., stable isotopes and physical stratigraphy).
During the 1996-1997 season, four 2-m snowpits were sampled on a transect from 30 km north to 30 km south of the ice divide (figure 1). In addition, a 4-m snowpit and a 100-m, 10.16-centimeter-diameter ice core were collected approximately 0.5 km south of the summit, at the deep-core site (figure 1). All snowpit and core sample collection was performed by workers using nonparticulating suits, polyethylene gloves, and particle masks to avoid chemical contamination. Snowpits were sampled in conjunction with other investigators (C. Shuman, J. McConnell) so that all measurements are co-registered. The 100-m core is being sampled at high resolution (subannual sampling in the upper 15 m; biannual sampling in the bottom 85 m) to provide accurate firn measurements that overlap the deep core. Concentrations of major anions, cations [sodium (Na ), calcium (Ca2+), magnesium (Mg2+), potassium (K+), ammonium (NH4+), chloride (Cl ), nitrate (NO3 ), and sulfate (SO )], and methanesulfonic acid (MSA; measured in core samples by the University of Miami) are measured via ion chromatography at the University of New Hampshire.
An example of the well-preserved glaciochemical signals present in the Siple Dome snowpack is given in figure 2. Concentrations of both excess (xs) SO4= and MSA (both byproducts of the oxidation of phytoplankton-produced dimethylsulfide) peak in the summer in the antarctic atmosphere (Wagenbach 1996). Therefore, xsSO4= maxima in the Siple Dome snowpack likely record peaks in summer biogenic activity. Such annual glaciochemical peaks can be used to assign dates to strata in snowpits (figure 2) and ice cores. Cores collected from Siple Dome thus far have been dated using a combination of high-resolution discrete chemical sampling, continuous measurements of Cl-, NO3-, and liquid conductivity, and physical properties (Kreutz et al. 1997). This technique will be used in conjunction with other measurements (e.g., electrical conductivity, dielectric properties, and stable isotopes) to date the Siple Dome deep core.
Based on the dating technique outlined above, a gradient in the number of years contained in each snowpit along the 30-km north/30-km south transect over Siple Dome is apparent (figure 2). This gradient in years, likewise, suggests a gradient in accumulation rate (b) whose dominant moisture source is from the north. Average chemical concentration values for 1994-1995 and 1996-1997 pits are similar (Kreutz et al. in preparation); however, flux (concentration*b) calculations also show a distinct gradient in all species going from north to south across the dome (figure 3). It appears that the major source of marine [seasalt (ss) Na+, MSA, and xsSO4=] species, like moisture, is from the north. This finding is consistent with previous work (Kreutz and Mayewski in press; Kreutz et al. 1997) and suggests the source of marine chemical species at Siple Dome is the Amundsen/Ross Sea region, with advection of lower tropospheric marine air across the Ross Ice Shelf to Siple Dome. Statistical investigation of glaciochemical variability on a range of spatial and temporal scales is currently being investigated (Kreutz et al. in preparation).
We thank our colleagues, the Siple Dome Science Coordination Office, Polar Ice Coring Office, Antarctic Support Associates, and U.S. Navy Squadron VXE-6 for field assistance at Siple Dome. This work was supported by National Science Foundation grant OPP 95-26449.
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