SNOW CHEMISTRY ACROSS ANTARCTICA - page 2
"Ice cores provide the most direct and highly resolved records of (especially) atmospheric parameters for the last 1,000,000 years" (EPICA Community Members, 2004). While ice core chemistry analyses have revolutionised our knowledge on the working of the climate system and its variability through time (Legrand and Mayewski, 1997; Mayewski and White, 2002), an improved understanding of the fundamental factors that ultimately control the chemistry of a snow or ice sample will allow even more detailed and accurate interpretation of glaciochemical records reconstructing past climate conditions with near-instrumental quality.
To reach this understanding it is necessary to determine individual sources and pathways of aerosols, mechanisms that control precipitation efficiency as well as post-depositional effects (Legrand and Mayewski, 1997). Comparing snow chemistry at different sites and investigating the processes leading to spatial differences in snow chemistry help to improve our understanding of temporal variability and teleconnections. Here, we provide an up-dated summary of available data from 520 sites in Antarctica, developed by the International Trans Antarctic Scientific Expedition (ITASE), with the goal of providing this new dataset along with research recommendations to the wider ice core community and thereby stimulate and focus the discussion towards a more comprehensive data interpretation.
ITASE has as its primary aim “… the collection and interpretation of a continental-wide array of environmental parameters assembled through the coordinated efforts of scientists from several nations” (Science and Implementation Plan, 1990). During the ISAG 7 meeting in Milano, Italy, 2003, the ITASE community established seven synthesis groups, of which this group – the ITASE Chemistry Synthesis Group – is coordinating the compilation and interpretation of the spatial variability in snow and ice chemistry across the continent to address the knowledge gap on factors governing variability of ice core chemistry in Antarctica. A two-step approach was adopted. Firstly, broad patterns in Antarctic snow chemistry are investigated using all available reliable data (this paper). This will allow formulating the strategy for the second step, in which the group will focus on individual time periods in order to investigate the causes for changes in chemistry patterns (future papers). This will be achieved by contrasting for example El Niño with La Niña years or studying the years before and after volcanic eruptions, such as the recent Mt Pinatubo event.
In this first step we summarise new and previously published data and provide recommendations for future common efforts. The new data are provided by the national ITASE programmes of Australia, Brazil, China, Germany, Italy, Japan, Korea, New Zealand, Norway, United Kingdom, United States of America, and the national Antarctic programme of Finland.
Data Selection Criteria
Previous glaciochemical surveys showed that careful data selection for an Antarctic-wide comparison is important (Mayewski and others, 1992; Mulvaney and Wolff, 1994; Wagenbach, 1996; Legrand and Mayewski, 1997; Wolff and others, 1998a; Wolff and others, 1998b; Kreutz and Mayewski, 1999; Kreutz and others, 1999; Stenberg and others, 1999). Data from 520 sites are summarised here and can be downloaded as a pdf file. (Data Table) While the laboratory procedures of the individual groups are of high standard, no cross-evaluation has yet been undertaken. To obtain further information on individual data sets contact details are provided along with the data.
Because Antarctic glaciochemistry shows large seasonal variability (Gow, 1965; Sigg and Neftel, 1988; Solomon and Keys, 1992; Legrand and Mayewski, 1997; Curran and others, 1998; Wolff and others, 1998b; Kreutz and others, 1999; Bertler and others, 2004b) it is desirable for any continent-wide comparison to use either well-dated (sub-annual) records or multi year averages. The achievable level of age control of ice core records is dependent on many factors, but particularly on annual accumulation (and sampling resolution) and therefore varies greatly across Antarctica. Of the 520 available data sources, 194 records are reliably identified as multiyear samples. The remaining records are predominantly surface snow samples collected along transects and thus are an important contribution to determine aerosol sources. For the comparison of new ITASE data, however, we decided to aim in this first step for 5-year averages. This allows short records to be included while eliminating seasonal variability. A survey of ITASE meta data indicates that the 5-year interval that is represented the most in the currently available data set is 1992 to 1997. At present 45 sites provide well-dated chemistry measurements for this time period. This interval coincides with the Mount Pinatubo volcanic eruption, and provides therefore an opportunity to study the effect of volcanic eruptions in future papers when time series are considered.
A second fundamental decision is whether to use concentration or flux data (Kreutz and others, 2000). Due to the spatially variable influence of dry and wet deposition across Antarctica and the difficulty to obtain reliable, high-resolution annual snow accumulation measurements, concentration data are preferred over flux. However, as more accumulation data will become available, the influence of spatially and temporal varying snow accumulation leading to varying contributions of wet versus dry deposition should be investigated further. This can be achieved by merging the data of this group with the currently compiled data sets of the ITASE/ISMASS Mass Balance and Atmospheric Chemistry Synthesis Groups.
In the meta data survey, information on all glaciochemical analyses have been compiled. Here, we focus on major ions – sodium (Na), magnesium (Mg), calcium (Ca), potassium (K), chloride (Cl), nitrate (NO3), sulphate (SO4), and methane sulfonate (MS). An Antarctic-wide comparison of otherspecies, such as trace elements, organic acids, and particles is hampered due to the limited number of data points currently available. However, growing interest and improved analytical methods will enable us to incorporate such data in the near future.