COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP

1.2 SPECIALTY GROUP REPORT: TRAPPED GAS COMPOSITION

Justification

Analyses of air bubbles embedded in polar ice reveal the composition of the pre-industrial and ancient atmospheres. So far, extensive measurements of carbon dioxide, methane, nitrous oxide and some chlorocarbons have been made on ice cores from both polar regions. The results provide a remarkable record of the magnitude and timing of human influences on the global cycles of these gases. Except for the chlorocarbons, for which there is no evidence of any substantial pre-industrial concentrations, the other gases (CO2, CH4 and N20) started increasing only during the last 200 years with the growing population and increasing needs for energy and food. The increase of N20 probably started only a few decades ago. The record shows that C02 concentrations were about 280 ppmv 200 years ago while methane and nitrous oxide concentrations were about 700 ppbv and 285 ppbv respectively. Today there is 25% more C02, 8% more N20, and 100% more CH4 in the atmosphere.

Measurements on existing ice cores provide longer records for C02 and CH4 which show large natural variations during glacial and interglacial periods. The Bern and Grenoble groups have published data that provides a convincing case that the C02 content of the atmosphere during glacial time (~ 200 ppmv) was substantially lower than that for the interglacial time (~ 280 ppmv).

Recent experiments by the Bern and Grenoble ice core groups show that the concentration of CH4 dipped to a low of about 350 ppbv during the last ice age. Khalil and Rasmussen's (in press) data spanning the Little Ice Age between 1450 and 1750 show a proportionate decrease in methane (about 40 ± 30 ppbv/°K) and also a decrease of N20 (about 5 ± 3 ppbv/°K). These decreases are believed to be a measure of the response of emissions from the Earth's soils, oceans, and high northern wetlands to global climatic change. The character and details of the transition of the atmospheric concentrations of C02, CH4 and N20 during the last deglaciation have yet to be well documented. Nevertheless, it is clear that concentrations of the radiatively active gases in air influence climate and are in turn influenced by climate.

The large role which varying levels of radiatively active gases play in climate change emphasizes the importance of understanding the global-scale interactions between climate and the biosphere. Studies of the d13C of C02,d13C and dD of CH4, d15N of N20, d180 Of 02, and the 02:N2:Ar ratio in the trapped gas can help in achieving this understanding. The importance of studying these variables lies not in their environmental influence, but in their role as tracers of selected geochemical processes that influence global climate. d13C of C02 serves as a tracer for studying the roles of the ocean and terrestrial biosphere in changing atmospheric pCO2. d13C and dD of CH4 reflect the relative production rates by the different sources. The same is true for the d15N of N20. d180 of 02 is governed by isotope fractionation during photosynthesis, respiration, and hydrologic Processes. Hence it reflects global scale interactions between the hydrosphere, biosphere and atmosphere. The atmospheric 02 concentration (expressed as the 02/N2 or 02/Ar ratio) indicates changes in the magnitude of the reduced carbon reservoirs, as well as the metabolic C02 content of the deep sea. N2/Ar, the d15N of N2, and 3He/4He must have been constant in the ice age atmosphere, and serve as indicators of the integrity of trapped gas samples.

In summary, studies of the composition of trapped gases in ice cores inform us directly about changes in the atmospheric concentrations of the radiatively active gases. They also reveal the composition of various tracers, which can help us unravel the nature and causes of Pleistocene climate change.

Objectives

In this section we review in more detail what can be learned from studies of trapped gas composition. There are three basic kinds of information which emerges from such studies. The first is the concentration of the radiatively active gases, C02, CH4 and N20, which directly influence Earth's heat balance. The second bears on the causes of changes in radiatively active gas concentrations and other alterations of geochernical cycles. This information comes from the following chemical and isotopic tracers: d13C of C02, d13C of CH4, d180 of 02, and the atmospheric 02 concentration. The third type of information is the integrity of trapped gas samples. This is reflected by several conservative parameters, d15N of N2, the ratio of N2/Ar, the He concentration and isotopic composition and the Ne concentration all of which must have been constant in the Holocene and Pleistocene atmosphere.

C02

The reconstruction of the atmospheric C02 concentrations during past periods of different climatic conditions is one of the most important and fundamental pieces of information to be obtained from ice cores. A detailed C02 record reveals the natural disturbances in the carbon cycle and is necessary to ultimately understand the relationship and interaction between C02 and climate. The modem C02 increase and the change upon deglaciation are well documented from different cores, Arctic and Antarctic. 'Me rapid variations observed in the Dye 3 core during the later Wisconsin have yet to be confirmed. If confirmed, this would mean that climate changes could occur on short time scales, and it would provide important information on the probable outcome of man's dangerous "experiment" of increasing the atmospheric C02 by releases from biospheric and fossil carbon. Also large portions of the Holocene have never been analyzed for C02/air ratios. New cores will extend into the previous interglacial, providing information on the atmospheric conditions at that time, and on what happened during the transition into the last glacial.

CH4 and N20

As discussed in the previous section, CH4 and N20 concentrations in air have varied over both anthropogenic and glacialfinterglacial time scales. It is important to measure the variability in much more detail, understand its causes, and gauge its effects.

d13C of C02

Atmospheric C02 exchanges with the biosphere and the oceans. The size of the biosphere may vary as climatic changes, and the uptake or release of C02 by the oceans is governed by pCO2 of the ocean surface waters, which depends on a number of factors.

d13C and the 14C/12C ratio of C02 can be used to learn whether atmospheric C02 concentration changes are due to biospheric or oceanic exchange. Atmospheric d13CO2
(~ -7°/oo PDB) is closer to that of the oceans (+2 °/oo) than to the more depleted biosphere (~ - 25°/oo); the radiocarbon in the biosphere and atmosphere are about equal, while the surface waters of the ocean are somewhat lower (~95%). Thus if, for example, an atmospheric increase in C02 were caused by a net flux from the biosphere, the 13C/12C ratio would decrease, with almost no change in 14C/12C. On the other hand, if a C02 increase is the result of a predominant influx from the oceans, the 13C/12C ratio would be minimally affected and the 14C/12C ratio would decrease. The expected variations in d13C are small and thus extreme care is required in the experimental techniques.

At the proposed drilling site in Central Greenland one expects to encounter the ice conditions most favorable for obtaining a detailed C02 concentration record during glacial and interglacial. times. High depth resolution measurements can be performed and compared to d180 of H20 (and particulate content, chemical species, and the concentrations of cosmogenic radio-nuclides), in order to determine the relative timing of C02 and climate variations, and therefore the causal relationship. Ultimately the achievable resolution is determined by the inherent age difference of enclosed air and surrounding ice, which can vary with time.

d13C of CH4

Isotopic measurements are the major constraint to understanding the rapid increase in methane in the atmosphere during the past several hundred years, as well as the change associated with glacial/interglacial cycles. Methane is produced by a variety of sources, including ruminant animals, uses of fossil hydrocarbon fuels, biomass burning and even termites. It is labelled with an isotopic signature that reflects relative fluxes from these sources and the fractionation effect of hydroxyl radical removal. If the sink for methane is relatively constant the sources will produce the greatest impact on the isotopic signature. Evidence from measurements on recent Holocene ice collected at DYE-3 and Site A indicate that isotopic methane concentration changes mirror atmospheric changes. Further study of d13CH4 in ice cores throughout the Holocene and into the Pleistocene should reveal the nature and relative importance of methane sources and sinks, as well as causes of concentration changes associated with major climate change in the past.

d15N of N20

The change in d15N of atmospheric N20 accompanies changes of N20 production in oceans and soil by nitrification and denitrification, and, to a lesser extent by fossil fuel combustion. N20 in the atmosphere and oceans exhibit similar enrichments of 15N relative to atmospheric N2. This enrichment coincides with the 15N enrichment of oceanic nitrate and ammonia and implies that soil ammonia and nitrate may be enriched in the same manner. The N20 record in ice cores may very likely reflect these isotopic changes. An isotopic N20 record in the Holocene would be particularly important in understanding any recent isotopic source and consumption changes.

d180 of 02

d180 of atmospheric 02 is governed by the d180 of seawater (which is the ultimate source of all photosynthetic 02), as well as isotopic fractionations during photosynthesis, respiration and hydrologic processes (which transport seawater to the sites of terrestrial photosynthesis). The first order control on the d180-time record of atmospheric 02 is the seawater d180, which of course changes with sea level. Respiratory and hydrologic fractionations are very different for terrestrial and marine ecosystems. The second order signal in the record is thus the ratio of terrestrial to marine productivity, and its variation through time. The 180 change of atmospheric 02 lags the seawater change because of the time required for photosynthesis to replace the 02 in the atmosphere (~2000 yrs.). The lag itself is an indicator of the planetary rate of primary production. d180 of atmospheric 02 is, like other gases, homogeneous throughout the atmosphere. It thus serves as a time stratigraphic marker for the correlation of ice cores, and may also allow correlation of ice cores in the seawater d180 record.

02 Concentration

The atmospheric 02 concentration, expressed as the 02/Ar ratio, is affected by the carbon cycle via photosynthesis and respiration. The processes thought to be responsible for changing atmospheric C02 levels leave different imprints on the atmosphere C02 content. The burial of organic carbon or production of terrestrial biomass raises the 02 concentration in air, erosion or destruction of biomass decreases 02. Changes in the transport of organic carbon to the deep sea have the same effect. Reactions between C02, CaC03 and oceanic HC03- can change atmospheric C02 but have no effect on the 02/Ar ratio. The measurement of the variable thus provides an important constraint for unravelling the behavior of the carbon system and understanding the causes of variations in the atmospheric C02 content.

N2/Ar and d15N of N2

The N2/Ar ratio in air must have been constant during the Pleistocene (geochemical fluxes are too small to have changed the concentration of either parameter). The N2/Ar ratio thus gives a measure of the integrity of ice core samples. For some existing deep ice cores, this ratio can vary by as much as 4% or more. Such a variation must be accompanied by changes in the concentrations of C02, CH4, N20 and 02/Ar from the true paleoatmospheric values. It is thus important to measure N2/Ar as an index of sample integrity.

d15N of N2, which must also have been constant during the Pleistocene, provides an independent and very important control on sample integrity. The N2 in trapped gases are uniformly enriched in 15N. This enrichment is a primary feature resulting from the more rapid expulsion of light isotopes as bubbles seal under pressure. Mass-dependent fractionation has an important affect on the d13C of C02, d13C of CFU, and the d180 of 02. It is essential to measure d15N of N2 and use the values to correct the isotopic composition of the nonconservative compounds back to their original paleoatmospheric value.

He and Ne

Helium is one of very few substances soluble in ice (10% of the concentration in bubbles) which makes it unique for studying gas diffusivity. Helium in cut cores diffuses out of the ice primarily along the C-axis (five times less perpendicular to the axis) down to the level of unrelaxed equilibrium solubility. Owing to the higher diffusivity of 3He relative to 4He the depletion of 3He is reversed by exchange with atmospheric helium in samples stored for several months accompanying relaxation processes. Similar observations have been made on neon.

A detailed study of helium and neon in situ by sampling in a manner which preserves the in situ concentrations should reveal much about diffusion processes of trapped gases and help unfold fractionation effects which may be diffusion controlled. Depending on the actual diffusive transport it might be possible to observe geomagnetic reversals in the proposed Laschamps and Blake events by large changes in the 3He/4He isotope ratio. Another effect which may be observed is the modification of helium contents in ice by the gravitational field effect on solubility.

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