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

1.4 SPECIALITY GROUP REPORT: TRACE METAL STUDIES

Objectives for studies of trace metals in ice cores are:

(1) Large meteoritic impacts probably modify global climate and ecosystems, but the size and frequency of these events is not well established. Ice cores preserve the record of cosmic dust input and meteorite impact, which can be measured by determining concentration pulses of Ir and changes in the 1870s/1860s ratio.

(2) Anthropogenic emissions into the atmosphere have increased dramatically over the last 200 years. Long range transport of anthropogenic emissions of Pb and Cd through the atmosphere and their preservation in ice cores allow us to determine the timing and magnitude of anthropogenic emissions to the global atmosphere by Pb and Cd back to 1750 A.D.. Parallel investigations on ancient ice extending through and beyond the present glacial cycle will enable a better understanding of the natural sources of trace metals, and their bio-geochemical cycles. They will yield a firm perspective for assessing the significance of anthropogenic increases, and a basis for predicting future trends.

(3) Ice cores preserve a record of volcanic eruptions, so they can be used to determine the extent of volcanic emissions of Pb, Bi, T1, and Cd (and evaluate possible Ir and Os emissions) to the troposphere and stratosphere.

(4) Terrestrial atmospheric dusts are linked to global climate change during the last several hundred thousand years. Ice core studies can determine sources and magnitudes of metals contributed by atmospheric dusts and their relation to global climate change. They will enable more thorough tests of the ability of climate models to predict the effect of such perturbations of the earth-ocean-atmosphere system.

(5) The intercorrelation of ice core climate records is difficult because absolute age dating is not possible in most cases. Global trace metal event horizons (e.g. from volcanic or impact events) whose occurrence is globally synchronous could be used as stratigraphic horizons for the intercomparison of different ice cores, and hence provide important chronological information.

Rationale for these objectives are:

(1) Polar ice contains a record of the accretion and variability of materials from outer space. The occurrence of "cosmic spherules" in polar ice is well known. The element iridium, which is highly enriched in extraterrestrial debris relative to crustal materials, has been determined in Antarctic ice as a measure of the steady state cosmic debris influx, and a pulse of cosmic iridium has been observed to coincide with the 1908 Tunguska impact event. This measurement indicates that the Tunguska object was 0.2 Ian in diameter. It has been estimated that less frequent impacts of 0.51an objects (which would produce a 10 km crater on land) occur every 100,OW years. If this estimate is COIT ct, it is likely that many events of Tunguska magnitude would be preserved in a 250,000 year ice core. A complete record of Ir in a long polar ice core would allow us to determine the size-frequency history of previous cosmic impacts. It would also allow us to determine whether the more uniform background influx of small particles is truly constant. To observe these pulses, it is necessary to distinguish them from normal background crustal and cosmic dust Ir deposition.

While it is believed that Ir found in ice cores is dominantly of cosmic origin, there is still room for doubt on this point. Ir is also known to be enriched in volcanic emissions. Since volcanic acids are observed in ice cores, it is possible that volcanic Ir may also be found in ice cores. In order to use Ir as a tracer of extraordinary cosmic impacts, it is necessary to correct the ice core signal for the possible influence of volcanic emissions. A study of Ir in the proximity of known volcanic events recorded in ice cores is needed to provide a basis for this correction.

Similarly, while the concentration of Ir in crustal dusts is low, the total concentration of crustal dusts in ice cores is high relative to the abundance of cosmic dusts. To allow for the possible influence of these high concentrations of crustal dusts on the cosmic Ir signal, it is necessary to examine the Ir concentration in relation to changes in the terrestrial dust record.

Further confirmation of the extraterrestrial nature of Ir can be provided by measurement of the isotopic composition of osmium. Due to fractions of radioactive parent 187Re from radiogenic daughter 1870s during magmatic processes, the ratio of 1870s/1860s is 400 in crustal rocks while it is only 3 in meteorites. Hence variation of the osmium isotope ratio can help to distinguish cosmic (or mantle-derived) Os from meteoritic Os.

(2) Chronological variations in emissions of lead (and its isotopes), cadmium, and thalliurn to the atmosphere from dust, volcanic emissions, and sea salts recorded as concentration variations in ice will be measured with focus on three periods: (A) the most recent two centuries; (B) the "Little Ice Age" from 1300 A.D. to 1700 A.D., and (C) the last 1/3 of the Wisconsin and the first 1/2 of the Holocene. 

(A) During the past several centuries global fluxes of industrial lead emissions to the atmosphere increased 100-fold above natural global lead emission fluxes. Profound contamination of the earth's biosphere and oceans has obliterated original, natural levels of lead. In the oceans, the switchover from major natural fluvial input pathways to major industrial eolian input pathways has established a transient equilibrium situation for the chemical oceanographic cycles of lead. This concentration transient combined with the use of isotopic compositions of lead as tracers makes lead useful for defining and imposing contraints on models of oceanographic cycles of other trace metals. Knowledge of temporal changes in eolian fluxes of lead to the oceans provides parameters which allow transient equilibrium models of oceanic lead to be explicitly defined. Measurements of temporal changes in lead isotopic compositions will identify the lead as industrial and indicate the geographic source of the industrial emissions.

In a similar fashion, the waxing and waning of the input flux of industrial cadmium to the atmosphere should be determined from the ice record because eolian inputs of cadmium to the ocean may also be significant. Proper modeling of the chemical oceanography of cadmium requires quantitative knowledge of variations in the fluvial/eolian input ratio.

Present biochemical knowledge of the effects of metal pollution is founded on studies of systems highly contaminated with lead and cannot be referenced to a truly natural system. In terrestrial ecosystems, organisms are contaminated by atmospheric industrial lead aerosols which enter food pathways mainly by dry deposition on leaf surfaces. Corrections for effects from these fluxes of industrial lead can be evaluated from contemporary studies of atmospheric lead/plant and soil interface interactions, combined with historical studies of integrated flux inputs (as established by lead data from ice cores) to selected ecosystems. Natural levels of lead in organisms, inferred from such studies will allow new animal controls containing natural levels of lead to be grown which will serve as the basis for new biochemical studies of non-lead contaminated systems. 

(B) Measurements of global volcanic emission fluxes of lead, bismuth, thallium and cadmium indicate that this was the origin of major portions of these metals in the atmosphere during the pre-industrial Holocene period. Comparable proportions of these metals in the atmosphere were also contributed from soil dusts during this period. Temporal changes in these proportions have probably occurred during waxing and waning of global volcanic emissions and global wind velocities. Some modelers have proposed that volcanic activity may have triggered the "Little Ice Age". Ice from this period should be examined to see whether there is any evidence for enhance elemental tracers of volcanic emissions.

(C) Changes in lead, dust, and seasalts have recently been shown to be substantial in the Antarctic tropospheric cell during the period of transitions from late Wisconsin to early Holocene. It is desired that studies of these substances be carried out in Greenland ice to see whether comparable temporal changes occurred in the Arctic tropospheric cell. Such studies will contribute to an understanding of natural controls on trace element concentrations in the atmosphere. These objectives can be met by undertaking the following experiments at ice core drill sites from both poles and in suitable mid-latitude high-elevation sites:

(I) Snow pit and hand auger collections, in which principle investigators collect their own large samples with extensive contamination precautions. High elevation sites should be supplemented by such collections from two other sites at elevations of 1 and 2 km above sea level.

(II) A series of ~100 meter deep firn cores (collected with pre-cleaned coring gear and with coring supervised by trace element principle investigators) at the sites of the snow-pit studies.

(III) 100 30-cm whole core sections (018 sampling from periphery OK) covering the period from the entire core. 25 of the core sections will be taken from the Holocene; 25 of the core sections would be taken from 13,000 to 40,000 yrs. bp.; the remainder would be taken from older ice.

(IV) These studies should be integrated with aerosol sampling and meteorological studies to document regional transport processes.

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