Past and recent tritium levels in Arctic and Antarctic polar caps

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Abstract

Tritium concentration was measured in snow deposited at the GRIP site (central Greenland) and at the Vostok station (east Antarctica) from snow pits covering the period 1980–1990. The objective of the study was to investigate tritium concentrations in polar regions several decades after the bomb peak of the sixties and to put them in the context of available data for environmental tritium in the Arctic and the Antarctic over the last five decades. The tritium content of the samples was measured by mass spectrometry using the helium-3 regrowth method. In Antarctica, the tritium concentrations are in the range 70–110 TU. The comparison of the bomb tritium history at different locations show that tritium levels increase moving inland, where vapour pressure becomes extremely low and therefore more sensitive to the intrusion of stratospheric air masses highly enriched in tritium. Although most tritium fallout occurred in the Northern hemisphere, the tritium levels in central Greenland in the 80's, in the range 10–40 TU, are significantly lower than at Vostok. Unlike Antarctica, no such continental effect is observed in Greenland, due to the higher water vapour content of the air masses, as evidenced by the much higher snow accumulation rate. Whereas tritium fallout in Antarctica appears to occur as a result of direct injections of stratospheric tritium during winter, Arctic fallout are the result of the dominant spring injection of stratospheric air at mid-latitude, in line with the deposition of other stratospheric tracers.

Introduction

Natural tritium (3H) is produced mainly by the interaction of cosmic radiations with the upper atmosphere, at a rate of ∼ 0.25 atoms/cm2/s [1]. This tritium enters the hydrological cycle in the form of tritiated water molecules (HTO). 3H has a radioactive half-life of 4500 ± 8 days. By equating the production and decay rates, a steady-state global inventory of tritium of ∼ 3.6 kg can be inferred.

Since 1954, vast amounts of thermonuclear tritium have been injected into the upper troposphere and the stratosphere. Estimates are that atmospheric detonations of thermonuclear devices produced ∼ 525 kg of tritium in the form of HTO [2]. Approximately 13 kg of tritium gas (HT) was also released by military tritium facilities and leaks from underground testing of nuclear weapons [3]. Over the same period (i.e., 1950–2000), the contribution of the civil nuclear industry to the tritium budget was comparatively quite small with modest airborne releases of 0.35 kg of tritium [2]. Since the Nuclear Test Ban Treaty came into force in 1963, tritium levels in precipitation have been steadily decreasing (Fig. 1) due to radioactive decay and dilution within the vast oceanic reservoir.

Natural tritium levels are very low. In pre-nuclear times (before 1950), the limited accuracy of measurement methods explains the sparseness and poor reliability of the data for tritium in natural waters. Ice cores provide an alternative and offer the opportunity to reconstruct with a good accuracy the recent history of tritium including pre-nuclear levels. Here we present tritium data measured in snow deposited in central Greenland at Summit (GRIP site, 72°N–37°W) and at the Vostok station, Antarctica (78°S–106°E). The objective of the study was to investigate tritium concentrations in polar regions several decades after the bomb peak of the early sixties and to put them in the context of available data for environmental tritium in the Arctic and the Antarctic over the last five decades. This allow us to draw a global picture of the history of the bomb tritium deposition in polar regions and to study its main characteristics.

Section snippets

Materials and methods

Eighty snow samples were taken at the GRIP site (Fig. 2), on the wall of a 5 m deep pit covering the period between 1984 and 1992. In the same manner, sixteen samples were also collected at the Vostok station (Antarctica) from a 2.5 m deep pit (Fig. 2). The tritium content of the ice was measured by mass spectrometry using the helium-3 regrowth method [4]. The principle of the method is to remove the 3He initially dissolved in the sample by degassing the water sample under high vacuum, then to

Results

The results from the GRIP site are shown in Fig. 3. Tritium concentrations, between 10 and 40 TU, are modulated by a clear seasonal cycle similar to the seasonal pattern already observed in precipitation (see Fig. 1). The mean accumulation at the GRIP site deduced from these annual oscillations for the period 1984–1992 is 57.75 cm of snow per yr (Fig. 4), in excellent agreement with that given by the GRIP group based on annual layers counting [6].

The Vostok data include 15 samples covering the

Stratospheric tritium input

Fig. 6 shows the time-evolution of the average tritium content in the stratosphere [7]. Tritium levels in the stratosphere are higher by several order of magnitude compared to tropospheric levels, due the large injection of the bomb tritium above the tropopause during the atmospheric nuclear tests.

In Antarctica, an increasing trend between the tritium content of the snow and the distance from coast is observed on two transects between the Adélie coast and Dome C [8] and between Terra Nova Bay

Conclusion

In Greenland, the extrapolation of the decreasing trend observed on the samples from Summit covering the period 1984–1992 shows that present-day levels are close to the natural background measured at Thule [31] before the nuclear tests, between 10 and 20 TU. In Antarctica, the comparison between our measurements on samples from Vostok covering the 80's and pre-bomb levels is more difficult since only one pre-bomb sample from Vostok was available for this study and data from other sites

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