Recent ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am accumulation in an ombrotrophic peatland from Amsterdam Island (Southern Indian Ocean)

Highlights

• First peat record of artificial radionuclides (¹³⁷Cs and ²⁴¹Am) in the Southern Indian Ocean.

• Updated world ²¹⁰Pb database with 47 new entries from the recent literature.

• ²¹⁰Pb inventory found in the peat core from Amsterdam Island is similar to those from South Africa and Madagascar.

Abstract

Over the past 50 years, ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am have been abundantly used in reconstructing recent sediment and peat chronologies. The study of global aerosol-climate interaction is also partially depending on our understanding of ²²²Rn-²¹⁰Pb cycling, as radionuclides are useful aerosol tracers. However, in comparison with the Northern Hemisphere, few data are available for these radionuclides in the Southern Hemisphere, especially in the South Indian Ocean. A peat core was collected in an ombrotrophic peatland from the remote Amsterdam Island (AMS) and was analyzed for ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am radionuclides using an underground ultra-low background gamma spectrometer. The ²¹⁰Pb Constant Rate of Supply (CRS) model of peat accumulations is validated by peaks of artificial radionuclides (¹³⁷Cs and ²⁴¹Am) that are related to nuclear weapon tests. We compared the AMS ²¹⁰Pb data with an updated ²¹⁰Pb deposition database. The ²¹⁰Pb flux of 98 ± 6 Bq·m⁻²·y⁻¹ derived from the AMS core agrees with data from Madagascar and South Africa. The elevated flux observed at such a remote location may result from the enhanced ²²²Rn activity and frequent rainfall in AMS. This enhanced ²²²Rn activity itself may be explained by continental air masses passing over southern Africa and/or Madagascar. The ²¹⁰Pb flux at AMS is higher than those derived from cores collected in coastal areas in Argentina and Chile, which are areas dominated by marine westerly winds with low ²²²Rn activities. We report a ¹³⁷Cs inventory at AMS of 144 ± 13 Bq·m⁻² (corrected to 1969). Our data thus contribute to the under-represented data coverage in the mid-latitudes of the Southern Hemisphere.

Introduction

Lead-210 (²¹⁰Pb, T_½ = 22.3 years) dating is the most common method employed to estimate short-term (from years to decades) chronologies in peat, estuarine, fluvial, and lacustrine environments (Le Roux and Marshall, 2011, Robbins and Edgington, 1975, Benoit and Rozan, 2001, Humphries et al., 2010). ²¹⁰Pb originates from the decay of gaseous ²²²Rn, which escapes from the Earth's continental crust to the atmosphere (Graustein and Turekian, 1990). ²¹⁰Pb adsorbs strongly to the surface of aerosols in the 0.1–0.5 μm diameter size range as soon as it is produced in the air (Knuth et al., 1983). ²¹⁰Pb-bearing aerosols are distributed globally by general atmospheric circulation and can be deposited on the Earth's surface mainly by precipitation, but also by dry fallout as well as convective updrafts (Knuth et al., 1983, Baskaran, 2011). The ²¹⁰Pb deposited from the atmosphere is called “unsupported ²¹⁰Pb” or “excess ²¹⁰Pb” (denoted ²¹⁰Pb_ex), which should be distinguished from the ²¹⁰Pb produced inside the matrix (e.g. lake sediment) and which is, named “supported ²¹⁰Pb” (Guevara et al., 2003).

The Constant Rate of Supply (CRS) model based on ²¹⁰Pb_ex flux, which could be validated by nuclear fallout studies (e.g. ¹³⁷Cs, ²⁴¹Am), is by now widely used (e.g. Appleby et al., 2001, Appleby, 2008). Another radionuclide that is widely used to derive ages is ¹³⁷Cs. With a half-life of 30.2 years, ¹³⁷Cs is considered as one of the important radionuclides among those from nuclear emissions (e.g. atmospheric nuclear weapon tests in the 1950s-1970s with the peak in 1963 in the Northern Hemisphere and the fallout from the Chernobyl accident in 1986), with respect to being a persistent tracer and an indicator of single-event chronology (Aoyama et al., 2006, Rodway-Dyer and Walling, 2010). In contrast, ²⁴¹Am - another artificial radionuclide - is strictly related to nuclear bomb testing in remote areas.

Up to now, most studies about the inventory of sediment radionuclides and radiochronology have been conducted in the Northern Hemisphere. Limited work has been carried out in the Southern Hemisphere, especially in the Indian Ocean. The scarcity of studies conducted in the Southern Hemisphere is partly due to the lower fallout of ²¹⁰Pb and ¹³⁷Cs, fewer continental surfaces and fewer man-made radionuclide emissions and fallout, which generally result in lower activities bordering on analytical detection limits (Owens and Walling, 1996, Bonotto and De Lima, 2006). In the Southern Hemisphere nuclear weapons fallout is about three times lower than that in the Northern Hemisphere. Consequently, the ¹³⁷Cs fallout peak is usually more difficult to identify in cores due to the relatively high measurement uncertainties associated with the low ¹³⁷Cs concentrations (Hancock et al., 2011). Southern Hemisphere investigations on ²¹⁰Pb and ¹³⁷Cs have been confined primarily to large land masses, such as South America (Sanders et al., 2006, Guevara et al., 2003, Cisternas et al., 2001), Australia and New Zealand (Pfitzner et al., 2004, Hancock et al., 2011) and South Africa and Madagascar (Humphries et al., 2010, Kading et al., 2009, Ivanovich and Harmon, 1992, Rabesiranana et al., 2016).

No studies on terrestrial sediment radionuclides and radiochronology exist for the Southern Indian Ocean, although the area is an important part of the global atmospheric and oceanic circulation patterns. Amsterdam Island (AMS, 37°S) is located just north of the Subtropical Front (at approximately 40°S, Orsi et al., 1995), where cool, low-salinity subpolar water submerges beneath warm, saline subtropical water (Prell et al., 1979). The island is located at the northern edge of both the Southern Westerly wind belt and the Antarctic Circumpolar Current.

The main objectives of this study were to investigate (1) to what extent observations in the Southern Indian Ocean could define Southern Hemisphere mid-latitude ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am background conditions for the last 100 years and (2) how ²¹⁰Pb levels are comparable between different matrices (e.g. wetland, sediment, glacier and atmospheric deposition) at different latitudes of the world, which could allow us to draw a new global sketch of ²¹⁰Pb flux.

In this paper, for the very first time the inventories and fluxes of ²¹⁰Pb and ¹³⁷Cs together with a ²¹⁰Pb-based peat accumulation rate for AMS are reported. This island is located in Southern Indian Ocean at 37°S and at 3400 km from the nearest land mass. The atmospheric conditions at this location offer the possibility to potentially define ²¹⁰Pb background concentrations, in a place with minimal perturbation from anthropogenic influences (Gaudichet et al., 1989, Angot et al., 2014). AMS is therefore an ideal site to investigate the background levels of ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am in the Southern Hemisphere, as well as to detect long-range transport of anthropogenic radionuclides.