Actinium-227 as a deep-sea tracer: sources, distribution and applications

Abstract

Actinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope ²²⁷Ac (half-life 21.77 yr) for the first time in the water column of the Southeast Pacific, the Central Arctic, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre (WG). Besides the profile in the Southeast Pacific, which confirms earlier findings about the role of diapycnal mixing for ²²⁷Ac distribution, we found three other different types of vertical profiles. These profiles point to a prominent role of advection for ²²⁷Ac distribution, especially in the Southern Ocean. Depending on the type of profile found, ²²⁷Ac is proposed as a tracer for different oceanographic questions. In the Southern Ocean, up to 4.93±0.32 dpm m^{−3 227}Ac is found close to the sea floor, which is the highest concentration ever observed in the ocean. Close to the sea surface in the WG, 0.46±0.05 dpm m^{−3 227}Ac_ex (²²⁷Ac in excess of its progenitor ²³¹Pa) is detected. We use ²²⁷Ac_ex there to determine the upwelling velocity in the Eastern WG to be about 55 m yr⁻¹. In the ACC, Upper and Lower Circumpolar Deep Water (UCDW and LCDW) are found to differ clearly in their ²²⁷Ac_ex activity. High ²²⁷Ac_ex activities are therefore a promising tracer for recent inputs of LCDW to the sea surface, which may help to understand the role of deep upwelling for iron inputs into Antarctic surface waters. The expected release of ²²⁷Ac is compared with ²²⁸Ra to make sure that the large near-surface excess in the water column of the Southern Ocean is not due to lateral inputs by isopycnal mixing. Data from the Central Arctic and from a transect across the ACC confirm that ²²⁸Ra and ²²⁷Ac_ex differ strongly in their sources. The first measurements of ²²⁷Ac on suspended matter (less than 1.7% of total ²²⁷Ac close to the sea floor) indicate that the particle reactivity of ²²⁷Ac is negligible in the open ocean, in agreement with earlier findings [Y. Nozaki, Nature 310 (1984) 486–488]. Despite the extremely low concentrations of ²²⁷Ac, new measurement techniques [W.S. Moore, R. Arnold, J. Geophys. Res. 101 (1996) 1321–1329] point to a comfortable and comparably simple determination of ²²⁷Ac in the future. Finally, ²²⁷Ac_ex may become a widely used deep-sea specific tracer.

Introduction

Understanding and modelling of the global oceanic circulation requires a detailed knowledge about mixing and advection. Information about vertical exchange is of particular importance, as this is the only process that redistributes nutrients from the deep-sea to the sea surface [3], draws down atmospheric inputs to the deep-sea or transports energy between deep and surface waters. Recently, diapycnal mixing in the deep-sea was shown to be an important term in the global heat budget [4], [5].

The vertical exchange in the Southern Ocean is of particular interest for climate research. There, the role of deep upwelling combined with formation of new bottom water in the Weddell Gyre (WG) for CO₂ transfers from surface to deep waters or vice versa is in the focus of interest (e.g. [6]). Deep upwelling also plays a crucial role in the supply of the growth-limiting micronutrient iron into the Antarctic Circumpolar Current (ACC) ([7], [8] and references therein).

The rates of deep upwelling or rates of diapycnal mixing cannot be obtained in the required accuracy by direct measurements of current velocities. Radionuclides provide a solution for this problem. They contain a time information that integrates the process of interest on a scale depending on their half-life. A variety of radionuclides, both natural and anthropogenous, have been used to derive rates of exchange in the ocean on different spatial and temporal scales. However, none of them perfectly met the requirements of a tracer for vertical exchange in the upper deep-sea.

Some short-lived nuclides released from the sea floor (several radium and radon nuclides with half-lives from a few seconds to several days [9], Fig. 1) give appreciable information about mixing close to the sea floor. Due to their short half-lives, they do not experience large lateral transport and they give reliable information about vertical mixing close to the sea floor. ²²⁸Ra (half-live 5.75 yr) is also supplied by the sea floor, mainly independent of the depth of the overlying water column. As isopycnal mixing (which is in first approximation horizontal) is several orders of magnitude stronger than diapycnal mixing, ²²⁸Ra released from the continental slope can reach the interior of the ocean basins, thus influencing the vertical distribution of ²²⁸Ra there [10]. Therefore, no reliable values for diapycnal mixing can be obtained by ²²⁸Ra alone.

²²⁶Ra (half-life 1600 yr) is mainly supplied by deep-sea sediments as a consequence of the distribution of its progenitor ²³⁰Th (Fig. 1), so lateral inputs by continental slopes will be smaller than for ²²⁸Ra. However, its relatively long half-life compared to the velocities of mixing and advection makes additional tracers necessary if full use of its information is to be made [11].

In 1984, Nozaki [1] proposed ²²⁷Ac (half-life 21.77 yr) as a tracer for diapycnal mixing in the deep-sea. This nuclide is produced by decay of ²³¹Pa, which is found in high concentrations in slowly accumulating deep-sea sediments. Indeed, ²²⁷Ac combines the advantage of a source in the deep-sea (much like ²²⁶Ra) with a favorable half-life comparable to ²²⁸Ra. In spite of its obvious advantages, ²²⁷Ac was never extensively used as a tracer. The main reason for the scarcity of data on its distribution and geochemical behavior must be seen in its extremely low concentrations and in difficulties in its detection. A measurement technique via α-scintillation proposed for ²²³Ra [2] now promises a strongly simplified detection of ²²⁷Ac.

To evaluate the potential and the limits of this tracer, we first set out to show that the distribution of ²²⁷Ac is indeed governed by sources in the deep-sea, and is not, as in the case of ²²⁸Ra, obscured from shallower depths.

We then present a series of water column profiles including the first measurements of ²²⁷Ac in the Southern Ocean and the Arctic and discuss how these results can be used to estimate mixing and upwelling rates for the deep ocean.

The longest-lived isotope of actinium, ²²⁷Ac, is produced by decay of ²³¹Pa (half-life 32500 y), a particle-reactive daughter of ²³⁵U (Fig. 1). To give an impression of the potential sources for ²²⁷Ac, we first discuss the distribution of ²³¹Pa in the marine environment. The parent nuclide of ²³¹Pa, ²³⁵U, occurs in the ocean at a virtually constant concentration as a stably dissolved complex. When ²³⁵U decays to ²³¹Pa in the water column, the latter is rapidly scavenged to the sea floor much like ²³⁰Th but with a slightly longer residence time in the water column of about 100 yr [12], [13]. Because the vertically integrated production of ²³¹Pa from ²³⁵U in the water column increases with depth, the flux of ²³¹Pa to the sea floor theoretically also increases in a linear way with water depth, leading to highest ²³¹Pa concentrations in deep-sea sediments (Fig. 2a). For very shallow oceanic regions like continental shelves, ²³¹Pa can be expected to be in secular equilibrium with ²³⁵U. Besides this general increase of ²³¹Pa with depth, further effects have to be taken into account.

Because the flux of ²³¹Pa to the sediment is at first approximation constant at equal water depths, the ²³¹Pa concentration decreases with increasing sediment accumulation rates due to a dilution effect (Fig. 2c). This effect also results in higher ²³¹Pa concentrations towards the deep-sea floor as accumulation rates are generally lower in the deep-sea. As a consequence of the long oceanic residence time of ²³¹Pa, not all of it is deposited at its production site, but it is preferentially scavenged at locations with high particle fluxes [13], slightly opposing the above-mentioned effect of dilution at sites with high accumulation rates.

For comparison, the distribution of ²³²Th, the progenitor of ²²⁸Ra, is also shown in Fig. 2b,d. In contrast to ²³¹Pa, ²³²Th concentrations at the sediment surface show no increase with water depth and no dependency on the accumulation rate.

It has been shown that ²²⁷Ac is much less particle-reactive than ²³¹Pa and part of the ²²⁷Ac produced in sediments by decay of ²³¹Pa is consequently released to the overlying seawater [16]. To distinguish between ²²⁷Ac released from marine sediments and ²²⁷Ac produced in situ in the water column (Fig. 3), the term ‘excess ²²⁷Ac’ or ‘²²⁷Ac_ex’ has been introduced [1]. Assuming steady state, ²²⁷Ac produced in the water column is in radioactive equilibrium with ²³¹Pa. All ²²⁷Ac exceeding the equilibrium with ²³¹Pa must be supplied by release from sediments because no other source of ²²⁷Ac to the seawater is known. A depletion of ²²⁷Ac with respect to ²³¹Pa is found in the sediment, which approximately corresponds to the integrated ²²⁷Ac_ex in the overlying water column [16]. The process of ²²⁷Ac release is schematically shown in Fig. 3.

Very few data of ²²⁷Ac in the ocean are available. To our knowledge, previous to this work only three vertical profiles of total (=dissolved+particulate) ²²⁷Ac in the ocean were reported, all of them from the North Pacific [1], [17]. All profiles revealed ²²⁷Ac_ex (up to 2.5 dpm m⁻³) close to the sea floor and no ²²⁷Ac_ex in the upper water column, except some weak signals close to the sea surface.

Much additional information was obtained by measurement of its daughter ²²⁷Th [18], which is assumed to be close to radioactive equilibrium with ²²⁷Ac if particle fluxes are not leading to enhanced scavenging of thorium, as shown for ²³⁴Th close to the sea floor and in productive surface waters ([19] and references therein). By means of ²²⁷Th, indications for release of ²²⁷Ac from continental slopes in the vicinity of Japan were found [18].

Only one value of ²²⁷Ac on sediment trap material is known [20]. There, ²²⁷Ac was found to be below equilibrium with ²³¹Pa. From the sediment trap value, the lower limit of the mean residence time of actinium in the ocean was estimated to be 540–3250 yr [20], which was later refined with water column data to 3000 yr [1]. In any case, ²²⁷Ac will behave conservatively in the open ocean as its mean residence time is very long compared to its half-life. Besides the data from the open ocean, some ²²⁷Ac data from the mixing zone between the Amazon freshwater and seawater are known, which indicate that ²²⁷Ac might be more particle-reactive than ²²⁸Ra in this particle-rich environment [21].

Conclusively, the currently available knowledge about the sources and the behavior of ²²⁷Ac points to a continuous release of ²²⁷Ac from the sea floor with a much stronger source in the deep-sea than in shallow areas.

A quantitative estimate for the flux of radium and actinium from sediments to the overlying water column would go beyond the scope of this work. Yet, far-reaching similarities in the behavior of both nuclides allow an estimate of the difference in ²²⁸Ra/²²⁷Ac ratios released by different sediments.

Comparing the ratio of the respective progenitors of ²²⁸Ra and ²²⁷Ac, some preliminary conclusions can be drawn. On continental shelves, ²³¹Pa can be expected to be in equilibrium with ²³⁵U as the overlying water column is too shallow to produce significant amounts of ²³¹Pa. Nozaki [17] estimates the average ²³¹Pa content of marine sediments for this area to be as low as 0.1 dpm g⁻¹. ²³²Th contents on shelves can be expected to be very variable. However, based on Fig. 2b, we would propose an average of about 2 dpm g⁻¹ for ²³²Th, from which an average ²³²Th/²³¹Pa activity ratio of about 20 on continental shelves results. This ratio is simply a function of the ²³²Th/²³⁵U ratio in the terrigeneous component of shelf sediments.

The situation in the deep-sea is completely different for ²³¹Pa. Yang et al. [22] estimate the average ²³¹Pa content in deep-sea sediment surfaces of the Pacific to be about 2.8 dpm g⁻¹. Most of this ²³¹Pa is ²³¹Pa_ex derived from production in the water column. The ²³²Th content of sediments does not show a trend with depth of the overlying water column, so its activities in the deep-sea can be expected to be similar to the 2 dpm g⁻¹ on the shelf. Therefore, a preliminary estimate of ²³²Th/²³¹Pa activity ratio for the deep-sea deposits of about 0.7 results, compared to a ratio of 20 on the shelf. Consequently, one must expect a change in the released ²²⁸Ra–²²⁷Ac activity ratio between shallow and deep water of about a factor 28. Implicitly, we assume that both ²²⁷Ac and ²²⁸Ra are governed by similar parameters on shelves and in the deep-sea.

Additionally, it has to be considered that ²³¹Pa_ex (bound adsorptively to particles) will eject its daughter ²²⁷Ac more effectively to the sediment porewater than ²³²Th (contained within the crystal lattice) does with ²²⁸Ra. Based on analogy of the ²³¹Pa_ex–²²⁷Ac pair to the ²³⁰Th_ex–²²⁶Ra pair, for which these ejection efficiencies have been determined [23], ²³¹Pa_ex can be expected to eject its daughter twice as effectively as ²³²Th does. This would result in a change in the released ²²⁸Ra/²²⁷Ac activity ratio between shelf and deep-sea deposits of up to a factor 54.

Our considerations imply that ²²⁷Ac/²²⁸Ra ratios released from shelf regions must be one or two orders of magnitude lower than those released from deep-sea sediments. ²²⁸Ra is known to build up to high activities in the shallow water column at continental shelves. Combining this knowledge with the low ²³¹Pa/²³²Th ratio in shallow water, we conclude that a detectable ²²⁷Ac input from shallow water sediments must be accompanied by a high ²²⁸Ra signal. This signal may be somewhat obscured with increasing distance from the source because the ²²⁷Ac/²²⁸Ra ratio slowly increases with time as a consequence of the different decay constants. However, our data presented below imply that even far from their source, ²²⁸Ra signals from shelves still exceed ²²⁷Ac_ex signals by far.