²³⁰Th and ²³¹Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantic transect, and implications for modern and paleoceanographic chemical fluxes

Abstract

The long-lived uranium decay products ²³⁰Th and ²³¹Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of ²³⁰Th and ²³¹Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using ²³⁰Th-normalization and to demonstrate enhanced ²³¹Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of ²³¹Pa is apparent. The ²³¹Pa/²³⁰Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation.

Introduction

The motivations to quantify chemical fluxes in the ocean are manifold. For instance, marine biological productivity is set by the balance between nutrient sources and sinks in surface waters and global climate is influenced by the redistribution of heat and salt associated with the ocean׳s overturning circulation. The well-known rates of radioactive production and decay of ²³⁰Th and ²³¹Pa (half-lives 75.69 kyr (Cheng et al., 2000) and 32.76 kyr (Robert et al., 1969), respectively), in addition to their insoluble nature, make them attractive tools to quantify the rates of the marine processes in which they are involved. These include removal from the water column by adsorption to particles (scavenging, related to biological productivity), redistribution by ocean circulation (related to heat transport), and sedimentation to the seafloor (providing a record of past biological productivity, ocean circulation, and more). Unfortunately, the influences of these processes on radionuclide distributions are potentially convolved. This study aims to utilize the spatial distribution of ²³⁰Th and ²³¹Pa across the U.S. GEOTRACES North Atlantic Transect (Fig. 1) to characterize the modern cycling of these isotopes in an effort to more completely calibrate their use as flux tracers in the modern and past ocean.

Because their production (²³⁴U and ²³⁵U decay, respectively) is uniform throughout the ocean (Andersen et al., 2010, Delanghe et al., 2002, Robinson et al., 2004, Weyer et al., 2008), the key question in ²³⁰Th and ²³¹Pa cycling in the water column is the balance between removal mechanisms. These are primarily (1) the downward flux by scavenging onto sinking particles and (2) lateral fluxes by advection and eddy diffusion. If lateral fluxes can be neglected, the concentration of the scavenged nuclide is expected to increase linearly with depth, representing an “equilibrium” between adsorption onto, and desorption from, vertically homogeneous sinking particles, a concept known as reversible scavenging (Bacon and Anderson, 1982, Krishnaswami et al., 1976, Nozaki et al., 1981).

Deviations from linearity in the radionuclide profiles therefore signal where this vertical equilibrium is perturbed by lateral fluxes or where the scavenging intensity has changed. This is admittedly a simple approach, as relatively linear depth profiles are not inconsistent with some lateral flux by dispersion (Roy-Barman, 2009, Venchiarutti et al., 2008). In a basin-scale view, nonetheless, characterizing anomalies to the predictions of reversible scavenging is our first step in deconvolving the oceanic ²³⁰Th and ²³¹Pa cycles. Three such anomalies, boundary scavenging, the effects of recently ventilated deep water, and bottom scavenging, appear in unprecedented detail in our North Atlantic section (Fig. 1). We now provide a context for these findings.

Boundary scavenging (Bacon, 1988, Bacon et al., 1976, Spencer et al., 1981) is the enhanced removal of scavenged-type elements (Bruland and Lohan, 2003) at ocean margins. When lateral gradients in particle flux exist, as between biologically productive ocean margin regions and oligotrophic ocean interior regions, insoluble elements are removed from the water column by scavenging to a greater extent at the margin versus the interior. The resulting gradient in radionuclide concentration produces a dispersive flux toward the margin from the interior. Lateral transport in the water column toward ocean margins is more significant for ²³¹Pa than for ²³⁰Th because it is more slowly removed downward by scavenging. The residence time with respect to scavenging of ²³¹Pa is 50–200 yrs while that for ²³⁰Th is 10–40 yrs (Henderson and Anderson, 2003). On the basis of the boundary scavenging concept alone, elevated ²³¹Pa/²³⁰Th ratios in both the dissolved and particulate phase at ocean margins are expected (Fig. 2). Prior to this study, the lateral gradients in the dissolved ²³¹Pa/²³⁰Th ratio or in dissolved ²³⁰Th (²³¹Pa) concentrations, predicted by the boundary scavenging concept, have not been definitively observed in the North Atlantic.

Modeling efforts have concluded that in ~70% of the ocean, ²³⁰Th is redistributed laterally by no more than 30% of its in situ production in the water column (Henderson et al., 1999), consistent with available observations from sediment traps (Yu et al., 2001). However, on the basis of sedimentary records some authors have argued that water column ²³⁰Th redistribution could be much greater than 30% due to boundary scavenging-type mechanisms, specifically along the equator in the Pacific (Broecker, 2008, Lyle et al., 2005, Lyle et al., 2007). This claim derives from a concern regarding ²³⁰Th-normalization, a method for calculating sediment accumulation rates on the basis of sedimentary ²³⁰Th concentrations (Bacon, 1984, François et al., 2004). This method assumes that the burial flux of ²³⁰Th is equal to its rate of production by ²³⁴U decay in the overlying water column, which allows one to correct for the lateral redistribution of sediments at the seafloor (sediment focusing). Because glacial–interglacial changes in sediment focusing have enhanced or diminished apparent accumulation rates by more than a factor of 2 (François et al., 1990, Suman and Bacon, 1989), the approach has been defended on the basis that neglecting a relatively small bias in the assumption that ²³⁰Th burial is equivalent to its production in the overlying water column is justified (François et al., 2007, Siddall et al., 2008). One aim of this study is to quantitatively estimate the magnitude of ²³⁰Th redistribution due to boundary scavenging.

While the effect of boundary scavenging of ²³¹Pa is well-expressed in the Pacific (Anderson et al., 1983, Anderson et al., 1990, Walter et al., 1999, Yang et al., 1986), it is considered to be suppressed in the Atlantic. This is because this basin is ventilated by southward flowing North Atlantic Deep Water (NADW) on timescales (<100–200 yrs) (Broecker et al., 1991) shorter than the Pa residence time with respect to scavenging (Walter et al., 1999, Yu et al., 1996, Yu et al., 2001). This means Pa can be transported south by deep water flow before it can be dispersed to North Atlantic margins. Although some studies have found evidence, in the form of sedimentary ²³¹Pa/²³⁰Th activity ratios above that produced in seawater by uranium decay of 0.093, for the enhanced removal of ²³¹Pa in the upwelling area off Northwest Africa (Legeleux et al., 1995, Lippold et al., 2012b, Mangini and Diester-Haas, 1983), studies of the North American (Anderson et al., 1994, Lippold et al., 2012a) and the northern Brazil (Lippold et al., 2011) margins do not support boundary scavenging of Pa. The dissolved ²³¹Pa/²³⁰Th distribution toward the margins of our transect (Fig. 1) will be used to determine the significance of boundary scavenging in the North Atlantic in light of its recent ventilation.

The possibility of boundary scavenging notwithstanding, previous studies have demonstrated that deepwater distributions of ²³⁰Th and ²³¹Pa are significantly perturbed by the influence of the recent ventilation of NADW (Luo et al., 2010, Moran et al., 1997, Moran et al., 1995, Moran et al., 2002, Scholten et al., 2001, Vogler et al., 1998). Deep convection at sites of deep water formation results in the injection to depth and propagation along deepwater flow paths of ²³¹Pa and ²³⁰Th concentrations which are lower than predicted by reversible scavenging (Moran et al., 1997, Moran et al., 1995, Moran et al., 2002). As the water mass ages, isolated from further perturbations to scavenging equilibrium, dissolved ²³⁰Th concentrations increase due to exchange with sinking particles, reaching a steady-state distribution relatively rapidly (determined by the residence time of 10–40 yrs), while ²³¹Pa responds more slowly (residence time of 50–200 yrs) because of the differing scavenging rates of the two elements (Moran et al., 2001, Rutgers et al., 1993). The longer residence time of ²³¹Pa allows for its southward export with NADW, leaving a ²³¹Pa deficit in deep North Atlantic sediments (Yu et al., 1996). This is the basis for using the sedimentary ²³¹Pa/²³⁰Th ratio as an indicator of the strength of the Atlantic meridional overturning circulation (McManus et al., 2004). The present water column transect is also intended to document the impact of ventilation on ²³¹Pa and the ²³¹Pa/²³⁰Th ratio.

In the absence of variations in scavenging intensity, one expects ²³⁰Th and ²³¹Pa concentrations and the ²³¹Pa/²³⁰Th ratio to increase with water mass age or time since deep water formation. The strongest response to ageing occurs within 1 to 2 water column residence times after deep water formation. Our section is appropriate to test this prediction because deep water age, or the time since deep-water (as averaged below 2 km) has been isolated from the atmosphere, ranges from <50 yrs in the west to >250 yrs in the east (Broecker et al., 1991). We have extracted an estimate of mean age for our North Atlantic transect from a recent inversion of ventilation tracer observations (¹⁴C, CFCs, ${\mathrm{PO}}_4^{⁎}$, temperature and salinity) by Khatiwala et al. (2012). These ventilation ages, which represent the time since a water parcel was last at the surface, taking into account contributions from multiple pathways and source regions, are referred to in the text as mean ages.

In addition to consideration of water mass ageing, we put our transect into hydrographic context with the salinity and neutral density (γ_n) section in Fig. 1. The dome of salty subtropical mode water, also known as Eighteen Degree Water, is apparent in the upper 500–800 m and is roughly bound at depth by γ_n=26.65 kg m⁻³ (LeBel et al., 2008). The remaining density surfaces in Fig. 1 demarcate the boundaries between the various sources of NADW, which are defined most clearly in the Northwest section between Bermuda and Woods Hole, Mass., known as Line W (Toole et al., 2011). These are, in order of increasing density, Upper and Classic Labrador Sea Water, Iceland–Scotland Overflow Water, and Denmark–Strait Overflow Water, which is underlain by Antarctic Bottom Water (AABW, γ_n>28.125). While we name the densest layer of water in the western basin AABW, this water mass, far from its source, must have gone through significant mixing with the overlaying NADW.

The deep waters of the Northeastern Atlantic (>3 km depth) are not as clearly defined by the contributions to NADW and are characterized by a relatively homogeneous water mass called Northeast Atlantic Deep Water (NEADW). NEADW is sourced by a mixture of NADW and AABW which enters the Northeast basin largely through the Vema Fracture Zone at 11°N (McCartney et al., 1991), with some contribution from the Romanche Trench near the equator (Broecker et al., 1980, Schlitzer, 1987, Schlitzer et al., 1985). The intermediate water in the southeastern portion of the cruise track intersects the northern extent of the salinity minimum (and silicic acid maximum) originating from Antarctic Intermediate Water (AAIW) (Talley, 1999, Tsuchiya, 1989), outlined in Fig. 1. Lastly, the high salinity intrusion of Mediterranean Outflow Water (MOW) at ~1 km depth is well represented on the largely south–north part of the transect approaching Portugal.

Deep water ²³¹Pa and ²³⁰Th concentrations can also be perturbed by changes in scavenging intensity near the seafloor (bottom scavenging) associated with a change in particle concentration or particle composition. Nepheloid layers (Biscaye and Eittreim, 1977, McCave, 1986), or zones up to hundreds of meters above the seafloor of increased particle concentration caused by the resuspension of sediments, have been known to enhance the scavenging of the shorter-lived ²³⁴Th (half-life 24.1 days) in the northwest (Bacon et al., 1989, DeMaster et al., 1991) and northeast (Schmidt, 2006, Turnewitsch et al., 2008, Turnewitsch and Springer, 2001) Atlantic. Previous studies in the North Atlantic have suggested that bottom scavenging could reduce the ²³⁰Th concentration in deep water, but since the same effect can be achieved via recent water mass ventilation without invoking a change in scavenging intensity, ventilation was the preferred explanation (Moran et al., 1997, Moran et al., 1995, Vogler et al., 1998). However, recent results from the Pacific, where the ventilation effect is not large enough to produce observed radionuclide depletions in deepwater, have confirmed early observations (Bacon and Anderson, 1982, Nozaki and Nakanishi, 1985) that significant bottom scavenging indeed occurs for ²³⁰Th (Hayes et al., 2013, Okubo et al., 2012, Singh et al., 2013) and ²³¹Pa (Hayes et al., 2013). Furthermore, nepheloid layers in the South Atlantic have been found to significantly enhance scavenging of ²³⁰Th and ²³¹Pa (Deng et al., 2014).

Based on extensive observations in the northwest Atlantic of thick nepheloid layers (Biscaye and Eittreim, 1977, Brewer et al., 1976), our transect is well situated to determine the effect of sediment resuspension on ²³⁰Th and ²³¹Pa. In addition to increased particle loading, bottom scavenging may also be affected by a change in particle composition. This section is also well suited to test the hypotheses that ²³⁰Th and ²³¹Pa are scavenged especially efficiently by authigenic iron and manganese oxide phases associated with hydrothermal activity at the mid-Atlantic ridge (German et al., 1991, German et al., 1993) or by (oxy)hydroxide coatings of particles formed in regions of organic-rich sediment diagenesis at ocean margins (Anderson et al., 1983, Bacon et al., 1976, Shimmield et al., 1986). To infer likely changes in scavenging intensity in our transect, we utilize the distribution of the particle beam attenuation coefficient, C_p, as measured by transmissometer from CTD casts, which is, to first order, linearly related to particle concentration (Bishop, 1986, Gardner et al., 1985), although the sensitivity of C_p to particle concentration is known to vary with particle size and composition (Baker and Lavelle, 1984, Richardson, 1987).