The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacial-interglacial timescales and in the future due to anthropogenic forcing. Isotopes of thorium (²³⁰Th and ²³²Th) and protactinium (²³¹Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of high-resolution observations. In this study, we present measurements of dissolved ²³⁰Th, ²³¹Pa, and ²³²Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that ²³¹Pa and ²³⁰Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemical-physical connections in the past, present, and future.

Key Points

Large gradients in dissolved ²³⁰Th, ²³¹Pa, and ²³²Th are found along upwelling isopycnals in the Pacific Southern Ocean
Zonal variations in Th/Pa fractionation across the Southern Ocean are driven by particle composition
Isopycnal mixing is of primary importance for transporting ²³¹Pa to the Southern Ocean

1 Introduction

Southern Ocean circulation is the major global conduit connecting the physical and chemical properties of the deep ocean with the surface ocean and atmosphere. In the Southern Ocean, the divergence caused by easterly winds near the continent and westerly winds offshore upwells nutrient- and carbon-rich deep waters that are carried either southward to form Antarctic Bottom Water around the Antarctic continent (e.g., Marshall & Speer, 2012) or northward where they degas natural CO₂ and take up anthropogenic CO₂ from the atmosphere (Gruber et al., 2009) before subducting to form intermediate and mode waters. Because of limiting levels of iron and light, phytoplankton are unable to fully draw down upwelled dissolved inorganic carbon and nutrients in the surface of the Southern Ocean, causing both intermediate and deep waters to subduct with significant preformed nutrient contents (Sarmiento et al., 2004; Sigman & Boyle, 2000). It is hypothesized that greater nutrient utilization due to enhanced dust inputs and increased stratification in the Southern Ocean led to significant reductions of atmospheric pCO₂ during the last glacial period, stemming the “leak” of CO₂ from the Southern Ocean via reduced ventilation and an increased efficiency of the biological pump (Martínez-Garcia et al., 2014; Siegenthaler & Wenk, 1984; Sigman & Boyle, 2000). Thus, it is critical to study the interactions between physics, chemistry, and biology in the Southern Ocean to understand the global carbon cycle both in the modern ocean and in the geologic past.

The long-lived isotopes of thorium (²³⁰Th and ²³²Th) and protactinium (²³¹Pa) can be used as tracers of particle dynamics and physical transport in both modern and paleo settings with widespread application in the Southern Ocean (e.g., Anderson et al., 2009; Kumar et al., 1993). Primordial ²³²Th is delivered to the ocean by the dissolution of lithogenic materials, making it a useful tracer for the deposition of continental dust (e.g., Hayes et al., 2013). The radiogenic isotopes ²³⁰Th and ²³¹Pa are produced in situ by the decay of uranium (²³⁴U ➔ ²³⁰Th, ²³⁵U ➔ ²³¹Pa). While uranium is highly soluble and well mixed in the oceans, varying only as a function of salinity (Owens et al., 2011), Th and Pa are highly insoluble, with typical water column scavenging residence times of 10–40 years for ²³⁰Th and 50–200 years for ²³¹Pa (Henderson & Anderson, 2003) that are much shorter than their respective half-lives of 75,584 years (Cheng et al., 2013) and 32,760 years (Robert et al., 1969). As a result, the vertical scavenging removal of ²³⁰Th and ²³¹Pa is nearly quantitative relative to their known water column production rates from U decay. In the absence of lateral redistribution, the burial ratio of ²³¹Pa/²³⁰Th would be equal to their water column production ratio (0.093, activity ratio).

Protactinium and thorium have differing sensitivities to many variables that can affect their water column distributions, including particle composition, particle flux, and lateral transport by advection and eddy diffusion. The Southern Ocean is a somewhat unique locale for Pa and Th dynamics. Elevated diatom productivity occurs as nutrient-rich deep waters reach the surface (Nelson et al., 2002), causing diatom opal to dominate the bulk particle flux beneath these areas (Honjo et al., 2000). Biogenic opal has an exceptionally high affinity for removing ²³¹Pa compared to other particle types like lithogenics, CaCO₃, and organic matter (e.g., Chase et al., 2002; Geibert & Usbeck, 2004). As a result, ²³¹Pa/²³⁰Th ratios in Southern Ocean sediment trap samples (Chase et al., 2003b), in situ pumped particulate matter (Venchiarutti, Roy-Barman, et al., 2011; Walter et al., 1997), and surface sediments (Bradtmiller et al., 2009; Chase et al., 2003b; DeMaster, 1981; Walter et al., 1997) exceed the production ratio of 0.093, necessitating greater net transport of ²³¹Pa into the Southern Ocean compared to ²³⁰Th to maintain observed inventories. The close coupling of opal fluxes and ²³¹Pa/²³⁰Th ratios has also led to the use of sedimentary ²³¹Pa/²³⁰Th as a paleoproxy for past changes in diatom productivity, and thus nutrient supply from upwelling, in the Southern Ocean (Anderson et al., 2009; Kumar et al., 1995).

Previous studies of dissolved and/or total (i.e., dissolved + particulate) ²³¹Pa and ²³⁰Th in Southern Ocean waters found negligible meridional gradients along isopycnals for these nuclides, suggesting that southward advection of deep waters, rather than diffusive mixing, is responsible for ²³¹Pa transport into the Southern Ocean (Chase et al., 2003b; Roy-Barman et al., 2019; Rutgers van der Loeff et al., 2016; Rutgers van der Loeff & Berger, 1993; Venchiarutti, Roy-Barman, et al., 2011). However, most of these observations are from the Atlantic sector of the Southern Ocean. Measurements of total ²³¹Pa and ²³⁰Th from the Pacific sector had low vertical resolution in the upper 1,500 m (Chase et al., 2003b). Additionally, these studies typically have stations spaced ~5° of latitude apart, preventing identification of gradients on finer scales.

In this study, we measured dissolved ²³¹Pa, ²³⁰Th, and ²³²Th along the NBP1702 section at 170°W in the Southern Ocean. This section reoccupied the Antarctic Environment and Southern Ocean Process (AESOPS) transect from the U.S. Joint Global Ocean Flux Study (JGOSF) program (Smith et al., 2000), where measurements of total ²³¹Pa and ²³⁰Th were previously made (Chase et al., 2003b). We first use several measurements of size-fractionated dissolved samples to study the physical speciation of ²³¹Pa and ²³⁰Th in near-surface waters. We also present measurements from 12 profiles with an average meridional spacing of ~1° of latitude, with 10–12 measurements in the upper 1,500–2,000 m at each location. At our station furthest to the north, we collected a profile spanning the entire water column (5,247 m). We combine our data with historical sediment trap observations to determine the relative scavenging intensity of ²³¹Pa and ²³⁰Th. We document the first evidence for steep north-south isopycnal gradients in dissolved ²³¹Pa and ²³⁰Th and find that isopycnal mixing plays an important role in the transport of ²³¹Pa into the Southern Ocean.

2 Materials and Methods

2.1 Cruise Track

The NBP1702 cruise took place aboard the RV Nathaniel B. Palmer from McMurdo Station, Antarctica, to Lyttelton, New Zealand, between 24 January and 6 March 2017. Samples were taken along a meridional transect from 67°S to 54°S at 170°W. This section spanned a variety of physical and biogeochemical regimes, from the Ross Gyre poleward of the Southern Boundary of the Antarctic Circumpolar Current (SBACC); the Antarctic Zone (AZ) between the SBACC and the Antarctic Polar Front (APF); the Polar Frontal Zone (PFZ) between the APF and the Subantarctic Front (SAF); and the Subantarctic Zone north of the SAF (Figure 1).

Details are in the caption following the image — **Figure 1**
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Site map of sampling locations during NBP1702. Individual stations are shown as stars and labeled with their station number. GEBCO 6 × 6 min bathymetry is shown in color. The location of climatological fronts (Orsi et al., 1995) are shown as solid lines, including the Subtropical Front (STF) in white, the Subantarctic Front (SAF) in green, the Antarctic Polar Front (APF) in black, and the Southern Boundary of the Antarctic Circumpolar Current (SBACC) in red.

2.2 Sample Collection and Analysis for dSi and bSi

For biogenic silica concentrations, seawater (1.2 L) was collected from Niskin bottles into polypropylene bottles and filtered through a 1.2-μm pore size polycarbonate filter, placed in a cryovial and dried at 60°C, and analyzed using the alkaline digestion method of Krause et al. (2012). Silicic acid concentrations were measured at sea. Forty milliliters was drawn from the Niskin bottles through silicone tubing into a 50-ml screw-cap polypropylene tube and analyzed as described by Brzezinski and Nelson (1995).

2.3 Sample Collection and Analysis for Th and Pa Isotopes

Most Th and Pa samples were collected in Niskin bottles deployed on a stainless steel rosette. Each sample (~4–5 L) for Th and Pa isotope analysis was filtered through a 0.45-μm Acropak capsule filter into an acid-cleaned cubitainer and acidified to pH = 2 using 6 M hydrochloric acid at sea, following GEOTRACES protocols (Anderson et al., 2012).

In addition, several near-surface seawater samples were collected to determine the size fractionation of dissolved Th and Pa isotopes. These samples were collected using a Perfluoroalkoxy alkane (PFA)-coated Polytetrafluoroethylene (PTFE) bellows pump deployed at approximately 10-m depth, with seawater passed through tubing running from the pump to a clean bubble in the ship's lab. There, approximately 5 L of water was collected for Th and Pa analysis by filtration through either a 0.2- or 0.02-μm capsule filter, collected in an acid-cleaned cubitainer, and acidified to pH = 2 using 6 M hydrochloric acid at sea.

Samples were analyzed onshore using previously published methods for analysis of seawater Th and Pa isotopes at Lamont-Doherty Earth Observatory (Anderson et al., 2012; Pavia et al., 2018, 2019). Samples were spiked with ²²⁹Th and ²³³Pa, added to samples along with 15 mg of dissolved Fe as Fe(NO₃)₃, and allowed to equilibrate for 24 h. Sample pH was then raised to pH = 8.3–8.7 by adding ~8–10 ml of concentrated Optima-grade NH₄OH to precipitate Fe as Fe oxyhydroxide. The precipitate was allowed to settle for 2 days. The overlying water was drained, and the Fe precipitate was centrifuged and washed in Milli-Q water before digestion at 200°C in concentrated HNO₃, HF, and HCLO₄. After dissolution, the samples were dried down and brought back up in concentrated HCl for separation of Th and Pa fractions via anion exchange chromatography on Bio-Rad AG1-X8 100-200 mesh size resin. Concentrations of ²³⁰Th, ²³²Th, and ²³¹Pa were determined by isotope dilution, with measurements made on a Thermo Element XR Single Collector Magnetic Sector ICP-MS in peak jumping mode.

Procedural blanks were determined by processing 2 L of Milli-Q water in acid-cleaned cubitainers in the same manner as seawater. Blanks were taken to sea and acidified to pH = 2 at the same time as seawater samples and analyzed onshore using identical methods, with two blanks measured for every ~15 samples. Mean and 1σ procedural blanks for dissolved samples were ²³²Th = 3.7 ± 1.4 pg, ²³⁰Th = 0.21 ± 0.06 fg, and ²³¹Pa = 0.03 ± 0.02 fg. Reproducibility was assessed by measuring aliquots of two intercalibrated working standard solutions of ²³²Th, ²³⁰Th, and ²³¹Pa: SW STD 2010-1 (Anderson et al., 2012) and SW STD 2015-1, which has lower ²³²Th concentrations more similar to Pacific seawater conditions. For standards run alongside NBP1702 samples, the reproducibility for each isotope was 2.12% for ²³⁰Th, 0.87% for ²³²Th, and 1.76% for ²³¹Pa on SW STD 2010-1 and was 0.87% for ²³⁰Th, 3.78% for ²³²Th, and 2.69% for ²³¹Pa on SW STD 2015-1.

Dissolved ²³⁰Th and ²³¹Pa were corrected for ingrowth of ²³⁰Th and ²³¹Pa during sample storage using the uranium-salinity relationship of Owens et al. (2011), a seawater ²³⁴U/²³⁸U activity ratio of 1.1468 (Andersen et al., 2010), and a seawater ²³⁸U/²³⁵U molar ratio of 137.824 (Weyer et al., 2008). Dissolved ²³⁰Th and ²³¹Pa were also corrected for lithogenic contributions using measured dissolved ²³²Th and assuming a detrital ²³⁰Th/²³²Th molar ratio of 4.0 × 10⁻⁶ (Roy-Barman et al., 2009) and a ²³¹Pa/²³²Th molar ratio of 8.8 × 10⁻⁸, derived from assuming an average upper continental crust U/Th ratio (Taylor & McLennan, 1995) and secular equilibrium between ²³¹Pa and ²³⁵U. Typically, ²³⁰Th and ²³¹Pa data corrected in this manner are denoted by the subscript “xs,” but since all data shown in this paper have been corrected the same way, we will omit the subscript for clarity.

3 Results

3.1 Hydrographic Setting

The Antarctic Circumpolar Current (ACC) is forced by strong westerly surface winds driving divergence and Ekman upwelling, drawing up waters from the deep ocean along isopycnals sloping upward from north to south, resulting in eastward geostrophic flow around the Antarctic continent. The waters upwelling in the Southern Ocean are primarily Circumpolar Deep Water (CDW), which is further subdivided into Upper Circumpolar Deep Water (UCDW; low O₂, high nutrients) and Lower Circumpolar Deep Water (LCDW; higher O₂, higher salinity). On NBP1702, UCDW can be identified by an O₂ minimum between the neutral density surfaces 27.6 ≤ γⁿ ≤ 27.95, sitting above the higher-salinity LCDW found at γⁿ > 27.95 (Figure 2). UCDW and LCDW upwell from below 1,000 m at the northern end of the NBP1702 section to <250-m depth at 65°S where the isopycnals flatten southward (Figure 2).

Above 250 m, waters south of the APF are stratified into two layers typical of austral summer, with warmer, fresher Antarctic Surface Water (AASW) at the surface atop colder, saltier Winter Water (WW) that forms a subsurface temperature minimum at 100×200 m. The WW is a remnant from deep winter mixing (Toole, 1981), while AASW freshens during summer from precipitation and ice melt (Chaigneau et al., 2004; Park et al., 1998). The northward persistence of the subsurface temperature minimum (<2°C) associated with WW can be used to distinguish the northward extent of the APF (Orsi et al., 1995). On NBP1702, this temperature minimum is found at 59°S (Figure 2), putting the APF ~ 2° further north than in Orsi et al. (1995). This is unlikely to be due to seasonality, as satellite reconstructions of the APF location suggest poleward front migration during austral spring and summer (Freeman et al., 2016).

Ekman upwelling, isopycnal mixing, and deep wintertime diapycnal mixing supply macronutrients and micronutrients like Si(OH)₄, NO₃⁻, and Fe that fuel diatom productivity. During the winter, more intense mixing, increased ice cover, and low biological uptake allows for net accumulation of nutrients, while intense diatom blooms during spring and summer result in slight NO₃⁻ drawdown (Morrison et al., 2001) and nearly complete Si(OH)₄ drawdown (Sigmon et al., 2002). These blooms result in sharp latitudinal Si(OH)₄ gradients in the spring and summer (Brzezinski et al., 2001). On NBP1702, surface Si(OH)₄ was below 3 μmol L⁻¹ everywhere north of 62°S, sharply increased to 20 μmol L⁻¹ at 63°S, and then steadily increased poleward to 61 μmol L⁻¹ at 67°S (Figure 3). Continued diatom productivity was evident from ~50-m biogenic silica (bSi) peaks >10 μmol L⁻¹ at 62–63°S, with smaller peaks of ~5 μmol L⁻¹ at 61°S and 64°S (Figure 3).

3.2 Dissolved ²³⁰Th and ²³¹Pa Distributions on NBP1702

Profiles of dissolved ²³⁰Th and ²³¹Pa are controlled by the combined influence of circulation and scavenging removal. Profiles of dissolved ²³⁰Th and ²³¹Pa typically increase linearly with depth in the upper 2,000 m due to reversible scavenging, reflecting their uniform source in seawater from U decay and equilibrium adsorption/desorption on slow-sinking particles (Bacon & Anderson, 1982). On NBP1702, concave downward profiles of dissolved ²³⁰Th, ²³²Th, and ²³¹Pa occur south of 61°S at Stations 1–7 (Figure 4). These concave profile shapes have previously been found in both the Atlantic and Pacific sectors of the Southern Ocean and reflect upwelling of Th- and Pa-rich deep waters along upward-tilting isopycnals (Chase et al., 2003b; Roy-Barman et al., 2019; Rutgers van der Loeff et al., 2016; Rutgers van der Loeff & Berger, 1993; Venchiarutti, Roy-Barman, et al., 2011). While we were only able to collect samples from the upper 2,000 m for the majority of the cruise, at our station furthest to the north (Station 15, 54°S) we collected a full-depth profile of Th and Pa isotopes. The deep data from Station 15 are not discussed in detail in this paper, but the full-depth profiles of ²³⁰Th, ²³¹Pa, and ²³²Th are included in supporting information Figure S1.

For much of the section north of 61°S, isopleths of ²³⁰Th and ²³¹Pa follow neutral density surfaces (Figure 5). The 61–64°S region of most pronounced ²³¹Pa removal directly coincides with the highest levels of biogenic silica measured on the section (Figure 3). Concentrations of ²³⁰Th, ²³²Th, and ²³¹Pa below 100 m increase from north to south, with the largest lateral gradients at depth for ²³⁰Th and ²³¹Pa found between ~64°S (Station 4) and ~63°S (Station 5). Dissolved ²³¹Pa/²³⁰Th activity ratios decrease from the surface to ~150-m depth, below which the ratios stay constant to 1,000–1,500 m, with values at depth increasing northward from 0.35 at 67°S to 0.6 at 54°S (Figure 4). In the sections below, we explore the impacts of both scavenging and circulation in setting the observed distributions and removal fluxes of Th and Pa isotopes in different sectors of the Southern Ocean.

3.3 Size-Fractionated Dissolved Th and Pa Isotopes

Samples collected via the PFA pump were passed through either a 0.2- or 0.02-μm filter. The ²³⁰Th and ²³²Th concentrations in samples from a given station that were collected using the PFA pump and passed through either a 0.2- or 0.02-μm filter are statistically indistinguishable (Figures 6a and 6c); however, surface samples collected in Niskin bottles and subsequently filtered through 0.45-μm capsule filters were consistently offset higher than samples from the same station collected by the PFA pump and filtered at 0.2 or 0.02 μm, with an offset magnitude averaging 0.4 ± 0.08 μBq kg⁻¹ (1σ, n = 6) for ²³⁰Th and 0.006 ± 0.003 pmol kg⁻¹ (1σ, n = 5) for ²³²Th. Unlike Th, there is negligible Pa found in any colloidal size class in the NBP1702 samples (Figure 6b). At four of the six stations, %colloidal Pa is statistically indistinguishable from 0 (Figure 6d).

4 Discussion

4.1 Physical Speciation of Dissolved Th and Pa

We measured the physical speciation of Th and Pa isotopes by measuring size-fractionated dissolved samples passed through different filter sizes. Studying the size speciation of Th and Pa is important for understanding the role of colloids in scavenging (Honeyman et al., 1988; Honeyman & Santschi, 1989) and for testing whether the radiogenic (²³⁰Th) and primordial (²³²Th) are identically scavenged or differentially incorporated into colloids, which would affect estimates of dust deposition based on Th isotopes (Hayes et al., 2013).

Interpreting the partitioning of ²³⁰Th and ²³²Th in size classes <0.45 μm is challenging as the results varied between sampling methods, with the concentrations of both isotopes being higher in samples filtered at 0.45 μm compared to those filtered at either 0.2 or 0.02 μm. Oceanographic interpretation of these results would argue for a significant proportion (up to 60%, Figure 6d) of dissolved Th being present as colloids between 0.2 and 0.45 μm in size, with the rest being soluble (<0.02 μm). This is difficult to reconcile with results from ultrafiltration experiments, where only 10–20% of Th was found to be between 10 kDa and 0.45 μm in the North Pacific (Hayes, Anderson, Fleisher, Huang, et al., 2015) and <10% of Th was found in the same size class in the subtropical North Atlantic (Hayes et al., 2017).

An alternative explanation for our results is that samples collected by the PFA pump lost Th via adsorption to the sampling apparatus, including the pumping system and the tubing at a constant rate during sampling. During GEOTRACES intercalibration, a small difference in Th concentrations in a 2,000-m sample from the Bermuda Atlantic Time Series filtered at 0.2 and 0.45 μm was observed, though the authors could not conclusively rule out that the difference was due to Th adsorption to the 0.2-μm Osmonics filter cartridge (Anderson et al., 2012). We similarly have difficulty ruling out methodological artifacts as driving the apparently high Th concentrations we observed in the 0.2- to 0.45-μm size class. The concentration of Th in this size class stays mostly constant despite large decreases in surface Th concentrations between Station 1 and Stations 9–13 (Figures 6a–6c). This would imply large increases in the percentage of dissolved Th found as colloids from south to north along the section (Figure 6d).

We attempted to test for adsorption of Th to the plastic PFA pump tubing by analyzing a 0.1 M HCl leachate of tubing used in a subsequent PFA pump deployment. While this acid concentration is fairly dilute, we found vanishingly low concentrations of Th and Pa in the tube leachate, implying that there was insignificant adsorption to plastic tubing during sampling. Future experiments should test whether the apparent colloidal Th signal we observe is real or a methodological artifact by collecting samples using the PFA pump across 0.02-, 0.2-, and 0.45-μm filters or by directly measuring colocated samples collected from Niskin bottles and the PFA pump that are both filtered at 0.45 μm.

We did not detect colloidal Pa, which implies that if a sampling artifact due to adsorption to the apparatus is responsible for the colloidal Th results, that adsorption to tubing does not seem to be an issue for Pa, consistent with previous results finding 15–20 times greater adsorption of Th than Pa to plastic bucket walls during sampling (Anderson et al., 1983a). To the best of our knowledge, these are the first measurements of physical speciation for dissolved ²³¹Pa in the ocean and show that the vast majority of dissolved ²³¹Pa, at least in the Southern Ocean, is not colloidal. This suggests that Pa scavenging in the Southern Ocean is regulated by direct adsorption onto particle surfaces.

4.2 The Role of Particle Composition in Scavenging Th and Pa

Particle composition plays an important role in dictating the removal of Th and Pa from seawater and setting sedimentary ²³¹Pa/²³⁰Th ratios. Diatom opal has a particularly high affinity for scavenging ²³¹Pa, and diatom blooms near the APF drive high ²³¹Pa/²³⁰Th ratios both in surface sediments (Bradtmiller et al., 2009; Chase et al., 2003b; DeMaster, 1981) and sediment trap particulate material (Chase et al., 2002) in the Southern Ocean. Sedimentary and particulate ²³¹Pa/²³⁰Th activity ratios at or above the production ratio of 0.093 in the Southern Ocean are best explained by opal particles having nearly identical affinity for removing Th and Pa from solution (Rutgers van der Loeff & Berger, 1993).

To determine the impact of particle composition in scavenging Th and Pa, we utilize the fractionation factor F(Th/Pa) (Anderson et al., 1983b):

$urn:x-wiley:08866236:media:gbc21055:gbc21055-math-0001$ (1)

where K refers to the partitioning of the indicated isotope between the particulate (p) and dissolved (d) phases. F(Th/Pa) quantifies the relative scavenging of the two isotopes onto particles. The composition of suspended and sinking particles has been long established as a key variable regulating the relative removal rates of ²³¹Pa and ²³⁰Th as particles differ in their relative affinity for the two isotopes depending on their chemical composition. In regions of the ocean where the particle flux is dominated by phases other than opal-like CaCO₃, lithogenics, and/or particulate organic carbon, fractionation factors are typically >10, reflecting the greater particle reactivity and shorter scavenging residence time of Th compared to Pa (Hayes, Anderson, Fleisher, Vivancos, et al., 2015; Moran et al., 2002). Areas rich in metalliferous particles have lower fractionation factors, reaching F(Th/Pa) < 5 in the intense Southeast Pacific hydrothermal plume at 15°S (Pavia et al., 2018). The lowest oceanic F(Th/Pa) values have been found in the Southern Ocean, where the particle flux is dominated by opal, indicating that opal is the particle phase with the highest relative affinity for Pa relative to Th.

In the Atlantic sector of the Southern Ocean, F(Th/Pa) is ~2 at the sea surface (Rutgers van der Loeff & Berger, 1993; Venchiarutti, Roy-Barman, et al., 2011) and increases linearly with depth to ~7 at 1,000 m (Venchiarutti, Roy-Barman, et al., 2011). Walter et al. (1997) found lower values of F(Th/Pa) = 1.86 at 700 m near the APF in the Atlantic sector and generally found decreasing F(Th/Pa) from north to south. In the wake of the Kerguelen Plateau, Venchiarutti, Rutgers van der Loeff, et al. (2011) found F(Th/Pa) values as low as 0.06–0.08. Their results were driven by exceptionally high particulate ²³¹Pa concentrations, linked to the intense diatom blooms in this region. In the Pacific Sector on the same 170°W line as the current study, Chase et al. (2002, 2003b) combined annually averaged sediment trap ²³¹Pa/²³⁰Th ratios and total (unfiltered) ²³¹Pa/²³⁰Th ratios to compute F(Th/Pa) < 1 in the upper 1,000 m at 63°S and 66°S. However, unfiltered samples contain dissolved and particulate Th and Pa and thus are potentially subject to biases when calculating fractionation factors.

We have recalculated F(Th/Pa) values at 170°W using our filtered samples for dissolved ²³⁰Th and ²³¹Pa in combination with the sediment trap data from Chase et al. (2002, 2003b). The only location where we sampled at the exact location of the trap moorings is at 63°S. However, several of the other trap moorings were deployed at locations within 110 km of two of our sampling stations. Where this was the case, we interpolated dissolved ²³¹Pa and ²³⁰Th between the bracketing stations onto the location of the trap mooring.

We can calculate F(Th/Pa) values in two ways. The first uses the dissolved ²³¹Pa/²³⁰Th ratio at the depth of the sediment trap (near 1,000 m), which assumes that the particles in the sediment trap have equilibrated with waters at the same depth. The second uses the dissolved ²³¹Pa/²³⁰Th ratio vertically integrated to the depth of the trap, which assumes that the particles in the sediment trap reflect scavenging over the entire water column above the trap depth. The two methods give F(Th/Pa) values that are within 1.5% of each other at all sites—smaller than the relative error of the calculated F(Th/Pa) values. For simplicity we present the F(Th/Pa) results only from the first method in Table 1.

Table 1. Particle Composition From Annually Averaged Sediment Trap Deployments Near 1,000 m on NBP9802 (Honjo et al., 2000) and Fractionation Factors F(Th/Pa) Estimated From Pa/Th Values in the Sediment Trap and Dissolved ²³⁰Th and ²³¹Pa Measured on NBP1702

Mooring site	Latitude (°N)	Depth (m)	F(Th/Pa)	Opal%	CaCO₃%	C_org%	Lithogenic%
2	−56.90	982	2.67 ± 0.11	52.3	40.6	5.7	0.4
3	−60.28	1,003	1.94 ± 0.09	63.8	29.7	5.4	0.3
4	−63.15	1,031	0.77 ± 0.03	80.6	15.4	3.2	0.2
5	−66.16	937	0.77 ± 0.03	84.7	3.3	10.3	0.3

Note. The particle composition values are calculated as the proportions of each constituent relative to the sum of the measured constituents (also including N_org%, not shown here), as described in Honjo et al. (2000).

We find F(Th/Pa) values at depths near 1,000 m that decrease from 2.67 at 56.9°S to 0.77 at 63.15°S and 66.16°S (Table 1). The decrease from north to south in F(Th/Pa) coincides with increasing opal contents of the sediment traps. Identical F(Th/Pa) at mooring sites 4 and 5 south of the APF, despite decreasing opal% between the two sites, is likely due to the decrease in CaCO₃ (F(Th/Pa) = 33.3; Hayes, Anderson, Fleisher, Vivancos, et al., 2015) and subsequent increase in POM% (F(Th/Pa) = 4.9; Hayes, Anderson, Fleisher, Vivancos, et al., 2015). These F(Th/Pa) values are slightly lower than the original estimate of F(Th/Pa) = 0.88 at 63°S from Chase et al. (2003b) and consistent with preferential removal of Pa relative to Th near and south of the APF at 170°W.

Our results show that there is zonal variability in F(Th/Pa) between the Pacific and Atlantic sectors of the Southern Ocean. This variability is likely not due to greater diatom productivity in the Pacific sector. Annually averaged satellite net primary productivity at the APF (Moore & Abbott, 2002) near 170°W is not substantially higher than that at 0°W (Walter et al., 1997), 50°W (Venchiarutti, Roy-Barman, et al., 2011), or 10–20°E (Rutgers van der Loeff & Berger, 1993). Average sedimentary opal burial rates are statistically indistinguishable between the Pacific and Atlantic sectors of the Southern Ocean (Chase et al., 2015). Instead, we propose that higher F(Th/Pa) in the South Atlantic is due to greater lithogenic dust fluxes compared to that in the South Pacific. The Atlantic sector of the Southern Ocean is downwind of the Patagonian dust plume and is modeled to have ~3–5 times higher dust deposition than at 170°W (Mahowald et al., 2005). Dust flux reconstructions from sub-Antarctic Atlantic coretops are typically 1.5–4 g m⁻² year⁻¹ (Anderson et al., 2014), 3–5 times higher than in the more remote Pacific sector (Chase et al., 2003a). Lithogenic particles have significantly higher affinity for scavenging Th than Pa, with an end-member F(Th/Pa) = 10 (Hayes, Anderson, Fleisher, Vivancos, et al., 2015). Higher dust fluxes in the Atlantic Southern Ocean would cause an increased proportion of the particle flux to be made up of lithogenics than those in the Pacific Southern Ocean, driving higher F(Th/Pa) values.

4.3 Transport of Th and Pa to the Southern Ocean by Isopycnal Mixing

4.3.1 Isopycnal Fluxes of ²³⁰Th and ²³¹Pa

Observations of ²³⁰Th and ²³¹Pa in the Southern Ocean have been used to construct box models of the transport and burial rates of these nuclides across different fronts and regions of the ACC (Chase et al., 2003b; Rutgers van der Loeff et al., 2016). These models are important for the application of sedimentary ²³¹Pa/²³⁰Th ratios as paleoceanographic proxies for opal flux and upwelling in the Southern Ocean (e.g., Anderson et al., 2009; Bradtmiller et al., 2009) and/or deep water flow into the South Atlantic (Negre et al., 2010). The transport modes of ²³⁰Th and ²³¹Pa into the Southern Ocean are also important for understanding the processes that close the budgets of these nuclides in both the Pacific (Hayes et al., 2014) and the Atlantic (Deng et al., 2018; Yu et al., 1996). Box models to date typically only consider advection for the meridional transport of Th and Pa toward the Southern Ocean (Luo et al., 2010). This is because dissolved ²³⁰Th and ²³¹Pa data from the Southern Ocean have not shown significant gradients along upwelling isopycnals in the upper 1,500 m that would facilitate transport by eddy diffusion, perhaps due either to measurement of total (unfiltered) Th and Pa (Chase et al., 2003b) or to sparse meridional and depth sampling (Chase et al., 2003b; Roy-Barman et al., 2019; Venchiarutti, Roy-Barman, et al., 2011).

On NBP1702, we observe north-south isopycnal gradients for ²³⁰Th, ²³¹Pa, and ²³²Th (Figure 7). North-south gradients along upwelling isopycnals are particularly pronounced for ²³¹Pa, which decreases from north to south at each neutral density level sampled from Station 15 at 54°S to Station 3 at 65°S (Figure 7), with steepest gradients found between density levels 27.5 < γ_n < 27.7 kg m⁻³ and ²³¹Pa values converging between stations toward γ_n ~ 28.1 kg m⁻³. Gradients in ²³⁰Th and ²³²Th show similar structure to each other, though the gradients are larger for ²³²Th than for ²³⁰Th, with maximum north-south differences found at γ_n ~ 27.7 kg m⁻³. Unlike those in ²³¹Pa, the north–south gradients in ²³⁰Th and ²³²Th converge at γ_n ~ 27.9 kg m⁻³ and reverse at deeper density levels, with concentrations of both Th isotopes increasing from south to north at γ_n > 27.9 kg m⁻³ (Figure 7).

The one-dimensional mass balance for dissolved ²³⁰Th or ²³¹Pa (A), in steady state, along an isopycnal surface x, can be written as follows:

$urn:x-wiley:08866236:media:gbc21055:gbc21055-math-0002$ (2)

where P is the production rate from the decay of ²³⁴U or ²³⁵U, J is the removal rate by particle scavenging, and the third and fourth terms represent dispersive fluxes due to lateral advection along an isopycnal and by eddy diffusion, respectively, where u is the horizontal velocity and K_h is the eddy diffusivity. This equation has been used to calculate lateral fluxes of ²³⁰Th from eddy diffusion in boundary scavenging regions in the North Atlantic (Hayes, Fitzsimmons, et al., 2015).

We attempt to determine the role of isopycnal mixing on the budgets of ²³¹Pa and ²³⁰Th in the SW Pacific Southern Ocean using a similar approach. We estimate the role of eddy diffusion following the approach of Hayes, Fitzsimmons, et al. (2015); however, we also outline a few ways this approach may be too simplistic and could be improved upon. Our assumption of one-dimensional dynamics may not be appropriate in the ACC, which is dominated by strong zonal transport. Features observed on a given meridional section may be inherited in part from upstream biogeochemical processes. Calculating the second derivative $urn:x-wiley:08866236:media:gbc21055:gbc21055-math-0003$ is done on discrete density levels and requires determining the length scales over which eddy diffusion acts on the tracers, with diffusion parameterized as a random-walk process: $urn:x-wiley:08866236:media:gbc21055:gbc21055-math-0004$ , where τ is the water column residence time of ²³⁰Th or ²³¹Pa. Hayes, Fitzsimmons, et al. (2015) determined τ by dividing water column inventories of dissolved ²³⁰Th and ²³¹Pa by their integrated water column production rates. However, in boundary scavenging regions where there is net lateral influx (outflux) of a nuclide, this approach to determining residence times will overestimate (underestimate) the true residence time. Additionally, the integrated residence times of ²³¹Pa and ²³⁰Th increase with depth, which should increase the length scale on which eddy diffusion acts to transport ²³⁰Th and ²³¹Pa; however, in their approach, a single length scale was used for determining lateral gradients. A final caveat that becomes important in the Southern Ocean is that eddy diffusivities vary as a function of both depth and latitude (e.g., Abernathey et al., 2010; Chapman & Sallée, 2017). This makes the selection of a single eddy diffusivity to compute lateral fluxes extremely complex and possibly makes the formulation of the diffusive fluxes incorrect, depending on the significance of the meridional gradients in eddy diffusivity.

A few of these issues can be circumvented in the Southern Ocean, though not all. For instance, the depth-integrated residence time of ²³⁰Th and ²³¹Pa on NBP1702 cannot be determined a priori given the influence of lateral transport apparent in their concave profiles (Figure 4) and uncertainties in the local eddy diffusivities. However, since F(Th/Pa) is close to 1 averaged over the upper 1,000 m at 63°S (see section 4.2), we assert that the scavenging residence times and thus the diffusive length scales of ²³⁰Th and ²³¹Pa are similar in this region. We use an estimate of K_h = 1,000 m² s⁻¹ at 170°W on the γ_n = 27.9 kg m⁻³ isopycnal (Chapman & Sallée, 2017) as representative for each of the isopycnals chosen, while acknowledging that this is a simplification with ~50% uncertainty. For now, we also assume constant residence times on each isopycnal of ~15 years for both isotopes, giving a resulting diffusive length scale Δx ~ 1,000 km. On NBP1702, 1,000 km is the latitudinal distance between Stations 3 and 13, and we choose Station 8 as the intermediate station between the two. Were there more station combinations spaced at the appropriate diffusive length scale, the following calculations would be possible there as well.

We multiply the calculated second isopycnal gradients by the lateral eddy diffusivity of 1,000 m² s⁻¹ to derive the flux convergence of ²³⁰Th and ²³¹Pa due to isopycnal mixing at 63°S (Table 2). The volume-specific mixing fluxes (μBq m⁻³ year⁻¹) of ²³⁰Th peak on the 27.7 kg m⁻³ neutral density surface, with maximum values that are ~55% of the local production rate by ²³⁴U decay, suggesting that isopycnal mixing is a significant source of ²³⁰Th to the Pacific Southern Ocean. To quantify the net effect of isopycnal mixing on the depth-integrated ²³⁰Th budget, we integrate the discrete volume-specific isopycnal flux divergences at Station 8 with depth. This depth-integrated ²³⁰Th flux from isopycnal mixing at 1,360 m is 172.2 mBq m⁻² year⁻¹, which is 30% of the integrated production rate from ²³⁴U decay. Particle fluxes derived from ²³⁰Th-normalization performed at 1,360 m would thus be biased by 30% due to isopycnal diffusive ²³⁰Th transport alone. Future work characterizing the full-depth water column will be needed to determine the net effect of isopycnal mixing on ²³⁰Th burial rates in the Pacific Southern Ocean. If the supply of ²³⁰Th to the Southern Ocean were to vary through time, either due to changes in scavenging-driven meridional ²³⁰Th gradients or via wind-driven changes in isopycnal mixing rates (Abernathey & Ferreira, 2015), so too would its burial rate, potentially complicating its use as a constant flux proxy for sediment mass accumulation rates in this region (Costa et al., 2020).

Table 2. Second Isopycnal Gradients of ²³⁰Th and ²³¹Pa on Six Different Neutral Density Surfaces, As Well As the ²³¹Pa/²³⁰Th Diffusive Transport Ratio, Evaluated at NBP1702 Station 8

Neutral density level (kg m⁻³)	²³⁰Th lateral diffusive flux convergence (μBq m⁻³ year⁻¹)	²³⁰Th production rate (μBq m⁻³ year⁻¹)	²³¹Pa lateral diffusive flux convergence (μBq m⁻³ year⁻¹)	²³¹Pa production rate (μBq m⁻³ year⁻¹)	²³¹Pa/²³⁰Th transport ratio (activity ratio)
27.5	37.6	433.1	−4.0	40.28	−0.11
27.6	179.1	433.1	90.2	40.28	0.50
27.7	237.3	433.1	152.6	40.28	0.64
27.8	153.9	433.1	141.1	40.28	0.92
27.9	152.0	433.1	205.1	40.28	1.35
28.0	79.2	433.1	172.9	40.28	2.18

Isopycnal transport plays an even greater role in the budget of ²³¹Pa. The volume-specific mixing fluxes (μBq m⁻³ year⁻¹) on a given isopycnal peak at γ_n = 27.9 kg m⁻³, with maximum fluxes 5 times higher than its production rate from ²³⁵U decay (Table 2). The depth-integrated ²³¹Pa isopycnal mixing flux at 1,360 m is 179.5 mBq m⁻² year⁻¹, 3.3 times higher than its water column production rate. At each density surface below γ_n = 27.6 kg m⁻³, the ²³¹Pa/²³⁰Th transport ratio greatly exceeds the water column production ratio of 0.093. Together, these results suggest that lateral mixing along upwelling isopycnals is of primary importance in supplying ²³¹Pa to the Pacific sector of the Southern Ocean from lower latitudes.

4.3.2 Mechanisms for Interbasin Differences in Mixing Transport

The importance of mixing in transporting ²³¹Pa along isopycnals into the Pacific Southern Ocean can be seen in sedimentary ²³¹Pa/²³⁰Th ratios. To illustrate this, we compare the downcore relationship between ²³¹Pa/²³⁰Th ratios and opal burial rates between sediment cores from different sectors of the Southern Ocean, all south of the APF (Figure 8): TN057-13-4PC from the Atlantic sector (Anderson et al., 2009); E27-23 from the Indian sector (Anderson et al., 2009); and NBP9802-5PC, NBP9802-6PC, and NBP9802-7PC from the Pacific sector (Chase et al., 2003a). The slope of the ²³¹Pa/²³⁰Th vs. opal flux relationship is greatest for the Pacific sector (Figure 8), implying that for a given change in opal flux, more ²³¹Pa is removed from the Pacific sector of the Southern Ocean to be scavenged and buried relative to ²³⁰Th. This effect is not due to differential opal preservation, as opal preservation efficiencies are similar among the sectors of the Southern Ocean (Sayles et al., 2001). Instead, this difference reflects the unique capacity of isopycnal mixing to enhance the supply ²³¹Pa into the Pacific sector over the Atlantic sector of the Southern Ocean.

The large dissolved ²³¹Pa gradients in the Southern Ocean are due to scavenging by biogenic opal, which has an order of magnitude higher distribution coefficient (K_d) for Pa scavenging than any other major particle phase (Chase et al., 2003b). The very high affinity of opal for Pa scavenging and abundance of diatom productivity in the Southern Ocean is what establishes the larger meridional concentration gradient for Pa than for Th in the Southern Ocean. Mean opal burial rates are similar in both the Atlantic and Pacific sectors (Chase et al., 2015), so meridional dissolved ²³¹Pa gradients should be similar as well.

We argue that the interbasin difference in Pa transport is due to differences in particle composition in the Atlantic and Pacific Southern Oceans, most notably the much higher dust flux in the Atlantic discussed in section 4.2. We hypothesize that the higher dust flux in the Atlantic sector increases the scavenging rate of Th, driving larger meridional isopycnal gradients and greater southward supply of ²³⁰Th by lateral mixing compared to those in the Pacific sector. This difference in ²³⁰Th supply between the Atlantic and the Pacific is thus the driver of the lower ²³¹Pa/²³⁰Th ratios observed for a given opal flux in the Atlantic compared to those in the Pacific (Figure 8). In the modern water column, the higher dust fluxes in the Atlantic sector of the Southern Ocean are what drive the higher F(Th/Pa) there than in the Pacific sector, as discussed in section 4.2. We do not wish to imply that ²³⁰Th is not efficiently scavenged in the Pacific Southern Ocean, as Chase et al. (2003b) found that 70–90% of the ²³⁰Th production above 1,000 m was scavenged locally in bottom-moored sediment traps. Rather, we suggest that ²³¹Pa is scavenged exceptionally efficiently throughout the Southern Ocean and that ²³⁰Th is scavenged more efficiently in the Atlantic sector than in the Pacific sector because of higher dust fluxes. The source of this extra ²³⁰Th transported to the Atlantic sector is yet unclear—whether it is from the north, the south, or export out of the Pacific basin. Additional work will be needed to determine the meridional and zonal transport of ²³⁰Th into the Atlantic Southern Ocean.

Future modeling work to construct complete ²³¹Pa and ²³⁰Th budgets may provide unique constraints on the rates of isopycnal upwelling and mixing in the Southern Ocean. Additionally, our results suggest that reconstruction of the changing ratios between sedimentary ²³¹Pa/²³⁰Th and opal flux may provide an avenue to reconstruct the efficiency of isopycnal mixing in supplying tracers from the ocean interior to the surface of the Southern Ocean, a physical processes that is critical for ventilating the modern deep ocean (Jones & Abernathey, 2019) but has yet to be reconstructed for past climates.

5 Summary and Outlook

Our high-resolution measurements from 170°W offer new insights into the dynamics of scavenging and physical transport of ²³¹Pa and ²³⁰Th in the Southern Ocean. There is intriguing evidence for unique physical speciation of ²³⁰Th in large colloids, but methodological artifacts cannot definitively be ruled out as a cause of the finding. More work will be required to determine whether adsorption onto one or more components of the sampling apparatus caused samples collected by the PFA pump to have lower Th concentrations and whether this is an issue for other reactive trace metals. We found fractionation factors F (Th/Pa) less than 1 in the upper 1,000 m south of the APF, indicating that ²³¹Pa is more efficiently scavenged than ²³⁰Th. In combination with the lack of colloidal Pa, this is strong evidence for direct adsorption of Pa to opal surfaces as being the dominant driver of scavenging in the Pacific Southern Ocean. Lower F (Th/Pa) values in the Pacific sector than in the Atlantic sector are consistent with higher coretop ²³¹Pa/²³⁰Th ratios in the Pacific. This difference may be due to greater dust deposition in the Atlantic sector, with the higher dust/opal ratio of particulate material in the Atlantic sector driving higher fractionation factors. The data also suggest that the magnitude of the fractionation factor in the Southern Ocean is key in setting the gradients in ²³¹Pa and ²³⁰Th along neutral density surfaces from north to south, thereby modulating the role of isopycnal mixing in transporting these nuclides to the Southern Ocean. In the Pacific, fractionation factors <1 drive strong isopyncal gradients in ²³¹Pa, causing the isopyncal flux to greatly exceed the production by U decay. Future work incorporating ²³⁰Th and ²³¹Pa into global models would allow for quantification of the net transport of ²³⁰Th and ²³¹Pa to the Southern Ocean due to both advective upwelling and isopycnal mixing.

The importance of mixing in transporting ²³¹Pa toward the Southern Ocean should be taken into account both in mass balances of ²³¹Pa in ocean basins (Deng et al., 2018; Hayes et al., 2014) and in paleoceanographic studies of ²³¹Pa burial in the Southern Ocean (Anderson et al., 2009). More complex models of these nuclides could be developed by combining our measurements with transports from the Southern Ocean State Estimate (Mazloff et al., 2010) to constrain isopycnal diffusivities and removal rates. Our results present exciting new opportunities for applying measurements of ²³¹Pa and ²³⁰Th to study Southern Ocean physics and biogeochemistry in modern and paleo-oceans.

Acknowledgments

This work was supported by U.S. National Science Foundation grant OPP-1542962 to LDEO and an NSF Graduate Research Fellowship to F. J. P. (DGE-1644869). We thank the captain and crew of the R/V Nathaniel B. Palmer for their support in difficult conditions on the NBP1702 cruise. Discussions with Spencer Jones and Sophie Hines improved the quality of the paper, as did comments from Matthieu Roy-Barman and an anonymous reviewer.

Open Research

Data Availability Statement

Dissolved Th and Pa data are available online through BCO-DMO (at https://www.bco-dmo.org/dataset/813379). Dissolved and biogenic silica data can be found online (at https://www.usap-dc.org/view/dataset/601269), and CTD data can be found through MGDS (at http://www.marine-geo.org/tools/entry/NBP1702).

Supporting Information

References

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Citing Literature

Volume34, Issue12

December 2020

e2020GB006760

Isopycnal Transport and Scavenging of ²³⁰Th and ²³¹Pa in the Pacific Southern Ocean

Abstract

Key Points

1 Introduction

2 Materials and Methods

2.1 Cruise Track

2.2 Sample Collection and Analysis for dSi and bSi

2.3 Sample Collection and Analysis for Th and Pa Isotopes

3 Results

3.1 Hydrographic Setting

3.2 Dissolved ²³⁰Th and ²³¹Pa Distributions on NBP1702

3.3 Size-Fractionated Dissolved Th and Pa Isotopes

4 Discussion

4.1 Physical Speciation of Dissolved Th and Pa

4.2 The Role of Particle Composition in Scavenging Th and Pa

4.3 Transport of Th and Pa to the Southern Ocean by Isopycnal Mixing

4.3.1 Isopycnal Fluxes of ²³⁰Th and ²³¹Pa

4.3.2 Mechanisms for Interbasin Differences in Mixing Transport

5 Summary and Outlook

Acknowledgments

Open Research

Data Availability Statement

Supporting Information

References

Citing Literature

Figures

References

Information

RESOURCES

PUBLICATION INFO

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Isopycnal Transport and Scavenging of 230Th and 231Pa in the Pacific Southern Ocean

Abstract

Key Points

1 Introduction

2 Materials and Methods

2.1 Cruise Track

2.2 Sample Collection and Analysis for dSi and bSi

2.3 Sample Collection and Analysis for Th and Pa Isotopes

3 Results

3.1 Hydrographic Setting

3.2 Dissolved 230Th and 231Pa Distributions on NBP1702

3.3 Size-Fractionated Dissolved Th and Pa Isotopes

4 Discussion

4.1 Physical Speciation of Dissolved Th and Pa

4.2 The Role of Particle Composition in Scavenging Th and Pa

4.3 Transport of Th and Pa to the Southern Ocean by Isopycnal Mixing

4.3.1 Isopycnal Fluxes of 230Th and 231Pa

4.3.2 Mechanisms for Interbasin Differences in Mixing Transport

5 Summary and Outlook

Acknowledgments

Open Research

Data Availability Statement

Supporting Information

References

Citing Literature

Figures

References

Related

Information

RESOURCES

PUBLICATION INFO

Isopycnal Transport and Scavenging of ²³⁰Th and ²³¹Pa in the Pacific Southern Ocean

3.2 Dissolved ²³⁰Th and ²³¹Pa Distributions on NBP1702

4.3.1 Isopycnal Fluxes of ²³⁰Th and ²³¹Pa