4.1 Water mass distribution and influence

The presence of multiple water masses sampled by the GEOVIDE Section allows the influence of water mass (and age) on ²³⁰Th and ²³¹Pa to be assessed. Extended optimum multi-parameter (eOMP) analysis (García-Ibáñez et al., 2018) for the GEOVIDE section maps the presence of 10 water mass end-members in the section (Fig. 4), including three recently ventilated waters in the GEOVIDE section.

• i.

  Labrador Sea Water (LSW) is formed by deep convection (Talley and McCartney, 1982). It is the dominant deep water along the section, extending from 1000 to 2500 m depth in the east and from surface to 3500 m in the west of the section.

• ii.

  Iceland–Scotland Overflow Water (ISOW) is formed in the Norwegian Sea and subsequently entrains overlying warmer and saltier waters. This water mass initially flows along the eastern flank of the Reykjanes Ridge before spreading back northwards, after crossing the Charlie-Gibbs Fracture Zone, into the Irminger and Labrador seas (Dickson and Brown, 1994; Saunders, 2001). A pronounced layer of this water mass is observed immediately below the LSW and extends as deep as 4000 m west of 20^∘ W.

• iii.

  Denmark Strait Overflow Water (DSOW) is formed after the Nordic Sea deep waters overflow and entrain Atlantic waters (SPMW and LSW) (Yashayaev and Dickson, 2008) with dense Greenland shelf water cascading down to the DSOW layer in the Irminger Sea (Falina et al., 2012; Olsson et al., 2005; Tanhua et al., 2005). This water occupies the deepest part of the IR and LA seas.

In the east of the section, deep waters consist of the much older Lower North East Atlantic Deep Water (NEADW_L), which is formed with a significant southern component from Antarctic Bottom Water. A number of other water masses are also observed at shallow depths, including Mediterranean Water and various mode waters.

Figure 6Water mass age based on CFC data along the GEOVIDE section. (a) full water-column data for the entire section, showing waters from 10 to 800 years in age; (b) a rescaled version of panel (a) omitting the upper 1000 m and the older waters east of 35^∘ W to show age variation in recently ventilated deep waters. Water masses were indicated by contours (black: DSOW; blue: ISOW; white: LSW; red: NEADW) based on 50 % level percentage composition of source water types from eOMP analysis.

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Some control of water mass on ²³⁰Th and ²³¹Pa concentrations is evident in nuclide section plots (Fig. 5), particularly relatively low ²³⁰Th and ²³¹Pa concentrations in DSOW and high values in the old NEADW. In other places, the impact of water mass is less apparent. The challenge with these nuclides is that they are not conservative tracers of water mass, but evolve significantly during transport and water aging. In the GEOVIDE section, we can analyse this evolution, because the ages of the water masses can be assessed from CFC data.

CFC measurements are not available from the GEOVIDE cruise itself. De la Paz et al. (2017), however, measured CFC concentrations along the east of the same section (covering the WE Basin, IC Basin, and IR Sea) in 2012 (OVIDE/CATARINA cruise). This allowed the computing of CFC-based age with the transit time distribution (TTD) method. Using the water mass distribution along GEOVIDE given by García-Ibáñez et al. (2018) and the distribution for the same water masses in 2012 (García-Ibáñez et al., 2015), we derived CFC-based ages for GEOVIDE waters (Fig. 6; further details in Supplement Sect. S2). Uncertainties (1 standard error) associated with CFC-based age calculated with this approach range between 11 % and 40 %.

CFC-based water mass ages range from ≈10 years, observed in DSOW at the bottom of the LA Sea, to ≈800 years, observed for NEADW at the bottom of the WE Basin. Because this study focuses on understanding controls on ²³¹Pa and ²³⁰Th in recently ventilated waters, we omit detailed consideration of the upper 1 km in subsequent discussion, and restrict our analysis to water sampled west of 35^∘ W of the section, where young waters (<50 years) dominate. A rescaled version of the CFC age section indicates the variation in age of ventilated waters (Fig. 6b). DSOW, occupying the deepest LA and IR seas, is the youngest water mass in this region, with an average age of ∼19 years. ISOW and LSW are slightly older, with ages ranging from 26 to 45 years and 32 to 40 years, respectively.

4.2 Evolution of ²³⁰Th and ²³¹Pa with water age

The presence of recently ventilated deep waters with constrained CFC ages allows analysis of the rates at which ²³⁰Th and ²³¹Pa concentrations increase during transport and the rates of scavenging of these nuclides. To conduct this analysis, we define five components in the budget of ²³⁰Th and ²³¹Pa.

• i.

  Preformed component. The ²³⁰Th or ²³¹Pa transported from the surface into the interior. For this analysis, in the absence of measurements for the exact location of deep water formation during winter convection, we assume the same preformed value for all water masses and set this as the average of concentrations measured in surface waters <100 m depth along GEOVIDE section. This gives preformed concentrations of 1.66 µBq kg⁻¹ for ²³⁰Th and 1.31 µBq kg⁻¹ for ²³¹Pa. We recognize that true preformed values may differ from these values and between water masses, and discuss the implications of uncertainty in preformed values in the following section. Preformed ²³⁰Th and ²³¹Pa will decrease due to radioactive decay during transport. Although we take this decay into account in the following analysis, it is insignificant given the ages of waters involved and the much longer half-lives of ²³⁰Th and ²³¹Pa.

• ii.

  Ingrown component. The ingrown ²³⁰Th or ²³¹Pa from the radioactive decay of U since the water was last in contact with the surface. This component increases as the water mass ages. The concentration of this component in a water mass of age t can be calculated as

  $$\begin{array}{}\text{(4.1)}& {\displaystyle }{{}^{\mathrm{230}}\mathrm{Th}}_{\mathrm{Ingrown}}={}^{\mathrm{234}}\mathrm{U}\times \left(\mathrm{1}-e^{-{\mathit{\lambda }}_{\mathrm{230}}t}\right),\text{(4.2)}& {\displaystyle }{{}^{\mathrm{231}}\mathrm{Pa}}_{\mathrm{Ingrown}}={}^{\mathrm{235}}\mathrm{U}\times \left(\mathrm{1}-e^{-{\mathit{\lambda }}_{\mathrm{231}}t}\right),\end{array}$$

  where ²³⁰Th_Ingrown and ²³¹Pa_Ingrown are the ²³⁰Th and ²³¹Pa ingrown from their U parents, respectively; ²³⁴U and ²³⁵U are activities of ²³⁴U, and ²³⁵U in seawater (45 551.2 µBq kg⁻¹ or 2801.4 dpm 1000 L⁻¹, and 1823.8 µBq kg⁻¹ or 112.2 dpm 1000 L⁻¹, respectively, assuming a constant seawater ²³⁸U activity of 39 609.8 µBq kg⁻¹ or 2436 dpm 1000 L⁻¹ at salinity 35 psu, seawater ²³⁴U∕²³⁸U activity ratio of 1.15 and a natural ²³⁸U∕²³⁵U abundance ratio of 137.88); λ₂₃₀ and λ₂₃₁ are decay constants of ²³⁰Th and ²³¹Pa ($\mathrm{9.17}\times {\mathrm{10}}^{-\mathrm{6}}$ and $\mathrm{2.12}\times {\mathrm{10}}^{-\mathrm{5}}$ yr⁻¹, respectively).

• iii.

  Potential total component. The ²³⁰Th and ²³¹Pa expected in the water due to the combination of preformed and ingrown components, if there is no removal by scavenging.

• iv.

  Observed component. The ²³⁰Th and ²³¹Pa observed in the water column, i.e. dissolved ²³⁰Th and ²³¹Pa measured in this study (after correction for detritus and ingrowth from U since sample collection).

• v.

  Scavenged component. The net ²³⁰Th or ²³¹Pa removed from the water since it left the surface due to scavenging. For each depth, this component is the net of nuclide added from above by desorption from settling particles and the removal downwards by scavenging.

Figure 7Potential total, observed, and scavenged components of ²³⁰Th and ²³¹Pa in waters >1000 m water depth and west of 35^∘ W.

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Figure 8Relationship between water mass age and the observed and scavenged components of ²³⁰Th, ²³¹Pa, and ²³¹Pa∕²³⁰Th (colour coded by water depth). Least-square-fitting statistics were also given, i.e. slope and correlation coefficient r of the least square line, mean value, and number of the data points. Note the increase in observed concentrations for both nuclides with age. Comparison of average values indicates that about three quarters of ²³⁰Th produced by decay is scavenged, compared with about half of the ²³¹Pa.

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These components are related to each other as follows:

The difference between the potential total and the observed concentration of ²³⁰Th (or ²³¹Pa) therefore provides a measure of the amount of nuclide scavenged since the water left the surface (Fig. 7).

We examine the evolution of both the observed and scavenged components of ²³⁰Th and ²³¹Pa with water mass age (Fig. 8). Both ²³⁰Th and ²³¹Pa show an increase in observed concentration with age of water, with the increase for ²³⁰Th much more regular than for ²³¹Pa. This strong ²³⁰Th relationship, regardless of depth of the sample (Fig. 8a), indicates a primary control of water mass age on the increase in ²³⁰Th in these younger waters.

For ²³⁰Th, the rate of increase with age (i.e. slope in Fig. 8a) indicates that about one quarter of the ²³⁰Th formed from U decay remains in the water, with the other three quarters being removed by scavenging. This ratio is consistent with the average ²³⁰Th for these waters, which requires about 3 times more ²³⁰Th than remains in water to be removed by scavenging (Fig. 8a, b). The scatter between ²³¹Pa and age (Fig. 8c) precludes the use of the slope to assess the relative proportion of scavenged ²³¹Pa, but the average values (Fig. 8c, d) indicate that about half of the ²³¹Pa remains in the water, while the other half is removed by scavenging. The relative behaviour of ²³⁰Th and ²³¹Pa is consistent with previous expectations, with a higher fraction of scavenging for ²³⁰Th than ²³¹Pa.

The hypothesis that ²³¹Pa∕²³⁰Th ratios increase monotonically as water mass ages forms the foundation of the water mass evolution model for interpretation of sedimentary ²³¹Pa∕²³⁰Th in terms of the rate of deep water circulation. For these young waters, however, there is neither a clear relationship between observed ²³¹Pa∕²³⁰Th and age (Fig. 8e), nor between the ²³¹Pa∕²³⁰Th value scavenged to the sediment and age (Fig. 8f), calling the water mass evolution model into question.

4.3 The importance of preformed ²³⁰Th and ²³¹Pa in young waters

To assess the controls on ²³⁰Th, ²³¹Pa, and particularly the resulting ²³¹Pa∕²³⁰Th ratio, we apply a simple scavenging-mixing model following Moran et al. (1997). This model was first created to assess the evolution of ²³⁰Th in a 1-D water column as it ages following ventilation. Here we adopt it by modelling the nuclide evolution with age for each depth and by also modelling ²³¹Pa. This assumes that waters have remained at the same depth since ventilation, which, though not correct in detail, still allows the model to provide insights about controls on these nuclides.

Following Moran et al. (1997), dissolved concentration of ²³⁰Th and ²³¹Pa is given by

where c_d is the dissolved concentration of the nuclide; P is the production rate of ²³⁰Th and ²³¹Pa, 0.42 µBq kg⁻¹ yr⁻¹ ($\mathrm{2.57}\times {\mathrm{10}}^{-\mathrm{2}}$ dpm 1000 L⁻¹ yr⁻¹) and 0.039 µBq kg⁻¹ yr⁻¹ ($\mathrm{2.37}\times {\mathrm{10}}^{-\mathrm{3}}$ dpm 1000 L⁻¹ yr⁻¹), respectively; K_d is the distribution coefficient of the nuclide; λ is the decay constant of the nuclide; C_pre,t is the preformed total concentration of ²³⁰Th (or ²³¹Pa); SPM is the suspended particle concentration; S is the particle settling speed, which represents the net effect of particle sinking, disaggregation, and aggregation; τ_w is water mass age; and z is the water depth.

Figure 9Results from a scavenging-mixing model of ²³⁰Th, ²³¹Pa, dissolved ²³¹Pa∕²³⁰Th, and particulate ²³¹Pa∕²³⁰Th compared to observations. Preformed concentration (C_pre) were set at 0 (dashed line) and at the average surface concentration (C_{surface average}) from GEOVIDE section (solid line), i.e. ²³⁰Th = 1.66 µBq kg⁻¹ and ²³¹Pa = 1.31 µBq kg⁻¹. A version of this figure extending to older waters is available in Fig. S2.

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The model requires values for four parameters: particle settling speed (S), suspended particle concentration (SPM), and distribution coefficients for ²³⁰Th ($K_{\mathrm{d}}^{\mathrm{Th}}$) and ²³¹Pa ($K_{\mathrm{d}}^{\mathrm{Pa}}$). We select these parameters to give a good fit to the ²³⁰Th and ²³¹Pa observations at an open ocean station, Station 13, on the east of the section (i.e. a station sampling older waters, which are close to equilibrium) and use these values to interpret the younger waters to the west. Best fits to Station 13 suggested S=800 m yr⁻¹, SPM = 25 µg L⁻¹, $K_{\mathrm{d}}^{\mathrm{Th}}=\mathrm{1.1}\times {\mathrm{10}}^{\mathrm{7}}$ mL g⁻¹, and $K_{\mathrm{d}}^{\mathrm{Pa}}=\mathrm{1.4}\times {\mathrm{10}}^{\mathrm{6}}$ mL g⁻¹ (the first three of these are close to those of Moran et al., 1997). A fuller description of the model is given in Supplement Sect. S3.

We show two sets of output from the model, one with a preformed component (C_pre) equal to the nuclide concentrations observed in the upper 100 m of the GEOVIDE section (as in Sect. 4.2 above), and one with the preformed component set to zero for both nuclides. For both cases, the modelled evolution of nuclide concentrations with age between 0 and 50 years at 2000 and 3500 m water depths is plotted in Fig. 9, and compared to data. As expected, modelled ²³⁰Th and ²³¹Pa concentrations increase with age, with deeper waters having higher concentrations and ²³⁰Th increasing more rapidly initially (Fig. 9), but the preformed concentration is seen to be important in setting total nuclide concentration for several decades after ventilation. The fit of the model to observations in young waters from GEOVIDE is improved in the model run with zero preformed nuclide, particularly for ²³⁰Th. This is surprising, given that surface-water ²³⁰Th, and ²³¹Pa values, are generally non-zero, and are typically close to the values observed in the GEOVIDE surface waters. For ²³⁰Th in young deep waters, even the model with zero preformed nuclide overestimates the observed value, possibly indicating additional scavenging from these waters close to the seafloor, or as a result of differing biological productivity and particle fluxes between stations.

Figure 10Ratio of scavenged component to potential total components for ²³⁰Th and ²³¹Pa, providing an assessment of the relative importance of scavenging for the two nuclides and of the location of scavenging.

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The most striking effect of changing the assumed preformed values in the model is on ²³¹Pa∕²³⁰Th (Fig. 9c). When preformed values are set at zero, ²³¹Pa∕²³⁰Th ratios always increase with water age, but when set at the average surface value from GEOVIDE, ²³¹Pa∕²³⁰Th ratios initially decrease before increasing. The impact of preformed concentrations has a long-lasting impact on water-column and scavenged ²³¹Pa∕²³⁰Th, lasting for hundreds of years following ventilation (Fig. S2c, d in the Supplement). This indicates that knowledge of the nuclide concentration at the site of deep water formation is critical to understanding the early evolution of ²³¹Pa∕²³⁰Th in waters and their underlying sediments.

4.4 Scavenging of ²³⁰Th and ²³¹Pa

Knowledge of the CFC ages of the waters analysed on the GEOVIDE cruise allows an assessment of the scavenging rates of ²³⁰Th and ²³¹Pa. To do so, we compare the scavenged component to the potential total component (as defined in Sect. 4.2). The percentage of the scavenged component relative to the potential total component is higher for ²³⁰Th, at an average of 80 %, than for ²³¹Pa, at an average of 40 % (Fig. 10), which is consistent with the relatively higher particle reactivity of ²³⁰Th. For both nuclides, there is a higher fraction of scavenging in samples from near the seafloor, particularly those from DSOW in the deepest LA Sea. Bottom scavenging has been indicated in previous studies (e.g. Bacon and Anderson, 1982; Deng et al., 2014; Okubo et al., 2012), but this study indicates that this enhanced nuclide scavenging occurs even in the very young overflow waters at the start of the meridional circulation.

4.5 Uncertainty analysis of the assessment of the scavenging of ²³⁰Th and ²³¹Pa

Uncertainty in both CFC-based ages and in preformed values of ²³⁰Th and ²³¹Pa contributes to uncertainty when calculating the scavenging of ²³⁰Th and ²³¹Pa. Uncertainties associated with CFC-based age range between 11 % and 40 % (1 standard error). These uncertainties lead to an average uncertainty of 23 % and 13 % in potential total ²³⁰Th and ²³¹Pa, respectively, corresponding to an average uncertainty of 30 % in the scavenged component of ²³⁰Th and 40 % in the scavenged component of ²³¹Pa.

A 2-fold increase in preformed values results in an increase by a factor of 1.2 and 1.6 in the total potential ²³⁰Th and ²³¹Pa, respectively, leading to an increase by a factor of 1.2 in the scavenged component of ²³⁰Th and of 2.6 in the scavenged component of ²³¹Pa. The impact of preformed uncertainty is less significant when comparing the scavenging to the potential total components. The ratio of scavenged component to potential total component increases by factors of 1.1 and 1.4 for ²³⁰Th and ²³¹Pa, respectively. This sensitivity analysis indicates that a better knowledge of preformed values will benefit the assessment of the scavenging of both nuclides.

4.6 Meridional transport of ²³⁰Th and ²³¹Pa in the North Atlantic

Previous calculations have indicated removal of ²³⁰Th and ²³¹Pa from the North Atlantic by meridional transport southward. Deng et al. (2014) calculated net southward transport of 6 % of the ²³⁰Th and 33 % of ²³¹Pa, relative to production of these nuclides in the water column. That calculation, however, did not provide a complete budget for ²³⁰Th and ²³¹Pa for the North Atlantic, because observations at the time did not constrain input of these nuclides from the north. Data in this study allow this calculation, and therefore a more complete budget for the modern North Atlantic.

García-Ibáñez et al. (2018) calculated volume transports for the Portugal–Greenland section of the GEOVIDE section by combining the water mass fractions from eOMP analysis with the absolute geostrophic velocity field calculated using inverse model constrained by Doppler current profiler velocity measurements (Zunino et al., 2017). They separated northward flowing upper limbs and southward flowing lower limbs of the Atlantic Meridional Overturning Circulation (AMOC) at isopycnal σ₁ (potential density referenced to 1000 dbar) = 32.15 kg m⁻³, with $+\mathrm{18.7}\pm \mathrm{2.4}$ and $-\mathrm{17.6}\pm \mathrm{3.0}$ Sv flowing across the section above and below this value (positive value indicates northward transport). With average ²³⁰Th and ²³¹Pa concentrations in the upper limb (σ₁<32.15 kg m⁻³) of 1.60 and 1.32 µBq kg⁻¹, respectively, northward transport of ²³⁰Th is 3.07×10¹⁰ and of ²³¹Pa is 2.53×10¹⁰ µBq s⁻¹. Average ²³⁰Th and ²³¹Pa concentrations in the lower limb (σ₁>32.15 kg m⁻³) are 3.44 and 2.07 µBq kg⁻¹, respectively, indicating that transports of ²³⁰Th and ²³¹Pa are $-\mathrm{6.22}\times {\mathrm{10}}^{\mathrm{10}}$ and $-\mathrm{3.74}\times {\mathrm{10}}^{\mathrm{10}}$ µBq s⁻¹, respectively.

Net transport of ²³⁰Th and ²³¹Pa across GEOVIDE is therefore to the south, and supplies 3.15×10¹⁰ µBq s^{−1 230}Th and 1.21×10¹⁰ µBq s^{−1 231}Pa to the North Atlantic (Fig. 11). This is a smaller net transport than further south in the Atlantic (Fig. 11), due to the lower ²³⁰Th and ²³¹Pa concentrations in the water column close of the site of deep water formation.

The budget for these nuclides for the North Atlantic consists of the following: production in the water column, addition by advection from the north, loss by advection to the south, and removal to the sediment. The data from this study allow this budget to be fully assessed, and indicate that the flux to the sediment is equivalent to 96 % of the production of ²³⁰Th, and 74 % of the production for ²³¹Pa (Table S4). For both nuclides, these fluxes are higher than in previous calculations (Deng et al., 2014), which ignored advective fluxes from the north. There is, however, still a significantly higher advective loss of ²³¹Pa relative to ²³⁰Th. At a basin scale, therefore, ²³¹Pa∕²³⁰Th in the sediment must be lower than the production ratio. This lower value is generated by the meridional transport of the North Atlantic and likely to be sensitive to changes in this transport.

Figure 11Fluxes of ²³⁰Th (blue arrow) and ²³¹Pa (yellow arrow) across the GEOVIDE section (blue solid line), 4.5^∘ S (green solid line), and 45^∘ S (red solid line). Also shown is production of ²³⁰Th (blue box) and ²³¹Pa (yellow box) in the North Atlantic (between GEOVIDE section and 4.5^∘ S) and in the South Atlantic (between 4.5 and 45^∘ S), based on calculation in Deng et al. (2014). These fluxes indicate that 4 % of the ²³⁰Th and 26 % of the ²³¹Pa produced in the North Atlantic is exported southward by ocean circulation.

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Using the basin-scale advection model to interpret sedimentary ²³¹Pa∕²³⁰Th to assess meridional transport, as initially proposed by Yu et al. (1996), is therefore still supported by a full modern North Atlantic budget for these nuclides.