Evolution of 231 Pa and 230 Th in overﬂow waters of the North Atlantic

. Many palaeoceanographic studies have sought to use the 231 Pa / 230 Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, 230 Th and 231 Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231 Pa and 230 Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. 230 Th and 231 Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 µBq kg − 1 and between 0.37 and 4.80 µBq kg − 1 , respectively.

Abstract. Many palaeoceanographic studies have sought to use the 231 Pa/ 230 Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, 230 Th and 231 Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231 Pa and 230 Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. 230 Th and 231 Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 µBq kg −1 and between 0.37 and 4.80 µBq kg −1 , respectively. Both 230 Th and 231 Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Assessing this dataset using extended optimum multi-parameter (eOMP) analysis and CFC-based water mass age indicates that the low values of 230 Th and 231 Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. The importance of water age is confirmed for 230 Th by a strong correlation between 230 Th and water mass age (though this relationship with age is less clear for 231 Pa and the 231 Pa/ 230 Th ratio). Scavenged 231 Pa and 230 Th were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more 230 Th is scavenged (∼ 80 %) than 231 Pa (∼ 40 %), consistent with the relatively higher particle reactivity of 230 Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young over-flow waters. Calculation of the meridional transport of 230 Th and 231 Pa with this new GEOVIDE dataset enables a complete budget for 230 Th and 231 Pa for the North Atlantic. Results suggest that net transport southward of 230 Th and 231 Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicate that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230 Th and 74 % of the production for 231 Pa. This result confirms a significantly higher advective loss of 231 Pa to the south relative to 230 Th and supports the use of 231 Pa/ 230 Th to assess meridional transport at a basin scale.

Introduction
Several palaeoceanographic proxies have been proposed that rely on the 231 Pa/ 230 Th ratio in marine sediments; one of which is that 231 Pa/ 230 Th may record the rate of deep water circulation, particularly in the North Atlantic. Both 231 Pa and 230 Th are produced in seawater at a constant rate by the decay of uranium, but they have decay activities much lower than their parent uranium isotopes due to rapid removal by adsorption onto sinking marine particles. Both nuclides are also reversibly scavenged, leading to particularly low concentrations at the surface and increasing concentrations with depth (Nozaki et al., 1981). Advection of surface waters to depth transports water with low concentrations of 231 Pa and 230 Th into the deep ocean, where their concentrations subsequently increase towards an equilibrium value at a rate dependent on the residence time of the nuclide. The longer residence time of 231 Pa relative to 230 Th (∼ 130 years versus ∼ 20 years; Henderson and Anderson, 2003) means that the equilibrium concentration of 231 Pa is closer to that expected from uranium decay, and that the time taken to reach this equilibrium is longer.
This oceanic behaviour of 231 Pa and 230 Th suggests that their measurement in marine sediments may reveal information about the past environment, with one common use being a recorder of deep water circulation, particularly in the North Atlantic (e.g. Gherardi et al., 2005Gherardi et al., , 2009McManus et al., 2004;Roberts et al., 2014;Yu et al., 1996). The interpretation of sedimentary 231 Pa/ 230 Th ratios for such past ocean circulation is based on two end-member conceptual models.
-Basin-scale advection. The longer residence time of 231 Pa than 230 Th means that deep water contains more 231 Pa than 230 Th relative to production from decay. Advection of deep waters out of the North Atlantic therefore removes more 231 Pa than 230 Th, leaving sediments in the basin with a 231 Pa/ 230 Th ratio below the production ratio. If deep water ventilation ceases, 231 Pa removal from the North Atlantic also ceases, and sedimentary 231 Pa/ 230 Th values reach their production ratio. This approach was first proposed by Yu et al. (1996), who measured 231 Pa/ 230 Th in Holocene and Last Glacial Maximum (LGM) sediments from many core-top samples from the Atlantic and Southern Ocean. They found similar Holocene and LGM values at a basin scale, suggesting broadly similar overturning during the two periods. Subsequent application to sediments from Heinrich Stadial 1, initially in a single core (McManus et al., 2004) and progressively in a geographical range of cores (Bradtmiller et al., 2014), revealed reduced advection of 231 Pa out of the basin at that time, suggesting decreased overturning.
-Water mass evolution. The longer residence time of 231 Pa means that, following ventilation, it takes longer for deep water 231 Pa concentrations to reach equilibrium with respect to scavenging than is the case for 230 Th. This leads to a systematic evolution of 231 Pa/ 230 Th with age of the water. Sediments capture this ratio (with a fractionation due to different scavenging coefficients for the two nuclides), thus capturing information about the age of the water. Simple models suggest an increase in 231 Pa/ 230 Th with age of over about 400 years (e.g. several residence times of 231 Pa). This approach to interpreting sedimentary 231 Pa/ 230 Th allows for the possibility of calculating flow rates for a single water mass and from a single core, rather than at a basin scale. It has been pursued by Negre et al. (2010) to assess deep water flow in both southerly and northerly directions by comparing sediments in the North and South Atlantic, and allowed these authors to apply a simple model (Thomas et al., 2007) to calculate flow rates.
Recent water-column measurements of 231 Pa and 230 Th on GEOTRACES cruises shed new light on the chemical behaviour and controls on these isotopes in seawater and provided evidence to assess the validity of the models underlying the use of sedimentary 231 Pa/ 230 Th as a proxy for deep water circulation. These measurements have indicated that there is considerably more net advection of 231 Pa than 230 Th out of the North Atlantic (Deng et al., 2014), supporting the basin-scale advection model for 231 Pa/ 230 Th. But these measurements also have suggested that there is no simple relationship between increasing 231 Pa/ 230 Th and age of water, as would be expected for the water mass evolution model (e.g. Deng et al., 2014). Studies using 2-D and 3-D ocean models (e.g. Marchal et al., 2010;Siddall et al., 2007) have also supported the use of sedimentary 231 Pa/ 230 Th to constrain deep water circulation at a basin scale and suggested that the relationship between 231 Pa/ 230 Th and water mass age is more complex than assumed in earlier studies (e.g. Luo et al., 2010).
Observations and model studies of 231 Pa and 230 Th have also suggested that other controls complicate 231 Pa/ 230 Th as a dynamic tracer of deep water circulation, such as the effect of boundary scavenging at seafloor and ocean margins (e.g. Anderson et al., 1994;Deng et al., 2014;Rempfer et al., 2017) and the influence of particle flux and composition (e.g. Chase et al., 2002;Hayes et al., 2014;Siddall et al., 2005).
To fully assess the behaviour of 231 Pa/ 230 Th, and its potential as a dynamic tracer of deep water circulation, knowledge of the concentrations and variations of these isotopes as deep waters form and enter the deep Atlantic is required. Some measurements have placed initial constraints on 231 Pa and 230 Th values in young North Atlantic deep waters (e.g. Moran et al., 1997Moran et al., , 2002Rutgers van der Loeff and Berger, 1993), but there has not yet been a systematic study of the composition of waters in the far North Atlantic. The GEO-VIDE cruise allowed waters to be collected for such a study, along a line where significant other data are available, from both that cruise and from previous occupations of OVIDE. GEOVIDE provided an ideal opportunity to understand 231 Pa and 230 Th at the start of the ocean meridional overturning circulation, and to assess the hypotheses underlying the use of 231 Pa/ 230 Th as a palaeo-proxy for the rate of deep water circulation.  Full-depth water-column 231 Pa and 230 Th for this study were collected from 11 stations (Fig. 1). Sampling followed the procedure suggested by GEOTRACES intercalibration work (Anderson et al., 2012). Briefly, seawater samples of 5 L were directly filtered from Niskin bottles mounted on the stainless steel CTD rosette through AcroPak ™ capsules with Supor ® membrane (0.45 µm pore size). Filtered seawater samples were collected into acid-cleaned HDPE plastic bottles and sealed with a screw cap and Parafilm to reduce evaporation and contamination. Samples were then double bagged for storage in boxes for transport back to the shorebased lab for analysis.
Once returned to the laboratory in Oxford, samples were weighed and then acidified with quartz-distilled concentrated HCl to pH ∼ 1.7, shaken, and left for at least 4 days to ensure that Pa and Th was desorbed from the walls of the bottle. A mixed 229 Th-236 U spike and a 233 Pa spike were then added to each sample to allow measurement of Th, U (for another study), and Pa by isotope-dilution multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The 233 Pa spike was freshly made by milking from 237 Np (following Regelous et al., 2004) and calibrated against a known 236 U solution after the complete decay of 233 Pa to 233 U, i.e. four to five half-lives of 233 Pa (t 1/2 = 26.98 days; Usman and MacMahon, 2000) after spike production (Robinson et al., 2004); 50 mg of pure Fe as a chloride solution was also added to each water sample. Samples were left overnight to allow for spike equilibrium, after which the pH was raised to ∼ 8.5 using distilled NH 4 OH to coprecipitate the actinides with insoluble Fe-oxyhydroxides. At least 48 h were allowed for scavenging of the actinides onto Fe-oxyhydroxides. The precipitate was centrifuged and rinsed, and Th, Pa, and U were separated using anion exchange chromatography following Thomas et al. (2006).
After chemical separation, Pa and Th were measured on a Nu instrument (MC-ICP-MS) at the University of Oxford. Mass discrimination and ion-counter gain were assessed with the measurement of a U standard, CRM-145 U, before each sample measurement. The use of a U standard for this purpose minimizes memory problems that might be caused by use of a Th or Pa standard (Thomas et al., 2006). Measurements were also made 0.5 mass units either side of masses of interest to allow accurate correction for the effect of abundance sensitivity on small 231 Pa and 230 Th beams, and a correction for a small 232 ThH interference on the 233 Pa beam is made from assessment of the hydride formation rate on a 232 Th standard. Concentrations of 231 Pa and 230 Th together with 232 Th were obtained from the precise MC-ICP-MS measurement of 231 Pa/ 233 Pa, 230 Th/ 229 Th, and 232 Th/ 229 Th ratios together with well-calibrated concentrations of 233 Pa and 229 Th-236 U spikes.
Chemistry blanks were assessed by conducting the complete chemical procedure on ∼ 100 ml of Milli-Q water with each batch of samples. Based on six blank measurements, the average blanks for dissolved 231 Pa,230 Th, and 232 Th are 0.21±0.14 fg, 1.59±0.60 fg and 5.13±1.47 pg, respectively (uncertainties are 2 standard errors). Blank contributions account for 2 %-22 %, 2 %-26 %, and 0.2 %-16 % of the dissolved 231 Pa, 230 Th, and 232 Th, respectively (with the higher values being for surface samples due to their low concentrations).  231 Pa (e-h) in the water column along the GEOVIDE section. Colours correspond to the region (as in Fig. 1). LA is Labrador, IR is Irminger, IC is Iceland, and WE is West European. Uncertainties represent 2 standard errors (2 SE).

Results
Measured 230 Th and 231 Pa concentrations were corrected for blanks, ingrowth from U in seawater since the time of sample collection, and detrital U-supported 230 Th and 231 Pa concentrations. Measured and corrected concentrations of 230 Th, 231 Pa, and 232 Th, along with details of corrections, are provided in the Supplement Sect. S1. Although analysis was conducted in terms of fg kg −1 , results are converted to the SI units adopted by GEOTRACES data product, i.e. µBq kg −1 for 230 Th and 231 Pa and pmol kg −1 for 232 Th. This conversion uses half-lives for 231 Pa, 230 Th, and 232 Th of 32 760, 75 584, and 1.405 × 10 10 years, respectively (Cheng et al., 2013;Holden, 1990;Robert et al., 1969). Uncertainties were propagated, including the contribution from sample weighing, spike calibration, impurities in the spikes, blank corrections, and mass spectrometric measurement, and are reported as 2 standard errors (2 SE). Average total uncertainties for 231 Pa, 230 Th, and 232 Th are ±0.17 µBq kg −1 , ±0.17 µBq kg −1 , and ±0.0032 pmol kg −1 , respectively. Vertical profiles showing the results of corrected 230 Th and 231 Pa concentrations in the water column are plotted by region in Fig. 2. 230 Th concentrations in the water column range between 0.06 and 12.01 µBq kg −1 , and initially generally increase with water depth from surface to deep water. Towards the seafloor, 6 of the 11 stations show a prominent decrease in 230 Th, with this feature most pronounced in the LA and IR seas. 231 Pa concentrations in the water column range between 0.37 and 4.80 µBq kg −1 and also increase with water depth, but less rapidly than 230 Th. 231 Pa profiles also often exhibit a decrease near the seafloor at stations showing a 230 Th decrease. Station 38 at the Reykjanes Ridge distinguishes itself from other 231 Pa profiles, in that an increase in 231 Pa concentrations from low concentrations at 1000 m is observed, continuing towards the bottom.
Observed 230 Th and 231 Pa values at GEOVIDE are lower than those observed in intercalibrated GEOTRACES data from further south in the Atlantic. Figure 3 compares average depth profiles for 230 Th and 231 Pa in the western Atlantic, covering the high-latitude northwestern Atlantic (from GEO-VIDE, west of the Mid-Atlantic Ridge), mid-latitude northwestern Atlantic (GEOTRACES section GA03_w, Hayes et al., 2015), and southwestern Atlantic (GEOTRACES section GA02, Deng et al., 2014). A southward increase in both 230 Th and 231 Pa concentrations is observed below 1000 m.

Discussion
Early studies of water-column 230 Th and 231 Pa reported a linear increase in both nuclides with water depth (e.g. Anderson et al., 1983b;Nozaki et al., 1981), and introduced a reversible scavenging model with the exchange of both nuclides between their dissolved and particulate phases. Later studies observed a deviation of 230 Th and 231 Pa profiles from this reversible scavenging model, with the expected increase with depth often inverting near the seafloor (e.g. Anderson et al., 1983a;Bacon and Anderson, 1982). This feature has In this study, recently ventilated overflow waters are sampled at depth, particularly in the Labrador and Irminger seas. Low values of 230 Th and 231 Pa near the seafloor might be expected to relate to these young waters, but the effects of scavenging must also be considered.

Water mass distribution and influence
The presence of multiple water masses sampled by the GEO-VIDE Section allows the influence of water mass (and age) on 230 Th and 231 Pa to be assessed. Extended optimum multiparameter (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.
Some control of water mass on 230 Th and 231 Pa concentrations is evident in nuclide section plots (Fig. 5), particularly relatively low 230 Th and 231 Pa 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 GEO-VIDE 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 231 Pa and 230 Th 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 deep-est 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.

Evolution of 230 Th and 231 Pa with water age
The presence of recently ventilated deep waters with constrained CFC ages allows analysis of the rates at which 230 Th and 231 Pa concentrations increase during transport and the rates of scavenging of these nuclides. To conduct this analysis, we define five components in the budget of 230 Th and 231 Pa.
i. Preformed component. The 230 Th or 231 Pa 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 230 Th and 1.31 µBq kg −1 for 231 Pa. 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 230 Th and 231 Pa 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 230 Th and 231 Pa.
ii. Ingrown component. The ingrown 230 Th or 231 Pa 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 The difference between the potential total and the observed concentration of 230 Th (or 231 Pa) 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 230 Th and 231 Pa with water mass age (Fig. 8). Both 230 Th and 231 Pa show an increase in observed concentration with age of water, with the increase for 230 Th much more regular than for 231 Pa. This strong 230 Th relationship, regardless of depth of the sample (Fig. 8a), indicates a primary control of water mass age on the increase in 230 Th in these younger waters.
For 230 Th, the rate of increase with age (i.e. slope in Fig. 8a) indicates that about one quarter of the 230 Th formed from U decay remains in the water, with the other three quarters being removed by scavenging. This ratio is consistent with the average 230 Th for these waters, which requires about 3 times more 230 Th than remains in water to be removed by scavenging (Fig. 8a, b). The scatter between 231 Pa and age (Fig. 8c) precludes the use of the slope to assess the relative proportion of scavenged 231 Pa, but the average values (Fig. 8c, d) indicate that about half of the 231 Pa remains in the water, while the other half is removed by scavenging. The relative behaviour of 230 Th and 231 Pa is consistent with previous expectations, with a higher fraction of scavenging for 230 Th than 231 Pa.
The hypothesis that 231 Pa/ 230 Th ratios increase monotonically as water mass ages forms the foundation of the water mass evolution model for interpretation of sedimentary 231 Pa/ 230 Th in terms of the rate of deep water circulation. For these young waters, however, there is neither a clear relationship between observed 231 Pa/ 230 Th and age (Fig. 8e), nor between the 231 Pa/ 230 Th value scavenged to the sediment and age (Fig. 8f), calling the water mass evolution model into question.

The importance of preformed 230 Th and 231 Pa in young waters
To assess the controls on 230 Th, 231 Pa, and particularly the resulting 231 Pa/ 230 Th ratio, we apply a simple scavengingmixing model following Moran et al. (1997). This model was first created to assess the evolution of 230 Th 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 231 Pa. 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 230 Th and 231 Pa is given by where c d is the dissolved concentration of the nuclide; P is the production rate of 230 Th and 231 Pa, 0.42 µBq kg −1 yr −1 (2.57 × 10 −2 dpm 1000 L −1 yr −1 ) and 0.039 µBq kg −1 yr −1 (2.37×10 −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 230 Th (or 231 Pa); 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. The model requires values for four parameters: particle settling speed (S), suspended particle concentration (SPM), and distribution coefficients for 230 Th (K Th d ) and 231 Pa (K Pa d ). We select these parameters to give a good fit to the 230 Th and 231 Pa 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 Th d = 1.1 × 10 7 mL g −1 , and K Pa d = 1.4 × 10 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 230 Th and 231 Pa concentrations increase with age, with deeper waters having higher concentrations and 230 Th 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 230 Th. This is surprising, given that surface-water 230 Th, and 231 Pa values, are generally non-zero, and are typically close to the values observed in the GEOVIDE surface waters. For 230 Th 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.
The most striking effect of changing the assumed preformed values in the model is on 231 Pa/ 230 Th (Fig. 9c). When preformed values are set at zero, 231 Pa/ 230 Th ratios always increase with water age, but when set at the average surface value from GEOVIDE, 231 Pa/ 230 Th ratios initially decrease before increasing. The impact of preformed concentrations has a long-lasting impact on water-column and scavenged 231 Pa/ 230 Th, 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

Scavenging of 230 Th and 231 Pa
Knowledge of the CFC ages of the waters analysed on the GEOVIDE cruise allows an assessment of the scavenging rates of 230 Th and 231 Pa. 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 230 Th, at an average of 80 %, than for 231 Pa, at an average of 40 % (Fig. 10), which is consistent with the relatively higher particle reactivity of 230 Th. 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.    231 Pa, relative to production of these nuclides in the water column. That calculation, however, did not provide a complete budget for 230 Th and 231 Pa 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 σ 1 (potential density referenced to 1000 dbar) = 32.15 kg m −3 , with +18.7 ± 2.4 and −17.6 ± 3.0 Sv flowing across the section above and below this value (positive value indicates northward transport). With average 230 Th and 231 Pa concentrations in the upper limb (σ 1 < 32.15 kg m −3 ) of 1.60 and 1.32 µBq kg −1 , respectively, northward transport of 230 Th is 3.07 × 10 10 and of 231 Pa is 2.53 × 10 10 µBq s −1 . Average 230 Th and 231 Pa concentrations in the lower limb (σ 1 > 32.15 kg m −3 ) are 3.44 and 2.07 µBq kg −1 , respectively, indicating that transports of 230 Th and 231 Pa are −6.22×10 10 and −3.74×10 10 µBq s −1 , respectively.
Net transport of 230 Th and 231 Pa across GEOVIDE is therefore to the south, and supplies 3.15×10 10 µBq s −1 230 Th and 1.21×10 10 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 230 Th and 231 Pa 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 230 Th, and 74 % of the production for 231 Pa (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 231 Pa relative to 230 Th. At a basin scale, therefore, 231 Pa/ 230 Th 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.
Using the basin-scale advection model to interpret sedimentary 231 Pa/ 230 Th 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.

Conclusion
Measurement of 230 Th and 231 Pa in waters from GEOVIDE shows some control of water mass on 230 Th and 231 Pa concentrations, particularly low concentrations in DSOW and high values in the old NEADW. There is, however, no close mapping of nuclide concentration to water mass. With the availability of CFC-based ages in this section, the evolution of 230 Th and 231 Pa concentration with age is possible. A systematic increase in 230 Th concentration is observed over the first 50 years following ventilation, and a similar though more scattered relationship is seen for 231 Pa. There is no clear relationship between the 231 Pa/ 230 Th ratio and age for these young waters. The long-term evolution of 231 Pa/ 230 Th is found from a simple model to be highly dependent on the preformed concentrations for these nuclides. These results complicate the interpretation of sedimentary 231 Pa/ 230 Th as a palaeo-proxy for deep water circulation based on the systematic evolution of water 231 Pa/ 230 Th with age, and point to the importance of a better knowledge of preformed 230 Th and 231 Pa concentrations to improve interpretation. This analysis of the 230 Th and 231 Pa concentration relative to the age of the water not only demonstrates the influence of water mass aging on 231 Pa and 230 Th but also points to the influence of scavenging. Scavenged 230 Th is much more extensive than 231 Pa, as expected, and enhanced removal of both nuclides is seen immediately above the seafloor, particularly for young waters.
Calculation of the meridional transport of 230 Th and 231 Pa indicates a southward net transport of both nuclides across the GEOVIDE section. This advection is smaller than that further south in the Atlantic as a result of lower 230 Th and 231 Pa concentrations at GEOVIDE. Calculation of the flux across GEOVIDE allows a more complete budget for the North Atlantic to be constructed and demonstrates a significantly higher advective loss of 231 Pa to the south, relative to 230 Th, with 26 % of the 231 Pa produced advected southward (relative to only 4 % for 230 Th). This calculation supports the interpretation of sedimentary 231 Pa/ 230 Th measurements as a proxy for overturning circulation, when based on the advective loss of 231 Pa at a basin scale.
Author contributions. GMH conceptualized and acquired funding for the research. MC collected seawater samples for 231 Pa,230 Th,and 232 Th at sea and provided expertise of other measurements on the cruise. FD conducted the chemical analysis and MC-ICP-MS measurement of 231 Pa,230 Th, and 232 Th. FFP and RS conducted the analysis, calculation, and interpretation of CFC ages. FD conducted the interpretation and analysis of 231 Pa,230 Th, and 232 Th data, with an extensive contribution from GMH. FD wrote the paper and prepared all figures, with GMH contributing extensively and contributions from the other co-authors.
Competing interests. The authors declare that they have no conflict of interest.
Special issue statement. This article is part of the special issue "GEOVIDE, an international GEOTRACES study along the OVIDE section in the North Atlantic and in the Labrador Sea (GA01)". It is not associated with a conference.