Nuclear waste radionuclides unveil three decades of anthropogenic carbon in the Canada Basin of the Arctic Ocean

Domain: neuroscience
Published: 2026-04-08
Editorial outcome: major_revision

Abstract
Ocean-atmosphere CO2 exchange is key in modulating global climate, but the Arctic Ocean’s anthropogenic carbon (Cant) uptake is poorly constrained. Most studies in the Canada Basin using the Transit Time Distribution (TTD) relied on two years of paired gas tracers, CFC-12 and SF6. Here we use nuclear reprocessing products 129I, 236U, and 137Cs to estimate Cant in the Atlantic Water layer by the TTD. We present the evolution of Cant in the Canada Basin from 1993 to 2023, and a high-resolution distribution of Cant across the basin in 2022.

We observe a basin-wide inventory increase in the Atlantic Layer (400–1000 m) of 5.5 ± 0.9 Tg C yr−1. Cant content in water entering the Canada Basin tracks increasing atmospheric input, time-lagged by 15-20 years; we anticipate Cant content of up to 65 µmol kg−1 in core Atlantic Water entering the basin by 2050.

Introduction
The Arctic is experiencing profound impacts due to anthropogenically driven increases in atmospheric CO2 concentrations. The region is warming nearly four times faster than the global average [1]. "Atlantification" of the ocean in the Eurasian Basin is weakening the strength of the halocline [2], with a projected total loss of summer sea ice by the end of the century [3]. In addition, warming and freshening of inflowing Pacific Water to the Amerasian Basin [4], accelerated rates of acidification [5], and decreasing O2 solubility in Pacific Waters an order of magnitude greater than the global average [6] have all been observed.

The oceans play a critical role in the Earth’s climate by absorbing approximately 25–30% of anthropogenic emissions [7]. The Arctic Ocean stores an estimated 2–3% of the global inventory of anthropogenic carbon (Cant) [8]. However, anthropogenic carbon accumulation is a primary driver of ocean acidification, as the increased concentrations of CO2 lower seawater pH and reduce carbonate ion content. In the Chukchi and Beaufort Seas, the Mackenzie shelf and the Canadian Archipelago, aragonite undersaturation is already being observed as a result of Cant influenced acidification. Quantifying the accumulation of Cant in this region is therefore especially urgent [9, 10, 11, 12, 13, 14].

Atlantic Waters account for over 90% of total inflow to the Arctic Ocean [15], and form three main inflowing components: the Norwegian Coastal Current Water (NCCW); the Fram Strait Branch Water (FSBW); and the Barents Sea Branch Water (BSBW) (Fig. 1a). The FSBW and BSBW are subducted in the St Anna Trough and form the mid-depth Atlantic Water (AW, σθ = 27.7 - 27.97 kg m−3, temperatures > 0 ◦C), and underlying dense Atlantic Water (dAW, σθ > 27.97 kg m−3, temperatures > 0 ◦C) throughout the Arctic Ocean. The AW is isolated from the atmosphere by a cold halocline, enabling long-term carbon storage [16, 17]. The FSBW contributes to the temperature maximum (Tmax) seen in the core AW [18, 19, 20]. From here on in the text, we refer to core AW as being centered on the σθ = 27.93 kg m−3 isopycnal, corresponding to the Tmax of the layer. The dAW is centered on the σθ = 28.00 kg m−3 isopycnal. Following Rajasakaren et al. [21], we do not differentiate between Arctic Atlantic Water and Atlantic Water, or between dense Atlantic Water and dense Arctic Atlantic Water.

Once subducted to the intermediate layer, AW is transported cyclonically in the Arctic Ocean Boundary Current (AOBC) [22], distributing Cant throughout the Eurasian and Amerasian basins (Fig. 1b), with eventual export to the North Atlantic. Ericson et al. [17] found increasing trends of total dissolved inorganic carbon (DIC) concentrations, 0.7–0.9 µmol kg−1 yr−1, in the Atlantic Water layers of the Amundsen Basin, and slightly lower rates in the Nansen and Makarov Basins. These rates correspond well with earlier observations from inflow regions, such as the Norwegian Atlantic Current and West Spitsbergen Current, where increasing Cant content of up to 1 µmol kg−1 yr−1 has been reported [23, 24]. A recent assessment by Rajasakaren et al. [21] confirmed these trends, finding the greatest increases in the Nansen and Amundsen basins (up to 0.7 µmol kg−1 yr−1), and lowest rates of increase in the Canada Basin (up to 0.2 µmol kg−1 yr−1).

Since Cant cannot be measured directly, being chemically indistinguishable from natural carbon, quantifying its uptake by the ocean is challenging, necessitating the use of models or other indirect methods [25]. These latter include, for example, the extended multiple linear regression (eMLR) [26], the ΔC* [27, 28] or the Transit Time Distribution (TTD) method, a simple 1-dimensional model which we use for this work [29, 30]. There are only a handful of studies using these various methods for investigating Cant in the Arctic Ocean [e.g. 8, 17, 31, 21, 32, 33, 34, 35, 36]. While all conclude the Arctic Ocean is storing greater concentrations of Cant and shows increasing acidification, none so far have provided a detailed study in the Canada Basin; here, we present the most up-to-date assessment based on data collected up to year 2023.

The TTD method has often been used with transient gas tracers (e.g. chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6)), which the few previous studies of Cant in the Arctic Ocean have relied on [e.g. 8, 21]. These have a global input from air-sea gas exchange; however, their saturation in seawater is highly variable and, in the Arctic Ocean, can be strongly affected by physical conditions such as sea-ice extent, temperature and salinity, which lends considerable uncertainty to their input functions [8, 37, 38]. Ideally, two tracers are used in the TTD to provide a robust constraint on the resulting parameters. In the Canada Basin, there are larger datasets of individual gas tracers from the 1990s and early 2000s, but only two years of paired gas tracers covering the Canada Basin have been published to date (GLODAPv2.2023 data product (Global Ocean Data Analysis Project version 2, expocodes 77DN20050819 and 06AQ20150817; [39]), analysed in Tanhua et al. [8], Rajasakaren et al. [21] and Raimondi et al. [32]. Using single gas tracers does expand the spatial and temporal coverage somewhat. However, paired tracers with distinct input histories provide two independent constraints, as they can resolve the two moments of the TTD without assuming any degree of mixing.

1.1 Nuclear Waste Radionuclides as Tracers of Anthropogenic Carbon

Recently, the TTD method using a new dual-tracer pair, the long-lived anthropogenic, nuclear-waste radionuclides iodine-129 (129I) and uranium-236 (236U), has been shown to provide a robust approximation to the TTD estimates calculated in the Arctic Ocean using the gas tracers [32]. This work also provided a sensitivity analysis of the TTD results obtained from the gas tracers and the radionuclides and demonstrated that uncertainty in the radionuclide input function leads to smaller changes in the resulting TTD parameters than varying saturation of the gas tracers. The resulting Cant inventories across all the Arctic basins from gas tracers exhibit approximately 20% greater variability than the radionuclides, depending on the gas saturation state used.

The TTD addresses advection-diffusion processes along the water-flow path, producing a probability density function (PDF). In this model, the TTD is an inverse Gaussian distribution, which assumes one-dimensional flow and requires a well-defined input function. The parameters obtained from the TTD using the radionuclides then provide the constraints for the TTD calculation to estimate Cant. The TTD for Cant assumes that Cant behaves as a passive tracer; therefore, its concentration at a known location and time can be described as for any other transient tracer (see methods, equations 1-4).

The radioisotopes 129I, 236U and 137Cs were first released into the environment during nuclear weapons testing in the 50s and 60s. About 1000 kg of 236U, 300 kg of 137Cs and only 50 kg of 129I were released to the atmosphere [40, 41, 42], forming a low-level global fallout input to the global oceans. Since the 1970s, the Nuclear Reprocessing facilities (NRP) in Europe, La Hague (France), and Sellafield (UK) (see 1a. radioactive symbol) have also released these tracers into the North and Irish Seas in annually reported volumes, which now form the dominant input to the Atlantic Ocean. The time-dependent input functions (Fig. 2a) of all three radionuclides consist of two main components: the low-level global input and the nuclear reprocessing signal. The input function of 137Cs also incorporates the 1986 Chernobyl accident [43]. The NRP concentrations in Atlantic Water are clearly distinguishable from those in water labelled only with global fallout concentrations (e.g. Pacific Water) due to the several orders of magnitude difference.

137Cs was initially used in conjunction with 129I to investigate circulation in the Arctic Ocean through the 90s and early 2000s [44, 43]. Developments in Accelerator Mass Spectrometry (AMS) methods allowed measurements of 129I in small sample volumes (< 1 L), which coincided with the significant increase in volume releases of 129I from NRPs. In contrast, 137Cs is measured via decay-counting, a time-consuming process requiring large sample volumes (20–30 L) due to its steadily declining NRP release and short half-life (30.2 years), further reducing its already low concentrations. As such, 137Cs has fallen out of favour as a tracer in the Arctic Ocean. In the last two decades, further advancements in AMS technology led to 236U emerging as a new oceanographic tracer [40, 45]. 236U has all the same advantages as 129I: a well-defined input function, small sample volumes needed, long half-life, and conservative nature in the open ocean [46, 47, 48]. Given that 137Cs has similar tracer characteristics as 236U and 129I, the same well-defined, time-varying input from a point-source, and being conservative in the open ocean, we consider calculated TTD parameters between 137Cs- 129I and 236U- 129I pairs to be comparable, as already done in Casacuberta & Smith [49] (unfortunately, no data exists with all three tracers available for a direct tracer intercomparison study).

Here, we present a high-resolution study of the distribution of Cant across the Canada Basin in 2022, using anthropogenic radionuclides 236U and 129I in the TTD method, first demonstrated by Raimondi et al. [32]. We expand the available tracer data in the Canada Basin with a high-resolution compilation of 236U and 129I spanning 2015 - 2023. Historical data of 137Cs and 129I extends the record of dual-tracer radionuclides in the Arctic Ocean back to 1993, allowing us to investigate changes in water mass transport and mixing rates, as well as the sub-decadal evolution of Cant in the Arctic Ocean using a dual-tracer approach.

Results
2.1 Distribution of 129I, 236U and 137Cs in AW and dAW, 1993–2023

We present mean profiles of 129I, 236U and 137Cs (Fig. 2b, c and d) across the surface and Pacific Water (PW) (≈ 0–200 m), Cold Haloc), Cold Halocline (CHL) (≈ 200–300 m, temperatures <0 ◦C), AW (≈ 300 - 600 m), and dAW (≈ 600 - 1500 m). The southern and northern regions of the Chukchi Plateau are thought to be gateway regions of the AOBC [50, 51, 52]. In every year occupied, the highest concentrations of 129I were found at station CB10 (see Fig. 1b.), located at the northern tip of the plateau where the AOBC enters the basin. Lower concentrations were observed towards the central basin regions (see supplementary figures S1 and S2 for annual individual profiles of each station). All three radioisotopes exhibited similar vertical distributions, characterised by low, close-to-global fallout signals in the surface and PW. Currently, global fallout values are ≈ 1.6 x 107 at L−1 for 129I [53], and ≈ 6 x 106 at L−1 for 236U [54]. For 137Cs, Smith et al. [43] reported a global fallout value of 2.2 Bq m−3 at the time of publishing. All global fallout values are marked on the profiles in figure 2. We found that the highest concentrations are observed in the CHL. The CHL had not been treated as a separate layer in previous studies of the radionuclides, and refutes the conclusion that the highest concentrations of the tracers in the mid-depth Arctic Ocean are found in core AW (note that in 2020, the first high-resolution study of 129I and 236U in the Canada Basin, the CHL was not sampled[52]). High concentrations were also observed throughout the AW, which then decrease with depth through the dAW. Below 2500 m, any tracer concentrations approach zero, or are below the measurement detection limit, as the deep waters are old and isolated [55, 8, 56, 52, 54].

The observed changes in the concentrations of all three tracers through time are directly linked to the different dynamics of the input function of each tracer (Fig. 2a.). In Payne et al. [52], the mean age of water entering the basin at the northeastern edge of the Chukchi Plateau and Northwind Ridge was estimated to be ≈ 20-26 years throughout the Atlantic Layer, which means water sampled there in 2020 would have picked up its tracer signal when entering the Arctic Ocean on the Eurasian side in the late 1990s - early 2000s, immediately after both input functions of 137Cs and 236U show a dramatic decrease. 137Cs had the highest activities (≈ 9 Bq m−3) measured in 1993, which steadily declined with time until 2008 (≈ 4 Bq m−3), reflecting the decreasing input function. 236U shows a similar decrease in mean concentration from 2015 (≈ 30 x 106 at L−1) to 2023 (≈ 25 x 106), also due to its decreasing input function. In contrast, the input of 129I to the Arctic Ocean did not ramp up until the 1990s [44, 43], peaking in the early 2000s and observed as a steady increase of mean concentrations until 2023 (≈ 300 x 107 at L−1). This is particularly prominent in the CHL and AW (Fig. 2), although it is observed throughout both AW and dAW.

2.2 TTD Parameters derived from Radionuclides

While changes in radioisotope concentrations are closely related to changes in their input functions, changes in circulation and mixing can also significantly influence their spatiotemporal distributions. To assess changes in circulation and mixing through time, we illustrate all calculated TTD parameters from the AW and dAW layer in Figure 3. The most reliable comparison is possible using the 2003–2023 data, as all sampled stations from 2003 onward are located within the Canada Basin, whereas the 1993 stations are upstream to the west of the Chukchi Plateau; we will not discuss this year in the context of circulation changes. Due to the differences in the number of stations sampled each year within the basin and the depth coverage, we identify individual stations with the best temporal coverage in the core AW to detect trends that may be obscured by averaging across the entire basin or water column (see the map in Figure 3a). We use CB10 and CB9 as two stations located closest to the inflow of the AOBC (for CB10, we combine LS19 from 2003 with the other years as the nearest station to extend the temporal coverage). Station A is located in the south of the Canada Basin and is not within the AOBC, representing the interior basin. Similarly, CBP5, located to the west (combining CB3 and CB4 with CB5 for optimal temporal coverage) , also represents the interior basin. Individual station profiles of the TTD parameters and Cant across all stations for all years are available in the supplement (Figures S6 & S7).

Although a full description of the TTD calculation and parameters is presented in the Methods section, we provide a brief overview of the parameters here. Γ is the mean age from the PDF of the TTD. tmode is the mode age, or the ’most likely’ age, which is always younger than the calculated Γ ages. Higher Γ and tmode ages indicate slower transport rates. Δ is the width of the TTD and describes the degree of mixing, with higher Δ values indicating more mixing. The ratio Δ/Γ is another mixing parameter, directly related to the Péclet number, as Pe = (Γ/Δ)2 [57]. For Δ/Γ, the larger the number (from 0.1 to 2), the more diffusive the transport. However, nearly all TTD studies in the Canada Basin indicate transport in the core AOBC and across the basin never exceeds Δ/Γ = 1 [52, 56, 43, 21, 58, 32]. Importantly, our TTD parameters are derived without constraining the ratio of Δ/Γ, allowing both Δ and Γ to vary freely. This ensures that the shape and spread of each TTD are determined by the radionuclide observations, rather than pre-determined by assumptions about flow regime or mixing intensity, as is often done in TTD studies based on gas tracers [59, 8, 21, 38, 37], with the choice of Δ/Γ contributing 3% uncertainty to global Cant estimates [60].

At CB9 and CB10, transport times (mean ages, Γ) of core AW appear to remain relatively steady across all years (≈ 20 ± 4 – 27 ± 4 years), although CB9 does show a pronounced increase in 2020 (to 32 ± 2), before returning to a similar Γ as CB10 in 2022 (Fig. 3b & c). In the tmode ages, CB9 shows almost identical values and behaviour to CB10. CB9 tmode ages vary between 9 ± 1 and 16 ± 1 years, while CB10 tmode ages vary between 7 ± 2 and 15 ± 1. In contrast, our inner basin stations, CB5 and Stn A, while they have almost identical Γ and tmode ages to CB9 and CB10 in the early 2000s, (20 ± 2 – 26 ± 1 at Stn A, 23 ± 1 at CB5 for Γ, 15 ± 1 – 20 ± 1 at Stn A, 20 ± 1 at CB5 for tmode) they show clear increases in both Γ and tmode to 2015–2020 (up to 44 ± 1 at Stn A and 33 ± 1 at CB5 for Γ, up to 24 ± 1 at Stn A and 26 ± 1 at CB5 for tmode), with a slight decrease then observed again from 2020 to 2022–2023. This finding is corroborated by the gas tracers in Rajasakaren et al. [21], who also found a positive age trend in both the southern and northern Canada Basin from 2005 to 2015, but they did not distinguish between stations within the AOBC in the north of the basin. Similar trends across the Arctic Ocean are also described by Gerke et al. [37], Körtke et al. [61] and Wefing et al. [62].

Interestingly, while CB10 and CB9 exhibit lower Γ and tmode in the early 2000s, they both have higher Δ values than the inner basin stations during the same period (Fig. 3d). However, this trend reverses in 2020 for Stn A, with a sharp increase in Δ observed. This deviation at Stn A was attributed to the possibility of eddy activity in Payne et al. [52]. CB9 also shows a shift to a higher Δ in 2020, while CB5 shows a slight increase, which may suggest a change in mixing regime. CB10 appears to remain steady in Δ, except in 2006 and 2023 when a slight increase in both mixing parameters is seen. The consistently low Δ/Γ values (generally < 0.5 across the entire basin in all years, not illustrated in Fig. 3, see Supplement Figures S6 & S7) indicate that, in general, transport of core AW and even dAW is advective, a finding also corroborated by other works [43, 56, 52]. Rajasakaren et al. [21] found slightly higher Δ/Γ values of 0.6 from the gas tracers fit their data best. An increase in Δ and Δ/Γ with depth is also expected, as deeper water mixes more with ambient water and interacts with the shelf topography.

The difference in the behaviour of the TTD parameters at the individual stations may result from an increase of mixing in the Eurasian Arctic Ocean over time. In that basin, Atlantification is enhancing vertical mixing and reducing stratification. However, if this were the case, we would expect to see an increasing trend in Δ and Δ/Γ, and similar behaviour at the boundary current stations and inner basin stations. Given that none of our time-series stations show an increasing trend in the mixing parameters, we propose an alternative explanation, involving changes in transport related to the state of atmospheric circulation. Previous works using 129I to investigate the transport of AW into the Canada Basin have suggested a shifting of the AOBC and transport pathways of AW to the Canada Basin in response to the atmospheric circulation state [63, 64]. The Arctic Oscillation (AO) and Arctic Dipole (AD) are atmospheric modes describing Arctic variability [65, 66]. Recent work by Polyakov et al. [67] has shown that the relative proportions of AW making up the three branches vary on decadal timescales depending on the AD state. From 1992 to 2007, Polyakov et al. [67] finds that the inflow of the upper 50 m through the Fram Strait was 15% greater than in 2007 - 2019, with somewhat reduced inflows through the Barents Sea (5%) and also the Franz Joseph Land–Novaya Zemlya passage. These changes in AW branch volumes likely control the concentrations of each tracer carried by each branch, also leading to the observed changes in our TTD parameters. Given these circulation-induced controls on the relative proportions of isotope concentrations carried in each branch of AW, Cant content in each branch of Atlantic Water may be influenced similarly. In section 2.4, we discuss the Cant content in Fig. 3e in the context of the Arctic Ocean Oscillation (AOO), which, while being closely linked to the AO, is considered a more Arctic-centric index. The AOO is a shallow-water model, and as the Atlantic Water branches were at shallow depths in the Eurasian Arctic, where they picked up the tracer signals, it is therefore the most appropriate to use here [68] (Fig. 3f).

2.3 Distribution of Cant in the Atlantic Layer in 2022

We now consider the TTD-derived Cant content across the Canada Basin in 2022. We present 2022 as this is the year with the best sampling coverage across the basin. Vertical distribution of Cant along the 150 ◦W line is illustrated in Fig. 4a (we only illustrate one section as the vertical distributions are similar across the basin). In core AW Cant content ranges from 37.6 to 46.4 µmol kg−1, and in the dAW range between 35.0 and 47.7 µmol kg−1, with the lowest content observed below 1000 m. The highest content was observed at the northern end of the line (47.7 ± 1.8 µmol kg−1 at σθ = 28.00 kg m−3, 45.4 ± 2.2 µmol kg−1 in core AW), decreasing southwards to the mid-point station (34.9 ± 1.1 µmol kg−1 in dAW, 37.6 ± 1.3 µmol kg−1 in core AW) and then increasing at the southern end (35.4 ± 1.4 µmol kg−1 in dAW, 43.6 ± 1.8 µmol kg−1 in AW). These two regions are where water enters the basin; younger water should carry higher Cant content. Greater insight into differences in distribution becomes apparent when looking at the isosurface plot of core AW (Fig. 4b). The highest Cant content in the Atlantic Layer enters west of the 150 ◦W line, at station CB10. The highest Cant values observed at CB10 are in the denser AW (48.7 µmol kg−1) (Fig. 4c.). Atlantic Water also appears to be entering the basin from the north along the 140 ◦W line, with decreasing Cant content observed southwards, and this distribution of Cant follows the transport and distribution of water in the basin indicated by the tracers 129I and 236U (see supplementary figure S3 and Payne et al. [52]).

We do not calculate Cant content in water deeper than 1500 m as the deepest (> 2500 m) waters have been isolated in the basin for 450 - 600 years [55, 8]. In deep waters, tracer concentrations approach the detection limit [52, 8], making estimates of Cant unreliable. However, Cant content should be low [32, 21], and globally waters below 1500 m are estimated to contain less than 7% of the total Cant inventory [28]. We cannot estimate Cant in the Pacific layer, as the tracers used in 2022 (129I and 236U) are only present in Pacific Water in global fallout concentrations, and lack the time-dependent input function required for TTD calculations. In the cold halocline layer, concentrations of both 129I and 236U exceed those in the core Atlantic Water layer (see supplementary S1 & S2). These elevated concentrations have now been observed across the Arctic Ocean basin [62] and into the subpolar North Atlantic. Here, we excluded the cold halocline layer from Cant calculations as our input function is only defined for the surface and mid-depth layers. However, the CHL has a large lateral extent and should carry high Cant, being ventilated as it traverses the Russian shelf seas. A better understanding of halocline formation, including how it acquires tracer concentrations through the incorporation of high-concentration labelled surface waters, and its subsequent transport, should be a priority in future work to estimate accurate basin inventories.

2.4 Temporal Changes in Cant and Response to Ocean Circulation State

When we compare Γ and tmode from the early 2000s to 2015–2023, our boundary current stations CB9 and CB10 appear to have relatively consistent transport times (Fig. 3b & c), suggesting a constant presence of the boundary current to the north of the Chukchi Plateau, unaffected by atmospheric circulation. This is also supported by the fact that the highest tracer concentrations are consistently found in this region every year, indicating that this is the inflow region of the core AOBC. At CB10 and CB9, Cant in the AOBC increased from ≈ 33.7 to 45.2 µmol kg−1 and 33.8 to 46.3 µmol kg−1, respectively, over 2003–2022, yielding an accumulation rate of 0.76 ± 0.06 µmol kg−1 yr−1, comparable to estimates for total DIC in the Eurasian Basin [17]. In contrast, our interior basin stations show greater variability that may well result from the state of the AOO. In 2003 to 2008 (Fig. 3e), Cant content across the basin never exceeded 35 µmol kg−1, and the mean of Cant across these years showed little variation (31-32 µmol kg−1), with relatively young mode ages at the interior basin stations (Stn A and CB5) (< 20 years), suggesting relatively rapid transport. In 2015 to 2020, the estimated mode age of the core AW at both these stations increased to over 25 years, before declining again in 2023 (Fig. 3c), while mean Cant rose throughout this period (> 43 µmol kg−1), particularly sharply between 2020 and 2023. Basin-wide across the entire AW depth, the rate is lower (0.34 ± 0.09 µmol kg−1 yr−1) (Fig. 3e), aligning with prior Arctic-wide estimates (e.g., 0.2 µmol kg−1 yr−1 for dense Arctic Atlantic and upper Polar Deep Water [21]).

We report changes in column inventories from 400–1000 m only, as the 2003–2008 data does not extend past this depth, and data coverage from 300–400m is incomplete across several years. We find a rate of increase in column inventory of 0.30 ± 0.05 mol C m−2 yr−1. Ulfsbo et al. [31] report a slightly higher trend in the upper 1500 m of the Amundsen Basin (0.69 ± 0.09 mol C m−2 yr−1) and 0.5 ± 0.1 mol C m−2 yr−1 in the Nansen Basin for the 1996–2015 period using the eMLR method. Rajasakaren et al. [21] report column increases in the AW/AAW layer of 0.50 ± 0.05 mol C m−2 yr−1 and 0.32 ± 0.16 mol C m−2 yr−1 in the western and southern Canada Basins, and 0.17 ± 0.21 mol C m−2 yr−1 in the northern Canada Basin; however, their total column inventories decrease due to an apparent decline in Cant at depths greater than 2000 m. Woosley & Millero [69] report a column inventory increase of 0.13 ± 0.26 mol C m−2 yr−1, with changes only observed down to the 27.55 kg m−3 isopycnal. However, they observed a significant decrease in Cant content in the upper 100 m, linked to surface freshening that reduced both total alkalinity and DIC between 2005 and 2015. They estimate a basin-wide inventory increase of 2.34 Tg C yr−1 using a basin area of 1,524,664 km2. In comparison, using this same area, we estimate 5.5 ± 0.9 Tg C yr−1 in the 400–1000 m layer only. Because we exclude both the surface and deep waters, which other authors report to show decreasing trends, our value likely represents an upper bound on the true basin-integrated rate.

A broader view of the regional carbonate system may help explain why our observed mid-depth trends diverge from changes observed in the surface and the Pacific layers by Woosley & Millero [69]. Yamamoto-Kawai et al. [70] report increasing aragonite undersaturation in both surface waters and the underlying Pacific Winter Water from 2003–2019, reinforcing the observations of Woosley & Millero [69]. Yet they also note that the aragonite saturation profile in the Canada Basin exhibits a subsurface maximum in the intervening Pacific Summer Water and also near 400 m, coinciding with the temperature core of Atlantic Water. Given the stratification between the Polar Mixed Layer, Pacific Waters and Atlantic Water, we expect that local Cant uptake has always been a minor contribution to the mid-depth Atlantic Water compared to the fraction transported from upstream. Any observed changes in Atlantic Water Cant content will likely be due to Atlantification and freshening in the Eurasian Basin before subduction. The tracer ages estimated with CFCs and SF6 in the Atlantic layer in Raimondi et al. [32] agree well with the radionuclides, and do not indicate substantial recent ventilation of mid-depth water in the Canada Basin. If local ventilation had occurred at scale, we would expect to find significantly younger gas tracer ages than those observed. Yamamoto-Kawai et al. [70] conclude that the most plausible drivers of changes in the surface and Pacific Winter Water lie upstream on the Russian shelf, where water masses equilibrate with the atmosphere before being subducted into the interior. Recent work by Polyakov et al. [71] strengthens this interpretation by documenting recent Atlantification in the Makarov Basin. Characterised by sea-ice loss, warming and enhanced ventilation of shelf waters, these factors collectively alter carbonate chemistry in the upper halocline waters.

When we compare the mode ages to the approximate time the water would have been in the Eurasian Arctic Ocean (Fig. 3, panels (c) and (f) and arrows) higher mode ages appear to occur when the AOO is in a positive phase, which means the AOO is in an anticyclonic regime, forcing surface waters and sea ice to drift clockwise, strengthening the circulation and extent of the Beaufort Gyre, the volume of Pacific Water and location of the Transpolar Drift, which can in turn reduce the presence of Atlantic-sourced waters at the surface, and likely similarly affect the mid-depth AW (see arrows on the map in Fig. 3a). In contrast, when the interior basin stations had their youngest Γ and tmode ages, these appear to correspond to a negative AOO. The resulting cyclonic circulation weakens the Beaufort Gyre and reduces the extent of Pacific Water, allowing a greater influx of Atlantic-sourced waters to the region. However, regardless of the state of the AOO, mean Cant content across all years (1993- 2023) has increased, reflecting the atmospheric increase in Cant.

2.5 Implications for Future Cant Accumulation

To assess whether the resulting Cant values we obtained in the Canada Basin are realistic and to investigate if there was a change in circulation/uptake regime from the early 2000s to 2020s, we compared them to an estimated range of surface Cant uptake in the Barents Sea and St. Anna Trough. We varied temperature (-1.5 to 2 ◦C), practical salinity (32–35), and total alkalinity (2150–2325 µmol kg−1) according to real, observed ranges, [72, 73], and a constant air-sea Cant equilibrium (i.e. 100% saturation of anthropogenic CO2). This is represented as a shaded region in Fig. 5a, reflecting the uncertainty in the actual surface conditions during ventilation. For each Canada Basin sample, calculated Cant is based on the atmospheric CO2 history, and the sample’s observed temperature, salinity, and alkalinity derived from salinity (see Methods), and the TTD parameters Δ and Γ calculated from the radionuclides. We then projected the calculated Cant of the sample back to the estimated time of ventilation using the mode age (tmode). Because the same surface history was used to construct the Cant input function for the TTD, alignment of the calculated Cant values with the input curve is expected, particularly since most samples have low Δ and Δ/Γ values, implying limited mixing. This is supported by tracer observations at the CB10 gateway station, where concentrations of all three radionuclides in the core AW closely align with their respective input functions when plotted at the sampling year minus the mean mode age of core AW for this station (11 years; Fig. 5, panels b, c & d). This agreement indicates almost total conservation of the tracer signal along the core of the AOBC, providing evidence of advective flow that is not necessarily captured in the T-S of a water mass. The fact that most calculated values fall within or slightly below the possible surface uptake range supports the consistency of the resulting Cant with known surface conditions.

It appears that the samples ventilated between 1980 and the mid-1990s plot more centrally in the grey-shaded region. In contrast, samples from after this period plot closer to the lower edge of the shaded area. Whether this is due to different temperatures, salinity, alkalinity, saturation, or loss of Cant due to mixing and dilution along the flow path under different transport regimes is beyond the scope of this work. Still, it could be taken as evidence of varying uptake regimes in the Eurasian Basin. The difference may reflect the shift from a negative to a positive AOO regime, as occurred in the mid-1990s, or could reflect the increasing influence of Atlantification in the Eurasian Basin [74, 2]. Even given these possible drivers, our tracer observations support our assumption that Cant in core Atlantic Water is transported near-conservatively from the Barents Sea into the Canada Basin. Our results indicate that future Cant accumulation and associated acidification of these waters will continue to track atmospheric CO2 increases, time-lagged by approximately 15–20 years (mean tmode age across the basin). This means that in another 20 years we should observe the Cant content entering the AW in the Barents Sea today of between 50–65 µmol kg−1 in the core AOBC as it reaches the Canada Basin. However, recent observations by Ericson et al. [75] of increasing CO2 fugacity in the Barents Sea indicate that the uptake potential of surface water in the Eurasian Basin is rapidly declining, with implications for the future sequestration of Cant in Atlantic Water. The rate of Cant increase in mid-depth Atlantic Water throughout the Arctic Ocean in the following decades will most likely be influenced by the state of the Arctic Ocean circulation regimes, as well as the ongoing freshening of the surface Arctic Ocean and Atlantification in the Eurasian Basin, highlighting the importance of continuing monitoring efforts across the entire Arctic.

This study utilises nuclear-waste radionuclides (129I, 236U and 137Cs) to constrain transit times, mixing and anthropogenic carbon into the Arctic Canada Basin. Tracer signals measured in Atlantic Water over time reflect the evolution of the radionuclides’ input functions, and these signals undergo limited dilution in the core of AW transport. This supports the interpretation of advective transport pathways from the estimated Transit Time Distribution parameters Δ and Δ/Γ. Temporal shifts in age estimates of both mean and mode ages at stations CB5 and Stn A in the interior basin reflect changes in large-scale atmospheric forcing, most likely due to the Arctic Ocean Oscillation, which governs Atlantic inflow and tracer transport. However, the high tracer concentrations observed at the gateway stations CB9 and CB10, along with relatively invariable transport times in both mean and mode ages, suggest a constant presence of the Arctic Ocean Boundary Current north of the Chukchi Plateau, regardless of the AOO state. The distribution of Cant in the Canada Basin in 2022 reflects the transport and distribution of the radionuclides. Cant content increase is highest in the AOBC (0.76 ± 0.06 µmol kg−1 yr−1), with lower rates of Cant content increase across the basin 0.34 ± 0.09 µmol kg−1 yr−1. We find an increasing basin inventory of 5.5 Tg C yr−1. Our estimates of Cant across the basin closely track atmospheric CO2 growth, lagged by 15–20 years. Radionuclide-based TTDs therefore provide a robust framework for monitoring and projecting future Cant accumulation and its resulting impacts, such as acidification, in the region. Our findings underscore the importance of continued monitoring of Cant in the Arctic Ocean. A more comprehensive comparison across methods, including the eMLR and models, would be beneficial, particularly in light of changing oceanic and atmospheric conditions. The study suggests that further investigation is needed to understand the role of Arctic Ocean circulation shifts in modifying Cant uptake and distribution to provide better insights for predicting future ocean acidification and climate change impacts in polar regions.

Methods
3.1 Sampling, Isotope Analysis and Historical Data

Samples for analysis of 236U and 129I were collected during the 2022 (280 samples) and 2023 (60 samples) expeditions of the JOIS/BGOS monitoring program on the CCGS Louis St Laurent in the Canada Basin. Complete depth profiles were collected at the stations indicated in Figure 1. 3 L samples were collected from 10 L Niskin bottles attached to a 24-bottle CTD rosette. Hydrographic data, including conductivity, temperature, and depth, were recorded using SeaBird Electronics sensors. The samples were directly filled into rinsed 3 L containers, left untreated at ambient temperatures, and sent to ETH Zürich for analysis.

Uranium samples were processed following Christl et al. [48]. Samples were acidified to pH 2 with HNO3 and spiked with 1 pg of 233U (PTB 2014-1126). Uranium was concentrated by Fe co-precipitation, with 250 mg of purified Fe-solution added to each sample, allowed to equilibrate overnight and precipitated with NH3. After overnight settling, the supernatant was removed, and the precipitate was re-dissolved in 16 mL of 8 M HNO3 before uranium separation by ion-exchange chromatography using pre-packed Triskem UTEVA resin columns. The sample was loaded into the column, followed by 16 mL 8M HNO3. The Np fraction was removed by 8 mL HNO3 (2 M) + 0.05 M H2C2O4, U was then eluted by 15 mL HNO3 (0.01 M) + 10 mL MQ water. 1 mg Fe in solution was added and precipitated by NH3. Samples were then centrifuged, rinsed with ethanol, and transferred to glass crucibles. These were oxidised at 650 ◦C for 3 hours, and pressed into Al targets with 3 mg of Nb.

Measurements of 233U/238U and 236U/238U were performed on the 300 kV Milea AMS system at the Laboratory of Ion Beam Physics, ETH Zürich, normalised to in-house standard ZUTRI (nominal values: 233U/238U=33170 ± 830 x 10−12 at/at; 236U/238U=4055 ± 200 x 10−12 at/at). 236U and 238U concentrations were derived from the measured 233U spike concentration. Samples were corrected using chemistry blanks (n = 15) prepared at ETH Zürich using MilliQ water (18.2 MΩ). Analytical uncertainty is 1–10% for samples within the upper 1500 m.

Iodine samples were purified following Casacuberta et al. [46]. Samples were spiked with 1.5 mg of Woodward Iodine (127I). 5 mL of saturated Ca(ClO)2 was added and stirred for ≈ 15 minutes. 20 mL NH2OH·HCl (1 M) + 1 g Na2S2O5 was added and stirred for about 45 minutes. pH was adjusted by the addition of NaOH (7 M) in 0.8 mL increments. Iodine was separated by ion-exchange chromatography using 5 mL DOWEX 1 x 8 resin columns. Samples were loaded immediately after pH adjustment. The resin was then rinsed with 20 mL Milli-Q water, followed by 50 mL KNO3 (0.5 M), then 10 mL KNO3 (2.25 M). The iodine fraction was eluted with 40 ml of 2.25M KNO3 and precipitated directly as AgI in prepared Falcon tubes containing 200 µL HNO3 (65%) + 500 µL AgNO3 (0.1 M).

Precipitates were triple-rinsed, dried, and pressed into Ti targets with 4 mg of Ag for measurement on the 500 kV Tandy AMS system. Each run included an in-house standard C2 in both concentrated and diluted forms (nominal values: diluted 5.055 x 10−12 at/at, concentrated 38.995 x 10−12 at/at 129I/127I). Samples were corrected using chemistry blanks (n = 15) prepared at ETH Zürich using MilliQ water (18.2 MΩ). A 4% analytical uncertainty is reported, incorporating an ETH internal standard and a 1–2% instrument uncertainty.

This work includes all tracer data with at least two radionuclides to constrain the calculated mean ages. For the radionuclides 129I and 236U, 2015 data was collected on the GN01 section on board the USCGC Healy (HLY1502), published in Wefing et al. [56], and 2020 data was collected during the JOIS/BGOS cruise onboard the CCGS Louis St Laurent, published in Payne et al. [52]. In addition, we also include data from 2003, 2005, 2006, 2007 and 2008 JOIS expeditions, where 129I is paired with 137Cs, published in Smith et al. [43] and the 1993 CCGS Henry Larsen expedition, where 129I is paired with 137Cs, published in Smith et al. [44].

We present mean profiles of each tracer rather than time-series stations, as not all the same stations were occupied yearly (see Fig. 1b.). We also include the 1993 stations west of the Chukchi Plateau to extend the time series. We interpolated along 5 m intervals for each station sampled that particular year and then calculated the mean from all stations. However, it is worth noting that the 1993 stations are located upstream of the Canada Basin and, therefore, should not be directly comparable to the stations within the Canada Basin itself, as different transport times and circulation parameters are expected.

3.2 Input Functions

For this work, to ensure consistency across all the data, we used the 137Cs input function from Smith et al. [43], and calculated the 129I and 236U input functions at 60 ◦N using the same method as for 137Cs described in Smith et al. [43], based on the annual releases reported by Sellafield and La Hague. A transfer factor of 25 Bq m−3/PBq yr−1 at a 2 and 4 year time-lag after the release for Sellafield and La Hague respectively, converts the releases (Bq yr−1) to the surface ocean input function (Bq L−1 or at L−1). An additional input from modelled global fallout of 236U from Christl et al. [48], and of 129I of 1 x 107 at L−1. The Atlantic Layer input function is calculated from the surface input function using a dilution factor of 10, coupled with a 1-year time lag. The Atlantic Water 137Cs input function was taken directly from Smith et al. [43], also based on the global fallout, reported releases from the nuclear reprocessing facilities, and includes the input from the 1986 Chernobyl disaster.

3.3 Transit Time Distributions

The TTD method is based on the following equation, which represents the sampled concentration, c, of tracers at time t, at location x:

c(x,t) = ∫0∞ c0(t − t′)e−λt′G(x,t′)dt′ (1)

Here c0(t) is the tracer input function, and G(t) is the Green’s function, which is unknown to be solved. λ is the decay constant for radioactive tracers and is only required for the radionuclide 137Cs as it has a short half-life. For 236U and 129I with long half-lives, e−λt = 1. For steady-state 1D transport, the TTD has an inverse Gaussian shape described by G(x,t):

G(x,t) = Γ3 / (4πΔ2t3) exp [− Γ(t − Γ)2 / (4Δ2t)] (2)

Γ is the first moment of the TTD, giving the "mean transit time" or "mean age", Δ is the second moment, defining the TTD’s width, describing the tracer signal’s dispersal due to lateral mixing. The ratio of Δ/Γ estimates the advectiveness of the flow, with 0.1 being purely advective; however, open ocean conditions are typically closer to 1 [76], and may be as high as 1.8 (greater than 1.8 leads to large uncertainties in mean age) [77]. This ratio can be directly related to the Péclet number, as Pe=(Γ/Δ)2 [30]. We calculate an additional parameter, tmode, which is also considered the age that has the highest probability of occurring within the PDF [43, 56]:

tmode = Γ [ (1 + (3Δ2/Γ2))1/2 − 1 ] (3)

Here, we use "Smith’s TATD" [32], where rather than setting a fixed Δ/Γ, we allow Δ and Γ to vary. We plot all possible Δ/Γ combinations on a grid of 236U vs 129I concentration and assign the nearest point to the actual measured data. We exclude any data points that do not lie within the gridded TTD domain.

We calculated the TTD parameters for each station, but limited the data to sampled depths between 300 and 1500 m as these depths we consider true "core" Atlantic Water (σθ ≈ 27.70 - 28.05 kg m−3), as well as having concentrations high enough for the TTD calculation. Monte Carlo simulations (sample size = 1000) were used to estimate uncertainties for each sample based on the measurement uncertainty.

3.4 Estimation of Cant using the TTD Method

Following Waugh et al. [29], we assume that Cant behaves as a passive tracer. Therefore, its concentration at a known location and time can be described as for any other transient tracer using the following equation [30, 78]:

Cant(x,t) = ∫0∞ Cant,0(t − t′)G(x,t′)dt′ (4)

Here, Cant(x,t) is the only unknown, as it cannot be measured directly. We use the age spectrum G(x,t’) we calculated using equation 2 and the surface input history of Cant as Cant,0(t), here we repeat the method of Raimondi et al. [32]. To reconstruct the surface history of Cant, we used the mid-year atmospheric CO2 concentrations for the Northern Hemisphere. We subtracted the pre-industrial baseline value of 280 ppm, isolating the anthropogenic portion of the partial pressure of CO2. This pCant value, along with preformed total alkalinity estimated from salinity using the surface relationship established by Brewer et al. [79]:

Surface alkalinity = 547.05 + (50.56 x S) (5)

was input into the CO2SYS program (v1.1) [80, 81] to calculate the amount of dissolved inorganic carbon (DIC) attributable to anthropogenic CO2 uptake. The carbonate dissociation constants (K1 and K2) used were those of Roy et al. [82]. This method assumes a constant air-sea CO2 disequilibrium over time and complete equilibration of anthropogenic CO2 between the atmosphere and ocean (i.e. 100% saturation of CO2). Although this does add additional uncertainty to our TTD estimates, we are limited by available data and are unable to better constrain true saturation values within the Arctic Ocean at present. For the 1993 data, temperatures are unavailable, and we use an estimated temperature of 0.3 ◦C for all data points. Although this means the estimates of Cant in 1993 are less accurate, small temperature differences have a minute effect on estimated Cant, compared to the TTD parameters Γ and Δ, which are independent of temperature. The uncertainty in Cant was estimated by propagating Monte Carlo-derived uncertainties in the TTD parameters Δ and Γ specific to each measured sample through the TTD calculation to obtain an upper and lower bound on Cant at each depth. These uncertainty bounds were interpolated and averaged across the basin to provide upper and lower estimates of Cant content and inventory (equation 6). The input function assumes steady-state circulation. In separate tests, the 10 % uncertainty of the radionuclide input functions was used to generate upper and lower bounds on the TTD parameters, and subsequently on Cant content. Overall uncertainty in the mean Cant content at 500 m each year, after averaging across available stations, is 4–12%. Column inventories were calculated using the mean interpolated Cant profiles calculated for each year.

ICant = ∫0max(depth) Cant ρdz (6)

where ICant stands for the column inventory of Cant. Cant is in mol kg−1, ρ is the in situ density (kg m−3), and max(depth) represents the depth limit of the mean profile obtained for each basin. Basin inventories were calculated using the same basin area as Woosley & Millero [69] of 1,524,664 km2, and only calculated from 400–1000 m as this depth range has the most consistent data coverage across years.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of NCOMMS-25-62368\nNuclear Waste Unveils Three Decades of Anthropogenic Carbon in the Canada Basin of the Arctic Ocean\nAnnabel Payne, Lorenza Raimondi, Anne-Marie Wefing, Christof Vockenhuber, Habacuc Pérez-Tribouillier, Willam\nWillams, Sarah Zimmermann, Núria Casacuberta\nGeneral\nUsing artificial radionuclide tracers (129I, 236U, 137Cs) to constrain Transient Time Distributions over three decades, the\nauthors quantify anthropogenic carbon (Cₐₙₜ) in the Canada Basin. They find a boundary-focused increase of about 0.76\nµmol kg⁻¹ yr⁻¹ in the core of Atlantic Water boundary current while the basin interior shows more muted changes, implying\nthat advective transport dominates over local ventilation. This study makes a substantial and timely contribution to our\nunderstanding of anthropogenic carbon storage in the complex Arctic Ocean.\nThe manuscript is clearly written, the figures are informative, and the analytical framework is sound. General issues need\nfurther attention from the authors. Addressing these issues in a revision, I have no reservation for the manuscript to be\npublished in Nature Communications. Please find my major comments below, followed by some minor comments that are\nreferred to by the line numbers in the manuscript pdf.\nMajor comments\nThe authors state that: “The rate of increase in the following decades will most likely be influenced by the state of the Arctic\nOcean circulation regimes, as well as the ongoing freshening of the surface Arctic and Atlantification in the Eurasian Basin.”.\nI agree with this, but the manuscript would benefit from unpacking how these processes impact anthropogenic carbon\nstorage in the Atlantic core and the Canada Basin interior. Freshening in the western Arctic dilutes total alkalinity and\nstrengthens upper-ocean stratification, which (i) lowers buffer capacity and (ii) suppresses subduction/ventilation of newly\nequilibrated waters - both effects reduce the invasion of Cant into intermediate Arctic layers and weaken the regional sink.\nThis mechanism was demonstrated with observations by Woosley & Millero (2020) in the Canada Basin and should be\nexplicitly discussed and referred to in the manuscript. This would potentially also help strengthen the results of muted Cant\nincreases in the basin interior. Also, the authors should consider putting their results in the context of the previous findings\n(rates of change) by Woosley and Millero (2020) and Ulfsbo et al. (2018), both using the eMLR approach, and not just the\nstudy by Ericson et al. (2014) who indirectly attributed trends of increasing DIC (by subtracting the deep-water DIC means) to\nincreasing Cant.\nThe authors also state: “Whether this is due to different temperatures, salinity, alkalinity, saturation, or loss of Cant due to\nmixing and dilution along the flow path under different transport regimes is beyond the scope of this work. Still, it could be\ntaken as evidence of varying uptake regimes in the Eurasian Basin.”. While I understand that such an investigation is\nbeyond the scope of the study, I think the manuscript would benefit from including a discussion about the study by Ericson et\nal. (2023) as they document seasonally resolved, significantly positive surface fCO₂ trends (up to 4.2–5.5 µatm yr⁻¹) in the\nsea-ice–influenced northern Barents/Nansen sector, which is faster than the atmospheric rise and linked to sea-ice loss and\nincreasing Atlantification, implying a declining surface uptake capacity and spatially varying uptake regimes in the Eurasian\nBasin. What could this mean for the time-lagged accumulation of Cant in the Canada Basin?\nThe value of using radionuclide tracers to improve TTD-based Cant estimates should be made more explicit, rather than just\nstating they are “preferred for robustness”. Currently, the manuscript does not clearly show how much uncertainty reduction\nis achieved compared with previous CFC/SF₆-only approaches.\nLastly, it was unclear to me what the total propagated uncertainty is in relation to the main result that is presented in the\nabstract (~0.76 µmol kg⁻¹ yr⁻¹). Would it be possible to clarify this?\nMinor comments\nOverall: Please consider a light copy-edit focusing on comma placement. In several sentences, extensive use of commas\ninterrupts the natural flow.\nTitle: While I understand the eye-catching use of “Nuclear Waste” in the title, consider using “Artificial/Anthropogenic\nRadionuclides” instead as “Nuclear Waste” only appears in the title and the heading of Section 1.1.\nL33: In acid–base terms the ocean does not “buffer the atmosphere”; it buffers changes in seawater pH (and CO₂ speciation)\nvia the carbonate system (CO₂-HCO₃⁻-CO₃²⁻). That chemical buffering modulates how much anthropogenic CO₂ the ocean\ncan take up, but it’s imprecise to say the ocean buffers atmospheric CO₂ itself.\nL35: Consider using “amount content” instead of “amount concentration” for µmol/kg-seawater. The formal definition (by\nIUPAC) for “amount concentration” is mol/dm3 and “molality” for mol/kg-water. In their forthcoming 4th edition of the IUPAC\nGold Book, they recommend the use of “amount content” for µmol/kg-seawater. Amount content is becoming more frequently\nused in today’s literature. Suggest changing to “carbonate ion content”, and throughout the text whenever µmol/kg-seawater\nis referred to as “concentration”.\nL42: For consistency, use either “Atlantic Water(s)” or “Atlantic water(s)”\nL43: Supporting reference is needed for “Cant is primarily taken up by inflowing Atlantic Waters through air-sea gas\nexchange in the Nordic Seas”.\nL71: should be “uptake by the ocean”…\nL72: “These latter include, for example, the…” There are other indirect methods applied in the global ocean that is not\nmentioned here.\nL83: Suggest that you also use “transient gas tracers” in the Abstract.\nL117: “Chornobyl” should be “Chernobyl”\nL241-244: Long and complicated sentence. Please try rewriting for easier reading.\nL270: should “pass” be “passage”?\nL338: Figure 4 caption: try to avoid using contracted words like “can’t”, use “cannot” instead. Also, it would be instructive to\ninclude circulation arrows for a schematic Beaufort Gyre.\nL357: Please try to avoid the “psu” unit. By definition, practical salinity on the Practical Salinity Scale from 1978 (PSS-78) is\ndimensionless. Thus, “salinity (32–35 psu)” should read “practical salinity (32–35)”.\nL494: Please include the equation for the preformed total alkalinity. Brewer et al. (1986) is difficult to find and get access to.\nL495: Please include the version of CO2SYS that was used. Please also state which sets of dissociation constants that\nwere used in the calculations.\nL512: “European Research Commission” should perhaps be “European Research Council”?\nReferences\nEricson, Y., Ulfsbo, A., van Heuven, S., Kattner, G., and Anderson, L.G. (2014). Increasing carbon inventory of the\nintermediate layers of the Arctic Ocean. Journal of Geophysical Research: Oceans, 119(4), 2312-2326, doi:\nhttps://doi.org/10.1002/2013JC009514\nEricson, Y., Fransson, A., Chierici, M., Jones, E.M., Skjelvan, I., Omar, A., Olsen, A., and Becker, M. (2023), Rapid fCO2 rise\nin the northern Barents Sea and Nansen Basin. Progress in Oceanography, 217, 1-15. doi:\nhttps://doi.org/10.1016/j.pocean.2023.103079\nUlfsbo, A., Jones, E. M., Casacuberta, N., Korhonen, M., Rabe, B., Karcher, M., and van Heuven, S. M. A. C. (2018). Rapid\nChanges in Anthropogenic Carbon Storage and Ocean Acidification in the Intermediate Layers of the Eurasian Arctic\nOcean: 1996–2015. Global Biogeochemical Cycles, 32(9), 1254–1275. doi: https://doi.org/10.1029/2017GB005738\nWoosley, R.J. and Millero, F.J. (2020), Freshening of the western Arctic negates anthropogenic carbon uptake potential.\nLimnology & Oceanography, 65: 1834-1846. https://doi.org/10.1002/lno.11421", "reviewer_2": "Review of the manuscript NCOMMS-25-62368, “Nuclear Waste Unveils Three Decades of Anthropogenic Carbon in the Canada Basin of the Arctic Ocean”, by Payne et al., submitted to Nature Communications\nGeneral comments\nThe study utilises the spreading of the anthropogenic radionuclides I-129, U-236, and Cs-137, released from the nuclear\nreprocessing facilities in La Hauge and Sellafield, with the northward flowing Atlantic Water as it circulates into the Canada\nBasin of the Arctic Ocean. Adopting the transit time distribution (TTD) approach on these tracers the study present estimates\nof anthropogenic carbon (Cant) evolution over three decades, from 1993 to 2023. The work is interesting and well-written,\nwith clear figures. The Arctic Ocean in general and Canada Basin specifically is under sampled and consequently our\nunderstanding of this region is poor, not the least in terms of anthropogenic carbon evolution. Here this study is an important\naddition. My main concern is about the presentation of uncertainties. The authors performed Monte Carlo simulations using\nmeasurement uncertainties and uncertainties in the TTD parameters, but in addition there is an applied (or assumed?)\nuncertainty in the input functions of the radionuclides, and environmental variability. The results are typically presented as\nranges, and error bars are clearly seen in the figures, which may be all fine, but I am left wondering about the overall\nuncertainty, for example presented as a fraction of the mean values? Could you expand on this for clarity?\nOtherwise, I have no major concerns and recommend publication after minor revision. Detailed comments are given below.\nSpecific comments\nL42-44: It has been suggested in several studies that the main uptake takes place already before the AW enters the Nordic\nSeas. At the very least you should give a reference to your statement.\nL45-46: The NCC is not really formed in the Nordic Seas, but earlier, fed by waters from the Baltic Seas; you should in any\ncase give some reference for this. I also question your statement that the NCCW forms the surface waters in the Eurasian\nArctic. I may of course be mistaken, but again, some reference makes this clearer.\nL47-54: I believe you should add units to the density anomalies (in kg m-3). This goes for the whole manuscript. (See also\nL478-479.)\nL58-59: Rudels et al. (1999) described the Arctic Circumpolar Boundary Current, which could be cited here.\nL62: Neither the Amundsen, Nansen nor the Makarov Basins are marked in the map in Figure 1. This would be useful for\nmany readers.\nL91: Didn’t also Raimondi et al. (2024) use paired gas tracers (together with their focus on radionuclides) in the Canada\nBasin?\nL108: This may be more of a technical comment, but due to the focus on the radionuclides I believe they should be properly\ndefined/written out the first time. Maybe this is not typically done, but I would suggest considering it for full clarity.\nFigure 2: You may consider adding lines for the global fallout values to the profiles. This may be useful as a reference and\nadd clarity.\nL176: “Northwind Ridge” is not known to me, and I would suppose I am not alone on this. Please add a notation in the map\n(Figure 1 a).\nL287: It is not so easy to see detail in the section, with the applied colour range/map. Perhaps this could be helped by\nadding some isolines in the section plot?\nL371-372, and L383-385: This is maybe more of a comment, but it is quite remarkable that the AW core seems so unaffected\nby mixing along the still rather long circulation path. The radionuclides have in many cases proven to be almost perfect\ntracers for the circulation of the AW in the Arctic Ocean. Can this lack of erosion of the core also be seen/backed up by T/S\nproperties? If there would be any inconsistency in what is seen from the physical variables and the radionuclides this would\nbe an interesting observation worth mentioning.\nFigure 5: Add reference to CO2SYS in the caption, or at least refer to the method section.\nConclusions: Possibly consider writing out names the first time mentioned in the Conclusions. This may seem like an\nunnecessary repeat but may be helpful to all those that jump to this section.\nL467: You write that open ocean conditions in Delta/Gamma are typically closer to 2, but don’t you mean 1? Please add\nclear reference(s) for this.\nL479-481: You performed Monte Carlo simulations based on measurement uncertainties, but surely the uncertainty\nconnected to the TTD method itself, and environmental variability is larger than these? Please add some thoughts on this.\nL501: I am not sure I fully understand this; you say that temperatures are unavailable for these data? I am not sure I am\naware of any collected ocean data without available temperature and salinity. If this is true (I mean, I believe what you write)\nhow is this possible?\nTechnical comments\nL42: …Arctic Ocean… This is seen throughout the manuscript, and I would suggest going through the text and add “Ocean”\nin quite many instances.\nL61: DIC should be defined.\nL72: These latter methods include …\nL79: … Cant and shows increasing …\nL83: CFCs and SF6 should be defined/written out the first time mentioned.\nL193: Potential typo: should it be “possibly” (instead of “possible”)?\nFigure 3: The referred figures S5 and S6 should be S6 and S7 I believe.\nL239: one “and” too much (after Gerke et al. (2024)).\nL288: … range from 37.6 to 46.4 … between 35 and 47.7 …\nL296: … enter west of the ...\nL297: You mean “denser” instead of “deeper”?\nL298: … (Fig. 4c).\nL303: Either “deeper than 1500 m”, or, “>1500 m”.\nL332: (Fig. 3e)\nL337: (Fig. 3c)\nL410: This study suggests …\nL491: You here define “Cant”, which is used throughout the text, and paragraph, so this should not be needed.\nSupplement: I believe all figures should be marked as “S1”, S2”, and so on. Also in S4, the panel for 2003, there are some\nicons you likely want to remove on the top right.\nReferences\nRudels, B., H. J. Friedrich, and D. Quadfasel (1999), The Arctic Circumpolar Boundary Current, Deep Sea Research Part II:\nTopical Studies in Oceanography, 46(6), 1023-1062. https://doi.org/10.1016/S0967-0645(99)00015-6"}, "author_response": "Response to Reviewers\nWe sincerely thank both reviewers for their thoughtful and constructive feedbackon our\nmanuscript. The major comments have substantially improved the clarity of the study,and\nmost of the minor suggestions have also been incorporated. We are aware of the\ntightword limit available and so have tried to keep within it. Because we added two\nparagraphs discussing inventories, where some statements about things were questioned (e.g.,\nC\nant\nuptakebefore Atlantic water reaches the Nordic Seas), we removed those statements to\nkeep the wordcount down.\nReviewer #1\nMajor Comments\nThe authors state that: “The rate of increase in the following decades will most likely be\ninfluencedbythestateoftheArcticOceancirculationregimes, aswellastheongoingfreshening\nof the surface Arctic and Atlantification in the Eurasian Basin.” I agree with this, but the\nmanuscript would benefit from unpacking how these processes impact anthropogenic carbon\nstorage in the Atlantic core and the Canada Basin interior. Freshening in the western Arctic\ndilutestotalalkalinity. Itstrengthensupper-oceanstratification,which(i)lowersbuffercapacity\nand (ii) suppresses subduction/ventilation of newly equilibrated waters - both effects reduce the\ninvasion of Cant into intermediate Arctic layers and weaken the regional sink. This mechanism\nwas demonstrated with observations by Woosley & Millero (2020) in the Canada Basin and\nshould be explicitly discussed and referred to in the manuscript. This would potentially also\nhelp strengthen the results of muted Cant increases in the basin interior. Also, the authors\nshould consider putting their results in the context of the previous findings (rates of change) by\nWoosley and Millero (2020) and Ulfsbo et al. (2018), both using the eMLR approach, and not\njust the study by Ericson et al. (2014) who indirectly attributed trends of increasing DIC (by\nsubtracting the deep-water DIC means) to increasing Cant. The authors also state: “Whether\nthis is due to different temperatures, salinity, alkalinity, saturation, or loss of Cant due to\nmixing and dilution along the flow path under different transport regimes is beyond the scope\nof this work. Still, it could be taken as evidence of varying uptake regimes in the Eurasian\nBasin.” While I understand that such an investigation is beyond the scope of the study, I think\nthe manuscript would benefit from including a discussion about the study by Ericson et al.\n(2023) as they document seasonally resolved, significantly positive surface fCO trends (up to\n2\n4.2–5.5 µatm yr−1) in the sea-ice–influenced northern Barents/Nansen sector, which is faster\nthan the atmospheric rise and linked to sea-ice loss and increasing Atlantification, implying a\ndeclining surface uptake capacity and spatially varying uptake regimes in the Eurasian Basin.\nWhat could this mean for the time-lagged accumulation of Cant in the Canada Basin?\nResponse:\nWe have added a paragraph (lines 352 – 368) column inventories to allow better comparison\nwith the findings of Woosley and Millero (2020) and Ulfsbo et al. (2018). We added a sentence\nonthebasininventorychangetotheabstract(line14). Wealsoincludeanadditionalparagraph\n1\ndiscussing the recent publications of Yamamoto-Kawai et al. (2025) and Polyakov et al. (2025)\nwhich give a bit more nuance to the difference between the results presented in Woosley and\nMillero (2020) and our work (lines 370 – 390).\nIn the supplement we have included a table (Table S1) of annual column and basin inventories\nwith uncertainties, and Figure S8 illustrating the rate of basin inventory increase. The methods\nsection was also updated to reflect these changes (lines 579–586).\n\"We report changes in column inventories from 400–1000 m. We find a rate of increase in\ncolumninventoryof0.30±0.05molCm−2yr−1. Ulfsboetal.(2018)reportasimilartrendinthe\nupper 1500m ofthe AmundsenBasin (0.69±0.09molCm−2yr−1)and 0.5±0.14molCm−2yr−1\nin the Nansen Basin for the 1996–2015 period using the eMLR method. Rajasakaren et al.\n(2019) report column increases in the AW/AAW layer of 0.5±0.05molCm−2yr−1 and 0.32±\n0.16molCm−2yr−1 in the western and southern Canada Basins, and 0.17±0.21molCm−2yr−1\nin the northern Canada Basin; however, their total column inventories decrease due to an ap-\nparent decline in C at depths greater than 2000 m. Woosley and Millero (2020) report a\nant\ncolumn inventory increase of 0.13±0.26molCm−2yr−1, with changes only observed down to\nthe 27.55 kg m−3 isopycnal. However, they observed a significant decrease in C content in the\nant\nupper 100 m, linked to surface freshening that reduced both total alkalinity and DIC between\n2005 and 2015. They estimate a basin-wide inventory increase of 2.34TgCyr−1 using a basin\narea of 1,524,664 km2. In comparison, using this same area, we estimate 5.50±0.90TgCyr−1\nin the 400–1000 m layer only. Because we exclude both the surface and deep waters, which\nother authors report to show decreasing trends, our value likely represents an upper bound on\nthe true basin-integrated rate.\nA broader view of the regional carbonate system may help explain why our observed mid-\ndepth trends diverge from changes observed in the surface and the Pacific layers by Woosley\nand Millero (2020). Yamamoto-Kawai et al. (2025) report increasing aragonite undersaturation\nin both surface waters and the underlying Pacific Winter Water from 2003–2019, reinforcing the\nobservations of Woosley and Millero (2020). Yet they also note that the aragonite saturation\nprofile in the Canada Basin exhibits a subsurface maximum in the intervening Pacific Summer\nWater and also near 400 m, coinciding with the temperature core of Atlantic Water. Given the\nstratification between the Polar Mixed Layer, Pacific Waters and Atlantic Water, we expect\nthat local C uptake has always been a minor contribution to the mid-depth Atlantic Water\nant\ncompared to the fraction transported from upstream. Any observed changes in Atlantic Water\nC content will likely be due to Atlantification and freshening in the Eurasian Basin prior to\nant\nsubduction. The tracer ages estimated with CFCs and SF in the Atlantic layer in Raimondi\n6\net al. (2024) agree well with the radionuclides, and do not indicate substantial recent ventilation\nof mid-depth water in the Canada Basin. If local ventilation had occurred at scale, we would\nexpect to find significantly younger gas tracer ages than those observed. Yamamoto-Kawai et\nal. (2025) conclude that the most plausible drivers of changes in the surface and Pacific Winter\nWater lie upstream on the Russian shelf, where water masses equilibrate with the atmosphere\nbefore being subducted into the interior. Recent work by Polyakov et al. (2025) strengthens\nthis interpretation by documenting recent Atlantification in the Makarov Basin. Characterised\nby sea-ice loss, warming and enhanced ventilation of shelf waters, these factors collectively alter\ncarbonate chemistry in the upper halocline waters.\n\"\nIn the final paragraph (line 442) regarding the suggestion to discuss Ericson et al. (2023)\nfindings, we have added:\n\"However, recent observations by Ericson et al. (2023) of increasing CO fugacity in the\ndeclining, with implications for the future sequestration of C in mid-depth Atlantic Water\nant\n2\nacross the Arctic Ocean. The rate of C increase in mid-depth Atlantic Water throughout the\nant\nArctic Ocean in the following decades will most likely be influenced by the state of the Arctic\nOcean circulation regimes, as well as the ongoing freshening of the surface Arctic Ocean and\nAtlantification in the Eurasian Basin, highlighting the importance of continuing monitoring\nefforts across the entire Arctic. \"\nReviewer Comment: The value of using radionuclide tracers to improve TTD-based Cant\nestimates should be made more explicit, rather than just stating they are “preferred for ro-\nbustness”. Currently, the manuscript does not clearly show how much uncertainty reduction\nis achieved compared with previous CFC/SF -only approaches.\ntracer pair rather than a single tracer, because two tracers can resolve the two moments of the\nTTD (Gamma and Delta) without having to assume any degree of mixing (e.g. fixed value of\nD/G). However, to explain the advantages of radionuclides over gas tracers, we added a brief\ndescription of the sensitivity test results from Raimondi et al. (2024) in the following section.\n\"Recently, the TTD method used with a new dual tracer pair, the long-lived anthropogenic\nradionuclides iodine-129 (129I) and uranium-236 (236U), has been shown to provide a robust\napproximation to the TTD estimates calculated in the Arctic Ocean using the gas tracers\n(Raimondi et al., 2024). This work also provided a sensitivity analysis of the TTD results\nobtained from the gas tracers and the radionuclides. They demonstrate that uncertainty in\nthe radionuclide input function leads to smaller changes in the resulting TTD parameters than\nvarying saturation of the gas tracers. The resulting C inventories across all the Arctic basins\nant\nfromgastracersexhibitapproximately20%greatervariabilitythantheradionuclidesdepending\non the gas saturation state used.\"\n(lines 100–108)\nReviewer Comment: Lastly, it was unclear to me what the total propagated uncertainty is\nin relation to the main result that is presented in the abstract (∼0.76 µmol kg−1 yr−1). Would\nit be possible to clarify this?\nResponse: added ± 0.06 µmol kg−1 yr−1, a description of how the uncertainty in the TTD\nparameters from the monte carlo was propagated through the calculations has been added in\nmethods. (lines 567–579).\nMinor Comments\nCommentOverall: Please consider a light copy-edit focusing on comma placement. In several\nsentences, extensive use of commas interrupts the natural flow.\nResponse: Have checked and corrected some sentences for better flow.\nCommentTitle: WhileIunderstandtheeye-catchinguseof“Nuclear Waste” inthetitle, con-\nsider using “Artificial/Anthropogenic Radionuclides” instead as “Nuclear Waste” only appears\nin the title and the heading of Section 1.1.\nResponse: We agree that greater consistency in terminology is valuable. We have revised\nthe title to “Nuclear Waste Radionuclides” to highlight the unique origin of these tracers, while\navoiding too much repetition with “anthropogenic” used in the title and manuscript in the\ncontext of carbon.\nCommentL33: In acid–base terms the ocean does not “buffer the atmosphere”; it buffers\nchanges in seawater pH (and CO speciation) via the carbonate system (CO -HCO−-CO2−).\n2 2 3 3\n2\nit is imprecise to say the ocean buffers atmospheric CO itself.\nCommentL35: Consider using “amount content” instead of “amount concentration” for\nµmol/kg seawater. The formal definition (by IUPAC) for “amount concentration” is mol/dm3\nand “molality” for mol/kg-water. In their forthcoming 4th edition of the IUPAC Gold Book,\nthey recommend the use of “amount content” for µmol/kg-seawater. Amount content is becom-\ning more frequently used in today’s literature. Suggest changing to “carbonate ion content”,\nand throughout the text whenever µmol/kg-seawater is referred to as “concentration”.\nResponse: Changed throughout.\nCommentL42: For consistency, use either “Atlantic Water(s)” or “Atlantic water(s)”.\nResponse: Changed throughout.\nCommentL43: Supporting reference is needed for “Cant is primarily taken up by inflowing\nAtlantic Waters through air-sea gas exchange in the Nordic Seas”.\nResponse: Removed for word count.\nCommentL71: should be “uptake by the ocean”...\nResponse: Changed\nCommentL72: “These latter include, for example, the...” There are other indirect methods\napplied in the global ocean that are not mentioned here.\nResponse: Changed\nCommentL83: Suggest that you also use “transient gas tracers” in the Abstract.\nResponse: Changed\nCommentL117: “Chornobyl” should be “Chernobyl”.\nResponse: Corrected\nCommentL241–244: Long and complicated sentence. Please try rewriting for easier reading.\nResponse: Changed to\nComment\"Interestingly, while CB10 and CB9 exhibit lower Γ and t in the early 2000s,\nmode\nthey both have higher ∆ values than the inner basin stations during the same period (Fig.\n3d). However, this trend reverses in 2020 for Stn A, with a sharp increase in ∆ observed. This\ndeviation at Stn A was attributed to the possibility of eddy activity in Payne et al. (2024).\"\nCommentL270: should “pass” be “passage”?\nResponse: Changed\nCommentL338: Figure 4 caption: try to avoid using contracted words like “can’t”,\nuse “cannot” instead. Also, it would be instructive to include circulation arrows for a schematic\nBeaufort Gyre.\nResponse: Corrected, arrows added.\nCommentL357: Please try to avoid the “psu” unit. By definition, practical salinity on\nthe Practical Salinity Scale from 1978 (PSS-78) is dimensionless. Thus, “salinity (32–35 psu)”\nshould read “practical salinity (32–35)”.\nResponse: Changed\nResponse: Included the equation:\nSurfacealkalinity = 547.05+(50.56×S) (1)\nCommentL495: Please include the version of CO2SYS that was used. Please also state\nwhich sets of dissociation constants that\nwere used in the calculations.\nResponse: CO2SYSv1.1andtheK1K2constantsofRoyetal.(1993)followingthereasoning\nof Anderson et al. (2009) for the Arctic Ocean. Added to methods and the figure caption as\nrequested by reviewer 2.\nCommentL512: “European Research Commission” should perhaps be “European Research\nCouncil”?\nResponse: Corrected\nReview of the manuscript NCOMMS-25-62368, “Nuclear Waste Unveils Three Decades of An-\nthropogenic Carbon in the Canada Basin of the Arctic Ocean”, by Payne et al., submitted to\nNature Communications\nGeneral comments\nThe study utilises the spreading of the anthropogenic radionuclides I-129, U-236, and Cs-137,\nreleased from the nuclear reprocessing facilities in La Hague and Sellafield, with the northward\nflowing Atlantic Water as it circulates into the Canada Basin of the Arctic Ocean. Adopting\nthe transit time distribution (TTD) approach on these tracers the study present estimates of\nanthropogenic carbon (Cant) evolution over three decades, from 1993 to 2023. The work is\ninteresting and well-written, with clear figures. The Arctic Ocean in general and Canada Basin\nspecifically is under sampled and consequently our understanding of this region is poor, not\nthe least in terms of anthropogenic carbon evolution. Here this study is an important addition.\nMy main concern is about the presentation of uncertainties. The authors performed Monte\nCarlo simulations using measurement uncertainties and uncertainties in the TTD parameters,\nbut in addition there is an applied (or assumed?) uncertainty in the input functions of the\nradionuclides, and environmental variability. The results are typically presented as ranges, and\nerror bars are clearly seen in the figures, which may be all fine, but I am left wondering about\nthe overall uncertainty, for example presented as a fraction of the mean values? Could you\nexpand on this for clarity? Otherwise, I have no major concerns and recommend publication\nafter minor revision. Detailed comments are given below.\nResponse: Thank you for highlighting the importance of clearly describing the uncertainty\nbudget. Typically for radionuclides, we have not provided the uncertainty in the input function\non the resulting TTD parameters. However, as a separate test, by using the upper and lower\nbounds of the input function, we calculated an uncertainty range of the TTD parameters, and\nthe resulting C content (umol /kg). Averaging this across the basin for each year at 500 m\nant\nyields uncertainties of 4–12% across all years. We have clarified this in the methods (lines 570–\n582).\nSpecific comments\nComment L42–44: It has been suggested in several studies that the main uptake takes place\nalready before the AW enters the Nordic Seas. At the very least you should give a reference to\nyour statement.\nResponse: Since we added two new paragraphs in section 3.2, following the request to\ncompare inventories with other works, we went over the word limit. We have decided to cut all\nthe Nordic Seas and Atlantic parts. As a consequence, this sentence no longer exists.\nComment L45–46: The NCC is not really formed in the Nordic Seas, but earlier, fed by\nwaters from the Baltic Seas; you should in any case give some reference for this. I also question\nyour statement that the NCCW forms the surface waters in the Eurasian Arctic. I may of\ncourse be mistaken, but again, some reference makes this clearer.\nResponse: Also removed the statement (word count).\nComment L47–54: I believe you should add units to the density anomalies (in kg m−3).\nThis goes for the whole manuscript. (See also L478–479.)\nResponse: Added throughout\nrent, which could be cited here.\nResponse: Citation added.\nComment L62: Neither the Amundsen, Nansen nor the Makarov Basins are marked in the\nmap in Figure 1. This would be useful for many readers.\nResponse: Labels added in Figure 1a.\nComment L91: Didn’t also Raimondi et al. (2024) use paired gas tracers (together with\ntheir focus on radionuclides) in the Canada Basin?\nResponse: Raimondi et al. used gas tracers, but it was the previously published 2015 data\nused by Rajasakaren et al. Have updated the text to clarify that only two years of sampling\nexist with the GLODAP expo codes, and listed the publications using the data.\nComment L108: This may be more of a technical comment, but due to the focus on the\nradionuclides I believe they should be properly defined/written out the first time. Maybe this\nis not typically done, but I would suggest considering it for full clarity.\nResponse: Added\nComment. Figure 2: You may consider adding lines for the global fallout values to the\nprofiles. This may be useful as a reference and add clarity.\nResponse: Added lines to the figure and listed the references for the values in the figure\ncaption\nComment. L176: “Northwind Ridge” is not known to me, and I would suppose I am not\nalone on this. Please add a notation in the map (Figure 1a).\nResponse: Label added in Figure 1b rather than 1a to make it easier to see.\nComment. L287: It is not so easy to see detail in the section, with the applied colour\nrange/map. Perhaps this could be helped by adding some isolines in the section plot?\nResponse: Isolines added as dashed lines at 2.5 intervals from 35 to 40 umol/kg\nComment. L371–372, and L383–385: This is maybe more of a comment, but it is quite re-\nmarkable that the AW core seems so unaffected by mixing along the still rather long circulation\npath. The radionuclides have in many cases proven to be almost perfect tracers for the circu-\nlation of the AW in the Arctic Ocean. Can this lack of erosion of the core also be seen/backed\nup by T/S properties? If there would be any inconsistency in what is seen from the physical\nvariables and the radionuclides this would be an interesting observation worth mentioning.\nResponse: TracingoftheWarmTemperatureAnomalyinthelate90sintotheCanadaBasin\nperhaps is the best indication that T/S properties are conserved in the core AOBC (Shimada\net al., 2004). We have not added this to the text as we haven’t done a proper analysis T/S\nfrom the Barents Sea and Canada Basin, but we appreciate the comment and it is certainly\nsomething to look into in a future study. For this, a sentence has been added \"This agreement\nindicates almost total conservation of the tracer signal along the core of the AOBC, providing\nevidence of advective flow that is not necessarily captured in the T-S of a water mass. \" (lines\n422-424)\nComment. Figure 5: Add reference to CO2SYS in the caption, or at least refer to the\nmethod section.\nResponse: Added in both the figure caption and the methods.\nComment.Conclusions: Possibly consider writing out names the first time mentioned in the\nConclusions. This may seem like an unnecessary repeat but may be helpful to all those that\njump to this section.\nResponse: Added\nComment. L467: You write that open ocean conditions in Delta/Gamma are typically\ncloser to 2, but don’t you mean 1? Please add clear reference(s) for this.\nResponse: Agree that wasn’t clear, typical open ocean is 1 but ∆/Γ values can be as high\nas 2 (but above 1.8 uncertainties are high). Updated text and added references Waugh et al.\n(2004) and Stöven et al. (2015).\nComment. L479–481: You performed Monte Carlo simulations based on measurement\nuncertainties, but surely the uncertainty connected to the TTD method itself, and environmental\nvariability is larger than these? Please add some thoughts on this.\nResponse: This paper is a first proof that a combination of radionuclide tracers can be\nused to understand changes in anthropogenic carbon in the Arctic Canada Basin. The model\nassumes steady state in circulation to keep the approach simple. However, we are aware that\nnon-steady state conditions in circulation would add an extra uncertainty to our calculations.\nA separate paper tackling this is under preparation. For this manuscript, we would like to keep\nthe steady-state approach.\nAdded \"The input function assumes steady-state circulation. In separate tests, the ±10 %\nuncertainty of the radionuclide input functions was used to generate upper and lower bounds\non the TTD parameters, and subsequently on C . Overall uncertainty in the mean at 500 m\nant\neach year, after averaging across available stations, is 4–12%. Tests that perturbed the input\nfunction across its entire period or across a chosen time period corresponding to changes in the\nArctic Dipole (Polyakov et al., 2023), by as much as 30-40%, lead to changes in estimated C\nant\ncontent of each sample of 10-15%, although this goes beyond the scope of this paper. \"\nComment L501: I am not sure I fully understand this; you say that temperatures are\nunavailable for these data? I am not sure I am aware of any collected ocean data without\navailable temperature and salinity. If this is true (I mean, I believe what you write), how is\nthis possible?\nResponse: The 1993 137Cs and 129I data were taken from the supplementary material of\nKumamoto et al. (2024), and while salinity was listed, potential temperature was empty. I\nassumed these authors had tried to obtain the original hydrographic data. The paper in which\nthe radionuclide data were published - Smith et al. (1998), did not provide a table of data.\nTechnical comments\nL42: ...Arctic Ocean ... This is seen throughout the manuscript, and I would suggest going\nthrough the text and add “Ocean” in quite many instances.\nResponse: Added\nL61: DIC should be defined.\nResponse: Added\nL72: These latter methods include ...\nResponse: Added\nL79: ... Cant and shows increasing ...\nResponse: Corrected\nResponse: Added\nL193: Potential typo: should it be “possibly” (instead of “possible”)?\nResponse: Corrected\nFigure 3: The referred figures S5 and S6 should be S6 and S7 I believe.\nResponse: Corrected\nL239: one “and” too much (after Gerke et al. (2024)).\nResponse: Corrected\nL288: ... range from 37.6 to 46.4 ... between 35 and 47.7 ...\nResponse: Corrected\nL296: ... enter west of the ...\nResponse: Corrected\nL297: You mean “denser” instead of “deeper”?\nResponse: Corrected\nL298: ... (Fig. 4c).\nResponse: Corrected\nL303: Either “deeper than 1500 m”, or, “>1500 m”.\nResponse: Corrected\nL332: (Fig. 3e)\nResponse: Corrected\nL337: (Fig. 3c)\nResponse: Corrected\nL410: This study suggests ...\nResponse: Corrected\nL491: You here define “Cant”, which is used throughout the text, and paragraph, so this\nshould not be needed.\nResponse: Corrected\nSupplement: I believe all figures should be marked as “S1”, “S2”, and so on. Also in S4, the\npanel for 2003, there are some icons you likely want to remove on the top right.\nResponse: Corrected\nReferences\nAnderson, L. G., Jutterström, S., Hjalmarsson, S., Wåhlström, I., & Semiletov, I. P. (2009).\nOut-gassingofCO2fromSiberianShelfseasbyterrestrialorganicmatterdecomposition.\nGeophysical Research Letters, 36(20).\nBecker, M. (2023). 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(2025).\nVertical Expansion of Aragonite Undersaturated Waters in the Canada Basin of the\nArctic Ocean From 2003 to 2019. Journal of Geophysical Research: Oceans, 130(2),\ne2024JC021166. https://doi.org/https://doi.org/10.1029/2024JC021166\n11"}]

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