We use a 30‐year time series (1986–2016) of dichlorodifluoromethane (CFC‐12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (C_ant) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF₆) in newly‐formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time‐varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC‐12 and SF₆ in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave C_ant concentrations ∼7 μmol kg⁻¹ larger than estimates obtained assuming a constant saturation of 100%. The resulting C_ant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of C_ant increased at an average rate of 1.8 mol m⁻² y⁻¹ over the 30‐year period. However, the accumulation rate of C_ant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the C_ant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non‐steady C_ant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of C_ant dynamics.

In this study we investigated the physical characteristics of the Atlantic layer in the Arctic Ocean (AO) and its role in the distribution and storage of anthropogenic carbon (C_ant). The novelty of this work is to use the Transit Time Distribution method (TTD) with the radionuclides ¹²⁹I and ²³⁶U and its comparison to the commonly applied gas tracers, CFC‐12 and SF₆. Overall, our examination of two distinct tracer pairs, along with the novel TTD method in comparison to a classical approach, revealed a notable agreement, highlighting the robustness of these C_ant estimates. The TTD analysis based on radionuclides showed that whereas the Eurasian Basin has shorter transit times and is dominated by strong mixing conditions, the Amerasian Basin is characterized by longer transit times and a strong advective flow. Overall, the C_ant concentrations obtained from radionuclides confirm that the distribution in the AO follows its circulation patterns, with higher concentrations in the Eurasian Basin (∼50 μmol kg⁻¹) compared to the Amerasian one (∼36–42 μmol kg⁻¹). An estimated partial inventory of 0.85 ± 0.17 and 1.0 ± 0.03 Pg C was assessed for 2015 from the novel application of TTD with radionuclides and gas tracers, respectively. Finally, we identified the saturation of gas tracers as a larger source of uncertainty for C_ant estimation compared to the uncertainty associated to different radionuclides' input functions, thus remarking the importance of including non‐saturation dependent tracers, such as radionuclides, as an additional tool to support C_ant estimates in the AO.

At present, it is well-known that the fast increase in atmospheric carbon dioxide (CO2) concentrations resulting from human activities (Cant), drives the dramatic changes observed in our environment such as global warming and ocean acidification. The Arctic Ocean has been identified as one of the fastest-changing regions of the world ocean and can therefore be considered as a sentinel for future global scenarios.Here, Cant-rich waters coming from the Atlantic Ocean become isolated from the atmospheric input of CO2 as they flow at an intermediate depth below the mixed layer, making the Arctic Ocean a key region for intermediate-to-long-term storage of Cant. Despite having such an important role, the magnitude of the Cant inventory and its change over time in the region is yet not fully understood, particularly if we are to consider future changes in ice coverage and therefore ocean circulation.A way of estimating oceanic Cant inventories is by applying the so-called Transit Time Distribution (TTD) method, which implies the use of transient tracers such as the anthropogenically produced CFC-12 and SF6.In this work we present a new estimate of Cant inventory for the Arctic Ocean in 2015 assessed with the TTD method using both well-established tracers (CFC-12 and SF6, both having a global source) as well as novel ones (anthropogenic radionuclides 129I and 236U, both having primarily a point-like source represented by European nuclear reprocessing plants, as well as a global one represented by the global fallout from nuclear bomb testing).The TTD was here applied following a relatively novel approach to infer the statistical parameters that describe the age distribution within a water sample, the mean (G) and the width (D). Unlike the “classical TTD” approach, the one used in this study allows the statistical parameters of the TTD to be constrained for each individual sample rather than finding values that are most representative of the region and time studied. We first show a comparison of the two TTD approaches by comparing mean and mode ages as well D/G ratios of this study (new TTD method) to those presented in Rajasakaren et al. 2019 (classical TTD method), using CFC-12 and SF6 as our tracers’ pair. We then compare TTD results obtained from the two tracers’ pairs, CFC-12/SF6 and 129I-/236U, using the new TTD method.Finally, we estimate and compare Cant concentrations and inventories obtained with the two pairs of transient tracers to one-another as well as to previous estimates of Cant in the region by Rajasakaren et al (2019) obtained with the “classical TTD”. This study demonstrates for the first time the feasibility of using anthropogenically produced radionuclides with input functions and chemical properties different than CO2 as proxies for Cant estimates.

This study investigates the spatial and temporal distribution of the artificial radionuclides ¹²⁹I and ²³⁶U in the Western Mediterranean Sea, focusing on their connection to radionuclide sources and circulation dynamics. Taking advantage of unprecedented precision of accelerator mass spectrometry, both tracers were firstly investigated in 2013. Here, we examine tracer observations obtained along four stations (re-)visited during the TAlPro2022 expedition in May 2022. Distributions of both ¹²⁹I and ²³⁶U were related to water masses and clearly linked to local circulation patterns: a tracer-poor surface Atlantic inflow, a thining of the tracer minimum at intermediate depths, and a higher tracer signal in Western Mediterranean Deep Waters due to dense water formation in the Algero-Provençal basin. The comparison to 2013 tracer data indicated recent deep ventilation of the Tyrrhenian Sea, the mixing of deep waters and enhanced stratification in intermediate waters in the Algero-Provençal basin due to a temperature and salinity increase between 2013 and 2022. We estimate an overall ¹²⁹I increase of 20% at all depths between 0 and 500m with respect to 2013, which is not accompanied by ²³⁶U. This suggests either the lateral transport of ¹²⁹I from the Eastern Mediterranean Sea, or an additional source of this tracer. The inventories of ¹²⁹I calculated for each water mass at the four stations point to the deposition of airborne releases from the nuclear reprocessing plants (La Hague and Sellafield) on the surface Mediterranean waters as the more likely explanation for the ¹²⁹I increase. This work demonstrates the great potential of including measurements of anthropogenic radionuclides as tracers of ocean circulation. However, a refinement of the anthropogenic inputs is necessary to improve their use in understanding ventilation changes in the Mediterranean Sea.