The Atlantic Ocean is composed by a complex vertical stratification of water masses characterized by differences in properties such as salinity, temperature and oxygen. Liu and Tanhua (2021) have systematically characterized 16 main water masses that have been extensively considered and studied due to their significant impacts on ocean circulation patterns and global climate regulation (e.g., Hall and Bryden, 1982; Bryden et al., 2005; Kuhlbrodt et al., 2007; Clark et al., 2012). Ocean ventilation constitutes a fundamental thermohaline process responsible for the vertical displacement of surface waters into the deep ocean, effectively redistributing surface heat and salinity anomalies through density-driven stratification. This process inherently obeys the principle of mass conservation, simultaneously enabling the upwelling of deep-sea waters to the surface ocean. The resultant vertical exchange plays a critical role in maintaining global heat distribution patterns and sustaining the thermodynamic equilibrium of the ocean system (Talley, 2013; Armour et al., 2016; Talley et al., 2016). Beyond its thermodynamic significance, ocean ventilation emerges as a pivotal mechanism governing marine biogeochemical cycles. By mediating the downward transport of atmospheric oxygen and carbon dioxide to abyssal depths, this process fundamentally regulates dissolved oxygen concentrations and carbon sequestration rates in the deep ocean (Tanhua et al., 2006; Ziska et al., 2013; Skinner et al., 2017). Related studies have further elucidated ventilation's deterministic role in controlling abyssal redox conditions and organic carbon remineralization processes (van Heuven et al., 2011; Ito et al., 2016; Schmidtko et al., 2017).

Early studies of the water mass ventilation are proposed by Sandström already in the early 20th century (Sandström, 1908, 1916). Sverdrup and Bjerknes also reported the impacts of ventilation and currents on the climate through the air-sea interactions (Sverdrup, 1940; Bjerknes, 1964). Hereafter many studies have been focusing on the ventilation. For instance, Dickson and Brown (1994) estimated the formation and pathway of the North Atlantic Deep Water (NADW) based on the hydrodynamics, Orsi et al. (1999) investigated the circulation of the Antarctic Bottom Water (AABW) by using Chlorofluorocarbons (CFCs). Recent studies suggest that the intensity of ocean ventilation varies with environmental factors, such as intensity of the westerly winds (e.g. Rahmstorf, 2010; Purkey and Johnson, 2010; Morrison et al., 2015).Several studies investigate the regional scale or specific water masses, for instance, the upwelling of Circumpolar Deep Water (CDW) in the Southern Ocean (Tamsitt et al., 2017), the ventilation in the South China Sea (Wang et al., 2021), the decline of deep and bottom water in the Weddell Sea (Huhn et al., 2013), and changes in the North Atlantic over the past three decades (Guo et al., 2026). In parallel, advances in ocean modeling have provided new insights into water mass transformation and tracer transport on basin to global scales, offering valuable perspectives on the climatic drivers of ventilation variability (e.g., Li et al., 2023). However, few studies look at the ventilation over ocean basin scales, which is helpful to understand the effect of ocean-climate interactions, and in addition can provide a basis for the biogeochemical studies.

The water mass age is an important parameter to evaluate the ventilation and refers to as the elapsed time on the pathway. The CFCs and Sulfur hexafluoride (SF₆) are recognized as effective transient tracers for water masses and to estimate their ages, i.e. the time since the water left the surface ocean (Fine, 2011). The concentrations of CFC-12 in surface seawater increased continuously following the increase of the atmospheric concentration since their introduction in the early 20th century (Gammon et al., 1982; Warner and Weiss, 1985) up to about the turn of the century. The use of CFCs as a tracer in the oceanography is well established (e.g. Bullister and Weiss, 1983; Doney and Bullister, 1992; Fine, 2011), and transient tracers is a core variable in the GO-SHIP repeat hydrography program. Similarly, the transient tracer is an Essential Ocean Variable (EOV) in the Global Ocean Observing System (GOOS) framework. SF₆ has been used as a new tracer since mid-1990s complementing the CFCs due to the reduction of CFCs in the atmosphere (Maiss et al., 1996; Bullister et al., 2002). The application of SF₆ is generally appropriate for recently formed water masses in the upper ocean (Tanhua et al., 2008), i.e. for well ventilated, or young, waters. The CFCs are used to trace the relative deeper water masses (before mid-1990s or partial pressure of CFC-12 lower than about 450 parts per trillion, ppt), while the SF₆ is the better choice for recently formed shallow water masses (after mid-1990s or partial pressure of CFC-12 higher than about 450 ppt, Tanhua et al., 2008). The complementary nature of CFCs and SF₆ allows for more accurate tracing of water masses (Vollmer and Weiss, 2002; Tanhua et al., 2004, 2005; Bullister et al., 2006). In addition, the application of transient tracers also provides support to the hydrographic and biogeochemical field in calculating the upwelling velocity (Tanhua and Liu, 2015) or estimating the ventilation and anthropogenic carbon cycle (van Heuven et al., 2011; Tanhua et al., 2013; Patara et al., 2021).

As two typical representatives of transient tracers, the CFC-12 and SF₆ are commonly employed for determining water mass ages, verifying hydrological models, and estimating rates of marine biogeochemical processes, such as anthropogenic carbon storage and oxygen consumption. They exhibit advantages in tracing upper and middle water masses with relatively younger ages. However, challenges arise when attempting to obtain data from deep and bottom layers with old waters. The chemically inert isotope argon-39 (³⁹Ar), which has a half-life of 269 years, addresses this gap. ³⁹Ar is particularly advantageous for tracing deep and bottom water masses with ages exceeding 500 years (Broecker and Peng, 2000; Holzer and Primeau, 2010; Lu et al., 2014). Initially, its application was constrained by the requirement for large sampling volumes – typically around 1000 liters – and the limitations of low-level counting (LLC) detection methods (Loosli, 1983; Schlosser et al., 1994; Schlitzer et al., 1985). Consequently, ³⁹Ar was not widely utilized in oceanographic tracer studies. With advancements in Atom Trap Trace Analysis (ATTA) technology (Chen et al., 1999; Jiang et al., 2012), ³⁹Ar has gradually gained prominence as an effective tracer for investigating deep and bottom waters (Lu et al., 2014; Ebser et al., 2018), the accuracy and stability have also been significantly improved (Jia et al., 2025). Therefore, the multi-parameter combination of CFCs, SF₆, and ³⁹Ar can transcend the temporal limitations associated with single-tracer dating methodologies while providing more comprehensive insights into biogeochemical processes. In this study, we introduce ³⁹Ar to investigate water mass ages within the Atlantic Ocean, in particular for the older waters.

The apparent oxygen utilization (AOU, µmol kg⁻¹) is an important indicator in characterizing the respiration of organic matter consuming oxygen, and balancing the oxygen supply through ventilation and photosynthesis. The oxygen utilization rate (OUR, µmol kg⁻¹ yr⁻¹), which can be calculated from AOU and water mass age, is a significant parameter in estimating the biogeochemical processes such as primary productivity or remineralization (e.g. Jenkins, 1982; Bender, 1990). The OUR has been recognized to be a function of pressure for a long time (e.g. Tseitlin, 1992). However, the OUR is the integrated oxygen consumption rate along the pathway of a water mass and represents a regional view of export flux (e.g. Stanley et al., 2012; Koeve and Kähler, 2016), and thus an imperfect measure of local OUR. In this study, the OUR is calculated from water mass ages estimated from the CFC-12 and SF₆. The change of OUR over time in different water masses are presented and the impacts from the currents and topography are discussed.

In the study by Liu and Tanhua (2021), the characteristics of main water masses in the Atlantic Ocean are defined by six key properties, using the Global Ocean Data Analysis Project version 2 the annual update 2023 (GLODAPv2.2023, Lauvset et al., 2016; Olsen et al., 2016; Lauvset et al., 2024) data product as a source of unbiased data. The static distribution of the water masses is estimated with the Optimal Multi-Parameter analysis (OMP analysis, Karstensen and Tomczak, 1997, 1998). As a continuation of that work, here we estimate the ventilation of water masses in the Atlantic Ocean with the combination of the above mentioned three transient tracers (CFC-12, SF₆ and ³⁹Ar). The water mass ages are estimated and the OURs are calculated. The goal of this study is to further improve the report of water masses in the Atlantic Ocean by adding the age as a measure of ventilation, and to provide a hydrographic assistant for the biogeochemical researches.

In this article, we start in Sect. 2 with an overview of the data sources and methodologies employed. Section 3 presents the core results, illustrating the spatial distributions of mean and mode ages for the principal Atlantic Ocean water masses, stratified from surface to bottom. In Sect. 4, the derived water mass ages are applied to estimate Oxygen Utilization Rates (OUR) and to examine associated biogeochemical implications. Section 5 offers a comparative analysis of water mass ages obtained from traditional transient tracers (CFC-12 and SF₆) and the radioisotope ³⁹Ar. Finally, Sect. 6 provides a comprehensive summary of conclusions, discusses the broader relevance of the findings, and acknowledges the limitations of the present study.

In this study, we follow the division of vertical layers and distributions of water masses presented by Liu and Tanhua (2021), but now focus on the transient tracers and water mass ages. Three hydrographic sections are selected from the WOCE/GO-SHIP sections in this work to represent the mode-ages of the main water masses (Table 1). The A16 sections in 2013 (Expo-code 33RO20130803 and 33RO20131223) show the meridional distribution across the whole Atlantic Ocean, while the A05 section in 2010 (Expo-code 74DI20100106) and A10 section in 2011 (Expo-code 33RO20110926) show the zonal distributions in the North and South Atlantic Ocean respectively (Fig. 4). The partial pressures of CFC-12 and SF₆ along the above sections are shown in Fig. 4 (Only CFC-12 data are displayed along A05 section due to the lack of SF₆ data in this section). Both tracers show similar distributions with high values in the shallow layers, and a general decrease with increasing pressure. As detailed in Sect. 2.3, under our fixed mixing ratio (Δ/Γ=1) the mean- and mode-ages are linearly related; we therefore show only mode-age sections in the following figures (mean-age sections are provided in the Supplements).

The upper water layers have the lowest mode-ages within ∼ 30 years (Fig. 5), the surface water has, by definition, zero age. The distributions of mode-ages also show spatial differences in the horizontal direction. Relatively low ages are found in the high latitude regions. This indicates that the deep water masses are newly formed here, and thus have low ages. In the region between 20 and 40° N, the mode-age reaches the peak value up to ∼ 150 years, suggesting the sluggish water exchange and long residence time of old water masses (Fig. 5a). In the zonal direction, the mode-ages are significantly lower in the west in the general region below 1500 m dbar, suggesting that these regions are better ventilated due to the western boundary current (Fig. 5b and c).

In the analysis of water mass ages in each layer, maps with mode-ages in each station are firstly presented as a qualitative overview. The average values of the mode-ages at the core pressure of each water mass are plotted at certain station. In addition, the quantitative calculations are made along the three selected sections to represent the distributions of water mass ages in detail and estimate the impacts from currents and mixing.

The transit time distribution (TTD) provides a framework for using transient tracers to estimate the time scales governing ventilation, which in turn can be applied to quantify biogeochemical rates. For instance, based on the TTD determined from the tracer observations, Wang et al. (2021) estimated the OUR in the northern South China Sea.

The oxygen concentration in seawater under steady state is maintained by the balance of supply from oxygen-rich surface water and consumption by the respiration of organic matter. The Apparent Oxygen Utilization (AOU) shows the accumulated oxygen consumption in a water mass since isolated from the atmosphere, and is defined as the difference between saturated and measured oxygen concentration (i.e. AOU = Oxy_saturated - Oxy_measured, Redfield, 1942; Redfield et al., 1963; Pytkowicz, 1971). The Oxygen Utilization Rate OUR (µmol kg⁻¹ yr⁻¹) can be calculated from AOU and water mass age and shows the integrated oxygen utilization rate (oxidation rate) during the pathway of a water mass from surface of the formation area to the sampling location (Jenkins, 1987; Doney and Bullister, 1992; Sonnerup et al., 2013). In addition, the OUR is also an important indicator in characterizing the combination of ventilation and respiration of the interior ocean (Koeve and Kähler, 2016; Thomas et al., 2020).

The true local OUR is difficult to determine because water samples contain a mixture of paths with different transit times (Tomczak, 1999; Koeve and Kähler, 2016). Researchers have tried a variety of ways to define the age of a water mass. For instance, the tracer-age (CFC age) is used in Karstensen et al. (2008), while the mean-age (partial pressure ages) is used in Sonnerup et al. (2015) in estimating the “integrated” water mass age that effects the oxygen consumption. There is no direct evidence that one age definition is theoretically superior for OUR calculations; both the mean-age and the mode-age are approximations. The mean-age (Γ) averages over all pathways, including old water fractions that have experienced very low respiration rates in the deep ocean; using it would tend to lower the estimated OUR. The mode-age, in contrast, represents the most probable transit time of the dominant water fraction and is often interpreted as the advective timescale. As a working assumption, we adopt the mode-age to calculate OUR, reasoning that it may better reflect the respiration accumulated along the core pathway of the water mass. However, we emphasize that the resulting OUR values are indicative rather than absolute. If the mean-age were used instead, the computed OUR would be roughly six times lower (given τmode=Γ10-3)≈0.162Γ), bracketing the plausible range. Our choice of mode-age likely provides an upper-end estimate of the respiration rate for the dominant water mass. For example, if we use the mean-age instead of the mode-age for the NADW core in the western basin, the OUR would decrease from ∼ 2 to ∼ 0.3 µmol kg⁻¹ yr⁻¹. Both values are within the range reported in the literature (e.g., Jenkins, 1987; Stanley et al., 2012), highlighting the uncertainty associated with the choice of age metric.

Water masses with mode-ages larger than ∼ 50 years in some parts of their distribution, including the AAIW, NADW, AABW and NEABW in the Atlantic Ocean, are defined here as wide-spread water masses. The transports of these wide-spread water masses are significantly influenced by the currents and topography. Therefore, their mode-age shows regional differences. Such differences also exist in DO and AOU, and lead to the spatial differences in OUR, especially between the eastern and western basins (Fig. 14). In the meridional direction, the mode-age increases with the distance from formation area. In the zonal direction, significant differences exist between the east and west of MAR. The wide-spread water masses generally have lower mode-ages in the western basin, which is better ventilated by the WBC and the DWBC (Fig. 14, left). In addition, surface oxygen-rich water is transported by these water masses to the deeper layers and are gradually consumed during the path-way. As a result, the DO is highest in the formation area and decreases with the distance. Meanwhile, the western basin displays higher DO due to the better ventilation (Fig. 14, middle left). The OUR shows a similar distribution to DO indicating that higher oxygen consumption rate, or oxidation rate is co-located with the regions with high oxygen concentration. At the same time, Fig. 14 also shows that the OUR approaches 0 when the mode-age is larger than ∼ 50 years.

In the intermediate layer, the AAIW show low mode-ages, high DO and low AOU in the formation area near the Antarctic region. During the northward transport between 40° S and 20° N, the mode-ages increase and the DOs decrease with distance from the formation area (Fig. 14, upper panel). In general, the mode-ages are lower in the west, while the zonal difference for DOs is not significant except for some regions in the eastern basins. The mode-ages are mostly between ∼ 60 and ∼ 90 years and reaches up to ∼ 120 years in the poor ventilated eastern basin. The DOs are between 160 and 220 µmol kg⁻¹ in the south hemisphere and mostly between 100 and 160 µmol kg⁻¹ in the north hemisphere. The OURs in the Antarctic region obtain the highest value in the latitude region between 20 and 40° S and decrease on the pathway to the north until 20° N.

In the deep and overflow layer, the NADW is newly formed in the high north latitude. In most regions covered by the NADW between 30° N and 40° S, the differences between west and east exist with obvious low mode-age and AOU (high DO) and in the west, where is well ventilated by the DWBC (Fig. 14). The mode-age is between ∼ 30 and ∼ 60 years in the west and between ∼ 90 and ∼ 150 years in the east. The AOU is lower in the west and higher in the east. The OUR, which shows the opposite distribution as the AOU in the zonal direction, obtains the highest value in the formation area of NADW, maintains relative higher in the west along the DWBC region and shows a lower value in the eastern basin.

The bottom layer is occupied by the AABW and NEABW in the south and north hemisphere respectively. The AABW is formed in the Antarctic region with an original mode-age lower than ∼ 30 years and increases on the way to the north. Similar to the AAIW and NADW, the western basin is better ventilated, therefore the mode-ages are lower in the west at the same latitude (Fig. 14, lower panel). In contrast to the AAIW and NADW, the AABW obtains relative lower DOs and higher AOUs when formed (see also Fig. 15b). In the western basin of the South Atlantic, the DOs decrease to 220 µmol kg⁻¹ and the AOUs increase to 130 µmol kg⁻¹ on the pathway, while in the eastern basin, the DOs are 240 µmol kg⁻¹ and the AOUs are 110 µmol kg⁻¹. This is because the eastern basin is more influenced by the relative oxygen-rich NADW from the north. Similarly, the DOs and AOUs of NEABW in the North Atlantic also show similar states to the NADW. In west regions ventilated by the DWBC, the DOs reach the values of higher than 260 µmol kg⁻¹ and AOU is lower than 70 µmol kg⁻¹. This result can also be confirmed in Fig. 15. Although the distributions of DOs and AOUs are more complex in the bottom layer, the OURs still match the distribution of mode-ages. The highest OURs appears in the formation area near the Antarctic region with values higher than 4 µmol kg⁻¹ yr⁻¹. In the western Atlantic region with low mode-ages, the OURs show relative higher values (> between 1 and 3 µmol kg⁻¹ yr⁻¹) and low value (below 1 µmol kg⁻¹ yr⁻¹) in the east (Fig. 14, lower panel).

The above four properties are also represented along the A16 section (Fig. 15). The AAIW has the lowest mode-age among the three water masses, but also the largest variations in the DO and AOU. The mode-age indicates that in total of ∼ 40 to 60 years is needed for the main body of AAIW (fractions > 80 %) to be transported to the equator (Fig. 15a). The DO decreases rapidly from 260 to below 180 µmol kg⁻¹ at the oxygen minimum zone at 20° S, while the AOU increases from 60 to 150 µmol kg⁻¹ during the northward pathway (Fig. 15b and c). The OUR is higher than 8 µmol kg⁻¹ yr⁻¹ at the beginning and decreases to 3 µmol kg⁻¹ yr⁻¹ close to the equator (Fig. 15d). The mode-age of NADW is ∼ 140 years at 40° S and reaches a peak of ∼ 160 years in the region of 20° N due to the sluggish water exchange caused by the topography. The NADW contains the highest oxygen concentration and the lowest AOU and OUR in the abyssal Atlantic Ocean. The value of DO is 260 µmol kg⁻¹ at the starting point at 60° N and still remains 220 µmol kg⁻¹ at 40° S where it meets the AABW and AAIW. The AOU of NADW remains below 100 µmol kg⁻¹ during the entire pathway. The OUR of NADW is below 5 µmol kg⁻¹ yr⁻¹ close to the formation area, and below 1 µmol kg⁻¹ yr⁻¹ south of 60° N. The transport of the main body of AABW takes in total of ∼ 100 years from the Weddell Sea to the equator. The DO and AOU remain unchanged at 220 and 130 µmol kg⁻¹ respectively on the pathway to the equator, indicating very low OUR. Although derived from the AABW, the characteristics of the NEABW are more similar to the NADW in all the above properties.

The radioactive isotope ³⁹Ar (half-life 269 years) is a valuable tracer for dating old water masses. The ³⁹Ar data used in this study are compiled from the doctoral thesis of Rodriguez (1993), which tabulates 125 measurements collected during the 1980s and early 1990s in the Atlantic Ocean using low-level decay counting (Loosli, 1983; Schlosser et al., 1994). More recent ³⁹Ar data from ATTA technology (e.g., Lu et al., 2014; Ebser et al., 2018) are not yet available in sufficient numbers for basin-scale synthesis. Therefore, our analysis relies on the Rodriguez (1993) dataset. We note that these ³⁹Ar measurements were taken approximately two decades before most GLODAPv2 CFC-12/SF₆ data (which are from the 2000s and 2010s). This temporal mismatch, together with the assumption of steady-state circulation, introduces additional uncertainty into the comparison. Nevertheless, the paired analysis presented below (Fig. 18) is based on spatially collocated stations, recognizing that the ventilation state may have changed over the intervening decades. For ³⁹Ar we focus our attention on older water masses, as younger ones are better characterized by CFC-12 and SF₆. Because this study adopts a fixed mixing ratio (Δ/Γ=1), the ³⁹Ar-derived ages, which physically represent mean residence times, are converted to mode-ages by multiplying by 0.162 for consistent comparison with the CFC-12/SF₆-derived mode-ages. All ages reported in this section are therefore mode-ages.

For this purpose, we make a distinction between the western (green dots) and eastern (red dots) Atlantic, divided by the Mid-Atlantic-Ridge (Fig. 16a). The distribution of ³⁹Ar is shown in Fig. 16b. Near the formation area of each wide-spread water mass, the concentration of ³⁹Ar approaches approximately 100 %, indicating that the surface waters are in equilibrium with the atmosphere. As these water masses are transported over long distances and extended periods, the continuous decay of ³⁹Ar leads to concentrations decreasing to roughly 30 % at their furthest locations. By integrating this information with its half-life of 269 years, the water mass ages can be estimated. For ³⁹Ar we focus our attention to the older water masses, as the younger ones are better characterized by CFC12 and SF₆.

Similar to the findings from CFC-12 and SF₆ observations, the mode-ages derived from measurements using ³⁹Ar increase with transport from the source of formation (Fig. 17). At a latitude of approximately 40° S, the AAIW exhibits mode-ages ranging between ∼ 16 to ∼ 32 years (the corresponding mean-ages are ∼ 100–200 years). When transported northward to about 20° S, the mode-ages rise to ∼ 60 years (mean-age ∼ 300 years). Upon reaching approximately 10° N during further northward transport, these values escalate to around ∼ 100 years for mode-age (∼ 700 years for mean-age). However, near the region at about 20° N, the terminus for northward flow, the mode-age decreases back down to roughly ∼ 65 years (mean-age ∼ 400 years) due to mixing with younger upper NADW. In the deep and overflow layer, the newly formed NADW initiates at high northern latitudes, exhibiting both mean- and mode-ages approaching zero. During southward transport at around 30° S, the concentration data indicates corresponding mode-ages of approximately ∼ 65 years (mean-age ∼ 400 years). Similarly, the bottom water masses (AABW and NEABW) display mode-ages of around ∼ 80–100 years (mean-age ∼ 500–600 years) at the farthest end of the transport distance approximately 50° N.

The contrast between the western and eastern Atlantic Ocean is distinctly evident (Fig. 16b). Generally, higher concentrations of ³⁹Ar are observed in the west compared to the east at equivalent latitudes. For instance, in the vicinity of the AAIW formation area at 40° S, the ³⁹Ar concentration is approximately 100 % in the west and around 70 % in the east. In the intermediate layer near the equator, these concentrations decrease to about 70 % (west) and 50 % (east). These findings suggest that mode-ages are close to zero at 40° S. The mode-age is estimated to be ∼ 50 years in the west and ∼ 80 years in the east north of 40° S. The mean-age is estimated to be ∼ 300 years in the west and ∼ 500 years in the east north of 40° S. Similar results are also observed in the deep water and overflow layers. Based on elevated levels of ³⁹Ar near its formation area, it can be inferred that both mean- and mode-ages of NADW are close to zero at 60° N across both West and East Atlantic Oceans. Between 20° N and the equator, the mode-age is estimated to be ∼ 50 years in the west and ∼ 80 years in the east, respectively. For bottom water masses below a depth of 4000 m, concentrations of ³⁹Ar generally fall below 50 %, with higher levels observed in the west (50 %–60 %) compared to lower levels seen in the east (40 %–50 %) between 20° S and 40° N, indicating the mode-ages range from ∼ 65–80 years in the west, and ∼ 80–100 years in the east.

Linear regression of the 22 paired points yields: ³⁹Ar mode-age = 0.92 × (CFC-12/SF₆ mode-age) + 2.5 years (R2=0.98). The 1:1 reference line (dashed) is crossed at a CFC-12/SF₆ mode-age of ∼ 31.25 years, corresponding to a ³⁹Ar mode-age of the same value. This crossing point (∼ 30 years) marks the threshold below which the two methods agree within uncertainties. For mode-ages > ∼ 30 years, CFC-12/SF₆ ages are systematically higher than ³⁹Ar ages, as indicated by the slope of 0.92, suggesting overestimation by CFC-12/SF₆ in deep, old waters due to tracer concentrations approaching detection limits. For younger waters (< 30 years mode-age), the two methods agree within uncertainties, although for such young waters ³⁹Ar is not an ideal tracer. The high R2 confirms overall consistency, while the offset at high ages demonstrates the complementary value of ³⁹Ar for dating waters on centennial-to-millennial timescales. We note that the comparison assumes steady-state ventilation over the ∼ 20-year temporal mismatch between the two datasets; changes in circulation over that period may contribute to the bias.

As expected, the inferred ages of water masses differ depending on the tracer employed. These discrepancies arise not only from potential limitations in the assumed IG-TTD, but also from the distinct temporal validity ranges and detection limits of each tracer. Our comparative analysis (as summarized in Table 2 and Fig. 18) reveals a systematic pattern: mean- and mode-ages derived from ³⁹Ar are generally lower than those estimated from CFC-12 and SF₆ for deeper/older water masses. This is exemplified by the NADW in the eastern basin near 20° N, where CFC-12/SF₆ yield a mean-age of ∼ 1000 years and a mode-age of ∼ 150 years, whereas ³⁹Ar suggests younger ages of ∼ 600 and ∼ 90 years, respectively. Similarly, for AABW/NEABW near 50° N, CFC-12 and SF₆ based estimates (∼ 800 years mean, ∼ 120 years mode) exceed those from ³⁹Ar (∼ 500 years mean, ∼ 100 years mode). A likely explanation for this systematic tendency towards higher age estimates from CFC-12 and SF₆ in deep and bottom layers might be attributed to their extremely low concentrations at great depths and for older waters. These concentrations often approach or fall below analytical detection limits, which can amplify uncertainties and appear to skew age calculations towards higher values. In contrast, the ³⁹Ar with 269-year half-life appears to be better suited for dating waters with transit times on the order of centuries to a millennium, and thus could provide relative more robust age estimates for these older water masses, as its slower decay rate remains measurable over such longer timescales.

The geographic distributions of main water masses in the Atlantic Ocean are reported in Liu and Tanhua (2021). In the present work, the transports of water masses are traced by transient tracers (CFC-12, SF₆and ³⁹Ar) and their ventilation timescales (water mass ages) are quantified. Three distinct concepts of water mass age are discussed within the analytical framework. While the tracer-age concept (assuming purely advective transport) is introduced for context, the quantitative analysis and results presented herein are based on the mean-age and mode-age derived from the Transit Time Distribution (TTD) model. The application of a standard mixing ratio (Δ/Γ=1) and 100 % surface saturation is followed. The mean-age, representing the average transit time of all water parcels in a sample and providing an integrative timescale, is used to characterize the overall ventilation status. The mode-age, representing the most probable transit time and serving as a proxy for advective timescales, is applied to trace the dominant transport pathways and to estimate process rates like the oxygen utilization rate (OUR).

In general, the water mass age increases with the depth and distance from the formation area, so the central waters in the upper layer obtain the lowest ages of within ∼ 100 years. In the intermediate layer, the AAIW and MW are two conspicuous water masses and both take ∼ 300 years to spread from south to north (AAIW) and east to west (MW) respectively. As the dominant water mass in the deep and overflow layer, the NADW takes ∼ 500 years to spread along the DWBC in the west Atlantic Ocean, and takes ∼ 900 years eastward to cover the east part. The bottom waters origin from south of Antarctic Circumpolar Current region take ∼ 600 years to spread northward and cover the bottom layer. Similar as the NADW, the mean-age of both AABW and NEABW show spatial difference between the west (low mean-age) and the east (high mean-age). This is because the west Atlantic Ocean is well ventilated by the DWBC while the water exchange is sluggish in the east.

The transient tracers (CFC-12, SF₆ and ³⁹Ar), serve as effective tools for identifying and quantifying oceanographic ventilation, each tracer possessing distinct advantages and limitations in determining and tracing the depths and mean ages of water masses. CFC-12 and SF₆ exhibit optimal applicability for water masses range within 2000 dbar pressure (or mean-ages <∼ 500 years). However, their utility becomes constrained in deeper aqueous regimes (below 2000 dbar or mean-age >∼ 500 years) due to inherent limitations in tracer concentration sensitivity, which may induce systematic overestimations of water mass ages when applied to deep or bottom layers. The chemically inert radioisotope ³⁹Ar, characterized by a 269-year half-life, effectively addresses this observational gap through its unique temporal resolution. Consequently, the synergistic application of SF₆, CFCs, and ³⁹Ar in multi-tracer frameworks overcomes temporal constraints inherent to single-tracer approaches, enabling comprehensive investigation of coupled biogeochemical-climate interactions. This integrated methodology facilitates novel insights into climate-driven water mass reorganization, carbon cycle dynamics, and other critical marine processes essential for understanding oceanic responses to global change.

The ventilations and respiration rates (oxidation rates, OUR) of wide-spread water masses are estimated by the mode-ages. The currents and topography have significant impacts on the mode-ages and furthermore the apparent oxygen utilization rate (OUR). The western basins of the Atlantic Ocean are better ventilated with lower the mode-ages, while the OURs show the opposite distribution, suggesting that the mode-age is a more important determinant of OUR than AOU, and that higher respiration rates exist in better-ventilated regions.

The quantitative age framework and ventilation rates established in this study provide a crucial baseline dataset. This baseline is essential for detecting future changes in ocean circulation and biogeochemical cycles, and will serve to validate the next generation of climate and ocean models.

This study provides a quantitative, basin-scale assessment of Atlantic water mass ages using a multi-tracer approach. However, several methodological assumptions should be considered when interpreting the results. First, the calculated ages rely on the assumption of a steady-state circulation and an Inverse Gaussian Transit Time Distribution (IG-TTD) with a fixed mixing ratio (Δ/Γ=1). While this is a necessary and common simplification for basin-scale analysis, regional deviations from this idealized transport are likely. Consequently, the mean-age and mode-age presented in this study are linearly related (τmode≈0.162Γ) and do not provide independent statistical information; they are shown together solely for conceptual comparison of their different physical interpretations (integrative vs. advective timescales). Second, the complementary use of tracers (CFC-12, SF₆, ³⁹Ar) mitigates but does not fully eliminate the inherent uncertainties associated with each method, particularly in deep layers where tracer concentrations are very low. Third, the apparent oxygen utilization rates (OUR) derived here represent integrated rates along flow paths and are subject to the same age estimation uncertainties. Moreover, the choice of using mode-age for OUR calculations is a working assumption; the true local OUR remains difficult to constrain. Fourth, the ³⁹Ar data used for comparison originate from the 1980s–1990s (Rodriguez, 1993), whereas the CFC-12/SF₆ data are from later cruises (predominantly 2000s–2010s). The comparison thus implicitly assumes a steady-state circulation and ventilation, which is a simplification given observed multidecadal changes in the Atlantic Ocean (e.g., Talley et al., 2016). This temporal mismatch introduces additional uncertainty into the age comparisons, particularly for older water masses where ventilation may have varied over the intervening decades (e.g. Guo et al. 2026).

Notwithstanding these limitations, the comprehensive age and ventilation maps presented here establish a baseline of the contemporary Atlantic Ocean ventilation structure. This dataset can serve to validate and constrain the next generation of ocean circulation and biogeochemical models. Furthermore, by quantifying current ventilation timescales and their spatial patterns, this work provides a reference frame against which future observational studies can detect changes in ocean circulation and ventilation in response to climatic forcing.