# Dissolved inorganic Radiocarbon content of the Western Coral sea: Implications for Intermediate and Deep Water Circulation

Aymeric Pm Servettaz, Yusuke Yokoyama, Shoko Hirabayashi, Markus Kienast, Yosuke Miyairi, Mahyar Mohtadi

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## To cite this version:

Aymeric Pm Servettaz, Yusuke Yokoyama, Shoko Hirabayashi, Markus Kienast, Yosuke Miyairi, et al. Dissolved inorganic Radiocarbon content of the Western Coral sea: Implications for Intermediate and Deep Water Circulation. Radiocarbon, 2019, 61 (6), pp.1685-1696. 10.1017/RDC.2019.122 hal-04238788

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# DISSOLVED INORGANIC RADIOCARBON CONTENT OF THE WESTERN CORAL SEA: IMPLICATIONS FOR INTERMEDIATE AND DEEP WATER CIRCULATION

Aymeric PM Servettaz<fnref n="1" />,<fnref n="2" />* • Yusuke Yokoyama<fnref n="2" />,<fnref n="3" /> • Shoko Hirabayashi<fnref n="2" />,<fnref n="4" /> • Markus Kienast<fnref n="5" /> • Yosuke Miyairi<fnref n="2" /> • Mahyar Mohtadi<fnref n="6" />

(Received 30 September 2018; revised 09 August 2019; accepted 16 August 2019)

**ABSTRACT.** The South Pacific Ocean contributes to the global carbon cycle by exchanging CO₂ between the atmosphere and intermediate to deep water masses. The path of the Antarctic Intermediate Water (AAIW) in the South Pacific gyre has been inferred from salinity, oxygen, and nutrient measurements, but radiocarbon (¹⁴C) measurements—a direct tracer of the carbon cycle—remain sparse. Here, we present the first radiocarbon profiles in the western Coral Sea and compare our measurements with South Pacific stations from GLODAPv2, a database of ocean hydrochemistry. Surface and subsurface waters in the Coral Sea cannot be attributed to a single source based on their Δ¹⁴C signatures, and we observe a penetration of bomb-produced ¹⁴C. AAIW in the western Coral Sea shows Δ¹⁴C values comparable to those in the South Pacific gyre, consistent with circulation of AAIW in the lower part of the southern equatorial current. The deep waters of the western Coral Sea have significantly higher ¹⁴C than the South Pacific at the same isopycnal, consistent with a northward intrusion of Circumpolar Deep Water from the Tasman Sea, along with a westward influx of deep waters from the Central Pacific. In accordance with silicate concentrations published previously, this shows the dual origin of deep waters in the Coral Sea.

**KEYWORDS:** bomb-produced radiocarbon, deep water circulation, oceanography.

## INTRODUCTION

In the context of global warming due to radiative forcing of greenhouse gases, many studies of the carbon cycle have substantiated the primary role of the ocean in rapid CO₂ uptake, in turn lowering the atmospheric concentration (Brewer 1978; Sundquist 1993; Sabine et al. 2004; Volk and Hoffert 2013, among others). Further, variable CO₂ partitioning between the atmosphere and ocean reservoirs has been implicated in past and present climate variations. For example, during the last deglaciation, increased ventilation released CO₂ from the deep oceans to the atmosphere (Lorius et al. 1988; Anderson et al. 2009; Skinner et al. 2010). While high latitude ocean regions support most of deep water—atmosphere exchange, transport of intermediate and deep waters towards lower latitudes also needs to be accounted for in the carbon cycle (Bostock et al. 2004).

Bomb-produced radiocarbon released to the atmosphere in the 1950s and early 1960s from nuclear tests significantly increased the ¹⁴C content of atmospheric CO₂, making it a suitable tracer for rapid processes, and enabling us to estimate the transfer rate of anthropogenic CO₂ to the ocean (Hesshaimer et al. 1994; Levin and Hesshaimer 2000; Sweeney et al. 2007). Several global oceanographic campaigns included sampling of

<footnote n="1">¹Laboratoire des Sciences du Climat et de l'Environnement (IPSL/CEA-CNRS-UVSQ UMR 8212), CEA Saclay, F-91190 Gif-sur-Yvette, France</footnote>

<footnote n="2">²Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan</footnote>

<footnote n="3">³Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan</footnote>

<footnote n="4">⁴Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan</footnote>

<footnote n="5">⁵Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada</footnote>

<footnote n="6">⁶MARUM-Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany</footnote>

<footnote>*Corresponding author. Email: aymeric.servettaz@lsce.ipsl.fr.</footnote>
dissolved inorganic carbon (DIC) for ¹⁴C analysis: Geochemical Ocean Sections Study (GEOSECS) in the 1970s (Broecker et al. 1985), the Hydrographic Program of the World Ocean Circulation Experiment (WOCE, Key 1996; Key et al. 2002), and Climate Variability and Predictability (CLIVAR) in the 2000s (CLIVAR Scientific Steering Group, 1995). Repeat sections between CLIVAR and WOCE, separated by a decade, allowed the study of the evolution of DI¹⁴C through time, further constraining the CO₂ flux to the ocean (Graven et al. 2012), and confirming the penetration of radiocarbon down to intermediate water depths in the southern Pacific Ocean (Jenkins et al. 2010; Kumamoto et al. 2013).

The deep waters, however, remain free of bomb-produced radiocarbon, and only include natural cosmic-ray produced radiocarbon. It is thus possible to use radiocarbon as a semi-conservative tracer of deep ocean circulation, and its natural decay can be used to radiometrically date water masses (Matsumoto 2007).

Here, we focus on the western Coral Sea, off Queensland, northeast Australia. The Coral Sea is bounded by the Solomon Islands to the North, Vanuatu and New Caledonia to the East, and the latitude of 30°S to the South. The Coral Sea is a major outlet of the South Pacific gyre, with water masses entering the Coral Sea through the South Equatorial Current (Wyrtki 1960; Webb 2000). This current splits into zonal jets that circulate around the New Caledonia and Vanuatu islands, as attested by Argo floats (Maes et al. 2007) and Doppler velocity profiles (Gourdeau et al. 2008). The North Caledonian and South Caledonian Jets (NCJ and SCJ, respectively) extend down to at least 1500 meters below sea level (mbls), which implies that these jets also transport Antarctic Intermediate Waters (AAIW) toward the Coral Sea (Kessler and Cravatte 2013). AAIW is a low salinity, high oxygen water mass with a neutral density (γⁿ) comprised between 27.0 and 27.6 kg/m³ in the South Pacific, at depths of 700 to 1200 mbls in the Coral Sea (Sloyan and Rintoul 2001; Schneider et al. 2005; Macdonald et al. 2009). The circulation of AAIW into the Coral Sea is indicated by solid arrows in Figure 1. The pathway A, adopted from Sokolov and Rintoul (2000), extends from the NCJ, crosses the Coral Sea westward at a latitude of 18°S, then bifurcates at the Queensland Plateau. The northern branch circulates around the Coral Sea as a western boundary current (Kessler and Cravatte 2013), while the southern branch joins the pathway B at 24°S to form the southward East Australian Current (Ganachaud et al. 2014).

Circulation of water masses in the Coral Sea was also inferred from salinity and oxygen, supporting the above pathways of AAIW (Sokolov and Rintoul 2000). The chemical properties were also used to describe the deep water circulation, depicted by dashed arrows in Figure 1. Topographic barriers at 2000 mbls in the Southwest Pacific Ocean restrict the circulation of deep waters that can only enter and exit the Coral Sea via a few troughs. Sokolov and Rintoul (2000) attribute the deep water in the Coral Sea to a mixture of Circumpolar Deep Water (CDW) from the Southern Ocean transported northward through the Tasman Sea via the Cato Trough, and a westward flow of recircled CFW through the central Pacific underneath the South Equatorial Current. Deep waters are characterized by a weak salinity maximum, and span neutral densities of 27.4 to 28.2 kg/m³ in the South Pacific (Sloyan and Rintoul 2001) or 27.7 to 28.27 kg/m³ in the Southern Ocean (Orsi et al. 2002); these neutral densities are the highest measured in the Coral Sea. Sokolov and Rintoul (2000) estimated the transport of deep water into the Coral Sea in northward and westward direction (respectively arrows C and D in Figure 1) from geostrophic velocities across a meridional transect at 155°E (WOCE P11) to be 3.0 and 3.9 Sverdrup,
<figure><img src="image_3.png" /></figure>

Figure 1 Bathymetric map of the Southwest Pacific. Land surfaces are shown in black; grey shading and light grey shadings indicate bathymetry above 1000 mbl and 2000 mbl, respectively. Solid arrows show circulation of Antarctic Intermediate Water at 1000 mbl, dashed arrows indicate circulation of Circumpolar Deep Water at 2000 mbl. Capital letters A, B, C and D refer to pathways described in text. A selection of stations where ¹⁴C has been measured is indicated with symbols: open symbols (◊, △, ∇, □): this study; filled symbols (★, ●, ▼, ■, ◆, ▲): GLODAP database (Olsen et al. 2016). NC – New Caledonia; NCJ – North Caledonian Jet; SCJ – South Caledonian Jet; NZ – New Zealand.

respectively, with outflows to the north toward Solomon Islands and to the east around New Caledonia. The ¹⁴C content of deep Coral Sea waters is nonetheless lacking in the GLODAPv2 data, as the Coral Sea legs did not include DI¹⁴C analysis north of the Cato trough.

In this study, we extend the coverage of Δ¹⁴C with the first measurements in the Western Coral Sea, which was missing in previously published datasets. We use the Δ¹⁴C as a complementary tracer to discuss the circulation of intermediate and deep water in the Coral Sea.

## METHODS

Seawater samples were collected with a conductivity-temperature-depth (CTD)-Rosette water sampler at four stations (see Figure 1) during the R/V SONNE expedition SO-256 for the study of Temperature And Circulation history of The East Australian Current (TACTEAC, Mohtadi et al. 2017). The water for ¹⁴C analysis was subsampled into 50-mL glass bottles. To avoid contamination of ¹⁴C samples, the sample bottles were abundantly rinsed with sampled water, and overflowing to remove water that could exchange with air in the bottle.
About 300 µL of mercuric chloride was added to each bottle for preservation of DIC during transport and storage.

For the ¹⁴C analysis, about 100 mL of seawater is transferred to the debubbling line, under a noble gas atmosphere (argon for the transfer, and helium in the line). Phosphoric acid is added to the seawater to reduce pH, and thus reduce the solubility of CO₂ that is released in the line. The sample preparation line is composed as follows, with tubing in series: the bottle sample, two water traps in Dewars of cooled ethanol at -100°C, a CO₂ trap in a Dewar of liquid nitrogen, a connection to graphitization line that is closed during debubbling, and a pump that actively circulates gas in a tube with the end immersed in the seawater sample. After 15 min of debubbling, we pump out the noble gases while maintaining water and CO₂ fixed in cold traps. Finally, the liquid nitrogen Dewar is replaced with a cooled ethanol Dewar, to release CO₂ from the trap to the graphitization line. The CO₂ was then reduced following the graphitization procedure described by Yokoyama et al. (2007, 2010), and the graphite was analyzed on the single-stage AMS at the University of Tokyo (Yokoyama et al. 2019; Yamane et al. 2019). Hereafter, we use the Δ¹⁴C as a measurement of radiocarbon concentration normalized to δ¹³C, as defined by Stuiver and Polach (1977). The measurement sequence includes counting of ¹²C, ¹³C and ¹⁴C, so we use the AMS δ¹³C values for an on-line normalization (Yokoyama et al. 2019).

We provide our Δ¹⁴C data in Supplementary Table S1, with uncertainty given as 1σ standard deviation of at least 10 repeated measurements on the same sample. A reproducibility analysis was conducted, where 5 pairs of subsampled bottles underwent full treatment, and we concluded that the sample preparation only slightly increased the mean 1σ uncertainty for the 5 samples from 8.803‰ to 9.251‰ (details are provided in Supplementary Table S2). We chose not to include this source of uncertainty in our data as the pair of subsamples were not statistically different against a Student's t-test.

The new water column profiles presented here are compared to data from the Global Ocean Data Analysis Project version 2 (GLODAPv2); accessible from the National Oceanic and Atmospheric Administration database (Olsen et al. 2016, last accessed July 6, 2018). This database includes high quality CTD and hydrographic data from the different oceanographic campaigns presented above, in addition to other cruise data.

Water masses have characteristic densities; we ensure comparison within the same water mass by computing the neutral density, a conservative density definition introduced by Jackett and McDougall (1997). Unlike potential density, neutral density is defined without a reference layer and can be used for a large range of depths in the ocean.

## RESULTS

The profiles of Δ¹⁴C in the Coral Sea are presented in Figure 2. The three profiles in the northwestern Coral Sea are almost identical (within measurement error), and are thus merged into a single composite profile that we consider representative of the northwest (NW) Coral Sea. On the other hand, a single profile was measured in southwestern Coral Sea, hereafter named SW Coral Sea.

Both profiles show a constant Δ¹⁴C of 40 to 50‰ in the neutral densities lower than 26 kg/m³, which corresponds to surface and thermocline waters (Sloyan and Rintoul 2001; Gasparin et al.
<figure><img src="image_4.png" /><figcaption>Figure 2 Profiles of Δ<sup>14</sup>C in the western Coral Sea on a neutral density scale in (A) the northwestern Coral Sea and (B) the southwestern Coral Sea. Error-bars indicate 1σ measurement standard deviation. Location of stations is indicated in Figure 1 with corresponding symbols.</figcaption></figure>

2014). In the intermediate waters, with neutral densities from 26 to 27.4 kg/m³, Δ¹⁴C is decreasing to about -100‰ at 27.4 kg/m³.

For neutral densities greater than 27.4 kg/m³, characteristic of deep waters, theΔ¹⁴C keeps decreasing to a minimum of -205.6 ± 3.6‰ at 27.9 kg/m³ in the NW Coral Sea. The deepest water sampled, with a neutral density just above 28.0 kg/m³, has a slightly higher Δ¹⁴C with -171.4 ± 2.6‰. The SW Coral Sea was not sampled at this depth here, except for one bottle with a Δ¹⁴C of -150.9 ± 2.9‰ at 27.64 kg/m³. The upper part of the SW Coral Sea profile, however, is almost identical to the composite profile from the NW Coral Sea.

# DISCUSSION

## Intermediate Water Circulation

In order to compare subtle differences in Δ¹⁴C over a wide range of densities, we subtracted the NW Coral Sea profile from all profiles from stations along pathways A and C in Figure 3A and Figure 3C, and subtracted the SW Coral Sea profile from all profiles along pathway B in Figure 3B. If no point was available at the same density, we linearly interpolated the two closest points on a neutral density scale.

In Figure 3A, Thermocline Water and Sub-Antarctic Mode Water, with neutral densities lower than 27.0 kg/m³ (Hartin et al. 2011), show consistently lower Δ¹⁴C values in the NW Coral Sea by about -20‰ compared to profiles from CLIVAR, even though both profiles are taken along the same pathway of Antarctic Intermediate Waters (AAIW) (Sokolov and Rintoul 2000; Gasparin et al. 2014). Conversely AAIW, with neutral densities of 27.0 to 27.4 kg/m³, is ¹⁴C depleted in WOCE and CLIVAR, compared to our profiles in the western Coral Sea: there is a +10‰ difference between CLIVAR 2009 and the NW Coral Sea at a neutral density of 27.3 kg/m³ presented here. These differences are even greater for the WOCE profile sampled in 1992 than for the CLIVAR profile sampled in 2007 at a very similar location (Figure 3B).
<figure><img src="image_5.png" /><figcaption>Figure 3 Selected profiles of Δ¹⁴C along the pathways A, B, and C, plotted versus neutral density as the difference to NW Coral Sea (A and C) or SW Coral Sea (B). Error-bars represent a 1σ combined standard error resulting from the combination of original data uncertainty and the subtracted value uncertainty. See Figure 1 for the location of stations indicated with corresponding symbols. Profiles are from WOCE 1992 (Roemmich et al. 2014), CLIVAR 2003 (Uchida et al. 2013a), CLIVAR 2007 (Fukasawa et al. 2013), and CLIVAR 2009 (Uchida et al. 2013b).</figcaption></figure>

Assuming that the stations located along a specific pathway for intermediate waters have identical Δ¹⁴C at a given time, the Δ¹⁴C gradient described in the previous section used to be steeper than what we measured in 2017. This smoothing of the Δ¹⁴C gradient at intermediate water depths has been observed on a larger scale in the Pacific Ocean by comparing CLIVAR profiles to WOCE on repeated sections separated by 15 years (Jenkins et al. 2010). It is caused by diffusion of ¹⁴C enriched carbon dioxide produced extensively in the 1950s and 1960s that since has reached intermediate depths in the South Pacific Ocean. Our measurements show that the bomb-produced radiocarbon keeps diffusing further down the water column, with increasing ¹⁴C activity at AAIW densities, and decreasing ¹⁴C activity in the overlying water masses.

In the South Pacific at 150°W, Jenkins et al. (2010) estimated a +15 to +50‰ increase of Δ¹⁴C in AAIW between 1991 and 2005, with the maximum increase occurring at mid latitudes and in the upper part of the AAIW. At low latitudes, the increase is smaller, +5 to +35‰ in the AAIW over 14 years. This rate is rather consistent with the increase of up to +12‰ measured along pathway B between CLIVAR 2007 and the new SW Coral Sea profiles from 2017 presented here, supporting the hypothesis of homogenous Δ¹⁴C content at a given time. It is also consistent with an increase along pathway A between CLIVAR 2009 and NW Coral Sea profiles, but not with the CLIVAR 2007 profile that do not show any significant difference with the NW Coral Sea at AAIW densities. Considering the position of the CLIVAR 2009 profile, we can constrain the eastward extent of pathway A up to north of New Caledonia. The CLIVAR 2007 profile however might be slightly off the pathway A, but was still the closest approximation due to the sparsity of Δ¹⁴C profiles.

In the Western Coral Sea however, we do not have repeated profiles. Because the water pathways differ for each water mass at a specific density, we can only compare a slice of a density profile with an upstream profile for the water mass. Additionally, we suppose that there is no mixing, which is the closest approximation we can make for Δ¹⁴C, but is not exactly true as shown by temperature and salinity (Supplementary Figures S4 and S6). Therefore, non-repeated profiles from the western Coral Sea are not suited for the computation of a downward flux of ¹⁴C, and we decide not to attempt a modeling of the flux that would be highly speculative as a result of all the uncertainties stated above.
Kumamoto et al. (2013) estimated the extent of bomb-produced radiocarbon diffusion down to 1200 mbl in the Coral Sea, which corresponds to neutral densities of 27.5 kg/m³. Our measurements further confirm the difference between the western Coral Sea and profiles upstream along the three pathways to be indistinguishable from measurement error, arguing for a water mass unaffected by diffusion thus far.

## Deep Water Circulation

Below this limit of diffusion, the ocean is free of contamination, and radiocarbon can serve as a tracer for deep ocean circulation. The pathways A and B are limited to depths shallower than the topographic barrier formed by Colville and Kermadec ridges south of Fiji Islands, isolating the deep Coral and Tasman seas from the rest of the South Pacific Ocean. This results in relatively weak westward flow north of New Caledonia below 2000 mbl (Gourdeau et al. 2008).

Furthermore, the deep water sampled in the NW Coral Sea has significantly higher Δ¹⁴C than stations upstream along pathway A, with a 30‰ difference below 27.9 kg/m³ in the NW Coral Sea where we observed a reversion of the Δ¹⁴C gradient (Figure 2 A) that is not observed at other stations east of New Caledonia (Fukasawa et al. 2013; Uchida et al. 2013b). This suggests a different source of deep waters.

Previous studies of the deep circulation in the Coral Sea suggested a northward circulation of Circumpolar Deep Water (CDW) through the Tasman Sea (Rotschi and Lemasson 1967; Sokolov and Rintoul 2000), along with a westward flow between Vanuatu and Solomon Islands, as shown by the dashed arrows C and D in Figure 1. We compare the Δ¹⁴C data from the NW Coral Sea, where we sampled deep waters with CDW densities at stations upstream of pathway C for CDW in Figure 3C.

In the Southern Ocean, CDW is often considered as the water mass just underneath AAIW, with neutral densities from 27.4 to 28.2 kg/m³ (Sloyan and Rintoul 2001). While Δ¹⁴C values of the NW Coral Sea do not show any significant differences compared to CLIVAR profiles at 25°S and 30°S, on neutral densities 27.4 to 27.8 kg/m³, the NW Coral Sea deep waters have a 20‰ lower Δ¹⁴C around the density of 27.9 kg/m³, where we identified the Δ¹⁴C minimum. A decrease of Δ¹⁴C along the pathway C could be due to natural decay of radiocarbon. As an order of magnitude, natural decay from -180‰ to -190‰ would take approximately 100 years. Assuming that there is no mixing along the pathway, this would imply a mean circulation below 0.1 cm/s from the East Australian Basin to the west of the Coral Sea Basin, via the Cato trough, ten times lower than the 1 to 5 cm/s implied by the 3 Sverdrup flow given by Sokolov and Rintoul (2000). We thus refute the hypothesis of an unmixed southern source for water at neutral densities 27.9 kg/m³, and argue for a significant contribution of ¹⁴C depleted waters from another source. The difference of Δ¹⁴C between the NW Coral Sea and southern stations along pathway C decreases to less than 10‰ for the deepest points at 28.0 kg/m³, which could be explained by an increased relative contribution of CDW at greater depths.

To further investigate the water masses arriving in the Coral Sea, we provide a map of Δ¹⁴C on the isopycnal surface of neutral density 27.9 kg/m³ in the Southwest Pacific (Figure 4, details on the map generation are provided in the supplementary information). Given that this density is largely below the diffusion limit of bomb-produced ¹⁴C, this map can be considered at steady state, independent of the time of sampling.
<figure><img src="image_6.png" /><figcaption>Figure 4 Map of Δ¹⁴C on isopycnal surface 27.9 kg/m³ neutral density, interpolated between stations shown by open stars (this study) and filled stars (GLODAP database; Olsen et al. 2016). As in Figure 1, land surfaces are shown in black; grey shading and light grey shadings indicate bathymetry above 1000 mbsl and 2000 mbsl, respectively.</figcaption></figure>

The Δ¹⁴C map in Figure 4 shows a gradient with high values of more than –180‰ south of Tasman Sea decreasing towards the Coral Sea that contrasts with very low Δ¹⁴C values of lower than –200‰ east of Vanuatu. The relatively high Δ¹⁴C of –195‰ in the northwestern Coral Sea confirms a major contribution of ¹⁴C rich Circumpolar Deep Water from the East Australia Basin through the Cato Trough (see Figure 1), although the eastern Coral Sea might also receive ¹⁴C depleted CDW flowing westward after circulating through the central Pacific, where it could have mixed to some extent with ¹⁴C-poor North Pacific Deep Water (NPDW) and consequently has a lower Δ¹⁴C. Sokolov and Rintoul (2000) discussed the sources for deep water in Coral Sea using the silica distribution, and concluded that low silica in the Coral Sea basin indicates a southern source of CDW, while higher values in the eastern Coral Sea indicates an eastern source, which is in excellent agreement with Δ¹⁴C data presented here.

The profiles along pathway C (Figure 3C) also show that the two water masses do not mix completely, but rather occupy different density layers in the Coral Sea basin, with a larger contribution of ¹⁴C depleted CDW from the eastern source at 27.9 kg/m³, and CDW from the southern source beneath, at 28.0 kg/m³. The lack of samples along the inferred westward flow (arrow D in Figure 1) does not allow us to fully confirm this contribution and would require further investigation. The westward flow is nonetheless in good agreement with modeled transport stream functions (Macdonald et al. 2009), and the location of the dissolved silica maximum (Sokolov and Rintoul 2000).
NPDW formation during deglaciation may have modified the deep water circulation in the Pacific Ocean (Keigwin et al. 1987; Okazaki et al. 2010; Rae et al. 2014), and it is possible that the ¹⁴C content of deep water entering the Coral Sea was altered significantly at the time, although it remains hypothetical.

## CONCLUSION

We investigated the Δ¹⁴C of Dissolved Inorganic Carbon in the Coral Sea, and compared it to previously published records of ocean circulation and time evolution of bomb-produced radiocarbon in the study region.

Our results are consistent with a deepening of bomb-produced radiocarbon, as observed at other stations in the South Pacific Ocean (Jenkins et al. 2010; Kumamoto et al. 2011, 2013), based the hypothesis of a westward circulation of AAIW from the South Pacific gyre via the North and South Caledonian Jets.

Δ¹⁴C at greater depths remains free of bomb-produced ¹⁴C and is consistent with a dual origin of deep waters in the western Coral Sea, with Circumpolar Deep Water from the east through the Central Pacific, and directly from the south through the Tasman Sea, which are moderately mixed. The inflow of the eastern source cannot be tracked because of a lack of deep Δ¹⁴C measurements from this area. In general, the Δ¹⁴C map is in excellent agreement with silicate concentrations (Sokolov and Rintoul 2000), which have been used previously to illustrate the dual origin of deep water in the Coral Sea.

While the Δ¹⁴C water column profiles presented here further validate the circulation pathways for intermediate to deep water masses, the sparse data available limits their interpretation in comparison to other geochemical tracers such as nutrients or oxygen. Yet the unique nature of Δ¹⁴C scaling with the age of the water mass complements other tracers to distinguish water masses and clarify the deep circulation of oceans.

## ACKNOWLEDGMENTS

The authors would like to thank members of the R/V Sonne 256 cruise for providing high-quality samples. The R/V Sonne 256 cruise was funded by the German Research Ministry (BMBF) as project 03G0256A “TACTEAC: Temperature And Circulation History of The East Australian Current.” A part of this research was supported by Grants-in-Aid for Scientific Research, MEXT (No. KAKENHI JP15KK0151, JP17H01168) and JST CREST Grant Number JPMJCR13A4. We are grateful to Katsumi Matsumoto and an anonymous reviewer for their comments and constructive criticism on the manuscript.

## SUPPLEMENTARY MATERIALS

To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2019.122

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