Unique current connecting Southern and Indian Oceans identified from radium distributions

We examined the spatial variations in 226Ra and 228Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. 226Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4–2.9 mBq/L), exhibiting small vertical variations, while 228Ra decreased southward and became depleted in the Southern Ocean. These distributions indicated the ocean-scale northward lateral transport of 226Ra-rich and 228Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). 226Ra concentrations indicated that the fractions of the ACC at depths of 0–800 m decreased from 0.95 to 0.14 between 60° S and 30° S. The ACC fractions in the subantarctic western Indian Ocean were higher than those previously reported in the eastern Indian region, indicating preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian Ocean in the 1970s. This could be attributed to the minimal southward shift of the polar front due to global warming over the last 50 years.

www.nature.com/scientificreports/ features in the subtropical and subantarctic Indian and Southern Oceans, indicating unique and ocean-scale current circulations [24][25][26][27] (and the biological scavenging of 226 Ra) 28 . This study examined the spatial distributions of 226 Ra and 228 Ra concentrations at depths of 10-830 m from the northwestern Indian Ocean to the Southern Ocean and obtained a comprehensive understanding of temporal (over the last 50 years) and ocean-scale spatial variations (between the western and eastern Indian Ocean and the Southern Ocean), incorporating the results from previous studies [19][20][21][22][23][24][25][26][27] . Furthermore, it clarified ocean-scale current circulations in this area, focusing on the waters connecting the Southern and southern Indian Oceans.

Results
Characteristics of current layers. Surface salinity along the expedition route and sampling sites for MR19-18-141 waters are shown in Fig. 1a 29 . @@Cross-sectional observations of salinity and dissolved oxygen (DO) are presented in Fig. 1b,c, respectively. Water columns at sites MR19-18 and -49 contained high-salinity (> 34.5) North Indian Central Water formed by subduction at the subtropical front 4 , which was covered with low-salinity upper-layer waters from the surface to depths of 50-150 m (Fig. 1b). In contrast, low-salinity southward currents (e.g., Antarctic Intermediate Water; AAIW) had a lesser effect on site MR19-49 27 . The physicochemical characteristics above a depth of ~ 1000 m changed drastically northward of site MR19-104. The salinity of the water columns at sites MR19-104 and -141 was remarkably lower (< 34.7) than that at site MR19-73 (34.5-35.5). Based on the salinity and DO profiles, water columns from depths of > 100-800 m at sites MR19-73 and -104 predominantly comprised the Subantarctic Mode Water (SAMW) and AAIW, which is formed via the convection of Antarctic Surface Water, existing between the upper layer and Upper Circumpolar Deep Water (UCDW) 6,30,31 . The UCDW and Lower Circumpolar Deep Water (LCDW), which may be characterized as the oxygen-minimum and salinity-maximum layers, respectively, spread northward from Antarctica. These  [20][21][22][23][24][25] . At the surface, 226 Ra concentrations along the coasts of Southeast Asia were higher than those in the ambient areas; it decreased towards offshore areas (Fig. 2a). In contrast, the concentrations gradually increased from > ~ 40° S to the Southern Ocean. Similarly, the 226 Ra concentrations of surface waters were ~ 1.5 mBq/L at sites MR19-18 and -49; it increased sharply from 1.4 to 2.9 mBq/L between sites MR19-73 and MR19-141.
The 228 Ra concentrations at the surface along the coasts of Southeast Asia were remarkably high (5-10 mBq/L), overlapping with the high-226 Ra concentration areas, and decreased sharply towards the offshore, in the equatorial-subtropical area [21][22][23] (Fig. 2b). Owing to the short half-life of 228 Ra, high concentrations of this isotope can be attributed to the mixing of seawaters that have been in contact with the shallow continental shelf and coastal sediments, as observed in the East China Sea 32,33 . In this study, the high 228 Ra concentrations were predominantly ascribed to the continual supply of 228 Ra from the coasts of Southeast Asia. Subsequently, 228 Ra spread to the surrounding areas because of the southwestward monsoon currents, particularly in January 34  Ra for MR19 waters, along with previous data from the Indian and Southern Ocean 19,21-23 . The seasonal variations in 228 Ra and 226 Ra concentrations, particularly in the northeastern Indian Ocean, were unclear (e.g., due to monsoon currents) 34 ; however, the concentrations recorded at the surface from January to April exhibited a positive correlation (Fig. 5a). This predominantly reflects the supply of 226 Ra and 228 Ra from shallow shelf and coastal sediments along the coasts of Southeast Asia. However, surface waters with high 226 Ra concentrations at sites MR19-73, -104, and -141 and at a few sites in the subantarctic Indian Ocean 23 exhibited the minimum 228 Ra concentrations. The 226 Ra concentrations (< 2 mBq/L) of waters at site MR19-73, including the eastern area (sites PA4 and PA5) 22 , above depths of 800 m exhibited a negative correlation with 228 Ra concentrations (Fig. 5b). However, the waters in the eastern area, particularly at depths of 5 and 50 m at site PA5, reflect the contribution of 228 Rarich surface layer waters (Fig. 4). The 228 Ra concentrations in samples from sites MR19-104 and -141 with high 226 Ra concentrations (> 2 mBq/L), as well as a site in the western area (~ 0.01 mBq/L at M3) 19 , were below the detection limit (< 0.03 mBq/L). Additionally, low 228 Ra concentrations at site MR19-141 indicated that the contribution of waters affected by the 228 Ra-rich continental slopes and coastal sediments along Antarctica was minimal. Additionally, the transport of waters from the Antarctic continental shelf to the offshore area was slow (1.5 years) 37 . Based on the spatial distributions of 228 Ra and 226 Ra concentrations (Figs. 3 and 4), water columns at sites MR19-18 and -49 were similar to those typically observed in open oceans 15 and marginal seas 16,35 . In  20,24 . Based on the salinity and DO features (Fig. 1b,c), the water columns at sites MR19-73 and -104 are strongly stratified into the SAMW (26.5-26.8σ θ ) and AAIW (27.0-27.3σ θ ), respectively, which disrupts the intense vertical mixing between these layers. Additionally, the 226 Ra concentrations of MR19 waters were two orders of magnitude higher than those of the reactive and parent 230 Th 38 ; this indicates that 226 Ra is largely supplied from bottom and coastal sediments. Therefore, high 226 Ra concentrations and small vertical variations at site MR19-104 can be explained by the lateral transport of 226 Ra-rich waters rather than the upwelling of 226 Ra from the LCDW. The 226 Ra-rich waters at MR19-141 in the Southern Ocean are mainly from UCDW (~ 27.6σ θ ), which plays a key role in increasing the 226 Ra concentrations of water columns at sites MR19-104 and -73.
Water column inventories (depth of 0-800 m) of 226 Ra concentrations in the subantarctic Indian and Southern Oceans are plotted against latitude in Fig. 7 and compared with the values from previous studies 19,20,22,24,25 . The inventories increased sharply from site MR19-73 (1205 Bq/m 2 ) to MR19-141 (2550 Bq/m 2 ) via site MR19-104 (1900 Bq/m 2 ). Inventories at sites MR19-73 and -104 in the subarctic area were evidently similar to those obtained at nearby sites in 1978 (sites 427-428 and 429, respectively) 25 . In contrast, the inventories decreased from the western Indian Ocean to offshore western Australia in the eastern Indian Ocean (850 Bq/m 2 at PA4 and 940 Bq/ m 2 at PA5) 22 . Combined with the high 228 Ra concentrations (Figs. 2b and 4), the low inventories of 226 Ra at sites PA4 and PA5 can be attributed to the intrusion of southward currents with high 228 Ra and low 226 Ra concentrations (e.g., the Leeuwin Current) 39 . Furthermore, the inventory yielded a higher value at site MR19-104 than that at similar latitudes in southern Australia (1426 and 1616 Bq/m 2 at sites EL35-II and -III, respectively) 20 . Such wide variations in the 226 Ra inventories, particularly from 30° S to 50° S, indicate different transport patterns of ACC from the Southern Ocean to the western and eastern Indian Oceans.
The salinity at site MR19-141 in the Southern Ocean was at the same level as that at site 430, except for a lower value above a depth of ~ 50 m 40 , indicating a larger contribution from Antarctic water. High-density (> 27σ θ ) and high-226 Ra features in the upper-layer waters at site MR19-141, which are equivalent to those at sites 430 and 431     24 . Conversely, the 226 Ra concentrations above a depth of 800 m at site PA4 were equivalent to the lowest values recorded in the Indian Ocean, such as the upper layer in the northern Indian Ocean (Fig. 3). Therefore, we considered the water columns from depths of 0 to 800 m at sites 431 and PA4 (970 Bq/m 2 ) as the end-members in the ACC and the water from the northern Indian Ocean, respectively. Consequently, the fractions of the ACC (i.e., column waters at site 431) were estimated as 0.95 at MR19-141, 0.56 at MR19-104, 0.14 at MR19-73, and 0.44 at site M3, based on their inventories (Fig. 7). The fractions at sites EL35-II and -III in the eastern Indian Ocean (0.27 and 0.39, respectively) correspond to ~ 60% of that at site MR19-104, despite their similar latitude. Higher inventories in the western subantarctic Indian Ocean could be explained by the direct inflow of the ACC, compared to that in the eastern area, where the ACC has longer pathways (< 10 and < 30 years to the west and east, respectively, based on CFC age) 7,31 , and by the large-scale intrusion of southward currents observed offshore of western Australia 22 .
Decadal variations in seawater temperature have recently been recorded in and around the polar front, reflecting changes in global climate 11 . Anomalous warming below depths of 200 m is accompanied by density anomalies in the Southern Ocean 41,42 . The fraction at site MR19-141 was comparable to that at the closest site 430 (2593 Bq/m 2 , 0.97). Additionally, the ACC fractions at sites MR19-104 and -73 exhibited similar values to those observed at sites 429 (0.64) and 428-427 (0.17-0.11) in 1978 25 . This can be primarily attributed to the minimal effects of the southward shift of the polar front 10 and/or the short-term (e.g., annual) and local variations in the circulation of the ACC-dominated water 43 and eddy mixing 44 . Compared to earlier reports, our study of water currents using 226 Ra and 228 Ra did not indicate any significant decrease in the contribution of the ACC that could be attributed to the southward shift of the polar front due to global warming since the 1970s.    (Fig. 1a). All water samples were unfiltered. The experimental procedures employed to collect Ra in seawater samples have been previously described 45 . After adjusting the pH to ~ 1 using concentrated HNO 3 , a minimally Ra-contaminated Ba carrier (960 mg) was added to a ~ 40 L aliquot of each seawater sample, and BaSO 4 was precipitated with the Ra isotopes. The chemical yields of the Ra isotopes were 93-100%, based on the yields of the BaSO 4 fractions. Low-background γ-spectrometry was performed on all BaSO 4 samples using high-purity Ge-detectors located in the Ogoya Underground Laboratory, Japan 46 , over 3-5 counting days. The 226 Ra concentrations were evaluated from the γ-ray peaks of 214 Pb (295 and 352 keV) after reaching radioactive equilibrium (> 3 weeks after sample compression); they were calibrated from the peak ratios of mock-up samples with almost the same chemical composition as the water samples, including uranium standards issued by the New Brunswick Laboratory, USA (NBL-42-1). In addition, 228 Ra concentrations were characterized from 228 Ac (338 and 911 keV), based on the detection efficiency curve obtained from the mock-up samples.
Under the analytical conditions employed in this study, the minimum amount of 228 Ra that could be determined in a water sample was ~ 1.5 mBq. This corresponded to a detection limit of ~ 0.03 mBq/L when using ~ 40 L of sampled seawater. Based on the standard deviation, the analytical precision was 1-3% and 3-30% for 226 Ra and 228 Ra, respectively. Both the 228 Ra and 226 Ra concentrations in this study were decay-corrected to the sampling date.