Characteristics of current layers
A total of 35 seawater samples were collected between depths of 10 m and 830 m at sites MR19-04-18 and -49 in the northern Indian Ocean (2°N and 10°S, respectively) in December 2019 and at sites MR19-04-73, -104, and -141, which extended from the Indian Ocean to the Southern Ocean (30°S, 43°S, and 60°S) in January 2020, during the R/V Mirai expedition (hereafter, MR19-18, -49, -73, -104, and -141) (Fig. 1a). Sites MR19-18 and -49, MR19-73 and -104, and MR19-141 were located in the subtropical and subantarctic Indian Oceans and the Southern Ocean, respectively3. Cross-sectional observations of the salinity and dissolved oxygen (DO) along the expedition route are presented in Fig. 1b and 1c23. Water columns at sites MR19-18 and -49 exhibited high-salinity (> 34.5) North Indian Central Water formed by subduction at the subtropical front4, 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 current (e.g., Antarctic Intermediate Water; AAIW) had less effects on site MR19-4922.
The physicochemical characteristics above ~1000 m depth drastically changed toward the north of site MR19-104. The salinity of water columns at sites MR19-104 and -141 was remarkably lower (< 34.7) than that at MR19-73 (34.5–35.5). Based on the salinity and DO profiles (Fig. 1b and 1c), water columns from depths of > 100–800 m at sites MR19-73 and -104 predominantly comprised the Subantarctic Mode Water (SAMW) and AAIW, which formed via the convection of Antarctic Surface Water (AASW), existing between the upper layer and Upper Circumpolar Deep Water (UCDW)6,24,25. The UCDW and Lower Circumpolar Deep Water (LCDW), which may be characterized as the oxygen-minimum and salinity-maximum layers, respectively, spread from the Antarctic continent northward. These layers occupied the depths of ~200–700 m and ~700–1200 m at site MR19-141 and exhibited convective behavior at depths of ~1000–2000 m and ~2000–3000 m at MR19-73–104, respectively. The major components of water columns (depth of 0–800 m) are considered to be the SAMW at site MR19-73, AAIW at MR19-104, and UCDW at MR19-141.
Lateral and vertical 226Ra profiles
The lateral and vertical distributions of 226Ra concentrations in the Indian and Southern Oceans are shown in Fig. 2, along with the data from previous studies14-16,18-20. At the surface, 226Ra concentrations along the coasts of Southeast Asia were higher than those of the ambient areas and decreased toward offshore areas (Fig. 2a). In contrast, the concentrations gradually increased from the subantarctic Indian Ocean (> ~20°S) to the Southern Ocean. Exceptionally, the 226Ra concentrations for waters off the coast of western Australia were lower than those of other areas in similar latitude. Similarly, the 226Ra concentrations of surface waters were ~1.5 mBq/L at sites MR19-18 and -49 and sharply increased from 1.4 to 2.9 mBq/L from site MR19-73 to -141.
Gradual vertical increases have been observed in the 226Ra concentrations in northern (south equatorial and monsoonal) and subtropical areas (sites PA7–10 and 413–425)15,20 (Fig. 2b), as well as in the Pacific Ocean in both hemispheres12, the Sea of Japan13, the Sea of Okhotsk26, and the Bering Sea27. Similarly, the 226Ra concentrations at sites MR19-18 and -49 increased from 1.5 to 3.5 mBq/L between water depths of 10 m and 830 m. At sites MR19-73 and -104, minor vertical variations in 226Ra concentrations were observed with depth, particularly below 100 m, than those at sites MR19-18 and -49. The concentrations at MR19-104 were remarkably higher than those at MR19-73 (2.2–2.7 mBq/L and 1.4–1.6 mBq/L, respectively) (Fig. 2c). Notably, the 226Ra concentrations offshore of western Australia (sites PA4 and PA5)15 were considerably lower than those above 800 m in the ambient area. Furthermore, 226Ra concentrations offshore of southern Australia (sites EL35-II and -III) above 800 m depth17 were similar to those at site MR19-73 (~1.5 mBq/L).
Above 800 m depth, the 226Ra concentrations at site MR19-141 were comparatively higher than those in the Indian Southern Ocean at all depths (Fig. 2d) and were higher than those at sites EL35-I and EL37-I–III in the eastern Indian section17. The variations in concentrations were small in the Southern Ocean, including the western Pacific and eastern Atlantic sections of the Southern Ocean28,29. However, the 226Ra concentrations were not closely homogeneous despite the vigorous lateral circulation of the ACC. The 226Ra concentrations at site MR19-141 were approximately same levels with those recorded at the nearest site 430 and at site 431 nearer to Antarctica (64°11'S) in February of 1978 at the same depth19.
Notably, the 226Ra concentrations of water samples gradually increased from the subantarctic to the Southern Ocean (Fig. 2). Additionally, the 226Ra concentrations in the western Indian Ocean were higher than those in the eastern area, and the 226Ra concentrations in samples from sites MR19-73, -104, and -141 seemed to be equivalent to the highest groups at similar latitude, although the variations in/around each latitude were less conspicuous.
Spatial 228Ra profiles
The spatial distributions of the 228Ra concentrations in the Indian and the Southern Oceans are shown in Fig. 3. The 228Ra concentrations at the surface along the coasts of Southeast Asia (5–10 mBq/L) were remarkably higher than those in the offshore areas and overlapped with the high-226Ra concentration areas14,18 (Fig. 3a). The 228Ra concentrations at the surface sharply decreased toward the Southern Ocean (~0.1 mBq/L) via the subantarctic region. Owing to the short half-life of 228Ra, 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 Sea30,31. In this study, the high 228Ra concentrations observed were predominantly ascribed to the continual supply of 228Ra from the large and shallow continental shelves and coastal sediments. Subsequently, 228Ra was spread to the ambient areas through seawater by southwestward monsoon currents, particularly in January32. 228Ra in surface waters at sites MR19-18 and -49 (1.8 mBq/L and 1.2 mBq/L, respectively) were likely due to the transport of the southward surface waters, and then, the concentrations further decreased from site MR19-73 to -141 (0.2 to < 0.1 mBq/L).
Vertically, the 228Ra concentrations at sites MR19-18 and -49 sharply decreased from the surface to 100 m depth, showing stratification at a depth of ~100 m (Fig. 3b). Such vertical profiles have commonly been observed in the northeastern Indian Ocean15 and other oceans12 and marginal seas13. The 228Ra concentrations in MR19-18 and -49 waters were lower than those in the eastern side waters at sites PA7–10, reflecting smaller contribution of the coastal or shallow shelf waters. Combined with the higher 226Ra concentrations (Fig. 2b), it is considered that water currents in the northern Indian Ocean were not homogeneous, e.g., smaller contribution of aged seawaters in the northeastern Indian Ocean.
The concentrations and vertical gradient of 228Ra at site MR19-73 were remarkably lower than those at MR19-18 and -49; the concentrations below ~100 m depth were between those observed at sites MR19-18 and -49, and MR19-104 (Fig. 3c). The concentrations of 228Ra supplied from the Antarctic continental shelf were high along the coast of the Weddell Sea (~4 mBq/L at 20 m depth) and sharply decreased offshore33. The 228Ra concentrations in the waters from site MR19-141 were lowest in the study area at all depths (Fig. 3d), as observed at the same latitude in the Weddell Sea (< 0.1 mBq/L).
226Ra vs. 228Ra concentrations
The concentrations of 226Ra are plotted against those of 228Ra for MR19 waters in Fig. 4, along with previously collected data from the Indian and Southern Oceans in November–February14,15,18. Although the seasonal variations in 228Ra and 226Ra concentrations, particularly in the northeastern Indian Ocean (e.g., due to monsoon currents32), were unclear, the concentrations recorded at the surface were positively correlated (Fig. 4a). This predominantly reflected the leaching of 226Ra from shallow shelf and coastal sediments along the coasts of Southeast Asia with 228Ra. However, surface waters with high 226Ra concentrations at sites MR19-73, -104, and -141 and at a few sites in the subantarctic Indian Ocean18 exhibited the minimum 228Ra concentrations.
In contrast, the 228Ra and 226Ra concentrations of waters at sites MR19-73, PA4, and PA5, above 800 m depth exhibited negative correlation in waters with < 2 mBq/L of 226Ra (Fig. 4b), except for 5 m and 50 m depth waters at site PA5 reflecting the contriburion of 228Ra-rich surface layer waters15 as observed in Fig. 3b. 228Ra concentrations in samples from sites MR19-104, -141, and M3 with high 226Ra concentrations (> 2 mBq/L) were below the detection limit (< 0.03 mBq/L) or extremely low-levels (~0.01 mBq/L)16. Additionally, low 228Ra concentrations at site MR19-141 indicate that the contribution of waters that contacted the 228Ra-rich continental slopes and coastal sediments along Antarctica was minimum and that the exchange of waters from the Antarctic continental shelf to the offshore area was slow (1.5 y)33. Meanwhile, based on the spatial distributions of 228Ra and 226Ra concentrations (Fig. 2 and 3), water columns at sites MR19-18 and -49 were found to be under those typically observed in the open oceans12 marginal seas13,26,27. In contrast, the spatial distributions of 228Ra and 226Ra concentrations at sites MR19-73, -104, and -141 could be plausibly explained by different circulation systems, which we will discuss hereafter.