Signature of the Mekong River plume in the western South China Sea revealed by radium isotopes

Abstract

We investigated the distribution of 223Ra, 228Ra, and 226Ra in the surface water of the western South China Sea (SCS) during summer based on a 30 day cruise conducted in August–September 2007. The activities of 223Ra varied from almost undetectable to 0.74 disintegrations per minute (dpm)/100 L, and those of 228Ra varied from 12.2 to 61.5 dpm/100 L. Their spatial distribution was characterized by a jet of high 228Ra (>48 dpm/100 L) and 223Ra (>0.4 dpm/100 L) extending eastward from the Vietnam coast along ∼11°N, curling up in the vicinity of 112°E and swirling counterclockwise to form a cyclonic eddy with lower 228Ra (21–25 dpm/100 L) and 223Ra (0.04–0.14 dpm/100 L) at its center. High 226Ra (10–14 dpm/100 L) appeared in the eastward jet and decreased to 6.0–8.5 dpm/100 L along the track of the jet described above. The observed distribution of Ra isotopes was consistent with the pattern of the Southeast Vietnam Offshore Current in the western SCS in summer. The higher radium activities were in all likelihood derived from the Mekong River. Using a simple two‐end‐member mixing model based on the 228Ra/226Ra activity ratio and salinity, we calculated that approximately 53% of the surface water in the western SCS was originated from the Mekong River diluted water. Note that this estimation should be regarded as an upper limit due to the lack of sampling at its immediate source, the Mekong estuary. The data revealed that more than 2 weeks were required for the transportation of freshened water from the Mekong River's mouth several hundred kilometers to the western SCS.

Seasonal variations of sea–air CO<sub>2</sub> fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

“…In spring, offshore high pCO 2 values of 420 to 463 µatm were mostly detected in domain C in April 2005 (Table 5). In summer and autumn, most relatively high pCO 2 values of 420 to 440 µatm (Table 5) were associated with the cyclonic cold eddies observed during our August-September 2007 cruise in the southwestern SCS (Chen et al, 2010;Hu et al, 2011). It should also be noted that, in the four domains, very low sea surface pCO 2 values of 280 to 320 µatm were only observed in domain A and mostly in summer (Table 3).…”

“…Figure 9c shows that both the highest pCO 2 value (∼ 430 µatm) and the lowest pCO 2 value (∼ 380 µatm) during the September 2007 survey were observed in the MKRDW area. In the September eddy (eddy II in Chen et al 2010), a relatively narrow pCO 2 range of 393 to 423 µatm (Fig. 9c) was observed.…”

“…Domain C was located between 7 and 18 • N and 110 and 117 • E. As the deep basin of the SCS, its upper mixed layer is shallow all the year round. Both mesoscale eddies and the Mekong River plume have impacts on the southwestern part of this domain during the southwest monsoon period (e.g., Chen et al, 2010;Hu et al, 2011). However, regionally averaged primary production and sea surface chlorophyll concentrations vary within limited ranges (Tan and Shi, 2009;Sasai et al, 2013).…”

“…Most lowsalinity values of < 33.5 were observed in domains A and C, which were under the influence of river plumes and/or heavy precipitation. In September 2007, the southwestern SCS is highly influenced by the Mekong River Diluted Water (MKRDW) (Chen et al, 2010). Therefore a very low sea surface salinity of 32.5 ± 0.6 was observed in domain C (Table 5).…”

“…During the late August and early September surveys in 2007, the influence of MKRDW was coupled with two offshore cold eddies in the southwestern SCS (Chen et al, 2010). According to Chen et al (2010), the MKRDW was identified with a salinity less than 33.2 and an SST higher than 29.0 • C during the September 2007 survey, while the two cold eddies had a relatively high salinity of > 33.2 and a relatively low SST of ∼ 28.2 • C at the center. Figure 9c shows that both the highest pCO 2 value (∼ 430 µatm) and the lowest pCO 2 value (∼ 380 µatm) during the September 2007 survey were observed in the MKRDW area.…”