Utilizing ²¹⁰Po deficit to constrain particle dynamics in mesopelagic water, western South China Sea

Abstract

The 210Po‐210Pb pair is increasingly used as a proxy of quantifying organic carbon export from the euphotic zone. However, disequilibria between 210Po and 210Pb in mesopelagic water have been poorly studied. Here we present unusual deficiencies of 210Po with respect to 210Pb in mesopelagic water (200–1000 m) in the South China Sea (SCS). The total particulate matter (TPM) increased by up to 32% in the mesopelagic layer comparing with the euphotic zone. The total 210Po/210Pb ratio varied from 0.41 to 0.98 with an average of 0.72 ± 0.19, showing an enhanced removal of 210Po in mesopelagic water. On average, particulate 210Po and 210Pb increased by 23% and 32% at the slope stations, respectively. These results indicated that the 210Po deficits result from lateral transport, probably via benthic nepheloid layer. Based on the deficiency of 210Po, the residence times of particulate 210Po were estimated to range from 0.11 to 0.25 year (avg. 0.17 ± 0.07 year), allowing resuspended sediment to disperse over a long range. The export fluxes of 210Po varied from 68 to 121 dpm m−2 d−1 with an average of 96 ± 27 dpm m−2 d−1, which was 6 times that out of the euphotic zone. Using the 210Po deficits, the export fluxes of TPM out of the mesopelagic layer were quantified to vary from 4.19 to 10.20 g m−2 d−1, revealing a large amount of particles from the shelf to the SCS basin. This study suggests that 210Po‐210Pb could be an effective tracer of tracking particle cycling in mesopelagic water.

Hydrological and Biogeochemical Controls on Absorption and Fluorescence of Dissolved Organic Matter in the Northern South China Sea

“…This variation is possibly due to different flux rates and lability of sinking particles between the two regions. Compared to the WPS, SCS has greater primary production, downward and lateral fluxes of particle organic matter (POM) facilitated by the shift of monsoon strength, and frequent occurrence of strong internal waves in the SCS (Alford et al, ; Hung et al, ; Ma et al, ). Therefore, the higher concentration and larger contribution of recently produced POM to the middle water of SCS may have led to greater remineralization and concomitant production of humic‐like FDOM, reducing the M:C value compared to the WPS (Figure ).…”

Source and Fate of Dissolved Black Carbon in the Western South China Sea During the Southwest Monsoon Prevailing Season

“…Photodegradation (Stubbins, Niggemann, & Dittmar, ; Ward et al, ) and incorporation of DBC to sinking particle in particle‐abundant seas (Fang et al, ) have been thought to be responsible for the DBC leaving behavior within the euphotic zone. Suspended particulate matter (SPM) in the euphotic zone is usually low in the western SCS (0.35 mg L −1 ; Ma et al, ). In contrast, the SPM concentrations averaged 0.58 mg L −1 on the Chukchi and Bering Shelves (Fang et al, ), which was much higher than the SCS.…”

Particulate Export of PAHs Firstly Traced by 210Po/210Pb Disequilibrium: Implication on the “Shelf Sink Effect” in the Arctic Ocean

“…In addition, for analysis of particulate PAHs, 20 L of seawater samples were collected in the upper 0–30 m water column at stations BL13, BL16, and BM06, and 10 L seawater samples were collected for analysis of 210 Po and 210 Pb in the upper 0–30 m water column at stations BL13 and BL16. The sampling volumes are same as those given in our previous studies (Cai et al., 2016; Hu et al., 2014; Ke et al., 2017; M. Liu, Hu, et al., 2021; Ma et al., 2017). Details of the stations and sampling depth are shown in the Supporting Information in Table .…”

“…Besides the pair of 238 U/ 234 Th, the pair of 210 Pb/ 210 Po are also particle reactive and have quick sorption and sinking in seawater (Bacon et al., 1976; He et al., 2015). 210 Po (T 1/2 = 138.4 d), mainly produced from the decay of its grandparent 210 Pb (T 1/2 = 22.3 a), is easier to be absorbed onto biogenic particles than 210 Pb, hence deficient to its grandparent due to the insufficient accumulation time (Ma et al., 2017; Yang et al., 2015). Regarding their different biogeochemical behavior and particle affinity, 210 Po is regarded as a better tracer for biogenic particles, while 234 Th is more suitable for the total mass (Stewart et al., 2007; Verdeny et al., 2009; Yang et al., 2009).…”