Differential scavenging behavior of anthropogenic Pb revealed by sediment traps in the northern South China Sea basin
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Abstract: Trace metals emitted from human activities may have penetrated into the deep seas, and the underlying control mechanisms remain poorly understood. Sinking particles collected by moored time-series sediment traps from the northern South China Sea (NSCS) basin showed significant enrichment of anthropogenic aerosol Pb relative to lithogenic Fe. Total mass flux (TMF) was primarily driven by seasonal primary production, and significant positive correlations were found between Pb/Fe flux and major biogenic components, indicating the crucial role of the biological pump in Pb/Fe scavenging in the water column. Notably, Pb exhibited 30−50x higher affinity to biogenic components than Fe. A comparison was made between the enrichment factors of Fe and Pb in aerosols, euphotic particles, and sinking particles, which revealed that Pb exhibited significantly higher particle reactivity than Fe. This higher particle reactivity may encompass processes such as adsorption/desorption, bioaccumulation and decomposition release. The differential scavenging behavior of Pb suggested that the majority of Pb was rapidly scavenged in the euphotic zone and was preferentially released for accumulation in the twilight zone. This accumulation may further outflow through the Luzon Strait and result in the high dissolved Pb concentration observed in the subsurface water columns in both the NSCS and western Pacific Ocean. The rest of anthropogenic Pb in sinking particles tended to penetrate into deeper water layers and continue to be released below the twilight zone. These findings provide new insights into the biogeochemical cycling of trace metals originating from anthropogenic aerosols in marginal seas and serve as an example of the fate of other anthropogenic atmospheric pollutants.
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Key words:
- anthropogenic aerosol /
- Pb /
- sinking particle /
- biological pump /
- northern South China Sea
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Figure 1. The location of the SCS-N (18.5°N, 116°E, water depth
3736 m) sediment trap mooring station in the northern South China Sea with the diagram of the three synchronized traps array structure to the right (a). The time-series of wind (b), 8-day average AOD, daily precipitation and surface Chl a data (c) at the SCS-N station during June 2009 to May 2010. Blue shade in b and c indicates northeast monsoon, the grey dashed line denotes the dust storm event in 19th− 21st March 2010.
Figure 2. The temporal variations of total mass flux (TMF) (a), Al flux (b), Fe flux (c), Pb flux (d), EFFe (e) and EFPb (f) of sinking particles. The date labels on the x-axis indicate the mid date of each sampling period. For all depths full and hollow symbol denote, respectively, the low and high particle flux period. The arrow with numbers 1, 2 and 3 represent the three uncommon samples with abnormal biogenic relationships.
Figure 3. The vertical distribution of EFFe (a), EFPb (b) and POC : Al molar concentration ratio (c). Full and hollow symbol stand, respectively, for the low and high particle flux period. The reference values of EFFe and EFPb were calculated by dividing the reported trace metal to Al ratios in aerosols, 30−160 m sinking particles and sediments by the trace metal to Al ratios in the average crustal abundance (Table S3). The reference values of POC : Al in 30−160 m sinking particles and sediments were also listed in Table S3. Dissolved Pb concentration data in b were collected from Kuroshio transect (Chen et al., 2023b).
Figure 4. Correlation between the EFFe, EFPb and the ratios of OM : LM, Opal : LM, and CaCO3 : LM, respectively. OM and LM represent organic matter and lithogenic matter, respectively. Full and hollow symbol denote, respectively, the low and high particle flux period. The arrow numbers 1, 2 and 3 represent the three uncommon samples with abnormal biogenic relationships. Shades of dark grey and light grey represent 95% confidence band and prediction band, respectively.
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