Particle Dynamics In Mesopelagic And Bottom Water As Revealed By The ²¹⁰Po- ²¹⁰Pb Pair In The South China Sea

Biogeochemical cycling of ²¹⁰Po and ²¹⁰Pb in seawater is one of the important contents given the ongoing GEOTRACES Program in order to extend their applications in oceanic environments.Recently,the disequilibria between ²¹⁰Po and ²¹⁰Pb has been used to trace the export of biogenic particles out of the euphotic zone,while only a few studies on particle recycling and sinking in mesopelagic water have been reported.Similar to ²¹⁰Po and ²¹⁰Pb,Be and 234Th are both radioactive isotopes with strong particle-reactivity.7Be/²¹⁰Pb ratio has been used to estimate sediment resuspension,and 234Th-238U has also been applied to constrain particle dynamics in bottom nepheloid layer.However,there is still not report on the application of ²¹⁰Po-²¹⁰Pb disequilibrium in tracing particle dynamics in mesopelagic and bottom water.In order to explore the applications of ²¹⁰Po-²¹⁰Pb in these oceanic settings,the South China Sea?SCS?was chosen as the target area to study the geochemical cycling of ²¹⁰Po and ²¹⁰Pb in mesopelagic and bottom water.The SCS is the largest semi-closed marginal sea in the tropical West Pacific Ocean.In this study,the differences in particle dynamics between the continental shelf and slope of the northern SCS were observed based on the ²¹⁰Po-²¹⁰Pb disequilibria.The sediment resuspension and its influence on particulate organic carbon export,for the first time,were quantified using the ²¹⁰Po-²¹⁰Pb pair.In addition,the scavenging and removal rates,export fluxes and residence times of ²¹⁰Po were evaluated in terms of the mesopelagic layer in the western SCS as well as the particle sinking.The primary results indicated that the ²¹⁰Po-²¹⁰Pb pair could be effective to constrain particle dynamics in aphotic zone.?1?Particle dynamics in mesopelagic water of the western SCS as revealed by ²¹⁰Po-²¹⁰Pb disequilibriaThe geochemical characteristics of ²¹⁰Po and ²¹⁰Pb in the upper 1500 m were examined in the western SCS.In the upper 50 m,²¹⁰Po is deficit with respect to ²¹⁰Pb,revealing a rapid removal of ²¹⁰Po.At the bottom of euphotic layer?100-200 m?,²¹⁰Po reached an equilibrium status with ²¹⁰Pb.In the mesopelagic layer?300-700 m?,the deficiency of ²¹⁰Po was observed,and the ratios of ²¹⁰Po/²¹⁰Pb were even as low as 0.4,revealing the strong scavenging and removal of ²¹⁰Po.Meanwhile,the concentrations of total particulate matter?TSM?in mesopelagic water were comparable to the euphotic layer,also indicating the strong sinking of particulate matter from the mesopelagic layer.Based on the deficiency of ²¹⁰Po,we figured out that the export fluxes of ²¹⁰Po in mesopelagic water ranged from 67.8 to 120.8 dpm m-2 d-1,higher than the euphotic layer?1.4-17.5 dpm m-2 d-1?.The residence times of ²¹⁰Po in mesopelagic water are shorter than the euphotic layer,reflecting rapid particle cycling in the mesopelagic layer.The export fluxes of TSM out of the mesopelagic layer ranged from 3.61 to 10.20 g m-2 d-1,which were higher than the euphotic layer?0.68-1.14 g m-2 d-1?,indicating the lateral transport of particulate matter in mesopelagic water of the western SCS.These results revealed that the ²¹⁰Po-²¹⁰Pb disequilibrium can be used to study the particle dynamics in mesopelagic water of the SCS.?2?Characteristics of particle dynamics on the continental shelf and slope of the northern South China Sea²¹⁰Po and ²¹⁰Pb were examined in the water columns on the shelf and slope of the north SCS.Elevated concentrations of TSM in bottom water indicated resuspension of sediments on the continental shelf.Meanwhile,the lower POC/TSM ratios and higher ²¹⁰Po/²¹⁰Pb ratios in bottom water than those observed at similar depths on the slope,further supporting the sediment resuspension.Based on the traditional model of ²¹⁰Po-²¹⁰Pb disequilibrium,the average POC export fluxes on the shelf and slope were 3.56±0.54 and 3.10±0.39 mmol m-2 d-1 respectively.While on the continental slope of the north SCS,the ²¹⁰Po/²¹⁰Pb ratios were higher than 1,and POC or PN export fluxes at 100 m were lower than 50 m at some stations,indicating that remineralization occurs in the thermocline on the continental slope of the north SCS in spring.The remineralization fluxes of POC and PN ranged from 0.97 to 1.19 mmol nm-2 d-1 and from 0.10 to 0.38 mmol m-2 d-1.On average,POC and PN remineralized for about 24%in the thermocline.?3?Sediment resuspension constrained by ²¹⁰Po-²¹⁰Pb disequilibrium on the continental shelf of north SCSThe bottom water on the continental shelf of north SCS affected by sediment resuspension,was considered as a mixture of general seawater and resuspended surface sediment.Combining the ²¹⁰Po/²¹⁰Pb ratio,a resuspension model was proposed to quantify the flux of sediment resuspension.The resuspension flux of TSM ranged from 28 to 494 mg m-2 d-1?averaging 264±124 mg m-2 d-1,?.Resuspension made up about 30%of the TSM sinking flux.The resuspension flux of ²¹⁰Po ranged from 0.20-3.56 dpm m-2 d-1?averaging 1.82±0.87 dpm m-2 d-1?,which made up 9.8±4.9%of the sinking flux.The resuspension fluxes of POC,PN and PBC ranged from 0.013 to 0.173 mmolC m-2 d-1?averaging 0.092±0.043 mmolC m-2 d-1?,from 0.003 to 0.020 mmolN m-2 d-1?averaging0.011±0.005 mmolN m-2 d-1?and from 0.004 to 0.052 mmolC m-2 d-1?averaging 0.026±0.012 mmolC m-2 d-1?,respectively,and accounting for about 3-4%?POC and PN?and 7.1 ±3.9%?PBC?of their sinking fluxes,respectively.These results indicated that the disequilibria between ²¹⁰Po and ²¹⁰Pb could be used to quantify the resuspension dynamics in bottom water.