On The Distributions,Fractionation,and Controls Of Stable Barium Isotopic Compositions In The Upper Water Column Of The South China Sea

The element barium(Ba)has been widely used for tracing present and past biogeochemical processes in the ocean.Dissolved Ba(DBa)is a useful tracer of ocean circulation,water mass mixing,and river inputs.Excess particulate barium(pBaxs)or barite fluxes to the deep ocean and their accumulation rates in sediments have been widely applied to reconstruct biological productivity.However,several problems fundamental to understanding biogeochemical Ba cycling remain exclusive.The actual mechanisms resulting in a mesopelagic pBaxs maximum in the water column are under debate and the poor preservation of barite in anaerobic sedimentary environment also limits the application in paleooceanography.The recently established stable Ba isotopic systematics can be used as a new proxy for better understanding key processes in the oceanic Ba cycling,which is also important to reconstruct carbon cycle in the past ocean.This dissertation systematically investigated the distributions and controls of both dissolved and particulate Ba isotopic compositions and the fractionation between them in the upper ocean.In addition,the remineralization flux of particulate organic carbon(POC)in the twilight zone was estimated by pBaxs concentrations.In January 2010 from the outer shelf to the lower slope of the northern South China Sea(NSCS),the vertical profiles of DBa in the upper 150 m displayed small variations between stations(31-37 nmol kg-1)and all of their Ba isotopic compositions(δ138BaDBa)values were within analytical uncertainty identical to+0.60‰.The isotopic signatures of suspended particulate Ba(δ138BapBa)were nearly constant at+0.1 to+0.2‰throughout the upper 150 m water column of the entire study area,which are lighter than δ138BaDBa by 0.4-0.5‰.This contrast indicates significant Ba isotope fractionation between particulate and dissolved phases in the NSCS euphoric zone.The primary DBa source in the upper NSCS during the sampling period was vertical mixing from the underlying waters below 150 m.Using the average of DBa and δ138BaDBa values collected from 300 m depth as initial values,the estimated in situ fractionation factor between particulate and dissolved Ba were-0.4 ± 0.1‰ following a Rayleigh model and-0.5±0.1‰ following a steady state model.DBa concentration variations in the upper 150 m of the water column were decoupled from phosphate,nitrate,and silicate.While the concentration of the major nutrients decreased from the surface mixed layer with increasing depth,DBa concentrations were almost constant.Moreover,pBaxs varied independently against chlorophyll a(Chl-a)in the upper 150 m of the NSCS outer shelf and slope,in which significant positive correlations were observed between particulate calcium,particulate organic carbon and nitrogen,and biogenic silica and Chl-a.The mechanism controlling pBaxs formation is thus different from that of biogenic particulate matter.We suggest that Ba isotope fractionation in the euphotic zone of the NSCS is primarily induced by preferential adsorption of the lighter Ba isotopes onto particles rather than by biological utilization.In June 2017(summer)and January 2018(winter)),we collected dissolved and particulate Ba samples at station SEATS in the NSCS and station SS1 in the southern SCC(SSCS),as well as at station SEATS in March 2018(spring).The vertical distributions of δ138BaDBa generally mirrored those of DBa concentrations throughout the water column and values of both at any depth were comparable at the two stations in different seasons.pBaxs concentrations were relatively low in the surface waters and reached a maximum at 100-150 m,which then decreased down to 1000 m water depth.At stations SEATS and SS1,the mesopelagic maximum pBaxs concentrations in summer were nearly twice of those in winter,which are corresponded by higher summer POC export fluxes from the euphotic zone at both stations.Based on an algorithm relating pBaxs contents and oxygen utilization rate in the twilight zone and the Redfield ratio,POC remineralization fluxes were estimated to be 4.61 mmol m-2 d1(SEATS summer),1.49 mmol m-2d-1(SEATS winter),5.59 mmol m-2d-1(SS1 summer),and 0.46 mmol m-2d-1(SS1 winter),respectively.The ratio of these flux to the POC export fluxes were estimated to be 177%,83%,238%and 27%,which are within the range reported by previous work.Moreover,we observed slightly lighterδ138BapBa signatures of+0.15±0.04‰ at the pBaxs concentration maximum layer at station SEATS in spring,suggesting potential Ba isotope fractionation during barite formation in mcroenvironments of the twilight zone.However,this isotope effect needs further dedicated examination.To sum up,stable Ba isotope fractionation observed in the euphotic zone of the NSCS is mainly controlled by passive adsorption onto particles,while the fractionation likely occurs during barite precipitation in the twilight zone.The formation of mesopelagic particulate barium maximum is mainly related to POC remineralization the fluxes of which estimated by particulate barium inventory are within a reasonable range.Therefore,stable Ba isotopes can be used to constrain the linkage between oceanic Ba and carbon cycles and as a potential proxy of paleoproductivity.