Distribution And Behavior Of Dissolved Lead In The Antarctic Peninsula And The Northwestern Pacific Ocean

Lead(Pb)is one of the main trace elements studied in the international GEOTRACES program.Pb in the environment is mainly released from human activities,the study of Pb in the ocean could be used as a tool to explore human footprints in the marine environment.Since the 1970s,studies have focused on the temporal and spatial variations of the dissolved Pb(DPb)in the oceans as well as their relationship with the process of industrialization.However,due to that DPb is of low concentration and is easily contaminated,the geochemical study of DPb in the ocean remains insufficient,such as the response of DPb to the glacial degeneration in the polar marine environment,the fluxes and processes of continental run-off,and the transportation of DPb through physical circulation.This study focused on the distribution and constraints of DPb in three different areas,the Fields Peninsula in Antarctica,the shelf area of the East China Sea(ECS)and the tropical northwestern Pacific.On the premise of high-quality data,the export fluxes and processes of Pb from Antarctic glaciers and the East China Sea shelf,and the transportation of DPb in the Pacific Ocean through physical circulation were also discussed.In the method,the self-made X-Vane sampler was applied for trace samples,and the X-Vane sampler was calibrated with the MITESS sampler which was approved by GEOTRACES to ensure the accuracy of the collection of DPb samples.The Pb isotope ratios were measured using the Neptune MC-ICP-MS.The NTA resin and isotope dilution method were coupled in the DPb concentration analysis method to concentrate and isolate the DPb in seawaters.The process blank was 1.3 pmol/kg and the analysis results of intercalibration SAFe samples were consistent with reference values.In the analysis method of DPb isotopes,Presep^?Polychelate resin and AG1-X8 resin were applied to concentrate and purify the DPb in seawater.The recovery of the entire process was close to 100%,and no isotope fractionation was found.The Pb isotope method was cross-calibrated with MIT Boyle Laboratory to verify the accuracy of the analysis method.On the Fields Peninsula,the concentration of DPb in the surface ice and snow was3.6–8.0 pmol/kg,which was 7–9 times lower than the peak period of industrial Pb emissions between 1960 and 1980,but 2–4 times higher than that from natural sources(1.9–2.4 pmol/kg).The concentration of DPb in rivers ranged from 0.3 to 14.0 pmol/kg,and it was influenced by melting snow,ice,and glaciers,as well as local human activities,with underground waters having a less significant influence.The concentration of DPb(6.6–843.0 pmol/kg)in Maxwell Bay varied spatially due to local human activities such as vessel docking and shipping.Lower values were observed in coastal areas affected by subglacial melting waters.Combined with previous studies,a large amount of industrial Pb has been buried in the glacier since 1884,and this buried Pb will be released as a result of global warming,which could become an important factor affecting the Pb distribution pattern in the future for the Southern Ocean.The current Pb flux from the atmospheric deposition to the Southern Ocean was approximately 2.8–5.6 t/yr,while the glacial Pb flux was also estimated of 0.6–0.8 t/yr based on the current annual glacier loss rate in the Antarctic.Moreover,the diverse isotopes of Pb at different times in history were also well preserved in the glacier,which are proposed to become a new challenge in the Pb source identification studies as the release and mixing process of the glacier Pb into the Ocean interior.With the intensification of global warming,the projected release of buried Pb will increase significantly.In the shelf area of the ECS,DPb concentration was 41.9–96.7 pmol/kg,which was higher than that in the upstream of the Kuroshio Current(20.0°N,40.8–70.8pmol/kg)and the Kuroshio Current(KC)in the ECS shelf break(28.2°N,23.8–71.8pmol/kg).The distribution of DPb in the shelf area was mainly affected by the atmospheric deposition and the physical mixing of water masses.DPb in the ECS was mainly from the atmospheric deposition,the intrusion of open ocean waters and the inputs of the Changjiang diluted water,and they accounted for 56%,44%and<1%of the total annual Pb inputs,respectively.The output of DPb from the shelf of the ECS to the northwestern Pacific Ocean was estimated,of which approximately(1.1–1.7)×10³t/yr of DPb is exported through the across-shelf transport,and approximately 1.4×10³t/yr of DPb is exported through the Tsushima/Korea Strait.The increase of DPb concentration in the Kuroshio subsurface water was significant,which revealed the main layers of the cross-shelf DPb export,but in the analysis of Pb isotopes,the changes of Pb isotope influenced by the shelf sources were also found in the Kuroshio intermediate water and Kuroshio bottom water.We suggested that it may be related to the influence of sediments,which is consistent with the previous findings in the deep Indian Ocean that‘in some cases,the isotope exchange of DPb and Pb in particulate matters could be carried out without the change of concentration'.In the tropical northwestern Pacific,the total dissolvable Pb(TDPb)concentration varied from 16 to 70 pmol/kg in the upper 750 m,and its vertical distribution was constrained under hydrological structures.In the area occupied by the North Equatorial Countercurrent(NEC)and KC,Pb vertical profile were characterized by the significant subsurface maximum.In addition,the area occupied by the New Guinea Coastal Undercurrent,TDPb concentration was significantly reduced and uniformly distributed in the vertical profile.There were also significant differences in Pb isotopes between these two areas,indicating the Pb sourced from Asian ores and Australian ores,respectively.In the convergence area of currents,TDPb behaved consistently with the salinity,revealing the conservative behavior of TDPb in the transport of currents.The‘subsurface maximum'characterize was also found in other studies of the North Pacific subtropical gyre.Combined with previous studies,it is found that dissolved Pb peaks appeared only in the modal waters and in the intermediate water of the North Pacific.These waters link up to the mix layers of 30°N–40°N in the Northwest Pacific which receives the highest amount of Asia-sourced atmospheric Pb depositions.Then,the high surface Pb was subducted and southward isopycnal transport along with mode waters and the intermediate water.Although the Pb concentration was reduced along the transportation pathway,the ratios of Pb isotopes remained consistent and clearly distinguished from those of other water masses.The lateral transport carried in currents was also estimated.The lateral transport of TDPb in the NEC is approximately(16.8–22.5)×10³ t/yr,which could be considered as the convergence budget of the advection transport of the North Pacific.With the bifurcation of the NEC,approximately(4.5–9.0)×10³ t/yr of Pb is distributed into the northward-flowing Kuroshio Current,which will rejoin the subtropical circulation of the North Pacific,and another(7.8–18.0)×10³ t/yr of Pb is distributed into the southward-flowing Mindanao Current,which will join the tropical circulation of the Pacific or enter the Indian Ocean via the Indonesian through-flow.This process also completes the trans-basin export of TDPb in the subtropical gyre of the North Pacific.Water fluxes in the western boundary current system is also affected by climate change,so the residence time of TDPb in the subtropical gyre of the North Pacific as well as the trans-basin export pattern may also change.In the deep water of the tropical northwestern Pacific,Pb isotopes showed mixed characteristics of Chinese ores and natural sources.Combined with previous observations in other areas,anthropogenic Pb has reached every corner of the deep Pacific.Anthropogenic Pb accounts for 40%,63%and 20%of the total dissolvable Pb in deep waters of the tropical northwestern Pacific,mid-and high-latitude northwest Pacific and high latitude north Pacific,respectively.In this study,the distribution and behavior of DPb in the studying areas under the forces of human activities were discussed,and the Pb contribution of glacial and marginal sea inputs to open oceans was also discussed and quantified.This study also explores the conservative transport of Pb carried in open ocean currents and quantifies the trans-basin export of Pb in the subtropical gyre of the North Pacific.Moreover,the fingerprint characteristics of Pb can be used to trace the transport pathways of water masses.In the case of traceable anthropogenic Pb emissions,the behavior of Pb in the ocean can provide more effective information about climate change and physical ocean processes.