| The Indo-Pacific Intersection(IPI)is where the tropical western Pacific Ocean and the eastern Indian Ocean intersect.The Indonesia Seas(IS)and South China Sea(SCS)are adjacent to each other,and are the two main regions in Indo-Pacific Intersection.The IS are located between Pacific Ocean and Indian Ocean.The numerous islands and interlaced pathways within the IS,together with climate sensitivity,make the circulation and biogeochemical activities of IS complex,intense,and variable.The water transfer between the Pacific and Indian Oceans,commonly referred to the Indonesian Throughflow(ITF),is the only tropical inter-basin oceanic pathway.As an integral element of the global meridional overturning circulation,the ITF connects these two warm pools,which plays a critical role in the redistribution of heat and mass,affecting both the regional and global climate systems.SCS,as the largest marginal sea in the North Pacific Ocean,has a complex water mass exchange processes with the Western Pacific Ocean and other marginal seas.This results in a complex physical circulation structure and active biogeochemical cycles within SCS.The SCS Throughflow(SCSTF),a branch connecting Pacific and Indian Oceans,provides a source of fresh water and heat to the IS.The IS and the SCS together constitute the core area of the Coral Triangle and the Indo-Pacific Triangle that have an intense sea-landair interaction,and a key zone of global climate change and global biodiversity.Thus,due to the complex circulation structure and various material sources and sinks in the two regions,the physical and biogeochemical processes in the two regions are variable and difficult to be observed directly.Anthropogenic radionuclides are the radionuclides produced as a byproduct of the development and utilization of nuclear energy and technology.The definite source and constant radioactive decay rate are the inherent advantages of using anthropogenic radionuclide as a tool to trace some environment problems.Tritium(3H),a radionuclide primarily released from nuclear weapon testing that has a half-life of 12.32 years,can be used as a transient tracer to constrain the timescale of various ocean processes.Assessing 3H with a combination with its decay product helium-3(3He)permits calculation of the age of a specific water mass ranging from half year to 50-60 years(45 times the half-life).This range encompasses the residence time of the IS and SCS.Plutonium(Pu),an another primarily anthropogenic radionuclide that is released from Pacific Proving Grounds(PPG)in Pacific Ocean,is a powerful tool for source tracing due to the various isotopic compositions of Pu that are released from specific sources.The sources can be determined by examining the ratio of the Pu isotopes such as 240Pu/239Pu(global fallout and close-in fallout)in Pacific Ocean.In this study,Pu isotopes and 3H describe the oceanic circulations and biogeochemical processes in the two key regions of Indo-Pacific Intersection(IS and SCS)on a spatiotemporal scale.The results are summarized below:The ITF transfers water masses with high 3H from the north Pacific to the Indian Ocean,resulting in an imbalance of 3H reserves between the south Pacific and the south Indian Ocean.As such,the long-term mean water mass transport of the ITF is calculated,based on the distributions and changes in the tritium inventory and budgets in and between the South Pacific and Southern Indian oceans from the Geochemical Ocean Sections Study(GEOSECS)and the World Ocean Circulation Experiment(WOCE)Program datasets.The total through-flow of the ITF is estimated as 16.2±3.5 Sv.The North Pacific contributes 14.1±2.7 Sv to the total throughflow,of which the layers between 0-250 m,250-650 m,and 650-1000 m transport 7.3 ± 0.6 Sv,5.4 ± 1.3 Sv,and 1.4 ±3.0 Sv,respectively.Along with the water transport,the North Pacific component of the ITF transfers 0.84 ± 0.14 PW of heat into the Indian Ocean.Heat loss before the ITF waters exit the Indian Ocean from 30°S is minimal.The horizontal transport of nutrients by ITF is limited,and vertical diffusion plays an important role in nutrient supply to the photic zone in Indonesian seas.Then,we focus on the vertical processes of substance using 3He as the tracer in the Indonesian seas.Helium isotopic composition(?3He)in Indonesian seas shows that the full water column,including surface water,contains excess 3He.Helium in the upper water is in dynamic equilibrium with respect to the interaction of the air-sea exchange and water mixing.Therefore,based on the mass balance between air-sea exchange and vertical mixing processes of excess 3He in the IS mixed layer,we estimate the vertical mixing coefficient is between 5.5×10-5?4.9×10-3 m2/s,with a mean value of 8.3×104 m2/s.In the entrance and exit of Makassar Strait and the Lifamatola Passage,the Kz value is higher,and the Kz value is lower in the central of Makassar strait and the Banda Sea.On this basis,we estimate the vertical flux of nutrients in the IS has a similar spatial distribution with Kz.Overall,the average vertical flux of nitrate in the IS is 5.8 mmol·m-2·d-1,which supports the new productivity of 461 mg-c·m-2·d-1.Different sources of Pu have distinct Pu isotope compositions in the IS surface circulation pathway.Therefore,based on the distribution pattern of Pu concentration(240+239Pu)and isotope ratio(240Pu/239Pu)in the IS,we found that the water mass from SCS enters the western Sulawesi Sea and Makassar Strait through the Sibutu Passage,and its contribution to Pu in this region is 28%-48%.The SCS water mass also flows into the main route of ITF through the Karimata Strait,Java Sea and Flores Sea and then enters the Banda sea and Lifamatola strait,contributing 21%-41%to the Pu in this region.The SCS water mass also be found at the outflow of ITF(Ombai and Timor),contributing 20%-75%to the Pu for surface water in this region.In the SCS,the water column will accumulate 3He at a constant rate due to the decay of tritium when isolated from the surface.Therefore,by using the salinity-normalized potential alkalinity(NPA),we distinguished the(tritiugenic)3He from hydrothermal 3He,and thereby estimating the tritium/3He ages.Together with the observed oxygen concentrations,we were able to estimate the apparent oxygen utilization rate(AOUR)for water masses with ages spanning from several years to ca.50 years.Calculated AOURs are 4.15±0.27 ?mol·kg-1·yr-1 between 100-500 m and a much lower value(0.81±0.23?mol·kg-1·yr-1)between 800-1000 m.Based on depth-integrated AOURs between 100-1000 m,we derived a mean export flux of carbon with the time scale of several decades,1.97±0.18 mol C m-2 year-1.In conclusion,the ITF transfers 14.1 ± 2.7 Sv of water mass and 0.84 ± 0.14 PW of heat from North Pacific Ocean into the Indian Ocean.However,the nutrient transportation by ITF is limited.Vertical mixing is the main nutrient source for the upper layer water in Indonesian Seas,which supplies 5.8 mmol·m-2·d-1 of nitrate.IS surface waters are affected by the inflow of SCS water masses,which contributes 20%-75%Pu to the southern IS.The SCS has a high production,which exports 1.97±0.18 mol C m-2 year1 carbon out the upper 100m water.Thus,the high-flux horizontal transport,strong vertical mixing,and abundant material sources and sinks in the two key regions(IS and SCS)of the Indo-Pacific interaction provide the material and energy basis for the region as a climatological and biological center. |
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