Source And Fate Of Black Carbon In The South China Sea And Polar Oceans And Its Influences On Ocean Carbon Cycling

Black carbon(BC),or pyrogenic carbon,is the residue of incomplete combustion of biomass and fossil fuels.Globally,rivers and atmospheric deposition contribute about 60 Tg(1 Tg=1012 g,equivalent carbon content,hereinafter)of black carbon,including particulate BC(PBC)and dissolved BC(DBC),to the oceans annually,comprising more than 10%of the total riverine dissolved organic carbon(DOC)flux.In the open ocean,DBC comprises more than 2%of the DOC pool.However,the DBC cycling in the worldwide oceans and its role in affecting marine organic carbon cycle have remained unknown.In order to shed light on the sources and fate of BC in the oceans,DBC in the South China Sea and Prydz Bay,and PBC in the western Arctic Ocean were constrained using BPCA and CTO-375 methods,respectively.The South China Sea is located in the southeastern of Asia,where contributes more than one-third of the global BC emission.The DBC concentrations range from 0.49 to 1.60 ?mol L-1 in the western South China Sea.Spatially,the Mekong River plume(i.e.,MR-plume)shows higher DBC concentrations and(B6CA+B5CA)/(B4CA+B3CA)ratios(i.e.,RH/L)in the mixed layer than its ambient seawater.In addition,the DBC concentration has a good linear relationship with salinity in the mixed layer of the MR-plume,indicating that fresh water discharged by the Mekong River is the major source of DBC in the western South China Sea.More than half of DBC derived from the river input and atmospheric deposition is removed in the euphotic zone based on a steady-state box model.DBC shows homogenous distribution in mesopelagic water(500?1500 m),ranging from 0.70 to 0.85 ?mol L-1.In contrast,RH/L increases with depth and has an inverse function with salinity,suggesting that RH/L is a conservative proxy in mesopelagic water and can be used to trace the mixing process of water masses.The Prydz Bay and its adjacent sea are crucial areas in terms of the formation of Antarctic Bottom Water.In the study area,the DBC concentration decreases from the inner Prydz Bay to basin area.Dense shelf water(i.e.DSW),resided in the bottom of Prydz Bay,shows high DBC concentrations with an average of 1.17 ?mol L-1,which is higher than that of 0.87 ?mol L-1 in surface water on the shelf and 0.49 ?mol L-1 in the circumpolar deep water.These results indicate that sediments are likely an important source of DBC in the Prydz Bay.The DOC concentration shows a similar distribution pattern to DBC,but its highest value was observed in shelf surface water,ascribing to the elevated primary production in summer.Based upon a conservative mix model and oxygen isotope signatures(? 18O),the downward transport of DSW is expected to entrain DBC and DOC from the Prydz Bay to the deep Southern Ocean.A preliminary estimation indicates that the downward transport of DSW could contribute 4-9%(66?150 Gg yr-1,1 Gg=109 g)and 2?4%(3700?8500 Gg yr-1)of DBC and DOC concentrations in Antarctic Bottom Water,respectively.Therefore,we conclude that the downward transport of DBC from the Antarctic shelves is likely a significant source of DBC to the deep waters of the Southern Ocean and global oceans.The abundance of PBC in the western Arctic and subarctic oceans were examined for the first time.In the central Arctic Ocean,the PBC concentrations are high in the ice marginal zone(i.e.,IMZ),averaging 0.021 ?pmol L-1.A number of parameters,including temperature,salinity and 234Th/238U ratios,indicate that both the rapid release of PBC from sea ice melting and a slow sinking rate are responsible for the high PBC concentrations.Based upon the disequilibria between 234Th and 238U,The export flux of PBC out of the mixed layer was estimated to be 290?680 Gg yr-1 for the Chukchi and Bering Shelves(i.e.,CBS),which are comparable to the export and/or burial fluxes of PBC obtained in some coastal areas.This result suggests that the western Arctic Shelf is probably an effective location for burying BC.In addition,the export flux of PBC estimated here is higher than the total input of PBC from all river runoff and atmospheric deposition to the CBS,indicating additional source terms of PBC.Our results shows the different geochemical behaviors of DBC between surface and mesopelagic oceans.River is the major source of DBC for oceans with abundant fresh water input.DBC derived from river discharge and atmospheric deposition are significantly removed in the euphotic zone probably due to photo-degradation and/or absorption to sinking particles,resulting a short term cycling of DBC in the upper ocean.On the contrary,DBC is homogenous in mesopelagic water,implying its persistent character and long term cycling in the deep ocean.On the Antarctic shelves,the downward transport of DSW allows DBC to be rapidly transported away from the sunlit surface water to the deep ocean,bridging the gap between the rapid DBC cycling in surface water and slow DBC cycling in the deep ocean.These works offer a framework on BC cycles in our study areas and advance our understanding on BC and carbon cycling in the global ocean.