海洋储碳机制及相关生物地球化学过程研究策略

1碳中和国家战略需要海洋负排放支撑工业革命以来,人类活动导致大气CO_(2)激增,加剧了气候变化,引发一系列社会、经济和环境问题.第75届联合国大会以来,习近平总书记在一系列重大场合多次强调,我国二氧化碳排放力争于2030年前达到峰值,努力争取2060年前实现碳中和.这个重大举措不仅大大提升了我国的国际话语权,也吹响了我国科技-产业-政策联合攻关的“集结号”.

Carbon pools and fluxes in the China Seas and adjacent oceans

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km^2 from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semiclosed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea,Yellow Sea, East China Sea, and South China Sea, including different interfaces(land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems(mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO_2 sources, releasing about0.22 and 13.86–33.60 Tg C yr^(-1) into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92–23.30 Tg C yr^(-1) of atmospheric CO_2, respectively. Overall, if only the CO_2 exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO_2, with a net release of 6.01–9.33 Tg C yr^(-1), mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon (DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr^(-1),respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr^(-1), significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr^(-1), respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25–36.70 and 43.93 Tg C yr^(-1), respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36–1.75 Tg C yr^(-1), with a dissolved organic carbon(DOC) output from seagrass beds of up to 0.59 Tg C yr^(-1). Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr^(-1) and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr^(-1), respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72–104.56 Tg C yr^(-1). The DOC efflux from the East China Sea to the adjacent oceans is 15.00–35.00 Tg C yr^(-1). The DOC efflux from the South China Sea is 31.39 Tg C yr^(-1). Although the marginal China Seas seem to be a source of atmospheric CO_2 based on the CO_2 flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export,and microbial carbon pump(DOC conversion and output) indicate that the China Seas represent an important carbon storage area.

Carbon fluxes in the China Seas: An overview and perspective

This paper aims to provide an overview of regional carbon fluxes and budgets in the marginal seas adjacent to China.The "China Seas" includes primarily the South China Sea, East China Sea, Yellow Sea, and the Bohai Sea. Emphasis is given to CO_2 fluxes across the air-sea interface and their controls. The net flux of CO_2 degassing from the China Seas is estimated to be9.5±53 Tg C yr^(-1). The total riverine carbon flux through estuaries to the China Seas is estimated as 59.6±6.4 Tg C yr^(-1). Chinese estuaries annually emit 0.74±0.02 Tg C as CO_2 to the atmosphere. Additionally, there is a very large net carbon influx from the Western Pacific to the China Seas, amounting to ～2.5 Pg C yr^(-1). As a first-order estimate, the total export flux of particulate organic carbon from the upper ocean of the China Seas is 240±80 Tg C yr^(-1). This review also attempts to examine current knowledge gaps to promote a better understanding of the carbon cycle in this important region.

Carbonate dynamics in a tropical coastal system in the South China Sea featuring upwelling, river plumes and submarine groundwater discharge

This study examined carbonate dynamics in the northwestern South China Sea(NWSCS),an area jointly influenced by upwelling,river plumes and submarine groundwater discharge.Data were obtained from two cruises conducted in summer 2009 and 2012.In 2009,a high salinity-low temperature water mass occurred nearshore off northeastern Hainan Island,indicative of upwelling,commonly referred to as HNEU.A river plume fueled primarily by local rivers and characterized by low salinity and high temperature was observed in the NWSCS off the mainland roughly along the 30 m isobath.In 2012,coastal upwelling off northeastern Hainan Island was not detectable at the surface,but was observed at a different location off eastern Hainan Island(HEU).River plume waters in 2012 were patchily distributed,with a low salinity zone further westerly than that in 2009 and another on the mid-shelf of the NWSCS sourced from the Pearl River which reached out~250 km from the mouth of the Pearl River Estuary.In 2009,elevated dissolved inorganic carbon(DIC)and total alkalinity(TA)occurred in the coastal plume,where submarine groundwater discharge contributed DIC and TA additions of 38.9±20.5 and 42.5±22.3μmol kg^(−1),respectively,with a DIC/TA ratio of~0.92,which made a minor contribution to the variation of seawater partial pressure of CO_(2)(pCO_(2)),pH and the aragonite saturation state index(Ω_(arag)).Additionally,high surface phytoplankton production consumed DIC of 10.0±10.4μmol kg^(−1) but did not significantly affect TA,which dominated pCO_(2) drawdown in the coastal plume water and increased the pH andΩ_(arag) at surface.Submarine groundwater discharge was also observed in the region influenced by upwelling,but to a lesser degree than that impacted by coastal plume.Lower pH andΩ_(arag) and higher pCO_(2) values than in offshore waters were observed downstream of the upwelling system,attributable largely to organic matter remineralization with a DIC addition of 23.8±8.4μmol kg^(−1).In 2012,submarine groundwater discharge was not detected but high phytoplankton production dominated carbonate dynamics in the coastal plume water with a net DIC consumption of 104.2μmol kg^(−1),which markedly drew down sea surface pCO_(2) and increased pH andΩ_(arag).In the Pearl River Plume,the solubility-driven CO_(2) sink exceeded biological CO_(2) uptake,resulting in an additional decrease of pH andΩ_(arag) and increase of seawater pCO_(2).Taken together,this study demonstrated complex spatial and year-to-year variability,and the controls of the carbonate system under the joint modulations of upwelling,river plumes and submarine groundwater discharge.A first order estimate that considered the rise of atmospheric CO_(2) and seawater temperature further suggested a high risk of ocean acidification in this coastal area by the end of this century,which could be amplified under the stresses of river plumes,submarine groundwater discharge and organic matter remineralization.

Fundamental Research and Policy on Carbon Neutrality

The emission of anthropogenic CO_(2) into the atmosphere since the Industrial Revolution and its subsequent distribution across the oceanic,land,and atmospheric components of the Earth system have led to an unprecedented perturbation of the global carbon cycle with severe consequences,as demonstrated by the ongoing climate crisis(e.g.,Gruber et al.,2019;IPCC,2021,2022)[1-3].These human-induced changes of the climate system may have exceeded the safety threshold,resulting in profound impacts on the global economy and sustainable development.In response to the crisis,the Paris Agreement defines climate-warming targets of 2℃ and 1.5℃,associated with end-of-century atmospheric CO_(2) concentrations of ca.450 and 400 ppm,respectively.These scenarios approach carbon neutrality by ca.2070 and 2055,respectively,and remain negative thereafter[4].More than 130 countries have signed the Paris Agreement and proposed emission reduction roadmaps to achieve carbon neutrality.China has pledged to reach carbon emission peak by 2030 and to achieve carbon neutrality by 2060,showing China’s strong will to strengthen its national strategy of sustainable development and its ambition,as part of the global force,to fight against the ongoing climate crisis.