| Anthropogenic activities and climate change have a significant impact on the global carbon cycle,which can be reflected in the increase of anthropogenic carbon(Cant)storage and the change of temperature-dominated oxygen consumption process in the ocean.As excellent transient tracers,Chlorofluorocarbons(CFCs)and Sulfur hexafluoride(SF6)are important tools for marine Cant research.They are widely used in the study of water mass mixing,ventilation and other oceanographic processes due to their good conservatism and known time source functions.Combined with the Transit Time Distribution(TTD)method with one-dimensional advection diffusion model,CFCs and SF6 can be used to estimate the mean age of water masses,further estimate the Cant storage and apparent oxygen consumption rate(AOUR)on this basis,and explore the influence of temperature change on the above biogeochemical parameters.The water exchange process in the form of "marginal sea-ocean" has important research value.In this study,typical sea areas at different latitudes from the Bering Sea(BS)to the western Arctic Ocean(WA)and from the northeastern South China Sea(NSCS)to the western Pacific Ocean(WP),were selected to complete seawater sample collection,analysis and data-processing of CFC-12 and SF6,to obtain the distribution characteristics of transient tracers.On this basis,hydrological and biogeochemical processes and relevant influencing mechanisms,such as ventilation difference,water mass transit time,mean Cant column inventory,and reaction dynamics characteristics of oxygen consumption are explored.The main conclusions are as follows.(1)The ventilation intensity of the WA is stronger than that of the BS on the whole under the mixed disturbance of multiple ocean currents and water masses.There are significant differences in the distribution of CFC-12 between the BS and the WA,where the surface layer is mainly controlled by sea water temperature,while the intermediate and deep layers are affected by different ventilation processes.The differences show as overall CFC-12 in the WA is higher than that in the BS,and there exists a CFC-12 maximum in the subsurface layer of BS.The mean transit time of Pacific inflow from the surface layer of BS to the southwest of Canada Basin(24.7<ae<26.2)is about 14.6±4.6 years.Compared with the Atlantic inflow,the Pacific inflow has longer transit time due to the discrete tansmission path.(2)The well-ventilated Atlantic water flows through the intermediate and deep layers of the WA,making the mean Cant column inventory(about 44.7 mol m-2)here is higher than that of the BS(about 25.2 mol m-2).In the WA,the Canada Basin has the lowest mean Cant column inventory compared to the other areas.The main reason is that the rapid melting of sea ice in recent years makes the mixing layer warm and reduces the absorption capacity of CO2 on the whole.(3)Luzon Strait provides a good water exchange channel between the SCS and WP,which makes the characteristics of temperature and salinity of water masses and the distribution of tracers have a high similarity.The mean age of water masses estimated by CFC-12 and SF6 shows that the ventilation in the WP is stronger than that in the NSCS under the influence of ocean currents such as Kuroshio.By establishing reference endpoints on both sides of the Luzon Strait,the mean age difference of water masses,namely the mean transit time caused by the three-layer circulation structure,can be observed as 8±5 years in the upper layer,39±22 years in the intermediate layer,and 20±18 years in the deep layer.The longer transit time of intermediate layer indicates that there is an additional westward transport mechanism in the Luzon Strait besides the density gradient.(4)The AOUR is further estimated to be 0.66～8.4 μmol kg-1 yr-1 in the NSCS and WP from the mean age and dissolved oxygen data.The warm vortex and Kuroshio formed vertical and horizontal transport mechanisms of oxygen-rich water respectively,which resulted in the abnormal trend of AOUR above 500 m in the WP.Based on the Redfield ratio,the organic carbon flux in the water column from 100 m to 1 500 m is estimated to be 1.7 mol C m-2 yr-1 in the NSCS and 1.3 mol C m-2 yr-1 in the WP.(5)The AOUR has long been conventionally regarded as a depth-dependent function.The results of Arrhenius equation and Eyring equation show a significant correlation between the AOUR and temperature,indicating that temperature is the key factor driving the oxygen consumption process in the NSCS and WP.Arrhenius activation energy Ea ranges from 87.7 kJ mol-1 to 154.2 kJ mol-1,which is higher in the WP than in the NSCS.The Gibbs activation free energy Δ≠G is stable at 140.5±0.9 kJ mol-1 in both sea areas.By contrast,the Eyring equation is closer to biological properties,and more suitable for enzymatic based biological reaction system such as oxygen consumption reaction in the ocean.The above study results are helpful to reveal the actual impact of ocean warming caused by global climate change and CO2 emissions on marine ecosystems,and may also provide theoretical support for the further assessment of ocean negative emissions and the demand of carbon neutrality. |
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