Seasonal Variation Of ²³⁴Th And Its Constraint On Export Flux Of Biogenic Elements In Semi-enclosed Bays

Particles transport biogenic(i.e.,C,N,P,and Si)and particle-reactive elements from the surface ocean to the deep ocean.Thus,well-understanding of the particulate export over different temporal and spatial scales is crucial to evaluate the biological pump in the ocean.The coastal seas are characterized by high primary productivity.With the global climate change,many coastal seas show increases in the primary productivity.Therefore,the carbon cycle in coastal seas is of great importance for the global carbon budget.However,particulate organic carbon(POC)sinking in coastal seas has been less studied,partly due to the influence of resuspension of sediment and remineralization of POC.In order to evaluate the POC export flux and the efficiency of biological pump in coastal waters,two semi-enclosed shallow bays(i.e.,Daya Bay and Jiaozhou Bay)were selected to examine the variability of 234Th on the seasonal timescale,the interactions between 234Th scavenging and particulate components,as well as the constraint of 234Th-23’U disequilibria on POC export and other biogenic elements.In Daya Bay,the activity concentration of 234Th varied from 0.45±0.06 to 2.87±0.20 dpm L-1,averaging 1.33±0.44 dpm L-1.Autumn showed the highest value,followed by summer,spring,and winter(t-test,p<0.05).The abundance of different particulate components and their affinities to 234Th together determined the contribution of each component in scavenginl 234Th.In spring,summer,and winter,particulate organic matter(POM)and biological silica(bSi)were the main components of 234Th scavenging(>50%)with stronger affinity to 234Th.In autumn,the most abundant lithogenic particles accounted for the major contribution(68%)of 234Th.The residence times of 234ThT(38-72 d)and 234Thp(18-35 d)in Daya Bay were shorter than that of seawater(90 d),indicating that 234Th cycle in Daya Bay was mainly confined in the inner bay.The export flux of 134Th also showed evident seasonal variation in Daya Bay.October-January showed the highest flux of 374±25 dpm m-2 d-1,followed by January-April(300±29 dpm m-2 d-1)and April-August(260±27 dpm m-2 d-1).The lowest 234Th flux of 219±33 dpm mm-2 d-1 was observed from August to October.The POC flux in Daya Bay was the lowest(7.0±1.2 mmol-C m-2 d-1)from August to October,and it was close to 11.1±1.0 mmol-C m-2 d-1(mean±sd)in the other seasons.The annual POC flux in Daya Bay was 0.30±0.02 mmol-C cm-2 yr-1,higher than that in the South China Sea shelves and basins.On the annual timescale,the efficiency of biological pump in Daya Bay was 15±1%,highlighting the efficient carbon sequestration in the subtropical Chinese coastal seas.The average annual export fluxes of particulate nitrogen(PN),bSi and particulate phosphorus(TPP)in Daya Bay were 2.0±0.1 mmol m-2 d-1,2.5±0.2 mmol m-2 d-1,and 157±12 μmol m-2 d-1 respectively.The bSi flux was obviously higher than that in the South China Sea,reflecting an effective burial site for biological silica.In Jiaozhou Bay,the activity concentration of 234Th ranged from 0.88±0.07 to 3.30±0.23 dpm L-1 with an average of 1.66±0.54 dpm L-1.The highest activity was observed in winter,followed by spring,autumn,and summer(t-test,p<0.05).In spring,summer,and winter,POM played a dominant role in scavenging 234Th(>60%),with the strongest affinity to 234Th.In autumn,lithogenic particles showed the largest contribution(68%)to the adsorption of 234Th due to its higher content.The residence time of 234ThT in Jiaozhou Bay(79-88 d)was longer than that of seawater(70 d),implying the input of 234Th from the Yellow Sea.The residence time of 234Thp in Jiaozhou Bay(36-41 d)was much smaller than that of seawater,indicating that 234Th removal in Jiaozhou Bay was sinking rather than output to the Yellow Sea.The export fluxes of 234Th also presented seasonal variations with the highest value(160±21 dpm m-2 d-1)and lowest value(-89±29 dpm m-2 d-1)during May-August and February-May,respectively.POC fluxes during Novemeber-February and February-May are expected to be lower than that during May-August because of no net sinking flux.Assuming the same POC flux during Novemeber-February and February-May as May-August,the upper limit of POC flux was estimated to be 0.15±0.01 mmol-C cm-2 yr-1.On the annual timescale,the upper limit of biological pump efficiency was 15±1%.The average fluxes of PN,bSi,TPP and particulate black carbon(PBC)in Jiaozhou Bay were 0.80±0.17 mmol m-2 d-1,2.05±0.43 mmol m-2 d-1,48±10μmol m-2 d-1,and 206±43 μmol m-2 d-1 respectively.Biological silica sinking seemed to be efficient than particulate nitrogen and phosphorus.