Redox Sensitive Elements(RSE) In One Sediment Core As The Environmental Change Indicators In Seasonal Hypoxic Waters Of The Inner Continental Shelf Of The East China Sea

As an important chemical parameter of marine ecosystem,dissolved oxygen content has a momentous impact on marine ecological environment.Therefore,the study of dissolved oxygen status in marine environment is of great significance.Redox sensitive elements?RSE?such as V,Cr,Mo and U are usually dissolved under oxic conditions in seawater.They are reduced to low-valent state in reductive sedimentary environment and transferred to sediments for enrichment and accumulation.Therefore,the enrichment of redox sensitive elements in sediments can be used to invert the redox condition of sedimentary environment.This paper systematically studied and analyzed the Zb7 sediment core?29.0167°N,122.2750°E?taken in September 2017 in the seasonal hypoxic sea area of the inner continental shelf of the East China Sea.The vertical distribution of the contents of redox sensitive elements V,Cr,Ni,Cu,Zn,Mo and U in the core sediment,RSE enrichment factors and RSE ratios were mainly studied to explore the changes of redox conditions in the sedimentary environment.Geochemical parameters such as grain size,TOC,TN,?¹³C of Zb7 sedimentary column were analyzed.The main conclusions were as follows:1.RSE enrichment factors,RSE ratios,RSE morphology analysis and correlation analysis of RSE/Al with Fe/Al and Mn/Al in different layers of Zb7sedimentary column indicated that the bottom seawater of the station was oxidative sedimentary environment.The oxidative sedimentary environment reflected by RSE of Zb7 sedimentary column was inconsistent with the seasonal hypoxia results reflected by the historical data of dissolved oxygen in this area,which may be related to the decrease of RSE enrichment during seasonal hypoxia in summer due to the recovery of dissolved oxygen and the re-release of RSE to seawater in autumn and winter.RSE enrichment factors of all layers of Zb7 sedimentary column were less than 3,and no obvious enrichment was observed.RSE ratios V/Cr<2,Ni/Co<5,U/Th<0.75,0.25<?Cu+Mo?/Zn<0.55 and Mo_EF/U_EF ratio were mainly distributed between 0.08and 0.3 times of seawater Mo/U ratio,indicating the oxidized sedimentary environment.RSE/Al were significantly correlated with Fe/Al and Mn/Al,which indicated that RSE entered sediments mainly by combining with Fe and Mn oxides or hydroxides after removing terrigenous debris inputs,and also indicated the oxidized sedimentary environment.RSE speciation analysis showed that V,Cu and Ni mainly existed in reducible state F2 in sediments,indicating that these RSE were enriched in sediments in the forms of binding with Fe,Mn oxides or hydroxides,reflecting the oxidized sedimentary environment,which was consistent with the results of correlation analysis between RSE/Al and Fe/Al,Mn/Al.RSE ultimately indicated the oxidized sedimentary environment,which was inconsistent with the seasonal hypoxia results reflected by the historical data of dissolved oxygen in the region.It may be related to the absence of RSE enrichment signals in sediments due to the recovery of dissolved oxygen levels in autumn and winter.2.The change trends of RSE enrichment were consistent with that of primary productivity reflected by?¹³C value and marine-derived carbon?Cm?since 1990s to now.The increase in 1990s and the decrease after 2009 of RSE enrichment,?¹³C value and Cm reflected to a certain extent that seasonal hypoxia had increased in the region since 1990s,and it had been alleviated after 2009,which further explained the increase of primary productivity and oxygen consumption of bottom organic matter mineralization caused by eutrophication may be one of the main causes of seasonal hypoxia in the sea area of Zb7.Although RSE could not effectively indicate the seasonal hypoxic environment in the East China Sea,the fluctuation trends of RSE indexes in the vertical distribution could reflect the intensity of redox signals eventually presented in the sedimentary environment,and then estimated the variation trends of seasonal hypoxic intensity in the range of corresponding years.The RSE/Al of Zb7 sedimentary column did not change significantly before the 1978.Since the 1978,the RSE/Al of Zb7 sedimentary column fluctuated upward.The high values mainly appeared in 3.5 cm?2009?,8.5 cm?2000?,12.5 cm?1995?,but showed a downward trend from 2009.The variation trend of?¹³C value and C_m can reflect the change trend of marine primary productivity.From1990s to 2009,the change trends of?¹³C value and C_m were consistent with that of RSE/Al and RSE enrichment factors,showing an upward trend,which may be related to coastal eutrophication and frequent occurrence of red tides,eutrophication and low oxygen in bottom waters were usually concurrent,which also reflected the intensification of seasonal hypoxia caused by eutrophication.From 2009 to now,the?¹³C value,C_m,RSE/Al and RSE enrichment factors had shown a downward trend,indicating that the ecological problems of red tide caused by eutrophication and seasonal hypoxia had been alleviated in recent years.This further indicated that the increase of primary productivity and oxygen consumption by mineralization of bottom organic matter caused by eutrophication may be one of the main causes of seasonal hypoxia in the sea area of Zb7.