Benthic Oxygen Consumption And The Transport Of Redox Sensitive Elements(Fe And Mn)Across The Sediment-water Interface In Coastal Seas

Dissolved iron(Fe)plays a vital role in primary production in open oceans.Previous research has focused on assessing its source and potential role in climate changes.Coastal sediments are an important reservoir of organic matters and iron and manganese(Mn)minerals.Substances(e.g.nutrients,iron and manganese)in sediments are transformed to solutes and enriched in porewater during the early diagenesis.The benthic fluxes of dissolved Fe and dissolved Mn that are derived from the solute exchange across the sediment-water interface become one of primary external sources for oceans.However,Fe and Mn are redox sensitive elements that yield to oxidative precipitation in oxic environment.The correlation between the redox environments and the migration efficacy of benthic Fe and Mn will help understand the relevant cycles between sediments and waters.Moreover,coastal sediments are a sink of dissolved oxygen(DO)and have a crucial influence on the redox environment of bottom waters.Accurate quantification on benthic oxygen consumption also helps uncover the dominant processes that release dissolved Fe and Mn.A novel isotopic method,224Rap28Th disequilibria in sediments,is utilized to quantify the fluxes of DO,Fe and Mn across the sediment-water interface under various coastal conditions.This thesis examines the role that sediments play in DO budget of bottom water,the variations of benthic Fe and Mn fluxes under different redox environments and the related mechanisms.In the Pearl River Estuary(PRE)and the East China Sea shelf adjacent to the Yangtze River Estuary(YRE),the benthic DO fluxes were 201-1570 mmol m-2 d-1 and 6.0-58 mmol m-2 d-1,respectively.Diuring the period of PRE cruise(November,2013),there was no hypoxia or anoxia in the well-mixed water column downstream off Humen.An area-weighted average rate of sediment oxygen demand(SOD)from water column is estimated to be 30 ?mnol L-1 d-1,comparable to the DO supply rate from the atmosphere.During the period of YRE cruise(August,2017),a strong vertical stratification formed as a consequence of the heated freshwater outflow from the Yangtze River and the intrusion of the cool high-salinity Kuroshio subsurface water.The average SOD from the bottom water in YRE was 3.6±0.7 ?mol O2 L-1 d-1.With a simplified model of DO budget,the amount of SOD is evaluated to account for 15-65%of the total utilization of DO in the bottom water,implying that other processes may dominate the seasonal hypoxia in YRE.The 224Ra/228Th disequilibrium and core incubation methods were compared to examine the benthic fluxes of both 224Ra and MeHg.The result indicates that the latter method has major limitations and is not applicable to non-conservative substances.It implies benthic fluxes of Fe and Mn based on the traditional incubation method in other coastal seas are likely underestimated by a factor of 30.In China coastal seas,there were considerable temporal and spatial variabilities in benthic Fe and Mn fluxes,which were in the range of 2-89000 ?mol m-2 d-1 and of 24-196000 ?mol m-2 d-1,respectively.We have identified a prominent trend showing that the benthic fluxes tend to decline exponentially with bottom depth.This trend is probably a result of kinetic energy and chemically bound energy gradients that culminate in the coastal seas.We compiled the benthic Fe and Mn flux measurements under different redox conditions,including reductive seepage environments with H2S and anoxic or oxic overlying waters with DO concentration levels of 60-250 ?mol/L.Large benthic Fe and Mn fluxes were more frequently observed within a narrow redox window of 100-150?mol/L DO in the bottom water.This probably suggests a compromise of oxygen-demanded irrigation and oxygen barrier.In oxygen-enriched waters,dissolved Fe and Mn are retained at the sediment-water interface with re-oxidation in oxic waters,while hypoxia leads to few bioactivities and stagnant water,resulting in little sediment-water exchange.The formation of pvrite in hypoxic zone also constrains Fe migration away from sediments.Under the conditions with relatively modest DO levels,oxygen barrier is less efficient for Fe and Mn retention near the sediment-water interface.In the meantime,irrigation proceeds to enhance the release from bottom sediments.Overall,the redox window with large Fe fluxes provides a hint that what the redox condition would be when the ocean became highly productive.It helps to better understand the mechanisms of productivity variability and climate changes on a time scale of geological periods.