Effects Of Ocean Acidification On The Biogeochemical Behavior Of Trace Elements At The Sediment-seawater Interface

Trace elements in the marine environment can not only closely participate in the material cycle and significantly regulate life metabolism,but also attract widespread attention because of their toxicity,refractory degradation and biological activity.Since the Industrial Revolution,the ocean has continuously absorbed atmospheric CO₂,resulting in ocean acidification on a global scale.While large-scale severe acidification is less likely to occur in the short future,seasonal acidification along banks and estuaries is more likely.At present,more studies have been carried out on trace element changes in marine sediments under p H reduction conditions,but most experiments use inorganic acids to directly reduce p H,and do not reflect changes in carbonate parameters such as[HCO₃^-],[CO₃^2-]and total dissolved inorganic carbon（DIC）.In this study,an indoor culture simulation unit was constructed,and two laboratory experiments were set up by continuously introducing CO₂ standard gas to situ-situ seawater to simulate the phenomenon of ocean acidification:the study of trace element behavior at the sediment-seawater interface simulated by laboratory simulation under static conditions,and the leaching experiment to evaluate the influence of trace element behavior in sediment under the continuous CO₂ injection process.The active state concentration of trace elements was obtained by gradient thin film diffusion（DGT）to explain the migration of elements under acidification conditions from the perspective of bioavailability.The main conclusions reached are as follows:（1）After 30 days of indoor static simulated incubation experiments,the total concentrations of Cd,Cu,Ni,Zn,Fe and Mn in the sediment under the acidification background were less than the control and less than the concentrations in the sediment at the beginning of the experiment.Using the interstitial water sampler（HR-Peeper）and the plate-type DGT,it was observed that the depth profiles of total trace element concentrations and active state concentrations in the interstitial water showed a greater change in depth than in the surface layer with time,while all the analyzed metal elements showed an increase in concentration at the sediment-seawater interface.The mobilization of P in the overlying water,sediment-seawater interface,and sediment is consistent with the variation of Fe,and the correlation analysis of each sampling time point confirms the good coupling between Fe and P.In a comparative study of the chemical forms of trace elements in sediments,it was found that the bioavailable fraction（F1）of elements decreased with decreasing p H and the residual fraction F4associated with the silica lattice increased,but the potential bioavailable fraction（F1+F2）was variable.（2）The dynamic changes of typical metal elements Cd,Cu,Ni,Pb,Fe and Mn in sediment during ocean acidification were simulated by laboratory leaching experiments.After 528 h incubation,the total dissolved concentrations of analyzed elements in water and sediment at the end of incubation were compared with those at the beginning of the experiment,and it was found that all analyzed elements were released from sediment to seawater and the release was significantly affected by acidification,and the metal leaching rate increased by 195%,404%,362%and 350%under air and The metal leaching rate increased by 195%,404%,362%,and 350%under the conditions of 5000ppm,3000 ppm,and 1000 ppm CO₂ concentration,respectively.The trend of metal effective state concentration and total dissolved metal concentration in the leaching experiment was consistent,showing an overall trend of C-5000 ppm>C-3000 ppm>C-1000 ppm.The final changes of the chemical forms of the metal elements under the influence of acidification were studied by BCR sequential extraction at the end of the experiment.The morphology and concentration of different metal elements leached under the influence of acidification were different,among which the acid extracted state（F1）was the most sensitive to the acidification environment,and the solubility of metals increased and the content of metals in the sediment decreased with the decrease of p H value.The dynamic response process and diverse ecological effects of trace metal elements to different levels of ocean acidification scenarios obtained in this work can provide a scientific basis for clarifying the risk of metal release in marine sediments under the background of global ocean acidification,evaluating the evolution trend of marine trace elements under different CO₂ emission scenarios in the future,and studying the new marine biogeochemical functions of trace elements attributed by global change.