The Study On Optical Characteristics Of CDOM In Sediment Pore Waters And Photochemistry Of CDOM In Estuarine Waters

Marine chromophoric dissolved organic matter(CDOM) was main component of dissolved organic matter(DOM),and played a key role in marine carbon cycle,widely attention was paid to by most oceangraphers recently.The paper enclosed the two potential sources,that was resuspension of sediments and rain contribution,and a key removal path,that was photochemical degradation.We employed new excitation-emission matrix fluorescence spectroscopy(EEMs),associated with absorption spectrum and dissolved organic carbon(DOC) investigate the optical chaeacteristics of CDOM in pore waters from Chinese estuarine and coastal sediments and analysed that it was possible that resuspension was a source of marine CDOM.And the study observed firstly characteristics of EEMs of CDOM in rainwater in china, and discussed it was significant to atmospheric waters and contrubition to marine CDOM flux.And,this study developed the photodegradation of CDOM from estuarine region exposure on solar radiation,pointed out it was important that photochemical reaction was a transformation approach of CDOM,provided the more complete references/information to further research into the source and sink,and removal and transfer, and cycle mechanism of dissolved organic matter in coastal an estuarine region.The main results were as follows:1) Characteristics of EEMs of CDOM in surface sediments were all similar from Xiamen Bay and Xinghua Bay and Meizhou Bay and Beibu Gulf,main fluorescence peaks including humic-like fluorescence peaks(e.g.Peak A and C) and protein-like fluorescence peaks(such as tryptophan-like fluorescence B and S and tyrosine-like fluorescence T and R ) were observed,these peaks appeared to occur in pairs.The emergence of intensive protein-like fluorescence indicated dissolved organic matter with aromatic character such as tryptophan and tyrosine deposited in surface sediment during early diagenesis.The humic-like fluorescence peak C reflecting terrestrial source varied with different bays and sites,while the humic-like fluorescence peak M reflecting marine source was not evident in these forementioned sites.2) The maxima of average concentration of CDOM in sediment pore waters occurred in Meizhou Bay, and subsequently Xinghua Bay,and then the minima of that appeared in Xiamen Bay and Beibu Gulf, indicating the most influence of the input of terrestrial organic matter on the sediments derived from Meizhou Bay and Xinghua Bay.The ratios of fluorescence intensities of fluorescence peaks in these regions existed in many differences,illuminating there were differences in the sources and constitutes.The fluorescence indexes of CDOM in pore waters from four coastal regions were all more than 1.5,closed to 1.9,showing properties of dissolved organic matter went through huge disturbance of microorganism, especially in surface sediments from Xinghua Bay.3) The simulative experiment in laboratory devided from Jiulong Estuary was employed to study fluorescence characteristics of CDOM readily released from sediment particles during episodes of resuspension,and bottom waters and surface sediment pore waters were compared in all corresponding sites.Results showed,for a given site,both relative intensities of CDOM fluorescence and concentrations of DOC have the same trend with the maxima in the pore waters and the lowest in the bottom waters. Relative intensities of CDOM fluorescence increased with salinity descreasing from marine to riverine environments in bottom waters and resuspended samples.EEMs showed several types of fluorescence signals observed,including humic-like and protein-like fluorescence for all samples,and simulated experimentation also showed humic-like and protein-like material were released to bottom waters by resuspension,indicating sediment resuspension would be an important source of CDOM from coastal waters.For a given site,positions of fluorescence peaks(e.g.peak A and peak C) in pore waters were red-shifted,compared to their respective bottom waters.EEMs for all resuspended samples exhibited mixed characteristics between corresponding bottom and pore waters,but Ex/Em-max of peak A and C was very close to that in bottom waters,and observed blue-shift of peak positions in comparison with pore-water samples.Peak M was strong in marine environment,and gradually disappeared with salinity descreasing from marine to riverine environments,and peak M and C ratios also lowered in bottomwaters and resuspended samples,indicating peak M was derived from marine origin.The fluorescence index of DOM varied from 1.61 to 1.93,suggesting DOM in marine environment was dominating biological origin,DOM in riverine environment was controlled by terrestrial input,or was associated with terrestrial substance and microbial activities.4) Photochemical degradation experiments of the samples from Jiulong estuary exposure on solar radiation were performed in summer and fall 2007,the humic-like and protein-like fluorescence observed in the samples photodegradated obviously when exposed on solar irradiation,and fluorescence loss could reach 90%approximately in the end of the experiment,while the fluorescence of any fluorescence peaks of the samples controlled hardly changed.Peak C was more liable to solar effect to photodegradate than peak A.Absorption coefficients a(250) and a(280) of CDOM decreased with increasing radiation time,and absorption loss could arrive at 50%when the experiment ended.The rate of the fluorescence loss was faster than that of absorption loss.The loss of fluorescence and absorption mainly took place at less than 320 nm.5) Compared with the controlled samples,the ratios of A₂₅₀ and A₃₅₀ reflecting molecular constitutes and average molecular weight of CDOM increased during exposure,making clear that photochemical reaction transfered the CDOM of large molecular weight into the CDOM of small molecule weight,and the average molecular size became small,explaining photodegradation was possibly an important sink to terrestrial CDOM entering into the coastal sea.The photodegradation of DOC was slight comparison with fluorescence and absorption of CDOM,suggesting DOC absorbing light of large molecular size was transformed into DOC nonabsorbing light of small molecular size.The photodegradation rate of CDOM samples with low salinity was faster than that of CDOM samples with high salinity.6) All rainwater samples contained CDOM collected in Xiamen Bay,the absorptance spectra of CDOM in the samples decreased exponentially with wavelength.The CDOM content was high during abundant rainfall.EEMs spectra showed 5 types fluorescence peak(2 humic-like fluorescence and 3 protein-like fluorescence) in rainwater samples,there was strong positive correlation between the two humic-like fluorescence and absortance coefficient at 300 nm,suggesting that these optical properties are directly interrelated and that the compounds responsible for absorbance might be the same as those responsible for fluorescence.Anthropogenic or terrestrial sources were impotant contributors to CDOM livels in precipitation.The pH values didn't affect the positions of the fluorescence peaks,but when pH value varied from 5 to 9,intensity of every fluorescence peak and absorption coefficient were both increasing.The presence of highly absorbing and fluorescing CDOM in rainwater had significant ramifications in atmospheric chemistry and might play a previously unrecognized role in the wavelength dependent spectral attenuation of solar radiation by atmospheric waters,at the same time,it also was an unneglectable source to CDOM from coastal sea.