| Methane (CH4) is an important greenhouse gas, which influences global climate by photochemical reactions in the atmosphere and absorbing infrared radiations. Recently, the continuously increasing of CH4 concentration in the atmosphere has caused more and more concern by researchers because of its effect on global climate change. Ocean is one of the natural sources of atmospheric CH4. Although ocean occupies about 71% of the Earth’s surface area, its contribution of CH4 to the atmosphere is very low. Reeburgh et al. assumed that most of CH4 (about 90%) has been oxidized in the sea water before releasing to the atmosphere. Therefore, the study of methane oxidation rates (MOX) is an important aspect of the oceanic methane biogeochemistry. In this study, the methods for measuring MOX were establishd by appling 13CH4 as tracers, and the MOX of the sea water samples collected from the Jiaozhou Bay in spring (March, April and May),2014, were systematically investigated. Series 13CH4 incubation experiments of the sea water were conducted and the δ13C-CH4 were measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The MOX of the samples were calculated based on the variations of δ13C-CH4 and the concentrations of methan of each sample. The main results are as follows:1. The method by incubating isotope labeled 13CH4 method in sea water samples was effective to measure MOX in a semi-closed bay strongly influenced by human activities. By applying this method, the average MOX of the Jiaozhou Bay in Jan.2014 was 71.4 nmol·L-1·d-1.Total amount of methane oxidized in the bay was estimated as 48.0×105 mol/mon by the grid method, which is equivalent to 3.2-6.9 times of the average methane air-sea flux in the Jiaozhou Bay, indicating that most of the methane in the seawater (about 90%) has been oxidized before entering into the atmosphere.2. We investigated the MOX in spring,2014 (March, April and May). The results showed that the average MOX in the surface water samples in March, April and May,2014, were (85.7±51.2) nmol·L-1·d-1, (104.7±89.7) nmol· L-1·d-1 and (73.2±46.8) nmol·L-1·d-1; and the average MOX in the bottom water samples in March, April and May,2014, were (100.5±59.8) n mol·L-1·d-1, (148.9±76.2) nmol·L-1·d-1 and (134.2±54.5) nmol·L-1 d-1, respectively. MOX in the bottom water were slightly higher than those in the surface water. According to the MOX, total amount of methane oxidized in the Jiaozhou bay in March, April and May of 2014 were 6.86×106 mol, 9.42×106 mol and 8.74×106 mol, respectively. Total amount of methane oxidized in the bay in the spring of 2014 was 2.50×107mol.3. In general, in the eastern and western area of the Jiaozhou Bay, the MOX in the surface sea water tend to be higher inshore and lower offshore in March, April and May,2014. In the western area of the bay, the maxmum MOX in the surface water was occurred around the estuary of the Dagu River in March and April. But the maxmum MOX in the surface sea water was detected between the estuaries of the Dagu River and the Yang River in May. The results showed that the distributions of MOX in the surface sea water were affected significantly by anthropogenic factors. In the eastern area of the Jiaozhou Bay, the MOX in the bottom sea water tend to be higher inshore and lower offshore in March, April and May,2014. On the whole, the maxmum MOX of the Jiaozhou Bay in March, April and May of 2014 tend to be occurred at its western area. Vertical variations of MOX of the Jiaozhou Bay showed that in the deep water of the southern and at the mouth of the Jiaozhou Bay, the maxmum MOX were detected at the middle or bottom water, and vaired a little at different time, maybe caused by vertical mixing of the sea water.4. The MOX and the variations of pH, TA (alkalinity) and DOC (dissolved organic carbon) had little relationship in the Jiaozhou Bay in March, April and May,2014. The MOX and the variations of DIC (dissolved inorganic carbon) had significantly linear relationship in the Jiaozhou Bay in March,April and May,2014, maybe related to aerobic oxidation of methane. |
PDF Full Text Request