Studies On The Biogeochemistry Of Carbon Monoxide In The East China Sea And The Yellow Sea

Carbon monoxide (CO) is an important atmospheric trace gas, which plays a significant role in the global warming and atmospheric chemistry. Global oceans are net natural sources of atmospheric CO. Although coastal regions such as continental shelves, estuaries and bays only occupy a small part of the world ocean, they appear to be responsible for a large part of the oceanic CO emission. Therefore studies on the biogeochemistry of CO in the coastal waters will be helpful to estimate the contribution of oceanic emissions to the atmospheric CO and the influence to the oceanic carbon cycling on a global scale.In the present dissertation, we choose the East China Sea (ECS) and the Yellow Sea (YS) as the study areas. The spatial and temporal variations of distributions of CO, sea-to-air fluxes, microbial consumption and photoproduction are systematically studied for the first time. The photoproduction of CO in Jiaozhou Bay that is affected seriously by human activities was also examined. The main conclusions are drawn as follows:1. The distributions and sea-to-air fluxes of CO are determined in the ECS and YS during April-May and Nov-Dec,2009. The surface water concentrations of CO in ECS in spring and autumn are 2.42±1.86 and 0.88±0.52 nmol L-1, respectively The surface concentrations of CO in YS in spring and autumn are 2.23±1.46 and 0.80±0.56 nmol L-1, respectively. CO concentrations show a notable seasonal variation, with those in spring higher than those in autumn, which corresponds with the solar photon irradiance higher in April-May than those in December. The spatial distributions of CO in the ECS and YS are obviously influenced by the Yangtze River import and the oligotrophic Kuroshio waters. When the distribution patterns are compared with each other in spring and autumn, they are nearly synoptic and decreased from inshore to offshore sites. Vertical profiles of concentration of CO along different transects and at different stations were similar. The highest concentrations of CO were generally found at the sea surface, concentrations of CO in the water column gradually decreased with depth. However, CO could be detected even at depths under euphotic zone, where the light intensity was under detection limit, indicating the existence of other sources of CO such as dark production, diffusion, deep water current, sediment, etc.2. CO concentrations exhibited obvious diurnal variations in almost the entire euphotic zone in the ECS and YS, with maximum values 5-50 folds higher than minimum values at two anchor stations. Minimal concentrations of CO all occurred before dawn. However, there were differences of the maximum concentrations of CO between the sea-surface and other depths. The peak of concentrations of CO was observed in the early afternoon (about 14:00), a time lag about 2 hours at the surface and at noon at other depths, respectively. Marked diurnal variation of concentrations of CO in seawater indicated that CO was produced primarily by photoreaction and the concentrations of CO in surface seawater were mainly related to the solar irradiance.3. CO supersaturation was ubiquitous at all investigated sites in the ECS and YS in spring and autumn, the supersaturation factors were 16.05±13.54 and 3.88±2.95, respectively, which indicated that the ECS and YS are net sources of atmospheric CO. A short-term estimate of near-instantaneous flux was based on the concentration of CO in surface water and in situ wind speed for each time point by the arithmetic of W92. The sea-to-air flux in spring (6.67±4.61μmol m-2 d-1) was much higher than that in autumn (0.84±0.82μmol m-2 d-1), mainly due to the higher supersaturation factors in spring (the average is 16.05μmol m-2 d-1) than those in autumn (the average is 3.88μmol m-2 d-1). In connection with the area of the ECS and YS, the preliminary CO emission from the ECS and YS is estimated to be 18.9±13.7 Gg CO-C yr-1. Extrapolation of the mean flux of the ECS and the YS to the global coastal surface area provides coastal emission of about 0.49±0.36 Tg CO-C yr-1, accounting for approximately 11% of the global oceanic CO emission, though it occupies a small part (7-10%) of the world ocean. Our results indicated that the coastal contribution to the global CO emissions should not be negligible.4. Microbial consumption of CO in sea-surface in the ECS and YS in spring and autumn typically followed first-order kinetics in most cases. The microbial CO consumption rate constants (kbio) covaried with chlorophyll a concentrations and diminished with salinity. CO consumption rates show obvious seasonal variations, with those in spring (0.22 h-1) higher than those in autumn (0.16 h-1), primarily due to relatively higher Chl-a concentrations and seawater temperatures.5. For CO photoproduction, effects of water temperature and the origin of CDOM on the apparent quantum yields of CO (AQYCO) were examined. AQYco showed a strong positive correlation with the dissolved organic carbon-normalized absorption coefficient at 254 nm (SUVA254), suggesting that terrestrial CDOM is more efficient at photochemically producing CO than marine origin CDOM. CDOM photobleaching dramatically decreased AQY on the most terrestrial CDOM, but had little effect on the most marine samples. An empirical equation was derived for predicting the CO photoproduction efficiency in the ECS and YS based on SUVA254 and water temperature. Annual CO photoproduction in the ECS and YS was estimated to be 246.32 Gg CO-C yr-1.6. CDOM photobleaching could dramatically decreased AQY on the most terrestrial CDOM samples in. AQYco in Jiaozhou Bay had a linear correlation with SUVA254 and water temperature. Based on the average solar photon irradiance in autumn, the photoproduction rate of CO in Jiaozhou Bay was estimated to be 36.16μmol m-2 d-1 in autumn. The total photomineralization of DOC in Jiaozhou Bay was estimated to be 12.58 mg C m-2 d-1, representing 5.1% of the primary production in autumn.