Quantification of carbon cycling within the aerobic zone in surface sediments of the Bohai Sea, China

Abstract

Evaluating the carbon cycling driven by aerobic oxidation of methane (AeOM) in surface marine sediments and calculating the CH4 flux to the seawater is critical for assessing the risk of greenhouse gas emission, as well as exploring for gas field. This study provides quantitative results of carbon cycling in the aerobic zone of the Bohai Sea, China, and the function of AeOM for CH4 interception via numerical simulations constrained by laboratory experiments. TOUGH + HR, an integrated simulation package, was used to numerically characterize the microbial AeOM, which couples water/gas transport and biogeochemistry. Based on the laboratory experiments and measured data in the Bohai Sea, a one-dimensional column conceptual model was constructed and solved by TOUGH + HR. The results show that when the upward CH4 flux into the aerobic zone is between 4.5 × 10−9 and 4.8 × 10−9 kg/s/m2, the simulated dissolved methane matched the measured data well. Under this condition, most of the CH4 source flux (18.63 mmol·carbon/m2/day) was oxidized to dissolved inorganic carbon (DIC), which finally discharged into the seawater with the flux of 18.44 mmol·carbon/m2/day, while only a tiny part of CH4 leaked into the seawater with the flux of 0.0003 mmol·carbon/m2/day. Therefore, it can be concluded that the aerobic zone in the sediments intercepts the majority of CH4, which mitigates the greenhouse gas emission from marine sediments. At the same time, the sensitivity analyses in this study indicate that carbon cycling is affected by methane flux and temperature, which influences the proportion of carbon sources and sinks. The results have important guidance for gas exploration and carbon cycling when the AeOM in the aerobic zone is considered.