Seasonal dependence of false-positives in detection of anomalous pCO2 using the covariance method with dissolved oxygen in monitoring offshore CO2 storage sites

To elucidate the dependence of frequency of false-positives on the season for the covariance threshold based on partial pressure of CO2 (pCO2) and the percentage dissolved oxygen saturation (DO%) for monitoring offshore CO2 storage, the deviation of observed pCO2 from a linear regression of pCO2 on DO% was investigated. The analyzed data were continuously observed for nearly 1 year in a semi-enclosed bay. Observed pCO2 tended to be higher than the linear regression in summer and fall, when false-positives would be more frequent. It is inferred that the difference in the air-sea equilibrium timescale between CO2 and O2 as well as high water temperature cause the higher pCO2 in fall while the higher pCO2 in summer is associated with hypoxic water. Continuous observations showed features of pCO2 and DO% variations more clearly than multiyear sporadic observations. This study provides data on anomalies necessary for developing strategies for marine monitoring.