Evaluating the impacts of drilling and extraction activities on the marine carbonate system in the natural gas fields of Beibu Gulf, Northern South China Sea.

Natural gas fields are typically located in shallow gulfs. Previous studies have predominantly focused on gas leakage and its subsequent toxic effects on marine organisms; however, the impacts of accidental CO2 leaks on carbonate dynamics during drilling and extraction remain poorly understood. In this study, we investigate carbonate parameters in two gas fields to elucidate the influences of extraction activities and natural processes on carbonate dynamics in Beibu Gulf, situated in the northern South China Sea (nSCS). Our findings indicate that Beibu Gulf acts as a CO2 source during the late spring season, with an air-sea CO2 flux ranging from 1.1 to 4.4 mmol m-2 d-1. Spatially, higher sea surface pCO2 and temperatures were recorded at 479 ± 17 μatm and 29.6 ± 0.3 °C respectively in the Dongfang gas field within the inner gulf, compared to values of 462 ± 20 μatm and 27.6 ± 0.6 °C observed in the Ledong gas field located in the outer gulf. In the Ledong gas field, carbonate dynamics are primarily influenced by mixing between offshore subsurface water and river plumes, with no significant contributions from extraction activities noted. Conversely, dissolved inorganic carbon (DIC) levels within the Dongfang gas field exhibited two extremes: a consumption of 13 μmol kg-1 in surface waters alongside an addition of 20 μmol kg-1 in subsurface waters. Although enhanced biological production may lead to decreased surface pCO2 levels, elevated surface pCO2 values observed at Dongfang can likely be attributed to higher sea surface temperatures. In subsurface layers, we quantify and determine the origin of excess DIC based on δ13CDIC analysis, attributing these carbon inputs to organic matter respiration following surface biological utilization of atmospheric CO2. This study provides the first evidence of high biological DIC consumption in surface waters and DIC generation in subsurface waters within a shallow semi-enclosed bay, a phenomenon previously seen only in river estuaries. We demonstrate that natural processes predominantly govern carbonate dynamics within these gas fields, while any potential influences from extraction activities appear negligible.