Considering the strong seasonality in carbonate system parameters in coastal ecosystems, a comprehensive understanding of seasonal carbonate cycling is critically important for deploying scalable, nature-based negative emission technologies. Located in the northern South China Sea (nSCS), the Beibu Gulf is a semi-enclosed bay hydrodynamically connected to the nSCS shelf. Previous studies have documented carbonate dynamics in the Beibu Gulf during spring and summer; however, the spatial distributions and controlling mechanisms in fall remain unreported, as do comparative assessments with the adjacent nSCS shelf. Here, we present observations of the carbonate system in the Beibu Gulf and the adjacent nSCS shelf from a fall cruise conducted in 2024. Our results reveal that both the nSCS shelf and the Beibu Gulf act as CO2 sources with air-sea CO2 fluxes ranging from 0.4 to 1.6 mmol m−2 d−1. Although sea surface temperature exerts the dominant control on pCO2 variability, biogeochemical processes within the Beibu Gulf further elevate surface pCO2 by ∼20 μatm. In the water column, the spatial distributions of carbonate system parameters are primarily governed by the intrusion of SCS water and Kuroshio water. Notably, we report, for the first time, elevated dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations relative to background SCS water in certain bottom waters in Dongfang and Ledong within the Beibu Gulf, with increases of 94 ± 9 μmol kg−1 and 57 ± 3 μmol kg−1, respectively. Associated with these DIC and TA enhancements, we observed a concomitant increase in pCO2 of ∼92 μatm, alongside decreases in pH and aragonite saturation state (Ωarag) of 0.08 and 0.34 units, respectively. Qualitative analysis suggests that these DIC and TA additions likely originate from coupled CaCO3 dissolution and organic matter respiration, processes consistent with influences from submarine groundwater discharge (SGD) and/or sediment porewater release. However, these interpretations require further quantification and tracer-based validation.
Yin H., Yang W., Zhang M., Peng X., Tang H., Ye J. & Lu C., 2026. Distributions and controls of carbonate parameters in offshore oil and gas fields of the Beibu Gulf and adjacent northern South China Sea (nSCS) shelf during the fall season. Marine Environmental Research 221: 108327. doi: 10.1016/j.marenvres.2026.108327. Article (subscription required).
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