Deep-sea cold seeps serve as key channels for seafloor methane release, but quantifying their emissions has long posed a challenge—creating uncertainties in the global ocean methane budget. To address this knowledge gap, a research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has discovered that methane emissions from cold seeps in the South China Sea are far higher than previously estimated, providing scientific evidence for accounting for oceanic greenhouse gas sources and sinks.

The findings were recently published in The Innovation Geoscience.

The research team focused on Site F, an active cold seep area located on the northern continental slope of the South China Sea. They combined deep-sea in-situ detection technologies with geochemical steady-state models to quantitatively characterize the migration and transformation of methane from the seafloor to the water column in cold seeps.

"The technical breakthrough of this work lies in the integration of multi-parameter collaborative detection equipment and fine-scale in-situ environmental monitoring on the seafloor," said Dr. CAO Lei, first author of the study.

During three research cruises in 2017, 2018, and 2020, the team used the remotely operated vehicle (ROV) "Discovery" aboard the research vessel "Kexue" to conduct fine-scale detection of the near-bottom seawater environmental field across different habitats in the active cold seep area. Their setup included a multi-parameter sensing system equipped with methane sensors and dissolved oxygen sensors, as well as synchronous precision sampling equipment—enabling continuous, high-resolution observations of key environmental parameters such as methane and dissolved oxygen.

Based on the in-situ observation data, the researchers established a steady-state box model to quantify the migration and transformation pathways of methane in the cold seep area. The results showed that methane flux in the active seepage zone was three orders of magnitude higher than the diffusive flux across the cold seep sediment-water interface—a discrepancy indicating that previous studies have underestimated cold seep methane emission intensity.

Meanwhile, the model results revealed that horizontal advection is the primary factor controlling methane migration and removal in the South China Sea's seafloor cold seeps.

The team estimated that annual methane emissions from South China Sea cold seeps range from 0.70 to 4.22 Gmol. Extrapolating this result to active cold seeps on global continental slopes, they found that the annual amount of methane entering the hydrosphere in dissolved form is equivalent to 126 Tg of carbon. This figure is twice the estimated methane emissions from shallow continental shelves and significantly higher than previous estimates of global mud volcano methane release—highlighting the long-overlooked critical role of deep-sea cold seeps in the global methane cycle.

These findings not only reshape the understanding of deep-sea methane cycle mechanisms but also provide key data support for assessing the global methane budget.

In situ detection of methane fluxes from active cold seep in the South China Sea. (Image by IOCAS）