Hydrate formation at sea-level lowstands during glacial cycles: Temporal constraints from overpressure evolution

Marine gas hydrate is a huge carbon sink on Earth. Understanding how past climate change affected it is crucial for predicting its environmental impacts in a warmer future. A major challenge, however, is that the hydrate formation time in geological systems remains poorly constrained. Previously, we could only infer the formation time of gas hydrates formed within the past tens of thousands of years. Here, we present a novel approach and the first successful attempt to date hydrates formed in the geological past using overpressure evolution in overlying sediments. Overpressures of ~0.58 MPa and ~0.5 MPa were observed in hemipelagic deposits overlying a hydrate-bearing sand in the updip and downdip directions, respectively, in the Qiongdongnan Basin, South China Sea. The inferred high pore pressures caused an abnormally high porosity of ~70% at ~136 m below the seafloor. Overpressure evolution during sedimentation shows that updip hydrates formed at ca. 135 ka, coinciding with the formation time of earliest cold seep in this region, and downdip hydrates formed at ca. 110 ka. Both formations occurred during sea-level lowstands and were separated by an ~25 k.y. interval matching glacial cycle. We interpret that sea-level falls triggered upward migration of deep gas, successively inducing hydrate formation across the sand. Thus, sea-level falls may not only dissociate shallow-water hydrates but also promote new hydrate formation in deep-water settings by solidifying gas that migrated from below into the overlying sedimentary system. Gas hydrate may be a carbon sink rather than the seeping methane source proposed by the “clathrate gun hypothesis” during sea-level lowstands.