Evidence of intense methane seepages from molybdenum enrichments in gas hydrate-bearing sediments of the northern South China Sea

Abstract

The application of molybdenum (Mo) enrichment to sediments can potentially provide unique constraints on the methane seepage dynamics at continental margins. We report herein elemental and isotopic measurements for authigenic carbonates and sediment samples in drill cores (~95m long; Site GMGS2-08 at ~800m water depth) from gas hydrate-bearing sediments in the northern South China Sea. Five intervals that were impacted by methane seepages in the sediment profile were identified by the presence of strongly 13C-depleted carbonates (δ13C values as low as −56.8‰). The carbonates often cement chemosynthetic bivalve shell fragments and have anomalous δ18O positive values. These observations suggest that methane seepage was intense during those time intervals and that carbonate precipitation occurred in the uppermost sediments, probably due to gas hydrate dissociation. Interestingly, the five intervals were all characterized by strong Mo enrichments. We thus hypothesize that the presence of strong Mo enrichments is a good indicator of past episodes of methane seepage. Such an environment results in sulfidic conditions that occur in a narrow zone in proximity to seawater close to the seafloor, leading to resultant Mo enrichments in associated sediments. In contrast, environments with low methane flux would lead to the distribution of Mo throughout a wide sulfidic zone and, consequently, an absence of Mo enrichment in the sediments. The insights into the environment for Mo enrichments at seeps have significant implications for tracing past methane seepages, their intensities, and even possible occurrences of gas hydrate dissociation.