Hydrocarbon seepage in the mid-Cretaceous greenhouse world: A new perspective from southern Tibet

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained delta C-13 ear b value of -30.5 parts per thousand is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including C-13 depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates - the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system - agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous.

Automated summary

This study focuses on the analysis of sedimentary rocks from the mid-Cretaceous inland basin, investigating methane oxidation and authigenic carbonate formation mechanisms to reveal the extreme climate conditions of that period. The results show the presence of methane seepage in the Gamba area of southern Tibet, suggesting it may be a major factor contributing to the formation of carbonates in the sedimentary rocks.