Orbital pacing of the Early Jurassic carbon cycle, black shale formation and seabed methane seepage
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The Early Jurassic (ca 201 to 174 Ma) was marked by a series of rapid perturbations in climate, the environment and global geochemical cycles, which have been linked to volcanic outgassing and the release of biogenic or thermogenic methane into the ocean–atmosphere system. The state of the global carbon cycle and prevailing climatic and environmental conditions that existed at this time are, however, poorly constrained. Here, mudrocks of the Lower Sinemurian Arietites bucklandi ammonite Biozone at coastal exposures at Kilve, Somerset, UK, have been studied. This succession includes laminated organic-rich black shales, which are present throughout the Bristol Channel Basin, and coincides with a 2 to 3‰ negative carbon-isotope excursion, distinct changes in inferred land vegetation, and abundant marine prasinophytes (green algae). The event itself does not represent a single perturbation of the regional environment, but follows in a sequence of eccentricity-modulated, precession-paced perturbations that occur throughout the preceding Hettangian stage, with the periodic formation of organic-rich laminated black shales in the Bristol Channel Basin. However, the Early Sinemurian event studied herein is more extreme in nature, with sedimentary total organic carbon values of 5 to 11% persisting over about 2 m, representing ca 100 kyr, possibly in phase with short (ca 100 kyr) and long (ca 405 kyr) eccentricity forcing. The formation of methane seep carbonate cemented mounds took place relatively soon after the deposition of laminated black shales. Biogenic methane probably formed in response to microbial methanogenesis in the organic-rich black shale, which was subsequently channeled to the sediment–water interface approximately 5 m above the source bed, and ca 200 kyr after cessation of formation of the black shale.
Accepted manuscript online: 3 October 2016
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.