Organic carbon burial by large Permian lakes, northwest China

Permian strata of the Junggar-Turpan-Hami Basins represent one of the thickest and most laterally extensive lacustrine deposits in the world, yet they are very poorly known outside of China. Deposition spanned approximately 30 m.y., from the Sakmarian through Changhsingian epochs. Continuous intervals of organic-rich lacustrine mudstone may exceed 1000 m, and the total thickness of lacustrine and associated nonmarine strata locally exceeds 4000 m. Early Permian basin subsidence coincided with regional normal faulting and associated volcanism, interpreted to result from extension or transtension of newly amalgamated accretionary crust. In contrast, relatively uniform regional subsidence occurred during the Late Permian, most likely due to flexure caused by renewed regional compression. The maximum expansion of Permian lakes during the Wordian to Capitanian postdates any evidence for significant normal faulting or volcanism. Organic-rich mudstone facies cover an area at least 900 × 300 km, indicating that at their maximum, the Permian lakes were comparable in size to the Caspian Sea. An organic-rich profundal mudstone section in the south Junggar Basin has been ranked as the thickest and richest petroleum source rock interval in the world, with total organic carbon content averaging 4% and commonly exceeding 20%. Total Late Permian carbon burial is estimated at 1019 gC. Maximum organic carbon burial rates are estimated at 4 × 1012 gC/yr, equivalent to approximately 4–8% of estimated global carbon burial rates during this time. Permian lakes in northwestern China were broadly synchronous with other large lakes (or inland seas) in South America and Africa that also formed due to the amalgamation of Pangea. Collectively, these basins represent a large and heretofore unrecognized organic carbon sink that may have influenced atmospheric CO2 concentrations.