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We develop and compare a semiquantitative plot showing the distribution the Phanerozoic record of kerogen accumulation in organic-rich black shales (TOC > 3%), to variation in atmospheric CO2, volcanism, sea-level change, and tectonic degassing. Our results suggest that at the first and second stratigraphic order scale, kerogen accumulation is related to periodic tectonically driven increases in atmospheric CO2, enhanced land productivity (CO2 fertilization), aggressive chemical weathering through accelerated soil formation, and flux of nutrients to aquatic bodies, promoting increased marine productivity at the global scale. Moreover, periods of high CO2 are coeval with sea-level rises, forming large anoxia-prone epicontinental seas, which provided conditions favorable for the preservation of organic matter. In this respect, the deposition of major source rocks at this time scale can be speculated as being a result of periodic global “CO2-induced eutrophication”.

At the first-order scale, a time shift is observed between the period of maximum accumulation of prolific source rocks and maximum accumulation of coals. This offset is interpreted by assuming that the depositional settings most favorable for coals were due to global tectonic relaxation at the ends of the two megacycles.

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