Complex sentence: This combustion of prodigious quantities of peat was once postulated, because there was likely a greater mass of carbon stored as living terrestrial biomass during the Paleocene than there is today since plants in fact grew more vigorously during the period of the PETM.
Split into simpler sentences: This combustion of prodigious quantities of peat was once postulated, because there was likely a greater mass of carbon stored as living terrestrial biomass during the Paleocene than there is today. Since plants in fact grew more vigorously during the period of the PETM.