A Late Paleozoic climate window of opportunity

The highest rates of global organic carbon burial (up to 6.5 × 1018 mol/Myr) over the past half billion years occurred during the Carboniferous–Permian (330–260 Myr), in large part because of the accumulation and burial of peat in broad tropical lowland basins (1). Atypical rates of organic carbon sequestration led to low atmospheric pCO2 and anomalously high pO2 (Fig. 1), which in turn triggered the longest-lived and perhaps most severe icehouse of the Phanerozoic (1, 2) and, possibly, unusual physiological innovations, such as insect flight and gigantism (3). This unique atmospheric composition arose with the radiation of the Earth’s most expansive wetland tropical forests (4), introducing into the terrestrial realm a vast supply of biodegradably resistant organic matter. A hypothesized temporal gap of ∼120 Myr between the emergence of lignin biosynthesis by plants and the evolution of lignin-degrading fungi has been argued to have created a window of opportunity for substantially increased organic carbon sequestration in the Carboniferous (4⇓–6). This long-standing evolutionary lag paradigm becomes even more compelling with a recent fungal genome study suggesting that Agaricomycetes fungi, with the enzymatic capacity to efficiently degrade lignin, did not evolve until the early Permian (7). In PNAS, Nelsen et al. (8) invoke multiple lines of evidence to refute this paradigm, documenting: (i) the low lignin content of some of the most important Carboniferous peat-forming plants, (ii) the lack of correspondence between peak coal accumulation rates in North America and anticipated periods of abundant lignin production in late Paleozoic tropical forests, (iii) that lignin resistance to decay was not limited to Agaricomycetes fungi, and (iv) phylogenomic data that close the hypothesized evolutionary gap. Late …