Effects of Syn-pandemic Fire Reduction and Reforestation in the Tropical Americas on Atmospheric Carbon Dioxide During European Conquest

A new reconstruction of the Late Holocene biomass burning history of the tropical Americas is consistent with expanding fire use by Mesoamerican and Amazonian agriculturalists from 2000-500 BP and a subsequent period of fire reduction due to indigenous demographic collapse. Our reconstruction synthesizes published data from 50 charcoal accumulation records obtained from stratified lacustrine sediments and from soils, including soil charcoal records recovered from archeological sites. Synthesis of stratigraphic charcoal records yields indexes of the mean rate of regional charcoal accumulation and of variability in charcoal accumulation among sites during 500-year increments since 3500 BP. The age distribution of dated soil charcoal particles from non-archeological sites provides an independent measure of variation in regional charcoal accumulation; whereas age distribution of soil charcoal dates from archeological sites records variation in charcoal accumulation related to anthropogenic biomass burning. We observe that the charcoal accumulation indexes derived from stratigraphic records begin to increase at 2000 BP, remain high until 500 BP, and then decline to near-minimum values during the 500-year period subsequent to European contact. Similarly, the age distributions of soil charcoal dated from both non-archeological and archeological sites indicate increases in charcoal accumulation from 2000 to 500 BP followed by decline. An index of the inter- site variability in charcoal accumulation obtained from the stratigraphic records attains a maximum during the time period between 1000 and 500 BP and a near-minimum value afterward. We interpret the covariation between measures of charcoal accumulation derived from archeological and non-archeological sites as a consequence of the expansive influence of anthropogenic activity on the regional fire regime. Increases in regional charcoal accumulation apparent in both the stratigraphic and soil charcoal records beginning at 2000 BP correlate with expanding indigenous population, agriculture, and fire use in the tropical Americas. The rise in inter-site variability in charcoal accumulation after 2000 BP is consistent with a demographic shift toward sedentary agrarian communities and localized increases in charcoal accumulation in densely populated centers. Declines in regional charcoal accumulation and inter-site variability after 500 BP suggest a correlative cause related to reduction in anthropogenic biomass burning resulting from pandemic-driven population collapse. Published reconstructions of Pre-Columbian demography indicate that during European conquest, pandemics killed ~90% of the indigenous American population (~60 million), estimated to represent ~20% of the 16th century global population. Our predictive calculations suggest that fire reduction in the tropical Americas is associated with massive forest regeneration on ~5 x 105 km2 of land and sequestration of 5-10 Gt C into the terrestrial biosphere, which can account for 13- 50% of the ~2% global reduction in atmospheric CO2 levels and the 0.1‰ increase in δ13C of atmospheric CO2 from 1500 to 1700 CE recorded in Antarctic ice cores and tropical sponges. New archeological discoveries revealing extensive networks of geoglyphs and urban polities in Pre-Columbian Amazonia suggest that our estimates of reforestation, and consequent effects on atmospheric CO2, may be conservative.