BPCA-derived PyC may reflect fire signals over regional scales from the western Amazon Basin fire record

Fire alters the biogeochemical cycling of important elements, plays a role in climate change, and shapes the composition of global biological communities. Detection of past fires has long been used to reconstruct human settlement and climate records. Charcoal and phytolith abundance has been the most commonly used paleofire proxies but may only represent evidence of local fires. Chemical analyses of pyrogenic carbon (PyC) have been more recently used, but are also not without controversy. Thus far, very few intercomparisons of these proxies have been conducted. Here, the fire records contained in soil and lake sediments of Western Amazon (at lakes Ayauchⁱ, Parker, Gentry, and surrounding regions) were determined by charcoal microscopy, chemical thermal oxidation (CTO), and benzene polycarboxylic acids (BPCA) molecular biomarkers. Charcoal represented a smaller portion of PyC and, with its patchy distribution, likely indicated local or larger regional fire events. With a median value of about 15% of organic carbon, PyC via CTO oxidation was of the highest concentrations, which suggests a larger PyC detection window and lower sensitivity of reflecting regional fire. With a median value of about 3% of organic carbon, the BPCA-derived PyC distributions bore the closest resemblance to both spatial and temporal regional fire variations, established via archeological, pollen and phytolith records, thus may be a more sensitive indicator of fire over larger regional scales. Molecular ratios of BPCA molecules in Lake Ayauchⁱ soils indicated higher temperature fires (> 600°C) and suggested a history of more human occupation and human-caused fire in the Lake Ayauchⁱ region compared with the Lake Gentry & Parker region. However, our findings suggest that the use of a combination of fire proxy methods provides a fuller picture of the fire history of a region than any single approach. Establishing a better understanding the differences in the information provided by various paleofire proxies will allow a more complete understanding of the drivers, history and ecological and biogeochemical effects of fire, both regionally and globally.