Holocene land cover dynamics in the Curuai Floodplain inferred from lacustrine biomarkers

This study on Curuai Floodplain (Central Amazonia) sediments (last 5600 cal yr BP) focused in Rock‐Eval [RE; hydrogen (HI) and oxygen (OI) indices, Tmax, S2 curve and TpS2], organic petrography and molecular biomarkers [n-alkanes, hopanes, pentacyclic triterpene methyl ethers (PTMEs) and derivatives of pentacyclic triterpenes such as des-A-lupane and aromatic derivatives] analyses. Between 5600 and 5100 cal yr BP, the environment was waterlogged and dominated by emersed plants (Paq 0.27-0.38). The influence of the Amazon River was low. Terrestrial vs aquatic ratio (T/A, ~ 0.8), CPI values (3-4) and high contents of PTME (0.22-1.54 μg/g sed) and aromatics (21-53 μg/g sed) indicated a strong contribution of terrestrial material and anoxic OM deposition conditions. Between 5100 and 5000 cal yr BP more humid conditions due to heavier rainfalls were observed. Early in this period, terrestrial biomarker contents (253 μg/g sed of aromatics, Paq ~ 0.17) increased due to input of watershed erosion material. This provoked a rapid burial and good preservation of terrestrial OM (CPI 3-4.5). From 5000 cal yr BP a strong sedimentation of carbonate minerals (OI ~ 1600 mg CO2·g− 1) masked the OM signal and affecting RE parameters. Paq (~ 0.18), T/A ratio (~ 0.4) and the decrease of C31αβ/C31ββ hopanes (1.18) indicate increasing aquatic conditions up to 2700 cal yr BP. CPI ratio (~ 1.2) and degraded phytoclasts suggest intense degradation during transport to the sedimentation location. The PTMEs results showed that, from 3000 yr cal BP, the Poaceae genera were different from those found at the bottom (middle-Holocene), suggesting a change of vegetation. The last 600 cal yr BP were marked by the seasonal influence of the Amazon River; during periods of low water level, the organic petrography and CPI values (~ 1.0) evidenced OM degradation and the return of rather drier conditions.