Organic geochemical evaluation of organic acids to assess anthropogenic soil deposits of Central Amazon, Brazil

Terra Preta de Índio (TPI) and Terra Mulata (TM) are anthropogenic soils from the Amazon region and are rich in stable organic matter (OM). The formation and incorporation of OM in these soils has recently been under investigation. Organic geochemical analysis is an appropriated tool for the assessment of the sources of OM. Therefore, we have used the distribution of different acid classes preserved in the free and bound soil fractions of 12 samples from two contrasting anthropogenic soils (TPI, TM) and an adjacent soil, in order to infer the sources of OM and the magnitude of non-cultural influence on the formation of anthropogenic soils. The major acids in both fractions (i.e. free and bound) were n-saturated, branched and unsaturated alkanoic acids, hydroxyalkanoic acids, bile acids and lignin/suberin derived aromatic acids. In general, the acids in the free and bound fractions appeared to be complementary and together provided valuable information about OM incorporation into anthropogenic soils. Different incorporation of ω-hydroxyalkanoic acids (C22, C24 and C28) and 9(10),16-dihydroxyhexadecanoic acid, and presence/absence of bile acid showed a distinct genesis for the soils. The influence of modern vegetation was revealed by bound ω-hydroxyalkanoic acid (C22, C24 and C28) distributions only in the topsoil profiles of TPI and TM, indicating that organic geochemical analysis is a useful approach in the investigation of ancient human deposits in tropical archaeological soils.