Late Quaternary vegetation dynamics in the southern Amazon Basin inferred from carbon isotopes in soil organic matter

Carbon isotopes of soil organic matter (SOM) were used to evaluate and esta blish the chronology of the vegetation dynamics of an ecosystem presently c omposed of savannas surrounded by forests. The study was carried out on a 2 00-km transect along highway BR 319, on the border of Amazonas and Rondonia states, in southern Amazon, Brazil. Large ranges in delta C-13 values were observed in SOM collected from profiles in the savanna (-27 to -14 parts p er thousand) and forest regions (-26 to -19 parts per thousand), reflecting changing distribution of C-13-depleted C-3 forest and C-13-enriched C-4 sa vanna vegetation in response to climate change. These results indicate that from about 17,000 to 9000 C-14 yr B.P., the study area was covered by fore st vegetation. Between approximately 9000 and 3000 C-14 yr B.P., savanna ve getation expanded at the expense of the forest. Although the expansion of s avanna did not occur with the same intensity along the study transect, this process was very clearly registered by C-13- enrichment in the SOM, Since 3000 C-14 yr B.P., the carbon isotope data suggest that forested regions ha ve expanded, This study adds to the mounting evidence that extensive forest ed areas existed in the Amazon during the last glaciation and that savanna vegetation expanded in response to warm and dry conditions during the early to middle Holocene. (C) 2001 University of Washington.