A Fresh Look

Abstract

GEOLOGY Most of the Amazon rain forest, even in western Peru and Brazil, is not far above sea level, and it has been proposed that during the Miocene (about 10 to 20 million years ago), much of this region was part of a shallow inland sea or seaway connected to the Atlantic or Caribbean. This notion is controversial, however, and resolving the geography has implications for the extent and development of rainforest flora and fauna during this and later periods. Vonhof et al. used carbon, oxygen, and strontium isotope measurements of mollusks in the dominant Pebas Formation to analzye the composition of the waters across western Amazonia during this time. Together, these isotopes reflect and fingerprint the origins and salinity of waters. The results show that most of this formation represents deposition in a shallow freshwater lake and swamp, where most of the water was derived from snowmelt in the Andes and was sufficient to prevent any marine incursion and large enough to have tides. In one period, about 11 million years ago, outcrops to the northeast show evidence of brackish water, implying a limited marine incursion, perhaps from a connection to the Caribbean, that was insufficient to produce a marine seaway. — BH Geol. Soc. Am. Bull. 115 , 983 (2003