Gustavo Sarmiento

Andean tectonics as a cause for changing drainage patterns in Miocene northern South America

NewdatafromNeogenestratainnorthernSouthAmericasuggestthatMiocenetectonisminthenortheasternAndeswasresponsible for the genesis of the Amazon River and changes in the drainage patterns of other major rivers such as the Magdalena and the Orinoco. Here we present a new model for the paleogeographic evolution of northern South America during the Miocene. In the early Miocene, a large part of the drainage of northwest Amazonia was directed northward along the paleo‐Orinoco river system to a deltainLakeMaracaibo.UpliftoftheEasternCordillerainthelate middle Miocene caused thefirst development of the Amazon River; however, no connection with the Atlantic was established, and the Amazon fed the paleo‐Orinoco river system, which drained toward the Caribbean. Substantial Andean uplift in the late Miocene resultedinmajorchangesinpaleogeography:theOrinocochangedits course, the Amazon established a connection to the Atlantic, causing the drowning of carbonate platforms, and the Amazon-Caribbeanconnectionwasclosed.Thusthedrainageandpaleogeography ofnorthernSouthAmericaintheMiocenewerestronglycontrolled by tectonic movements in the northeastern Andes.

Terrestrial record of methane hydrate dissociation in the Early Cretaceous

Reconstruction of changing C isotopic composition of Early Cretaceous atmospheric CO2 from fossilized C3 vascular land-plant tissue revealed a brief and striking negative excursion (D 25‰) in atmospheric d 13 C, followed by a rapid positive compensation (D 15‰) during the Aptian (ca. 117 Ma). Mass-balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion; this would also result in an increased CO2:O2 mixing ratio as O2 is consumed during CH4 oxidation to CO2, spurring the exponential phase of angiosperm biogeographic expansion.