Arkosic sands of latest Wisconsin age from deep-sea piston cores taken in the Guiana Basin off northeast South America between lat 20° N. and lat 10° S. support previous conclusions that an arid to semiarid climate dominated large portions of equatorial South America during the Pleistocene glacial phases in complete contrast to the present-day and Pleistocene interglacial humid tropical climate. Thirty-nine cores showed stratigraphic relations that characterize the transition from Wisconsin to Holocene. Sand beds from 23 of the cores were of latest Wisconsin age and contained 25-60 percent feldspar. In contrast, a sample of Holocene sand taken from the continental shelf northeast of the mouth of the Amazon River contained only 17—20 percent feldspar.

The Quaternary history of Brazil appears to have been climatically controlled by a repeated displacement of the South Atlantic high-pressure cell by some 1500 km—northward during the glacial phases and southward during the interglacial phases. It was further influenced during glacial phases by a lowering of the snow line in the Andes by about 1000 m. In consequence, the glacial phase climates of the lower Amazon Basin were marked by cold, dry, southerly winds while the South Atlantic trade winds were deflected off the northeast coast. A semiarid to and climate ensued, coupled with a eustatic drop of sea level that caused degradation of the principal river valleys near the coast. In this way unweathered feldspars, chlorite, and other relatively coarse elastics were transported into the tropical Atlantic in contrast to the usual lateritic clays (gibbsite and kaolimte) of interglacial stages.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.