Terrigenous sediments from Ceara Rise in the western tropical Atlantic Ocean record Pleistocene Amazon Basin climate variability. Iron oxides and oxyhydroxides in this region originate mainly from chemically leached Amazon lowland soils. Concentrations of goethite and hematite in the terrigenous fraction consistently peak during transitions from glacial to interglacial periods, suggesting an increased proportion of erosive products derived from the Amazon lowlands compared to the physically weathered highlands. Lowland Amazon Basin precipitation changes, monitored by the percentage of goethite relative to total iron oxides, lead ice age extremes with maximum aridity during ice growth and maximum precipitation during ice melt. Rapid climate changes over the Amazon Basin may reflect shifts in the position of the Intertropical Convergence Zone forced by northern hemisphere insolation at precessional (1/23,000 yr(-1)) and obliquity (1/41,000 yr(-1)) frequencies, Variance in the orbital eccentricity bands (1/100,000 and 11413,000 yr(-1)) may be explained by nonlinear amplification of insolation forcing at precessional frequencies. The early response of Amazon precipitation to insolation, ahead of high-latitude ice volume (delta(18)O) at all orbital frequencies, suggests that tropical aridity is part of the chain of events leading to ice ages, rather than a response to glacier oscillations. (C) 1999 University of Washington.
