A 2-Million-Year Record of Amazon Climate

During the Quaternary Period, major fluctuations between wet and dry climate have been implicated in the expansion and contraction of the Amazon forests and associated changes in their biodiversity. Yet, most of the Quaternary Period in the Amazon remains poorly known because of the complete lack of long geologic records from the Amazon basin. Here we reconstruct Amazon climate spanning the last ~1.93 million years (myr) from a piston core collected offshore of the mouth of the Amazon River. High-resolution titanium-to-calcium (Ti/Ca) and iron-to-potassium (Fe/K) values of sediments register a secular increase with time, reaching highest mean values in the past ~400 thousand years (ka). We interpret these trends to indicate a secular increase of precipitation and runoff during the Quaternary integrated over the entire Amazon basin. On shorter, orbital, timescales higher mean values of Ti/Ca and Fe/K, thus higher precipitation and runoff, occur during glacial stages, likely forced by millennial North Atlantic cold events (such as Heinrich stadials) that were far more numerous during glacial stages than during interglacials. A similar explanation is valid for the secular wetting trend: the increased amplitude and number of millennial North Atlantic cold events in the late Pleistocene, especially after ca. 650 ka, forced a proportionately larger Amazon mean climate response. Our finding of wetter, cold glacial stages is contrary to the paradigm of a drier Amazon during glacial stages but is consistent with the direction of a future, warmer and drier Amazon predicted by global climate models.