LETTERS PUBLISHED ONLINE: 30 SEPTEMBER 2012 | DOI: 10.1038/NGEO1588 North Atlantic forcing of Amazonian precipitation during the last ice age Nicole A. S. Mosblech 1 * , Mark B. Bush 1 * , William D. Gosling 2 , David Hodell 3 , Louise Thomas 2 , Peter van Calsteren 2 , Alexander Correa-Metrio 1,4 , Bryan G. Valencia 1,2 , Jason Curtis 5 and Robert van Woesik 1 The last glacial period was marked by multiple, abrupt reorga- nizations of ocean and atmosphere circulation 1 . On thousand- year timescales, slowing of the Atlantic meridional overturning circulation was associated with cooling in the high northern lat- itudes, whereas strengthened circulation was linked to north- ern warming 1,2 . In the tropics, these millennial-scale events were primarily reflected in altered patterns of precipitation 3 . These hydrologic fluctuations induced ecological changes in the Atlantic seaboard and the high Andes 2 , but less is known about the Amazon Basin. Here we reconstruct precipitation over Amazonian Ecuador over the past 94,000 years using a δ 18 O record from speleothems collected in Santiago Cave in western Amazonia. We interpret the variability of the δ 18 O record as changes in the source and amount of precipitation. With the exception of the period between 40,000 and 17,000 years ago, abrupt, high-frequency changes coincide with shifts in North Atlantic circulation, indicating a high-latitude influence on Amazonian precipitation over millennial timescales. On longer timescales, the record shows a relationship to precessional changes in the Earth’s orbit. In light of the lack of extreme aridity in our records, we conclude that ecosystems in western Amazonia have not experienced prolonged drying over the past 94,000 years. Both long-term stability and profound Amazonian climate change have been invoked to explain the origins of Amazonian biodiversity. Suggestions of glacial–interglacial oscillations between wet and dry states have been replaced by recognition that precessional cycles (∼19–22 kyr) are probable pacemakers of precipitation change 4 . Shorter-term (millennial-scale) variation in precipitation, however, may have been controlled by external forcings on atmospheric reorganization within the tropics 5,6 . Such short-term variations are most likely even more critical to immediate ecosystem functions, niche availability and modern conservation strategies than millennial oscillations. Indeed, high- resolution palaeoclimate records from the North Atlantic Ocean Basin reveal abrupt climate changes occurring on decadal to centennial scales 7 . Abrupt Greenland warmings, marking the Dansgaard/Oeschger cycles, and rapid North Atlantic coolings, during Heinrich events, were reflected as dry and wet events, respectively, in the High Andes 2,8 . Yet the millennial-scale climatic history of Amazonia remains largely unknown, especially as models suggest that Andean regional climates diverged from Amazonia climates during the late Pleistocene 9 . 1 Florida Institute of Technology, Department of Biological Sciences, Melbourne, Florida 32901, USA, 2 Department of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AA, UK, 3 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EG, UK, 4 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, Mexico, 5 Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. *e-mail: nsublett@my.fit.edu; mbush@fit.edu. Temperature reconstructions for ice-age Ecuadorian Amazonia indicated maximum cooling of 4–6 ◦ C (ref. 10). Precipitation reconstructions for the western Amazon have been limited to only the very late Pleistocene epoch (post-dating 14 kyr ago; ref. 11), or were based on indirect ecological responses to precipitation changes 12 . Using cave calcite as a proxy for precipitation, we provide independently dated evidence for 89,000 years of precipitation variability within the western Amazon. In 2007, we collected four speleothems from Santiago Cave within the lowland wet forests of western Amazonia, Ecuador (3 ◦ 1 0 S, 78 ◦ 8 0 W; 980 m above sea level). Simulations suggest that about 40% of moisture in the region is transported by the South American low-level jet, driven by Atlantic sea surface temperature (SST) gradients and equatorial easterly trade winds 13 . Both oceanic and continental moisture contribute to the strength and intensity of the South American summer monsoon (SASM), which controls precipitation variation in the Amazon Basin and in the Andes 14 . Consequently, variation in SST in the Atlantic, and the position and strength of wet-season Amazonian convection, probably determine the amount of precipitation falling over Amazonia and over the Andes on centennial and millennial timescales 15 . Austral summer and autumn (January–May) were the wettest months, as the intertropical convergence zone (ITCZ) reached its southernmost location and increased convection fuelled the SASM. However, even in the drier months, Santiago caught some rains at the southern edge of the Northern Hemisphere convective zone. The seasonal fluctuations in western Amazo- nian precipitation influenced precipitation δ 18 O values, with wet- season SASM-dominated months exhibiting more-depleted iso- tope values—because of the fractionation process during upstream rainout events—whereas dry-season months had less depleted isotope values—because droughts restricted fractionation (Supple- mentary Information). We created a composite stable oxygen isotope (δ 18 O) record spanning 94–6 kyr ago, based on four speleothems collected from the Santiago Cave (Fig. 1; also see Supplementary Information). The δ 18 O values spanned 3.53h (max =-4.52; min =-8.05), with little evidence of kinetic fractionation. The combined effects of oceanic and cave cooling during the last ice age on the isotopic record were probably 1–1.5h (ref. 8). Consequently, the observed variability has been assumed to reflect inter- annual changes in SASM strength and reflect the degree of NATURE GEOSCIENCE | VOL 5 | NOVEMBER 2012 | www.nature.com/naturegeoscience 817 © 2012 Macmillan Publishers Limited. All rights reserved.