Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin—one of Earth’s major centres of deep atmospheric convection1,2. For example, whether the Amazon basin was substantially drier3,4 or remained wet1,5 during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting6. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide7. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.
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This work was supported by a Singapore National Research Foundation (NRF) Fellowship (NRFF2011-08) and a Gary Comer Fellowship to X.W.; US National Science Foundation (NSF) grants 1103404 and 1317693 to R.L.E. and H.C.; a Brazil National Council for Scientific and Technological Development (CNPq) grant (540064/01-7) to A.S.A.; grants from the China National Basic Research Program (NBRP; 2013CB955902) and the National Natural Science Foundation of China (NSFC; 41230524) to H.C.; and a grant from the São Paulo Research Foundation of Brazil and US NSF Dimensions of Biodiversity joint program (FAPESP/NSF; 2012/50260-6) to F.W.C. Field travelling funds were partially supported by a National Geographical Society grant, 7574-03. We acknowledge the help of colleagues from the Grupo Bambuí de Pesquisas Espeleológicas with cave mapping and sampling. We thank R. Fonseca, S. Yuan, Y. Lu and Y. Djamil for assistance with the figures concerning wind fields and regional rainfall, and B. Wohlfarth and S. Hemming for discussions during manuscript preparation.
The authors declare no competing financial interests.
Reviewer Information Nature thanks M. Bush, J. Shakun and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, The locations of Paraíso Cave in the eastern Amazon (red rectangle), and of Diamante cave17 (blue rectangle) and Tigre Perdido cave9 (purple rectangle) in the western Amazon. Paraíso Cave is located between Belém and Manaus, next to the Tapajós River. Also shown are easterlies, which carry moisture to the lowlands from the tropical Atlantic. The Amazon basin and the Andes are shown in green and brown, respectively. b, 72-hour back-trajectories of moisture arriving at Paraíso and the western Amazonian cave sites (white stars), during the wet season (in red) and the dry season (in blue), averaged over 1981 to 2010. The background topographical map was created with grid files from the global multi-resolution topography (GMRT) synthesis (http://www.marine-geo.org/tools/GMRTMapTool). Moisture trajectories were derived using the US National Oceanic and Atmospheric Administration (NOAA) Hysplit model (http://ready.arl.noaa.gov/HYSPLIT.php). The moisture at the Paraíso Cave site is predominantly from the tropical Atlantic, whereas precipitation received in the western Amazon has largely endured recycling in the lowlands.
a, Depiction of horizontal winds over South America at 850 hPa (vectors, in metres per second), based on data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) (1981–2010; http://cfs.ncep.noaa.gov/cfsr/atlas/). Also shown is precipitation (blue shading, in millimetres per day) from the Tropical Rainfall Measuring Mission (TRMM) 3B43 dataset (1998–2010; http://trmm.gsfc.nasa.gov/3b43.html). Winds and precipitation are averaged over December to March. b, As in a, but for June to September. c, Monthly averaged temperature, precipitation and rainfall δ18O over Belém (blue dots) and Manaus (green triangles). The local climate at Paraíso Cave shares the same characteristics as those of Belém and Manaus. Data are from the International Atomic Energy Agency (IAEA) Global Networks of Isotopes in Precipitation (GNIP) database (http://www-naweb.iaea.org/napc/ih/IHS_resources_gnip.html).
a, Image of a Paraíso sample. The Paraíso calcite stalagmites typically have a high uranium concentration (up to 40 p.p.m.) but a low thorium concentration (<1 parts per billion, p.p.b.), almost ideal for uranium/thorium-based age determination. b, Age models for samples PAR01, PAR03, PAR06, PAR07, PAR08, PAR16 and PAR24. The chronology of the samples is established by linear interpolation between successive uranium/thorium dates. Dates are shown in black dots. Age uncertainties (2σ) are also included (most of the error bars are smaller than the symbols).
Extended Data Figure 4 Scatterplots of oxygen and carbon isotope ratios for the Paraíso stalagmites.
a, Relationship between the δ18O and δ13C data for Holocene Paraíso stalagmites. b, As in a, but for glacial Paraíso samples.
a, Monthly averaged precipitation (solid dots and triangles) and actual evapotranspiration (AET, open dots and triangles) over Belém and Manaus. We used the water-balance model44 as implemented in the US Geological Survey (USGS) Thornthwaite model45 to calculate monthly AET. b, As in a, but for LGM conditions. We assume that the cave temperature was ~21 °C during the LGM. Rainfall in the region was ~60% of today’s in each month, as calculated in Extended Data Table 1. The LGM and present-day patterns are essentially the same.
The cave δ18O record spans about 46,000 years, long enough to cover two precessional cycles. However, no obvious correlation can be observed between the cave record with local insolation in the months of January (blue), April (cyan), July (dark blue) and October (dark cyan). Insolation data are from ref. 50.
a, The Paraíso δ18O record is compared with ice-core records from Greenland53 (dark blue; North Greenland Ice Core Project (NGRIP)) and from Antarctica54 (blue; EPICA Dronning Maud Land (EDML) Ice Core) during the time interval from 25 kyr bp to 45 kyr bp. The NGRIP ice-core data are plotted in the Antarctic ice-core chronology 2012 (AICC12) timescale55, which is identical to the annual-layer-counted Greenland ice-core chronology 2005 (GICC05) timescale56 for the studied time interval. The EDML ice-core data are plotted in the AICC12 age scale55. D/O events are marked on the NGRIP record. The strong correlations between the Paraíso record and the ice-core records confirm the existence of rapid air–sea interactions between the high latitudes and the tropics on millennial timescales57,58, probably through the so-called bipolar seesaw mechanism59. b, As in a, but the Paraíso record is compared with ice-core records from Greenland53 (dark blue; NGRIP) and from Antarctica25 (blue; West Antarctic Ice Sheet Divide Ice Core (WDC)). The NGRIP and WDC data are plotted in the West Antarctic Ice Sheet Divide (WD) 2014 timescale25. The slightly enhanced correlations between the Paraíso record and the ice-core records, albeit visually, support the chronological method adopted in ref. 60. VSMOW, Vienna standard mean ocean water.
Contrary to the stalagmite δ18O record, the Paraíso δ13C record does not show an obvious shift from the last glacial period to the Holocene. In fact, the δ13C value reaches as low as about −10‰ during the LGM, similar to the observed minimum value in the Holocene. This suggests that the type of vegetation in the region has not undergone dramatic changes, remaining dominated by C3 plants37,61. The rainforest in the eastern Amazon might have become an open forest when the precipitation decreased substantially during the LGM. However, it was not replaced by savanna or grassland—that is, it has not become dominated by C4 plants. The δ13C spikes were probably caused by individual air–water–rock interactions during calcite precipitation.
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Wang, X., Edwards, R., Auler, A. et al. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature 541, 204–207 (2017). https://doi.org/10.1038/nature20787
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