letters to nature 18. Van der Voo, R. Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans (Cambridge University atmospheric circulation and convective intensity over South Press, Cambridge, 1993). America. We find that these variations in rainfall source and 19. De Wever, P., Aze´ma, J. & Fourcade, E. Radiolaires et radiolarites: production primaire, diagene`se et pale´oge´ographie. Bull. Centres Rech. Explor. Product. Elf Aquitaine 18, 315–379 (1994). amount are primarily driven by summer solar radiation, which is 20. Baumgartner, P. O. Age and genesis of Tethyan Jurassic Radiolarites. Eclog. Geol. Helv. 80, 831–879 controlled by the Earth’s precessional cycle. The Dansgaard/ (1987). Oeschger cycles can be detected in our record and therefore we 21. Bartolini, A., Baumgartner, P. O. & Guex, J. Middle and Late Jurassic radiolarian palaeoecology versus carbon-isotope stratigraphy. Palaeogeogr. Palaeoclimatol. Palaeoecol. 145, 43–60 (1999). confirm that they also affect the tropical hydrological cycle, 22. Bernoulli, D., Manatschal, G., Desmurs, L. & Muenterer, O. Where did Gustav Steinmann see the but that in southern subtropical Brazil, millennial-scale trinity? Back to the roots of an Alpine ophiolite concept. Geol. Soc. Am. Spec. Pap. 373, 93–110 (2003). climate changes are not as dominant as they are in the Northern 23. Richards, D., Butler, R. F. & Harms, T. A. Paleomagnetism of the late Paleozoic Slide Mountain Hemisphere. terrane, northern and central British Columbia. Can. J. Earth Sci. 30, 1898–1913 (1993). 24. Hagstrum, J. T. & Murchey, B. L. Deposition of Franciscan Complex cherts along the paleoequator and The study area in subtropics of southeastern Brazil is well- accretion to the American margin at tropical paleolatitudes. Geol. Soc. Am. Bull. 105, 766–778 (1993). situated for investigating changes in tropical and subtropical 25. Larson, R. L., Steiner, M. B., Erba, E. & Lancelot, Y. Paleolatitudes and tectonic reconstructions of the atmospheric circulation. Although wet and dry seasons are not oldest portion of the Pacific Plate: A comparative study. Proc. ODP. Sci. Res. 129, 615–631 (1992). observed, the region receives rainfall from two distinct sources in 26. Besse, J. & Courtillot, V. Correction to “Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr”. J. Geophys. Res. Solid Earth 108, 2469, doi:10.1029/ different seasons (see Supplementary Information S1). During the 2003JB002684 (2003). austral winter and early spring, equatorward incursions of mid- 27. McFadden, P.L. & Lowes, F. J. The discrimination of mean directions drawn from Fisher distributions. latitude cold dry air result in cyclonic storms that move moisture Geophys. J. R. Astron. Soc. 67, 19–33 (1981). inland from the nearby Atlantic Ocean7. During the late summer 28. Lottes, A. L. & Rowley, D. B. in Palaeozoic Palaeogeography and Biogeography (eds McKerrow, W. S. & Scotese, C. R.) 383–395 (Memoir 12, Geological Society of London, 1990). and early autumn, rainfall is related to the intense convection 29. Bullard, E. C., Everett, J. & Smith, A. G. The fit of the continents around the Atlantic. Phil. Trans. R. Soc. over the interior Amazon basin that is associated with the South Lond. A 258, 41–45 (1965). American summer monsoon (SASM)8. From December to March, a 30. Klitgord, K. D. & Schouten, H. in The Geology of North America, The Western North Atlantic Region northwesterly low-level flow, the Andean low-level jet (ALLJ), (eds Vogt, P. R. & Tucholke, B. E.) 351–377 (Geological Society of America, Boulder, 1986). transports Amazon tropical moisture from interior Brazil near the Supplementary Information accompanies the paper on www.nature.com/nature. Equator towards the South Atlantic Convergence Zone (SACZ) located over southern Brazil9. At the study site (278 S), a significant Acknowledgements E. Dallanave is thanked for assistance in the field and laboratory analysis. fraction of summer precipitation derives from this moisture flux10. Comments by D. Bernoulli and J. Hagstrum greatly improved the manuscript. Thus, although it is not directly beneath the centre of convective activity over the Amazon basin, summer rainfall in southeastern Competing interests statement The authors declare that they have no competing financial Brazil is strongly influenced by the southward progression of interests. convection across the Amazon basin through the summer. The Correspondence and requests for materials should be addressed to G.M. two sources of rainfall also have quite distinct oxygen isotopic 18 ([email protected]). ratios. The more locally sourced winter rainfall is enriched in O compared to summer precipitation, with average d18O values of precipitation during the winter of 23‰ as compared to average values of 27‰ in the early autumn at Porto Alegre City, southern .............................................................. Brazil11 (and see Supplementary Information S2). At present the mean annual isotopic composition of rainfall is mainly determined Insolation-driven changes in by the relative contribution of summer, monsoonal precipitation versus winter, extratropical precipitation and not by temperature or atmospheric circulation over the past amount effects. This interpretation is supported by the coincidence of more positive values of d18O with the rainiest period in Porto 116,000 years in subtropical Brazil Alegre during winter and early spring, which is contrary to the expected tendency if either temperature or precipitation amount Francisco W. Cruz Jr1,2, Stephen J. Burns1, Ivo Karmann2, were the major source of variation in rainfall oxygen isotope Warren D. Sharp3, Mathias Vuille1, Andrea O. Cardoso4, Jose´ A. Ferrari5, ratios12. Pedro L. Silva Dias4 & Oduvaldo Viana Jr2 Stalagmite BT2 was collected from Botuvera´ Cave (278 13 0 24 00 S; 498 09 0 20 00 W, 230 m above sea level, a.s.l.). On the basis of 20 U/Th 1Department of Geosciences, University of Massachusetts, Amherst, analyses, the 70-cm-long stalagmite was deposited from ,116 01002 Massachusetts, USA 2 thousand years (kyr) ago to the present without detectable hiatuses Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Rua do Lago 562, Sa˜o Paulo, (Fig. 1). Samples for stable isotopic analyses were taken every 1 mm, SP, 05508-080, Brazil which represents an average resolution of ,150 yr. Values of d18O 3Berkeley Geochronology Center, 2455 Ridge Rd, Berkeley, California 94709, USA 4Departamento de Cieˆncias Atmosfe´ricas, Instituto de Astronomia, Geofı´sica e for stalagmite BT2 range from 20.5 to 25.0 with an apparent Cieˆncias Atmosfe´ricas, Universidade de Sa˜o Paulo, Rua do Mata˜o 1226, Sa˜o Paulo, cyclicity of ,20 kyr (Fig. 2). The lowest values are observed around SP 05508-090, Brazil 14–20 kyr ago and around 40–45 kyr ago. Superimposed on the 5Instituto Geolo´gico, Av. Miguel Stefano 3900, Sa˜o Paulo, SP 04301-903, Brazil longer-term cyclicity are more abrupt millennial-scale variations ............................................................................................................................................................................. with an amplitude of ,1 to 1.5‰. During the last glacial period, large millennial-scale temperature Stalagmite BT2 appears to have been deposited in approximate oscillations—the ‘Dansgaard/Oeschger’ cycles—were the pri- isotopic equilibrium with cave drip water as indicated by the mary climate signal in Northern Hemisphere climate archives relatively small ranges of d18O along single speleothem (such as from the high latitudes to the tropics1–6. But whether the stalactites or stalagmites) layers13 (S3a) and absence of significant influence of these abrupt climate changes extended to the tropical correlations between d18O and d13C (Supplementary Information and subtropical Southern Hemisphere, where changes in insola- S3b and c). The relatively large range of variation in d18O, more than tion are thought to be the main direct forcing of climate, has 5‰ in the speleothem, also suggests that temperature, through its remained unclear. Here we present a high-resolution oxygen effect in the calcite–water fractionation factor, is not the primary isotope record of a U/Th-dated stalagmite from subtropical cause of the observed variation. The temperature-dependent frac- southern Brazil, covering the past 116,200 years. The oxygen tionation between calcite and water is 20.24‰ per 8C (ref. 14), isotope signature varies with shifts in the source region and requiring 20 8C of temperature change to explain the total range of amount of rainfall in the area, and hence records changes in variation in d18O, or more than twice what is thought to be the total NATURE | VOL 434 | 3 MARCH 2005 | www.nature.com/nature 63 © 2005 Nature Publishing Group letters to nature temperature change between glacial and interglacial climates in the in both SASM and the SACZ. Convective intensity could affect subtropics15,16. Furthermore, the colder temperatures of the glacial the isotopic composition of rainfall through an amount effect. period at ,18 kyr ago would result in more positive calcite d18O Speleothem d18O values, particularly those from the tropics or values, not more negative as is observed during the Last Glacial subtropics5,6, are often interpreted as indicating changes in the Maximum. We conclude that d18O of speleothem BT2 primarily amount of rainfall because rainfall amount and d18O are observed reflects changes in d18O of regional precipitation. to be inversely correlated in regions