HR: 1330h
AN: PP22A-1195    [PDF]
TI: Millennial-Scale ITCZ Variability in the Tropical Atlantic and Dynamics of Amazonian Rain Forest
AU: * Wang, X
EM: wang0452@umn.edu
AF: Department of Geology & Geophysics, University of Minnesota, Minneapolis, MN 55455-0219 United States
AU: Auler, A S
EM: aauler@zaz.com.br
AF: CPMTC, Instituto de Geociˆncias, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627, Belo Horizonte, MG 31270-901 Brazil
AU: Edwards, R L
EM: edwar001@umn.edu
AF: Department of Geology & Geophysics, University of Minnesota, Minneapolis, MN 55455-0219 United States
AU: Cheng, H
EM: cheng021@umn.edu
AF: Department of Geology & Geophysics, University of Minnesota, Minneapolis, MN 55455-0219 United States
AU: Shen, C
EM: river@ccms.ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, Taipei, 106 Taiwan
AU: Smart, P L
EM: p.l.smart@bristol.ac.uk
AF: School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS United Kingdom
AU: Richards, D A
EM: David.Richards@bristol.ac.uk
AF: School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS United Kingdom
AB: Precipitation in the Amazon Basin is largely related to the intertropical convergence zone (ITCZ) in the tropical Atlantic which undergoes a regular seasonal migration. We chose a site south of the present day rainforest in semiarid northeastern Brazil, in order to study the timing of pluvial periods when the southern extend of the ITCZ would have been much further south than today. Shifts in the ITCZ position may have influenced the dynamics of rain forest and species diversity. We collected speleothems from northern Bahia state, located southeast of Amazonia. Age determinations with U-series dating methods show that samples grew rapidly during relatively short intervals (several hundreds of years) of glacial periods in the last 210 kyr. In addition, paleopluvial phases delineated by speleothem growth intervals show millennial-scale variations. Pluvial phases coincide with the timing of weak East Asian summer monsoon intensities (Wang et al., 2001, Science 294: 2345-2348), which have been correlated to the timing of stadials in Greenland ice core records and Heinrich events (Bond and Lotti, 1995, Science 267: 1005-1010). Furthermore, these intervals correspond to the periods of light color reflectance of Cariaco Basin sediments from ODP Hole 1002C (Peterson et al., 2000, Science, 290: 1947-1951), which was suggested to be caused by a southward shift of the northernmost position of the ITCZ and decreased rainfall in this region. Abrupt precipitation changes in northeastern Brazil may be due to the southward displacement of the southernmost position of the ITCZ associated with atmosphere-ocean circulation changes caused by (1) an increase in northern high latitude-tropical temperature gradient (Chiang et al., 2003, Paleoceanography, in press), and/or (2) the bipolar seesaw mechanism (Broecker et al., 1998, Paleoceanography 13: 119-121) during these Heinrich events. Pluvial phases are also coincident with higher insolation at $10\deg$S during austral autumn. This association implies that insolation may contribute to the southward shift of the ITCZ and enhanced precipitation in northeastern Brazil. The mean latitudinal migration of the ITCZ in the tropical Atlantic may force Amazon rain forest dynamics. Fossils and geomorphologic evidence found nearby indicate much wetter climates and possible tropical rainforest expansion to this locality in the past (Auler and Smart, 2001, Quaternary Research 55: 159-167). Furthermore, low $\delta$$^{13}$C (around -11$\permil$) values in speleothems suggest an extensive C3 forest coverage, different from current drought-resistant caatinga vegetation. During time of high rainfall, northeastern Brazil may act as a migration corridor between two species-rich communities, the Amazon Rain Forest and the Atlantic Rain Forest.
DE: 1600 GLOBAL CHANGE (New category)
DE: 3344 Paleoclimatology
DE: 4231 Equatorial oceanography
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting