South American Summer Monsoon variability over the Late Pleistocene: a proxy perspective (Invited)
Abstract
Speleothem samples from tropical South America provide a high-resolution and precisely-dated history of South American Summer Monsoon (SASM) variability over the last 250,000 years. The combination of stalagmites collected from caves in northeastern Brazil (Cruz et al., 2009), western Amazonia (Cheng et al., 2013), the central Peruvian Andes (Kanner et al., 2012, 2013), and southern Brazil (Cruz et al., 2005, Wang et al., 2006) allows for comparison of climate variability along a moisture pathway. The SASM develops during the austral summer when deep convection carries moisture, originating from the tropical Atlantic, westward across the Amazon Basin and then southeastward through southern Brazil. We focus on the interpretation of SASM variability based on five stalagmites collected from the central Peruvian Andes (12°S, 76°W, ~3800m elevation) and compare these results from other samples collected throughout the region. Speleothem long-axis δ18O profiles represent changes in the intensity of the monsoon where enhanced monsoonal convection and upstream rainout cause precipitation to be depleted in the heavy isotope. Over orbital timescales, SASM variations coincide with austral summer insolation, following precessional cycles. In the central Peruvian Andes, δ18O shifts on glacial-interglacial timescales are on the order of 5-6‰, which is consistent with the magnitude of changed observed in northeastern and southern Brazil, and ~1-2‰ high than for the western Amazonia. These cycles in tropical South American climate are also in phase Antarctic temperatures and anti-phased with Asian Monsoon (AM) variability. Shifts in the mean position of the Intertropical Convergence Zone (ITCZ) in response to orbital forcing causes the monsoon in the respective hemisphere to strengthen while the monsoon in the opposing hemisphere weakens. This interhemispheric, anti-phased relationship of the monsoons exists over orbital timescales, and recent observations from the central Peruvian Andes and the western Amazonia demonstrate that this linkage also holds for abrupt changes into and during the last glacial period. The rapid millennial-scale shifts that characterize many Northern Hemisphere terrestrial and Atlantic Ocean marine climate records are also unequivocally present in South American speleothem δ18O, where shifts are on the order of ~1.5-2‰. Climate models suggest that cold periods in Greenland push the ITCZ southward, weakening the AM, and contemporaneously strengthening the SASM. Climate models equip with isotope tracers have the potential to relate the observed variations in the isotopic records to quantitative changes in monsoon variability and the South American hydrologic cycle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP43C..08K
- Keywords:
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- 1616 GLOBAL CHANGE Climate variability;
- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 1115 GEOCHRONOLOGY Radioisotope geochronology