Evapotranspiration and moisture recycling effects on the climate of the Amazon Forest during the Last Glacial Maximum and the Holocene

• Grant number: 20/09258-4
• Support Opportunities: Scholarships in Brazil - Doctorate
• Effective date (Start): July 01, 2021
• Effective date (End): June 23, 2025
• Field of knowledge: Physical Sciences and Mathematics - Geosciences
• Principal Investigator: Francisco William da Cruz Junior
• Grantee: Angela Ampuero Grández
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 17/50085-3 - PIRE: climate research education in the Americas using tree-ring speleothem examples (PIRE-CREATE), AP.TEM
• Associated scholarship(s): 23/04580-3 - Isotopic analysis of fluid inclusion and TEX86 applied in speleothems, BE.EP.DR

Abstract

The interdependence between the hydrological cycle and the vegetation cover over the Amazon Basin raises important questions about how the climate and the forest evolved over the climatic events during the quaternary glacial cycles. The South American Monsoon regime response to the temperature fluctuations of the last Glacial period and the resilience of the forest ecosystems under contrasting hydroclimate scenarios are still open questions for paleoclimatology. The knowledge of the paleo-precipitation in the Tropical Region of South America largely relies on the oxygen isotopic series of speleothems (d18Oc). In turn, d18Oc is interpreted as a direct function of the isotopic effects that take place during the hydrological cycle, from evaporation at the moisture source to the precipitation over the continent. Thereby, the effects of moisture recycling generated by the evapotranspiration of the forest are potentially important controls of the isotopic composition of rainfall over the Amazon Basin. Indeed, about 30% of the Amazonian rainfall originates from evapotranspiration within the Amazon Basin itself and accounts for more than half of the winter precipitation. Hence, the effects of evapotranspiration on the isotopic composition are at the center of a debate about the interpretation of the isotopic signal of Amazonian speleothems. This project takes a systematic approach based on the isotopic reconstitution of precipitation, including the analysis of fluid inclusions in speleothems, the integration of numerical simulations using stable water isotope coupled terrestrial models (CLAM4) and data of the isotopic composition of precipitation. With these tools, the project aims to assess the effects of the changes in the forest cover and moisture sources on the isotopic composition of rainfall over the Amazon Basin. Monitoring of the isotopic composition of rainfall is underway and the results will be compared with back-trajectory analysis and evapotranspiration rate along the moisture flux pathways. Numerical simulations of circulation models on the terrestrial surface will be used to help quantify the effects of evapotranspiration for different scenarios of vegetation cover. Finally, d18O and dD data from the paleo-precipitation of the forest obtained from fluid inclusions of speleothems in caves distributed in an east-west transect along the Amazon Basin will be evaluated comparatively. Overall, this study aims to understand the main factors of the spatial variability of the isotopic records in the Amazon Basin over the Last Glacial and Holocene, quantifying the isotopic effects of evapotranspitation across the forest. The results will be important to understand how the hydroclimate and the extent of the forest were related in the past. (AU)