Browsing by Author "Stríkis, N. M."
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Item Restricted Calibration of speleothem δ¹⁸O records against hydroclimate instrumental records in Central Brazil(Elsevier, 2016-04) Moquet, Jean Sébastien; Cruz, F. W.; Novello, V. F.; Stríkis, N. M.; Deininger, M.; Karmann, I.; Ventura Santos, R.; Millo, C.; Apaéstegui Campos, James Emiliano; Guyot, J.-L.; Siffedine, A.; Vuille, M.; Cheng, H.; Edwards, R. L.; Santini, W.δ¹⁸O in speleothems is a powerful proxy for reconstruction of precipitation patterns in tropical and sub-tropical regions. The aim of this study is to calibrate the δ¹⁸O record of speleothems against historical precipitation and river discharge data in central Brazil, a region directly influenced by the Southern Atlantic Convergence Zone (SACZ), a major feature of the South American Monsoon System (SAMS). The present work is based on a sub-annual resolution speleothem record covering the last 141 years (the period between the years 1870 and 2011) from a cave in central Brazil. The comparison of this record with instrumental hydroclimate records since 1921 allows defining a strong relationship between precipitation variability and stable oxygen isotope ratios from speleothems. The results from a monitoring program of climatic parameters and isotopic composition of rainfall and cave seepage waters performed in the same cave, show that the rain δ¹⁸O variability is dominated by the amount effect in this region, while δ¹⁸O drip water remains almost constant over the monitored period (1.5 years). The δ¹⁸O of modern calcite, on the other hand, shows clear seasonal variations, with more negative values observed during the rainy season, which implies that other factors also influence the isotopic composition of carbonate. However, the relationship between δ¹⁸O of carbonate deposits and rainwater is supported by the results from the comparison between speleothem δ18O records and historical hydroclimate records. A significant correlation between speleothem δ¹⁸O and monsoon rainfall variability is observed on sub-decadal time scales, especially for the monsoon period (DJFM and NDJFM), once the rainfall record have been smoothed with a 7–9 years running mean. This study confirms that speleothem δ¹⁸O is directly associated with monsoon rainfall variability in central Brazil. The relationship between speleothem δ¹⁸O records and hydroclimatic historical records allows approximation of the absolute changes in mean annual rainfall during the last millennia in the SACZ/SAMS domain.Item Restricted The forest effects on the isotopic composition of rainfall in the northwestern Amazon Basin(American Geophysical Union (AGU), 2020-02-27) Ampuero, A.; Stríkis, N. M.; Apaéstegui Campos, James Emiliano; Vuille, M.; Novello, V. F.; Espinoza, Jhan Carlo; Cruz, F. W.; Vonhof, H.; Mayta, V. C.; Martins, V. T. S.; Cordeiro, R. C.; Azevedo, V.; Sifeddine, A.In the Amazon basin, intense precipitation recycling across the forest significantly modifies the isotopic composition of rainfall (δ¹⁸O, δD). In the tropical hydrologic cycle, such an effect can be identified through deuterium excess (dxs), yet it remains unclear what environmental factors control dxs, increasing the uncertainty of dxs‐based paleoclimate reconstructions. Here we present a 4‐year record of the isotopic composition of rainfall, monitored in the northwestern Amazon basin. We analyze the isotopic variations as a function of the air mass history, based on atmospheric back trajectory analyses, satellite observations of precipitation upstream, leaf area index, and simulated moisture recycling along the transport pathway. We show that the precipitation recycling in the forest exerts a significant control on the isotopic composition of precipitation in the northwestern Amazon basin, especially on dxs during the dry season (r = 0.71). Applying these observations to existing speleothem and pollen paleorecords, we conclude that winter precipitation increased after the mid‐Holocene, as the expansion of the forest allowed for more moisture recycling. Therefore, forest effects should be considered when interpreting paleorecords of past precipitation changes.