RECONSTRUCTING LAST MILLENNIUM VARIABILITY IN THE SOUTH AMERICAN MONSOON USING SPELEOTHEM δ18O VALUES FROM THE AMAZON BASIN

The South American Summer Monsoon is a significant source of precipitation over much of tropical and subtropical South America. Reconstructing the response of the monsoon during past changes in global climate is pertinent to understanding how the monsoon might respond to future climate change. There are several existing reconstructions of last millennium monsoon variability based on δ¹⁸O records, however, many of the published records are derived from climate archives occurring outside the monsoon core (e.g., Andean Mountain range and east and southeast Brazil). To address this, we present a reconstruction of monsoon variability over the last 800 years from a composite (n=2) of speleothem samples collected from Paraiso Cave (4°S, 55.5°W) located in the Amazon Basin. Both samples show good overlap in δ¹⁸O values, ranging from -7 to -4‰. The samples appear to have consistent variation about the mean, with a distinct interval of low δ¹⁸O values (stronger monsoon) occurring from ~1750-1800, towards the end of the Little Ice Age. Other records from the region preserve low δ¹⁸O values during the Little Ice Age, although the timing of minimum δ¹⁸O values in existing records is variable (i..e., anywhere from ~1500 to 1650). Both Paraiso samples exhibit a decreasing trend in δ¹⁸O from ~1850 to 1920, although the trend reverses after 1920 in one of the samples. The decreasing trend in the stalagmites from Paraiso Cave (strengthening monsoon) contrasts with existing records from outside of the monsoon core that preserve an increasing trend in δ¹⁸O values (weakening monsoon) over the past century. However, the decreasing trend in our Paraiso samples is consistent with a trend of increasing speleothem δ¹³C values in Belize (Northern Hemisphere) interpreted to reflect reduced rainfall resulting from the southward shift of the Inter-Tropical Convergence Zone (ITCZ). A southward shift of the ITCZ enhances the intensity of the South American Monsoon, which is consistent with lower δ¹⁸O values. Intervals where δ¹⁸O records from the monsoon core differ from δ¹⁸O records from the periphery of the monsoon could reflect times where records from the monsoon periphery are more strongly influenced by non-monsoon processes (e.g., winter precipitation).