Stable Isotope Record of the MIS 12/11 Transition in a Tropical Speleothem Archive and Discussion of the Isotopic Evidence for the Evolution of Hydroclimate in the Eastern Andes over the last ~450,000 years.

The South American Summer Monsoon System (SASM) governs the hydroclimate of the Amazon basin and Eastern slope of the Andes, and through the development of paleoclimate proxy records has been demonstrated to be variable on annual to orbital timescales.  This system has been connected to many features of global climate, particularly glacial dynamics related to abrupt climate oscillations in the North Atlantic, variability in insolation, and Atlantic meridional overturning patterns. Therefore, these tropical paleoclimate archives in the Amazon Basin offer the ability to infer climate variability and linkages on multiple timescales. A principal way in which this system is understood is through the development of stable isotope timeseries records from speleothems, which have been demonstrated to respond to changing monsoon strength. Presented here is a new record from the Eastern Amazon basin at Huagapo Cave in Peru, which extends proxy archives to over 400,000 years and provides isotopic record of the MIS 12/11 transition. Oxygen and carbon isotope (δ¹⁸O and δ¹³C-) timeseries were developed from a stalagmite collected in May of 2019. The ~22 cm long stalagmite was sampled at a 350 μm resolution using a Merchantek micromill, yielding 573 samples, which were measured for stable oxygen and carbon isotopes using a Thermo Delta Advantage mass spectrometer in the Union College Stable Isotope Laboratory.  An age model was developed using U/Th disequilibrium dating of 7 samples from the stalagmite at the University of Minnesota and refined by tuning to published ice volume and insolation records. This new record provides isotopic evidence of the MIS 12/11 boundary at this site, which mirrors previously published speleothem records of subsequent deglaciations. Carbon and oxygen isotope correlations through time displays periods of inverse correlation around this deglacial indicating complexities in the moisture path at this time. In addition to this sample, others collected from this cave as well as previously published records provide a ca. 450,000 year composite record that can be used to illuminate long-term trends in hydroclimate at this site and assess the extent to which these isotopes are reliable records of climate through replication of previous proxy records. These long-term records show a distinct coupling to insolation, and gradual decline in peak δ¹⁸O values which suggests strengthened monsoonal conditions during progressive interglacials.