MARINE ISOTOPE STAGE 3 RECORDED IN A STALAGMITE FROM PUERTO RICO

In order to better understand and reconstruct the climate of the Caribbean, stalagmites from two caves of the northern karst region of Puerto Rico have been collected. Rainfall on the island is dominantly controlled by the phases of El Niño Southern Oscillation (ENSO) and Northern Annular Mode (NAM). These two coupled climate systems also affect or are related to the seasonal migration of the ITCZ, which imposes wet or dry conditions in the region. The island of Puerto Rico is located within the northern reaches of the Intertropical Convergence Zone (ITCZ) placing it in a transition zone making it sensitive to climate change.

This study presents the stable isotope and ICP-MS U/Th data of a selected section (~16 cm) from stalagmite ENS_1 recovered from Cuevas Ensueño (north-central Puerto Rico), which is developed in the Lower Miocene Aymamón Limestone. The entrance of the cave is located in a collapse doline at an elevation of ~310 meters asl. The total length of its linear passages is ~250 m. The sample was collected at the far end of the cave from a well-decorated chamber.

Six U/Th dates constrain the lower part of the ENS_1 stalagmite growth interval to be between 40.5 and 38.2 (±0.3) ka, and therefore coincident with Heinrich event 4 (H4). Stable isotope measurements (434 subsamples) were performed at a 500 µm-interval, which corresponds to a sub-decadal time resolution. Both δ¹⁸O and δ¹³C time-series show an overall small shift towards lighter values during 2000 years of growth. In addition, the stable isotope record exhibits abrupt shifts within this interval not observed in other terrestrial proxies. The most important of these are large (up to 2‰ and 5‰ heavier δ¹⁸O and δ¹³C values, respectively) oscillations of not more than 50 years that interrupt the apparently cool H4 interval. The timing of ENS_1 stalagmite growth seems to indicate a significant change in the North Atlantic that affected Puerto Rico, and the isotopic variations during the inter-Heinrich 4 might have been caused by perturbations in the thermohaline circulation, ENSO events or changes in the relative position of the ITCZ.