600 yr High-Resolution Climate Reconstruction of the Atlantic Multidecadal Variability deduced from a Puerto Rican Speleothem

Abstract:

A multi-proxy speleothem study tracks the regional hydrological variability in Puerto Rico and highlights its close relation to the Atlantic Multidecadal Oscillation. Our proxy record extends instrumental observations 600 years into the past, and reveals the range of natural hydrologic variability for the region. A detailed interpretation and understanding of the speleothem climate record is achieved by the combination of multi-proxy measurements, thin section petrography, XRD analysis and cave monitoring results.

The speleothem was collected in Cueva Larga, a one mile-long cave system that has been monitored since 2012. MC-ICPMS ²³⁰Th/U-dating reveals that the speleothem grew constantly over the last 600 years. Trace element ratios (Sr/Ca and Mg/Ca) as well as stable isotope ratios (δ¹⁸O and δ¹³C) elucidate significant changes in atmospheric precipitation at the site.

Monthly cave monitoring results demonstrate that the epikarst system responds to multi-annual changes in seepage water recharge. The drip water isotope and trace element composition lack short term or seasonal variability. This hydrological system creates favorable conditions to deduce decadal climate variability from Cueva Larga’s climate record. The speleothem time series mimics the most-recently published AMO reconstruction over the last 200 years with a time lag of 10-20 years. The time lag seems to results from slow atmospheric signal transmission through the epikarst but the effect of dating uncertainties cannot be ruled out. Warm SSTs in the North Atlantic are related to drier conditions in Puerto Rico. During times of decreased rainfall a relative increase in prior calcite precipitation seems to be the main process causing increased Mg/Ca trace element ratios. High trace element ratios correlate to higher δ¹³C values. The increase in both proxies indicates a shift towards time periods of decreased rainfall.

Over the past 600 years there are two intervals of increased Mg/Ca and δ¹³C values lasting several decades in our speleothem record. They are centered around 1680 CE and 1470 CE. The elevated ratios indicate that drier conditions than present occurred in the region during periods of warm Atlantic surface waters. This may be a precursor of conditions now and to come.