Five Centuries of ENSO and PDO from a South Pacific Stalagmite

Past speleothem studies have retrieved long-range climate archives that cover events of glacial-interglacial significance. Their resolving power was not required to be better than centennial thus matching the precision of the U/Th dating method. Recent interest in stalagmites as archives of late Holocene climate poses more stringent demands such as a finer resolution of the isotope proxy records and improved dating precision at annual to decadal level that stretches the U/Th dating method to its analytical limit. Important in this new endeavor is the removal of artifacts in oxygen and carbon stable isotopes proxy records by gaining a better understanding of their controlling ambient factors. Here we present the results of a study carried out on a continuously layered stalagmite from a mid-ocean island, and show the techniques we employed in order to tease out a detailed climate archive of ENSO and PDO for the past 500 years.

The first step for establishing a firm chronology of the stalagmite was laminae counting which yielded 524 annual couplets suggesting deposition between 1478-2002 AD. Results were verified by high precision AMS radiocarbon dating that confirmed our interpretation of the couplets as being annual. Our sampling strategy yielded sub-annually to annually resolved d¹⁸O and d¹³C time-series. Comparison between the measured records and forward models based on a century of overlapping instrumental data from the island indicate that ENSO-driven rainfall variability exerts the strongest influence on the isotope composition of the stalagmite-calcite. Strong annual ENSO events and decadal-long PDO variability are also discerned in the proxy records spanning the LIA interval. Coeval coral proxy climate data from the South Pacific are used for comparison and validation of our results.