Geochemical and climate modeling evidence for Holocene aridification in Hawaii: dynamic response to a weakening equatorial cold tongue

A 13.5 m sequence of Holocene limnic sediments from a sinkhole on Oahu, Hawaii provides a valuable paleoclimatic record for the central subtropical Pacific. δ13C analysis of plant leaf waxes (n-alkanes: n-C27, n-C29, n-C31 and n-C33) is used to infer vegetative changes. Average δ13C values of the suite of n-alkanes increase from approximately −31 ± 0.5‰ at 10 ca kyr BP (calibrated thousand years before present) to about −27 ± 0.5‰ by 6 ca kyr BP and then remain roughly constant until the Polynesian arrival (about 1.15 ca kyr BP). The increase in δ13C values of n-alkanes is interpreted to indicate a shift in the local vegetation from C3 to C4-dominated flora. Based on mass-balance calculations, the observed increase in the δ13C values translates to at least a doubling of the relative abundance of C4 plants. We argue that the expansion of C4 plants was a response to decreased overall water availability (aridification) due to reduced wintertime precipitation. Model simulations of an orbitally-induced increase in insolation along the equator during the Holocene provide evidence for a wintertime drying trend in the eastern subtropical North Pacific. This trend is associated with boreal fall to winter warming of the cold tongue in the eastern equatorial Pacific (EEP). These model results provide a conceptual framework to explain a dynamic link between the reconstructed Holocene drying trend in Hawaii and orbitally-forced climate change in the EEP that is analogous to the modern El Niño-Southern Oscillation teleconnection.