HOLOCENE HYDROCLIMATE CHANGE EVIDENCE FROM THE NORTH CENTRAL PACIFIC: WINDWARD MONTANE MOLOKAI, HAWAII

A wetland core from a windward mountain bog on the Island of Molokai, Hawaii shows hydroclimate and ecosystem changes since 10,000 years ago. Radiocarbon measurements indicate continuous accumulation through the Holocene. Stable carbon isotope value of organic matter shows a pattern of increasingly more-positive values from 10,000 to 8000 years ago, consistent with decreased stomatal conductance in woody vegetation and an early Holocene drying trend. In addition, an overall decrease in Holocene rainfall over the Pacific near Hawaii is observed in transient model simulations forced by insolation, greenhouse gases and ice sheets. Between 4000 and 2000 years ago, more-negative carbon isotope values and rapid carbon sequestration in the Hawaiian bog suggest a period of wetter climate. Conditions appear to have been drier since 2000 years ago. Molecular biomarkers of plant litter inputs from sedimentary n-alkane distributions, together with a new library of modern plant n-alkane distributions, suggest a lower relative abundance of woody species 8000 to 3000 years ago. The c29 stable hydrogen isotope value shows as shift toward much more negative values around 12,000 years ago, and shows a period of most negative values around 3000 years ago. Multiple proxies from this windward location show an overall decrease in rainfall during the Holocene in general agreement with other Hawaii proxy data and model simulations. These data from the central Pacific also reveal millennial-scale changes in hydroclimate and ecosystem responses. Comparison to montane peat-accumulating sites from several locations in Hawaii shows the onset of peat formation at around the same time and sometimes earlier during MIS3. An emerging network of sites in Hawaii shows the influence of both tropical and extratropical Pacific circulation on mountain bog development.