Central Pacific hydroclimate over the last 45,000 years: Molecular-isotopic evidence from leaf wax in a Hawai'i peatland

Long-term hydroclimate variability at low latitudes exerts a strong influence on global climate, yet its timing and mechanisms are poorly documented for the central Pacific region. Here we present the molecular and hydrogen isotopic compositions of long-chain n-alkanes in a montane wetland deposit at Mount Ka'ala on the Island of Oahu, Hawaii, which document hydrological conditions and related vegetation changes over the late Pleistocene and the Holocene. To aid the interpretation of fossil n-alkanes, we also determine chain-length distributions of the most common bog plants growing locally. Organic matter accumulation started around 45 ka BP, one of the oldest reported dates for peat-forming wetland ecosystems in Hawaii. Sedimentary n-alkane distributions suggest vegetation shifts driven by changes in rainfall regime, especially with enhanced abundances of shorter chain length homologues during wet periods. Depleted values of n-alkane delta H-2 dated to ca 45-40, 24-22.5, 19-16.5, and 10-6 ka BP generally coincide with faster carbon accumulation rates, and may indicate enhanced winter cyclonic storm precipitation, as suggested by modern rainwater isotope data. For the Late Pleistocene, the consistency in timing and direction of hydroclimate variability at Ka'ala, dynamics of the Mauna Kea ice cap, precipitation in the Cariaco basin, and East Asian monsoon intensity suggest that southward shifts in the mean position of the ITCZ contributed to a greater frequency of extratropical cyclonic storms, increasing precipitation at leeward and high-elevation sites. During the Holocene, the ITCZ-hydroclimate linkage at Ka'ala is inverted, consistent with a Pacific dominated by ENSO variability. (C) 2020 Elsevier Ltd. All rights reserved.