NOAA/WDS Paleoclimatology - Blackwood Sinkhole, Bahamas 3,000 Year Plant Wax Isotope Data

Plant wax hydrogen isotopic composition is commonly used to reconstruct the hydrogen isotopic composition of precipitation used by terrestrial vegetation. However, mangroves growing in coastal environments take up a mixture of freshwater and seawater. Biosynthetic fractionation (between source water and plant wax) differs between plant types and as a function of salinity, potentially complicating interpretations of past precipitation in coastal environments. In order to reconstruct Holocene hydrologic and ecologic changes archived within sediments from Blackwood Sinkhole on Abaco Island in The Bahamas, we adopt a multi-proxy approach using plant wax hydrogen isotopic composition (d2H) to reconstruct paleohydrology, together with plant wax carbon isotopic composition (d13Cwax), sterol biomarkers and pollen abundances to identify vegetation change. When pollen indicates a stable terrestrial plant community (2,950 to 850 cal yrs BP), variations of d2H values measured on the plant wax C28 n-alkanoic acid are interpreted in terms of precipitation isotope (d2Hprecip) changes, with 2H-depletion from 2,950 to ~2,100 cal yrs BP and ~1,700 to 1,000 cal yrs BP. However, interpretation is complicated at 850 cal yrs BP, when d2H values decrease (-50 per mil) concurrent with increased Laguncularia racemosa (white mangrove) and Conocarpus erectus (buttonwood mangrove), and the mangrove-derived biomarker, taraxerol. We develop a pollen-based correction for mangrove inputs, yielding reconstructed precipitation isotope estimates (d2Hprecip-corr). Low d2Hprecip-corr values are synchronous with increased abundance of pine pollen, both of which may indicate wetter conditions from 850 cal yrs BP to present. This study provides additional evidence that mangroves can complicate hydrologic reconstructions from n-alkyl terrestrial plant wax biomarkers, and that such complication can be removed by pollen-based correction. After correcting for mangrove inputs, we obtain estimates of d2Hprecip-corr from -33 to +25 per mil throughout the last 2,950 years, with uncertainties on the order of 10-20 per mil.