Interpreting plant wax records of precipitation on a changing Bahamian landscape.

Plant waxes carry a record of source water, commonly used to reconstruct precipitation isotopes. However, plants often exert a large fractionation that varies between plant types, which complicates interpretation of past precipitation. We take a multi-proxy approach (δD_{leaf wax ,} δ¹³C_{leaf wax}, δ¹³C_org, C:N, pollen) to reconstruct Holocene ecologic and hydrologic changes archived within a sediment core from a coastal sinkhole on Abaco Island in the Bahamas. Results of a three end-member mixing model (based on δ¹³C_org, andC:N signatures) reveal that sedimentary organic matter is derived from three primary sources throughout the last 3000 years: the wetland on the adjacent epikarst, authigenic primary productivity within the oligohaline meteoric lens, or the surrounding terrestrial landscape. When plant input communities are stable ( 3,000 to 1,000 cal yr BP), as evidenced by a stable pollen record, the variations of δD values measured on plant leaf wax (C₂₄, C₂₆, C₂₈, and C₃₀) n-alkanoic acid biomarkers can be interpreted in terms of precipitation isotope changes. But, at 1,000 cal yrs BP, shifts in δD_{leaf wax} values (up to -50‰), coincide with changes in the pollen record which point to a transition from palms and tropical hardwoods to pines and mangrove. Depleted δD_{leaf wax} values archived since that shift in the sinkhole are synchronous with an increased abundance of Languncularia (white mangrove) and Conocarpus (button mangrove) on the landscape, and by corollary, increased wetland-derived organic matter accumulating within the sinkhole. Previous work has suggested that δD values of mangrove-derived lipids from tropical ecosystems are affected by additional isotopic fractionation, due to variations of plant biosynthesis impacted by higher salinity in source water. As such, the increased wetland deposition to Blackwood sinkhole during the last millennia likely confounds the potential δD_{precipitation} signals. This work provides additional evidence that mangrove-derived lipids can complicate hydrologic reconstructions, calling for cautious hydroclimatic interpretations from coastal sedimentary archives influenced by wetland-derived organic matter.