PP11C-02
Isotopic Controls of Rainwater and Water Vapor on Mangrove Leaf Water and Lipid Biomarkers

Monday, 14 December 2015: 08:15
2003 (Moscone West)
Nemiah Ladd, EAWAG Swiss Federal Institute of Aquatic Science and Technology, Surface Waters Research and Management, Kastanienbaum, Switzerland; University of Washington, Seattle, WA, United States, Marta Wolfshorndl, University of Washington, School of Oceanography, Seattle, WA, United States and Julian P Sachs, University of Washington Seattle Campus, Seattle, WA, United States
Abstract:
Hydrogen isotope ratios (2H/1H or δ2H) of sedimentary mangrove lipid biomarkers can be used as a proxy of past salinity and water isotopes. This approach is based on the observation that apparent 2H/1H fractionation between surface water and mangrove lipids increases with surface water salinity in six species of mangroves with different salt management strategies growing at sites spanning a range of relative humidities throughout Australia and Micronesia.

In order to more robustly apply mangrove lipid δ2H as a paleoclimate proxy, we investigated the cause of the correlation between apparent 2H fractionation and salinity. We present results from two related experiments that assessed controls on isotopes of mangrove leaf water, the direct source of hydrogen in lipids: (1) Measurements of natural δ2H in precipitation, surface water, and mangrove tissue water from a series of lakes with varying salinity and water isotope composition in Palau, and (2) measurements of mangrove tissue water and treatment water from a controlled simulation in which mangroves were treated with artificial rain of varying isotopic composition.

Rainwater 2H/1H fluctuations of 30‰ over a one-month period explain up to 65% of the variance in leaf water δ2H for Bruguiera gymnorhiza mangroves from Palau despite lake water isotope differences among sites of up to 35‰. This indicates that in humid tropical settings, leaf water isotopes are more closely related to those of precipitation and water vapor than to those of lake surface water, explaining the observed change in apparent fractionation in B. gymnorhiza lipids with salinity. The relationship between leaf water and rainwater isotopes may be due to either equilibration of leaf water with water vapor in the nearly saturated air or direct foliar uptake of rain and/or dew.

Foliar uptake is an important water source for many plants, but has not been documented in mangroves. We tested the capacity for mangroves to perform this function by treating leaves of Rhizophora mangroves on the island of Kosrae with artificial rain that varied in δ2H from -30 to +500‰. The influence of enriched “rain” on leaf water demonstrates how increased apparent fractionation in mangroves at high salinity may be driven by greater differences between surface water and rainwater isotopes with increasing salinity.

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