Tropical paleoclimate reconstructions from stable isotopes of mangrove lipid biomarkers
Ladd, Sarah Nemiah
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Tropical oceans play an integral role in global climate dynamics, yet past tropical hydrologic variability is poorly constrained, as many established climate proxies are better suited to higher latitudes. Hydrogen isotope ratios (2H/1H) of leaf waxes are a promising new proxy. Previous field assessments have been conducted in temperate regions or in continental interiors. On tropical coastlines, mangroves are a major source of organic matter to sediments. Unlike other higher plants, mangroves grow in saline water, which is known to influence 2H fractionation in algal lipids. In order to use 2H/1H ratios of leaf lipids in coastal tropical sediments, it was thus necessary to assess salinity's effect on 2H fractionation in mangroves. In field calibrations of Avicennia marina from subtropical Australia and Rhizophora spp. from Micronesia, salinity strongly affected net 2H fractionation in mangrove lipids. A unit increase in salinity resulted in an increase of 1.5 ± 0.3 / in n-alkane fractionation in A. marina (Chapter 2) and 0.9 ± 0.2 / in the Rhizophora biomarker taraxerol (Chapter 4). In the same A. marina n-alkanes, carbon isotope fractionation decreased by 0.2 ± 0.05 / PSU-1 (Chapter 3). 2H/1H and 18O/16O ratios of leaf and xylem water from Micronesian Rhizophora and from a 9.5-month time series of 3 mangrove species from subtropical Australia were used to investigate mechanisms responsible for the H-isotope response. Salinity does not cause changes in leaf water enrichment. Rather, changes in net 2H fractionation are best explained by biochemical responses to salinity, such as compatible solute production or increased use of stored carbohydrates, or by the timing of production of leaves and leaf lipids (Chapters 4 and 5). Mangroves and algae have opposite H isotope responses to salinity. By pairing 2H/1H ratios of mangrove and algal biomarkers, it is possible to calculate past changes in salinity and water isotopes. Initial applications of this approach to an 800-year sedimentary record from Palau demonstrate significant drying during the Little Ice Age (~1400 - ~1800 AD) (Chapter 6).
- Oceanography