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Carbon and hydrogen isotopes of taraxerol in mangrove leaves and sediment cores: Implications for paleo-reconstructions

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Joseph M. Smoak

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Reconstructing past climate change in mangrove swamps contextualizes ongoing and future developments in these globally important ecosystems. Taraxerol, a well-recognized lipid biomarker for mangroves, is a promising target compound for calibration since it is relatively refractory and well preserved in sediments and since mangrove lipid δ2H and δ13C values have been shown to respond to salinity changes. Here we investigate the δ2H and δ13C values of taraxerol in leaves of two mangrove species (Rhizophora mangle and Laguncularia racemosa) and three dated mangrove cores along a spatial transect from the Shark River Estuary of South Florida, USA, to constrain its applicability for hydroclimate reconstructions. The net 2H discrimination between surface water and taraxerol increased by 1.0‰ ppt−1 over a salinity range of 0.7–32 ppt for both R. mangle and L. racemosa. Although the δ13C values of taraxerol showed a significant positive correlation with salinity in L. racemosa, the inverse trend was observed in R. mangle. The isotopic signature and spatial trends of taraxerol observed in mangrove leaves were well imprinted in mangrove surface sediments.

In addition, we further tested if the isotopic signal of taraxerol from mangrove leaves could be preserved in sediment cores on a time scale of ca. 300 yrs. No strong evidence of significant diagenetic alteration was observed for δ2H values of taraxerol. In contrast, an increase up to ∼1.1‰ was observed for δ13C, excluding the Suess effect. Considering the consistent salinity-dependent discrimination of 2H to salinity, and no significant diagenetic alteration of taraxerol δ2H values on centennial time scales, taraxerol H isotopes are a promising proxy for hydroclimate reconstruction in mangrove and mangrove-adjacent systems. However, the interpretation of δ13C values of taraxerol should be treated with caution since its correlation with salinity may be species-specific and a slight diagenetic enrichment in 13C may occur.





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