A taphonomic study of δ13C and δ15N values in Rhizophora mangle leaves for a multi-proxy approach to mangrove palaeoecology

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The response of mangrove ecosystems to environmental change can be examined with stable isotopic tracers of C and N. The δ 13C and δ 15N of a taphonomic series of Rhizophora mangle L. (Red mangrove) leaves were analyzed from Twin Cays, Belize, to facilitate reconstruction of past mangrove ecosystems. On Twin Cays, fresh leaves of dwarf R. mangle trees (∼0.5 m high) were found to have more negative δ 15N values (mean=−10‰) and more positive δ 13C values (mean=−25.3‰) compared to tall R. mangle trees (mean δ 15N=0‰, δ 13C=−28.3‰). These isotopic differences can be related to nitrogen and phosphorus availability [Ecology 83 (2002) 1065]. We investigated three taphonomic stages in the fossilization of R. mangle leaves into peat with the following: (1) senescent leaves; (2) fallen leaves on the surface of the peat; and (3) sub-fossil leaves found within mangrove peat. In addition, by examining natural leaf assemblages we established that δ 13C and δ 15N of R. mangle leaves were not altered during senescence, despite a significant (50%) decrease in the N%. Modern dwarf and tall trees could still be identified from δ 13C and δ 15N analyses of the leaf assemblages found directly below a tree. Dwarf and tall trees could also be identified from δ 13C analyses of leaves that had decomposed for four months. Although dwarf and tall trees could not be statistically separated after four months according to δ 15N analyses, leaves with very negative δ 15N (−7‰) were only collected below dwarf trees. Leaf fragments were present in ∼50 cm long cores of peat from four sites on the island, and their isotopic compositions were determined. The ranges of δ 13C (−29 to −22‰) and δ 15N (−11 to +2‰) values from sub-fossil leaves were similar to the ranges from modern leaves (δ 13C=−29 to −23‰, δ 15N=−11 to +1‰). The sub-fossil leaf isotopic compositions were independent values, in comparison to the uniform values of the surrounding peat. Because of the stability and persistence of the stable isotopic signals, they could contribute significantly to a multi-proxy approach to mangrove palaeoenvironmental reconstruction.