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

The response of mangrove ecosystems to environmental change can be examined with stable isotopic tracers of C and N. The δ13C and δ15N 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 δ15N values (mean=−10‰) and more positive δ13C values (mean=−25.3‰) compared to tall R. mangle trees (mean δ15N=0‰, δ13C=−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 δ13C and δ15N 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 δ13C and δ15N analyses of the leaf assemblages found directly below a tree. Dwarf and tall trees could also be identified from δ13C analyses of leaves that had decomposed for four months. Although dwarf and tall trees could not be statistically separated after four months according to δ15N analyses, leaves with very negative δ15N (−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 δ13C (−29 to −22‰) and δ15N (−11 to +2‰) values from sub-fossil leaves were similar to the ranges from modern leaves (δ13C=−29 to −23‰, δ15N=−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.