DECAY AND EARLY DIAGENESIS OF METASEQUOIA LEAVES: IMPLICATIONS FOR THREE-DIMENSIONAL PRESERVATION AND BULK ISOTOPE SIGNALS IN PLANT FOSSIL LAGERSTäTTEN

Plant remains from fossil lagerstätten have played a significant role in paleobotanical and paleoenvironmental studies, partly due to their remarkable preservation. While the mechanism for these exceptional preservations is still poorly understood, polysaccharides potentially offer insight, as they are more abundant in anatomically well-preserved plant fossils. In order to examine the role of polysaccharides in maintaining three-dimensional (3D) leaf structures and the impact of polysaccharide decay on bulk carbon isotope signals of leaf fossils, we investigated natural and laboratory decay of modern Metasequoia leaves, an important taxon in Cenozoic plant fossil lagerstätten, to mimic the early stages of diagenesis in leaf fossilization. Pyrolysis-gas chromatography-mass spectrometry, 3-methyl-2-benzothiazolinone hydrazine hydrochloride assay, and nuclear magnetic resonance analyses captured polysaccharide molecular abundances, while scanning electron microscopy qualitatively and a new Python-based method quantitatively revealed stages of morphological degradation. Our results show that polysaccharides do play a significant role in maintaining 3D structures of Metasequoia leaves; the lignin-strengthened tracheids and fiber cell walls in the xylem have remained almost intact throughout laboratory decay, whereas the cellulose-dominant phloem cell walls have gradually collapsed. Although bulk carbon measured by isotope ratio-mass spectrometry remain constant throughout the closed-system laboratory decay, the open-system natural decay showed a negative shift of ~1‰ in samples buried in pond sediment, suggesting that decomposition in the open aqueous environment led to carbon isotopic fractionations after partial removal of polysaccharides. Our morphological, molecular, and isotopic data from the natural and laboratory decay series provide benchmarks for estimating the degree stage of preservation in plant fossil lagerstätten that provide critical clues for ancient environments.