Preservation of ultrastructural features in Eocene <i>Metasequoia</i> of Axel Heiberg: Implications on senescence, diagenesis, and paleo-environmental conditions
The 45 million year old fossil forests of Axel Heiberg Island in the Canadian High Arctic have been preserved primarily as mummifications. The fertile and vegetative remains of the Taxodiaceae Metasequoia and Glyptostrobus dominate the megafloral remains in forest floor mats, although many other taxa are represented. In order to provide new information toward reconstructing the paleo-environmental conditions at the site, a cytological study of the preserved Metasequoia leaf tissues in a lignite layer of the Upper Coal member of the Buchanan Lake Formation was performed. The investigation initially involved a qualitative examination via transmission electron microscopy of a sampling of leaflets taken over a vertical gradient in a lignite segment. Based on the findings, a quantitative assessment of chloroplast preservation in the fossil leaflets was performed. The fossil samples were then classified as sun or shade-type leaves based upon several thylakoid features of the preserved chloroplasts, which required a separate study of these properties in modern Metasequoia glyptostroboides Hu et Cheng. A quantitative study on chloroplast preservation in litter of M. glyptostroboides decomposing under twelve different environmental regimes, evaluated in respect to the preservation observed in the Eocene samples, was also performed. Intact chloroplasts were observed in 41.6% of the fossil leaves examined, and in 66.6% of the 5mm intervals. In 27.7% of the leaves, chloroplasts were so well preserved that features related to thylakoid geometry were measurable. Significant differences in preservation were observed between individual leaflets as well as between vertical intervals. An apparent oscillation in preservation quality with an estimated period of 62.5 years was distinguished using the chloroplast features found to best discriminate between layers of lignite. The effect of light intensity on the ultrastructural chloroplast characteristics in M. glyptostroboides leaflets was found to be highly significant. Fifty-six percent of the fossil leaflets were classified as being of the “sun-type.” The results of this study strongly imply that senescence was not initiated by low-light conditions, a finding that supports physiognomic estimates of a cool Arctic climate during the mid-Eocene. The best chloroplast preservation in modern leaf litter was observed in samples decaying at pH 8.5 and 10°C, and in leaflets that had been subject to rapid freezing.