THE EXCEPTIONAL PRESERVATION OF LEAVES IN IRON-RICH SEDIMENTS FROM OCEANIA

The importance of iron in enhancing the fossilization potential of leaves has been documented in modern depositional environments, and many fossil leaves are preserved as iron-rich impressions in an otherwise iron-poor matrix. Leaves preserved in iron-dominated sediments, however, are rather uncommon. Exceptionally preserved fossil leaves were collected from fine- to medium-grained iron-dominated fluvio-lacustrine sediments in New Caledonia. Iron-rich concretions occur in layers at the outcrops, but only one of more than 15 such layers examined during a 2013 field expedition yielded fossil leaves. Seeds and stems, and other plant fossils are rare.

Electron-dispersive-X-ray spectroscopy shows that the fossil leaves are preserved mainly by replacement of the tissues by iron oxides, most commonly goethite. The iron that binds the sediments, forms the concretions, and preserves the leaves is sourced from the ultramafic basement. Organic components of the original leaf tissues very rarely survive. Several modes of preservation, including replacement by iron oxides, surface casts, organic preservation, and permineralization, were identified among the more than 35 leaves examined. Many of the leaves exhibit some three-dimensionality as a result of permineralization, particularly in the vascular tissues, even though void spaces associated with many of them show that they have undergone some shrinkage. Mesophyll tissues are also preserved in many of the leaves (by permineralization), a rare occurrence in the fossil record. In most instances where the mesophyll is preserved alongside the leaf vasculature, acicular goethite crystal bundles have replaced both kinds of tissues. This mineral replacement replicates cellular details in the vascular tissues, but the mesophyll is much less well preserved, most likely reflecting its susceptibility to more rapid decay.