RAMAN SPECTROSCOPIC INSIGHTS INTO BURGESS SHALE-TYPE PRESERVATION

Cambrian strata host more localities preserving soft-bodied fauna than any other time in the Phanerozoic. While these soft-bodied deposits, often referred to as Burgess Shale-type (BST) deposits, have greatly enhanced our understanding of some of the key events in animal evolution, the mechanisms by which they formed are still debated. The precise elemental and mineralogical composition of individual specimens has been investigated at different BST sites, but while elemental data allows inferences to be made about minerals preserved, on its own, it does not allow precise mineral identification, and thus cannot provide definitive characterization of the fossil’s diagenetic pathway. In contrast, Raman spectroscopy can reveal the chemical composition of materials at the molecular level, data which allow an identification of the mineral phases present, a delineation of the thermal maturity and molecular structure of the organic carbon, and an illustration of the spatial relationship between the organic carbon and the minerals. This information is crucial for revealing the pathways of preservation and weathering fossils have undergone.

Here, we present Raman spectroscopic data collected on a specimen from the Middle Cambrian Spence Shale, a BST deposit preserving soft-bodied fossils. Raman point spectra of different anatomical features reveal that, on a molecular level, the fossil preserves different types of organic carbon. Additionally, the Raman data reveal two different types of hematite present within the fossil, α-hematite and γ-hematite (maghematite), a mineral formed by weathering of ferrous iron. Furthermore, 2D Raman imaging, which allows an understanding of the distribution of minerals and carbonaceous material relative to one another within the fossil, reveals that carbon is found in its highest concentration in association with hematite. Therefore, on this BST specimen, at least, carbon is preserved, but only in association with hematite, which may have important implications for BST preservation. Raman spectroscopy is thus a powerful tool with which to investigate exceptional preservation not just in Cambrian lagerstätten, but in all lagerstätten through time.