Raman spectroscopy of individual fossils As illustrated above (Fig. 4f and o through q; Fig. 6e through j), 2- and 3-D Raman imagery provide selleck chemicals firm evidence of the carbonaceous composition

of cellularly preserved Precambrian microorganisms. In addition, however, the Raman spectra on which such images are based can themselves be analyzed to determine quantitatively the geochemical maturity of the preserved organic matter. Shown in Fig. 7 are Raman spectra acquired from the kerogenous cell walls of representative fossil microbes permineralized in eight Precambrian geological units ~720 to ~3,465 Ma in age. The spectra shown—ordered from less (top) to more (bottom) geochemically mature and representative of a much larger suite of kerogen-comprised microfossils for which such data are available (Schopf et al. 2005)—were acquired from microfossils preserved in rocks that range from relatively little metamorphosed (top) to being appreciably more geologically MLN8237 altered (bottom), metamorphosed to middle greenschist facies. As the spectra illustrate, the two principal Raman bands of kerogen change markedly

as its molecular structure, altered primarily by heat, progresses along a geochemical pathway toward graphite: as the carbonaceous matter becomes structurally more ordered, the left-most (“D”)

band becomes increasingly narrow Calpain and more peaked and the right-most (“G”) band narrows and, in partially graphitized kerogen, ultimately bifurcates. Fig. 7 Raman spectra of the kerogenous cell walls of representative Precambrian microfossils permineralized in cherts of the ~850-Ma-old Bitter Springs, ~1900-Ma-old Gunflint, ~775 Ma-old Chichkan, and ~1050-Ma-old Allamoore Formations, the ~3,465-Ma-old Apex chert, the ~760-Ma-old Skillogalee and ~720-Ma-old Auburn Dolomites, and the ~775-Ma-old River Wakefield Formation (Schopf et al. 2005, 2007), ordered by their RIP values (Schopf et al. 2005) from less (top) to more (bottom) geochemically mature For each of the eight spectra shown in Fig. 7 is listed its Raman Index of Preservation (RIP) value, a quantitative measure of the organic geochemical maturity of the analyzed kerogen that reflects the local geological (diagenetic and metamorphic) environment to which the fossil-containing unit has been subjected (Schopf et al. 2005). Of rapidly increasing use in paleobiological studies (e.g., Chen et al. 2007; Schopf et al. 2008; Schopf and Kudryavtsev 2009; Igisu et al. 2009) and derived directly from the Raman spectra measured, such RIP values are highly reproducible and easily calculated (Schopf et al. 2005).