NANOSIMS: NEW TECHNOLOGY FOR ASSESSING THE ORIGIN OF ORGANIC MATTER IN EARTH'S OLDEST SEDIMENTS

Recognition of the earliest, organic evidence of life on earth is extremely challenging. Archean organic matter not only tends to be poorly preserved, but it also may include remnants of both biogenic and non-biogenic processes and may co-exist with younger post-depositional contaminants.

The new technology of NanoSIMS, however, is providing sub-micron scale elemental and isotopic analyses that can address these complexities. For example, a recent study of Neoproterozoic organic microfossils¹ has demonstrated that metabolically important elements (e.g., C, N, and S) can be correlated with cellular structures identified by optical microscopy. That study also suggested that N/C ratios reflect precursor compounds and may be important biosignatures. These results illustrate nano-scale structure and elemental composition of clearly biogenic remains and thus can be used as guidelines for interpreting less well preserved organic fragments from older or even extraterrestrial materials. Moreover, since NanoSIMS allows materials to be analyzed in thin section, spatial relationships between organic matter, the mineral matrix, and any veins or cracks can be determined, allowing discrimination between indigenous material and possible post-depositional contamination by younger microbes, migration of soluble organic molecules, or hydrothermal alteration.

New NanoSIMS investigations of Early Archean cherts from the Pilbara of Western Australia demonstrate that organic carbon within the matrix of these cherts has morphological and elemental characteristics different from those of organic carbon that occurs in hydrothermal veins. In addition, the organic structures in the matrix have a spatial relationship to Si and O that seemingly reflects silica nucleation on organic surfaces,¹ arguing that those structures are the same age as the mineral matrix and thus are not post-depositional contaminants. Finally, some of the organic structures in the matrix have NanoSIMS element maps reminiscent of those from the undisputedly biogenic, Neoproterozoic microfossils.¹ While that similarity might suggest a biological origin for the Early Archean structures, additional analyses including isotopic determinations are planned to further evaluate this possibility.

¹Oehler, D.Z., Robert, F., Mostefaoui, S., Meibom, A., Selo, M. and McKay, D.S. (2006). Astrobiology 6 (6): 838-850.