TOWARD QUANTIFYING THE DIFFERENCE BETWEEN GEOLOGIC AND BIOLOGIC: A CASE STUDY FROM THE PALEOPROTEROZOIC TUREE CREEK GROUP IN WESTERN AUSTRALIA

Decades of research have been directed toward defining biosignatures in rocks from the early Earth, as a proxy for the search for life elsewhere. In cases of excellent preservation, morphology alone (e.g., a columnar stromatolite) may be used to differentiate features that are biologically influenced versus geologically formed. However, in most cases, there is ambiguity in discerning the origin of potential biosignatures in early Earth rocks.

Here, we interrogate the geologic ‘background’ processes recorded in carbonate rocks from a ~2.4 Ga microbialite reef in the Turee Creek Group. We combine field to micro-scale information, including quantitative compositional data, with the aim of objectively differentiating geological from biological features.

In one sample - a demonstrably biogenic columnar stromatolite (Barlow et al., 2016, Geobiology) - it is clear in both outcrop and hand sample where the stromatolite ends and the inter-columnar sediment begins. In thin section, the two can be delineated by changes in texture and grainsize. Yet, preliminary results from SEM-EDS analysis show that the stromatolite and the immediately adjacent sediment are composed of the same major elements: O, Ca, C, Mg, with minor Si, Fe, Al, K. Another sample - a coarsely bedded carbonate from an underlying, non-fossiliferous layer - contains the same elements.

So, what actually distinguishes sediment from stromatolite? Is it just the changes in texture and grainsize, or can more information be gleaned from finer-scale compositional analyses?

We are investigating the distribution of organic matter via confocal Raman imaging spectroscopy and using EPMA to determine quantitative elemental composition of these samples at higher resolution than EDS. Future work will utilize LA-ICP-MS to investigate the trace element composition and distribution in these samples. Ultimately, this research will reduce ambiguity in biosignature assessment by contributing to our understanding of what makes a feature not life.