TESTING THE BIOGENICITY OF ENDOLITHIC MICROBORINGS ON EARLY EARTH AND MARS: NEW DATA FROM WESTERN AUSTRALIA

Endolithic microborings provide an important but little studied biosignature with great potential for investigating the early biosphere on Earth and beyond. Here we report the discovery of a new assemblage of microtubular structures from early Archean sediments in Western Australia. These structures closely resemble those recently reported in pillow lavas of a similar age from South Africa, argued to represent endolithic etching of Archean seafloor (Furnes et al. 2004). Our structures differ, however, in several important and interesting regards: 1.occurrence within shallow marine sediments; 2.strongly selective distribution for specific clast lithologies; 3.presence of several distinct microtube morphologies; 4.association with marked geochemical gradients in earth’s oldest phosphatised sediments. A combination of petrographic, quantitative morphometric and geochemical criteria are used to test the biogenicity of these new remarkably well preserved microtubes (cf. Brasier et al 2002).

A syndepositional to early diagenetic origin for the microtubes is supported by mapping and petrography. Two contemporaneous tube morphotypes are recognised: type A tubes are straight with a modal width of c. 5µm and often occur in radiating arrays; type B tubes are branched, curved and tangled, with more variable widths. Many of the tubes are hollow, whilst some are partially or wholly infilled with vivianite (iron phosphate) or jarosite (iron sulphate) that can create pseudoseptation of the tubes. Type B tubes may also contain a terminal pyrite or jarosite crystal and quantitative morphometric analysis supports an origin from ambient inclusion trails. Type A tubes in contrast, occupy a narrow, well-constrained morphospace comparable with that of modern bacterial microborings. Preliminary geochemical results for both tube morphotypes will be presented and the implications for recognition of an early terrestrial biosphere will be discussed. The context and mineralogy of the assemblage also allows comparisons to be drawn with the results of recent Mars missions.

Brasier et al. (2002). Questioning the evidence for earth’s oldest fossils. Nature 416, 76. Furnes et al. (2004). Evidence for early life recorded in Archean Pillow lavas. Science 304, 578.