LIFE AT 3.4 GA: PALEOBIOLOGY AND PALEOENVIRONMENT OF THE STROMATOLITIC STRELLEY POOL CHERT, PILBARA CRATON, WESTERN AUSTRALIA

The Strelley Pool Chert (SPC), a ~30m-thick, 3.4 billion year old, stromatolite-bearing chert/carbonate horizon in the Pilbara of Western Australia, may contain the oldest fossil evidence of life on Earth. We test that hypothesis by studying the morphology of conical stromatolites in the formation. We also examine the environment of deposition of the SPC, particularly with regard to possible environmental influences upon biological activity.

Our data suggest that the complex suite of morphologic characteristics displayed by SPC stromatolites cannot be explained solely by known abiologic mechanisms. The characteristics include; 1) discrete cone shape, 2) laminae that are continuous across several cones, 3) vertical orientation of cones on paleoslopes 4) corrugated to branching laminae, 5) uneven distribution of linked, spaced and isolated cones, and 6) high degrees of geometric inheritance over stratigraphic thicknesses of up to a meter. The morphological evidence for microbial sedimentation encourages further investigation of the biogenic hypothesis by organic geochemical means.

Both marine and hydrothermal environments of deposition have been proposed for the SPC. New geological and geochemical evidence reveal a complex interplay of environments during deposition, grading from hydrothermal exhalative conditions in localized green chert/ breccia dike deposits at the base of the formation through to widespread shallow open marine conditions during stromatolite formation. Clastic and chemical sedimentation were dominant at different places and times. Stromatolites in the vicinity of the hydrothermal deposits display greater morphologic complexity than those documented elsewhere in the formation, suggesting a possible link between hydrothermal and biological activity. However, the position of stromatolites above the hydrothermal exhalites indicates that stromatolite formation did not commence until after hydrothermal venting had waned. Further mapping of geological and geochemical trends may help determine whether hydrothermal venting influenced local stromatolite development. The layers of the SPC are unique in providing such detailed insight to the establishment of one of Earth’s earliest ecosystems.