APPROACHES TO UNDERSTANDING AND EVALUATING THE QUALITY OF REDOX PROXY DATA IN ARCHEAN SEDIMENTARY SUCCESSIONS

The origin of oxygenic photosynthesis was the most important metabolic innovation in Earth history. However, the origins of Cyanobacteria remains controversial because of the absence of an Archean fossil record. A separate, widely applied, approach to date the evolution of Cyanobacteria centers on proxies for their metabolic product: molecular oxygen. A broad range of geological and geochemical observations support a minimum age for their origin to a time between ~2.4 and ~2.32 Ga, however a several studies have argued for much earlier appearance of oxygenic photosynthesis on the basis of subtle geochemical proxy data. These interpretations are debated because the cycles of these elements are not well understood, and they appear in conflict with other redox proxies in the same basin and in some cases the same lithologies. Furthermore, we do not always have a strong understanding of the mineral phases that control the proxies within rocks, or how they were affected by the various post-depositional metamorphic and metasomatic processes common to all Archean sedimentary basins. Drawing on examples from the Agouron South Africa Drilling Project and NASA’s Astrobiology Drilling Project, we will discuss the results of a several detailed studies to employ a complementary suite of microscale chemical techniques that enables one to directly image the redox proxy data and leverage cross-cutting relationships and geochronology to ordinate the (commonly) discordant signals held by Precambrian rocks. The techniques include: light and electron microscopy, electron microprobe and synchrotron XRF for elemental composition, synchrotron X-ray spectroscopy for redox state, secondary ion mass spectrometry (SIMS) to make trace metal and S isotope ratio measurements, and scanning SQUID magnetic microscopy and ion probe geochronology to ordinate mineralization in absolute time. Results show that the signals commonly interpreted as “whiffs of oxygen” appear in late stage minerals phases, suggesting that Cyanobacteria were not prevalent in these Late Archean marine basins. This is consistent with S isotope data, redox-sensitive detrital grains, the body and molecular fossil records of Cyanobacteria, and emerging data from comparative genomics that crown group Oxyphotobacteria emerged well after the rise of oxygen.