Monday, February 26, 2018, 12:15PM - 1:15PM
SEEC Sievers Conference Room (S228)
4001 Discovery Drive, Boulder
Before the rise of complex eukaryotic ecosystems the global biosphere is commonly assumed to have been less productive than today. However, direct evidence for this assertion is lacking. I will discuss how new triple oxygen isotope measurements (∆17O) from ca. 1.4 billion year old sedimentary sulfates from the Sibley Basin of Ontario Canada provide direct evidence for a less productive biosphere in the mid-Proterozoic. These sulfate minerals have some of most negative ∆17O values (down to -0.88‰) ever observed, excepting peculiar sulfate from the terminal Cryogenian period. This observation demonstrates that the mid-Proterozoic atmosphere was dissimilar to more recent ones, directly reflecting a unique interplay among pCO2, pO2 and the photosynthetic O2 flux at this time. As oxygenic gross primary production (GPP) is stoichiometrically related to photosynthetic O2 flux, under current estimates of Proterozoic pCO2 (2-30 times Pre-Anthropogenic Levels [PAL]), modeling of the new ∆17O measurements indicates that mid-Proterozoic GPP was ≈6% PAL if Proterozoic pO2 was between 0.1-1% PAL. If pO2 were higher (1-10% PAL), then GPP was ≈41% PAL. When compared to estimates of Archean and Phanerozoic primary production, these solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2 over geologic time.
Coffee hour at 11:45.
Free and open to the public.