The inhibition, rise, and maintenance of atmospheric O₂ each represent a distinct mode of the evolving relationship between geological processes and microbial activity on Earth.  This presentation will focus on sulfur isotopic evidence from Paleoproterozoic sedimentary rocks for the rise of O₂. A first-order similarity exists between S isotopic anomalies preserved in some Archean rocks and those associated with short-wavelength UV photochemistry of SO₂ (Farquhar et al., 2001). The Archean environment, therefore, likely permitted production and preservation of the isotopic signatures of UV-driven SO₂ photochemistry.  In particular, photochemical models indicate that only O₂ levels below 10^-5 PAL allow preservation of S isotopic anomalies (Pavlov and Kasting, 2002).  This implies that sulfur isotopic anomalies are a low resolution but high sensitivity proxy for O₂. In the Transvaal Supergroup, South Africa, S isotopic anomalies are found in the ~2.46 Ga Kuruman Iron Fm. (³³S anomalies from -2 to +2 per mil off of predictions from a reference mass-dependent fractionation relationship), while the ~2.32 Rooihoogte and Timeball Hill Fms. preserve ³³S abundances within -0.1 to 0.3 per mil of mass dependence (Bekker et al., 2004).  A significant threshold in O₂ abundance must have been crossed sometime between ~2.45 and ~2.32 Ga.  In the Transvaal Basin, this interval encompasses the Duitschland Fm., and we measured S isotopic compositions of carbonate-associated sulfate (CAS) and acid-volatile S (AVS) from this formation in order to more precisely define the nature of the Paleoproterozoic rise in O₂.  The lower Duitschland Fm. preserves significant ³³S isotopic anomalies in both CAS (up to 0.8 ä) and AVS (up to 1.4 ä).  In contrast, CAS from the upper Duitschland Fm. has ³³S abundances within 0.0 and 0.2 per mil of mass dependence.  This shift mimics the gross S isotopic transition that has been identified in Paleoproterozoic rocks (Farquhar et al, 2001; Bekker et al., 2004; Papineau et al., 2005) but occurs here over ~200 m of section.  Importantly, ³⁴S abundances in CAS jump by up to ~30 per mil when sizable ³³S anomalies disappear.  This dichotomy suggests that an abrupt re-organization of the S cycle accompanied the rise of O₂, and may imply an extreme sensitivity of the Paleoproterozoic atmosphere to microbial and geologic forcings.