Fig. 2.
A schematic view of feedbacks that acted to sustain Proterozoic environments on both short and long geologic time scales (A and B, respectively). The point of entrance into this cycle is the establishment of sulfidic conditions at ≈1,840 Ma (5) and possibly earlier. Dashed green and solid red arrows note the direction of the feedback. If an increase in one quantity is followed by a decrease in the next, the connecting arrow is red (a negative feedback). If an increase in one quantity leads to an increase in the next, then the connecting arrow is green (a positive feedback). For example, if we begin in A with an increase in OMZ sulfide, PO2 correspondingly decreases (thus a red arrow preceding the PO2 ellipse), propagating responses through the remainder of the system. The presence of sulfide increases the likelihood of anoxygenic (by cyanobacteria, purple S bacteria, and/or green S bacteria) contributions to primary productivity, which would then produce less overall O2, encourage N2 fixation, increase primary production and carbon export, and increase the degree of euxinia (a positive feedback). (B) A sulfide-rich ocean in which S0 is an oxidant byproduct of primary producers and provides sedimentary conditions conducive to burial of both pyrite and carbon, although the burial of anoxygenically produced carbon is not strictly coupled to residual O2 (no O2 left behind). The loss of sulfide through pyrite burial dampens the extent of ocean euxinia (a negative feedback). The result is a system that maintains both oxygenic and anoxygenic photosynthesis.