Opening Pandora's Box: The impact of open system modeling on interpretations of anoxia

The geologic record preserves evidence that vast regions of ancient oceans were once anoxic, with oxygen levels too low to sustain animal life. Becaus e anoxic conditions have been postulated to foster deposition of petroleum source rocks and have been implicated as a kill mechanism in extinction eve nts, the genesis of such anoxia has been an area of intense study. Most pre vious models of ocean oxygen cycling proposed, however, have either been qu alitative or used closed-system approaches. We reexamine the question of an oxia in open-system box models in order to test the applicability of closed -system results over long timescales and find that open and closed-system m odeling results may differ significantly on both short and long timescales. We also compare a scenario with basinwide diffuse upwelling (a three-box m odel) to a model with upwelling concentrated in the Southern Ocean (a four- box model). While a three-box modeling approach shows that only changes in high-latitude convective mixing rate and character of deepwater sources are likely to cause anoxia, four-box model experiments indicate that slowing o f thermohaline circulation, a reduction in wind-driven upwelling, and chang es in high-latitude export production may also cause dysoxia or anoxia in p art of the deep ocean on long timescales. These results suggest that box mo dels must capture the open-system and vertically stratified nature of the o cean to allow meaningful interpretations of long-lived episodes of anoxia.