Geoscience Reference
In-Depth Information
when we discussed how anaerobic microbes removed nitrate as N
2
gas in
oxygen-minimum zones of the oceans. We also discussed how nitrogen-
fixing cyanobacteria reconverted N
2
gas to ammonium, resupplying
nitrogen to the biosphere. One could imagine that as ocean anoxia in-
creased, the anaerobic processes producing N
2
gas would become more
important, potentially draining biologically available nitrogen (mostly
nitrate) from the sea. Therefore, in contrast to phosphorus, the expan-
sion of anoxia in the global ocean would tend to limit the availability
of N and potentially also limit primary production. The extent of this
limitation would depend critically on whether nitrogen fixation could
accelerate to resupply the missing nitrogen. If the resupply by nitrogen
fixation perfectly balanced the nitrogen loss as N
2
, then the phospho-
rus feedback described above would control primary production and
organic carbon burial rates. However, if nitrogen fixation could not keep
pace with nitrogen loss, then nitrogen concentrations would be drawn
down, and nitrogen would limit primary production. In this case, the
phosphorus feedback described above would be ineffectual.
My friend and colleague Paul Falkowski from Rutgers University be-
lieves that this would be the case. Indeed, in his models, ocean anoxia
has the effect of decreasing rates of primary production through severe
nitrogen limitation. If nitrogen did indeed limit primary production
during times of extended ocean anoxia, a rather interesting positive
feedback results. To see this, imagine that we have precisely the situa-
tion described above; there is widespread anoxia in the sea with severe
nitrogen limitation on primary production. Now, let's imagine that for
some reason atmospheric oxygen levels increase just a bit. This would
cause anoxia in the oceans to decrease, and following the logic outlined
above, this would also cause a decrease in rates of N
2
loss by denitrifi-
cation. Because less nitrate is removed from the ocean as N
2
gas, some
of the nitrogen limitation would be relieved, leading to increased rates
of primary production and organic carbon burial. This would produce
more oxygen, further reducing the extent of anoxia in the oceans, and
so on. In principle, this positive feedback would operate until nitrogen
was no longer the limiting nutrient. Then, the phosphorus feedback on
atmosphere oxygen regulation would take over.
There is one last feedback we need to consider. Those of us who work
with tanks of compressed gas know that pure oxygen is reactive stuff.
