You’re quite right that the algal blooms associated with fertilizer runoff consume oxygen in the water column. But some portion of the organic matter produced makes it through to the sediments, and becomes buried into the long-term carbon cycle. That portion of organic matter that is removed releases a stoichiometrically equivalent amount of oxygen to the ocean/atmosphere system.

You can think of it this way – photosynthesis generates organic matter and oxygen. Respiration runs the reaction in reverse, consuming both (this is why oxygen in the water column is consumed as organic matter settles). If all the organic matter produced during photosynthesis was consumed in this way, there would be no oxygen left to accumulate in the atmosphere. But a little bit escapes into sediments, releasing oxygen. In fact, many would argue that this burial process becomes more efficient if oxygen in the water column is consumed. So, in the simplest sense, the counterintuitive result of increasing photosynthetic production is that while water column oxygen concentrations decrease, there is ultimately more oxygen released into the atmosphere on longer timescales.

@Damian:

You’re right that there is a bit of a timescale mismatch! The paper doesn’t really make any links to the ‘Cambrian Explosion’, which, as you point out, occurred much later than the Cryogenian glacials. But there has emerged a pretty rich record of metazoan (animal) evolution prior to the Cambrian, and it appears to have really gotten going following these glacial events. This may or may not be the right mechanism, but there’s definitely a temporal relationship between the glacials and the initial expansion of metazoan life.

@TRJc, I’m no dead zone expert, but off the top of my head my guess would be that back then the atmosphere was very different (extremely lacking in oxygen), and so the algeal bloom would have reacted very differently to phosphorus than they are today in the dead zone in the Gulf. From the study’s abstract:

“An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere…. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.”

Anyone have any other ideas?

@Damian, I was wondering if someone would notice! I realized that when I put the post together, but it was the best picture I could find that we could use. I swear to you, I am NOT a plate tectonics denialist. I’m also not a witch.

Thanks for reading and commenting!

Jen