The levels of oxygen in seawater are just as important as on land. All aerobic aquatic organisms require oxygen to breathe and without sufficient oxygen, marine ecosystems collapse. As we’ll see in week 6, the detriment of ocean habitats can be just as dangerous for human beings as it is for marine communities. Deoxygenation of the oceans is a little known potential effect of climate change, so in this article I’ll discuss what we know about oxygen levels in the ocean past and present, and what we might see in the future.
Today, we can measure dissolved oxygen levels accurately using shipboard sensors, so we have a pretty good idea of how oxygen levels vary through the water column. Most of the oceans are sufficiently oxygenated for aerobic life, although suboxic zones where the breakdown of organic matter occurs are also quite normal in places. There are even some locations like the Black Sea where complete anoxia at depth is natural.
But how does this compare to the past? It’s very difficult to be certain about what the climate was like very early in Earth’s history, but evidence from sulphur isotopes tell us that between 3.85 and 2.45 billion years ago, there was virtually no oxygen in the atmosphere or oceans. It’s thought that some bacteria might have been producing oxygen by photosynthesis during part of this time, but not enough to significantly influence the overall composition of the atmosphere. So, whilst there might have been small oxygen ‘oases’ where early photosynthetic bacteria were active in surface seawater, the deep oceans were probably completely anoxic.
Somewhere around 2.0-2.4 billion years ago, one of the most important climate shifts in Earth’s history occurred: the Great Oxygenation Event. This is widely thought to have occurred due to the production of oxygen by photosynthetic bacteria, but was also related to other changes in environmental chemistry at the time such as reduced fluxes of hydrogen sulphide from hydrothermal sites. The most well-known evidence of the Great Oxygenation Event is Banded Iron Formations (BIFs). These rocks form when iron reacts with oxygen and precipitates from seawater, forming thick bands of red/orange sedimentary rock.
Atmospheric oxygen continued to increase after the Great Oxygenation Event until it reached the levels we have today (21%), although some major fluctuations did occur in the interval. In response, the surface oceans slowly absorbed more oxygen from the atmosphere, although the deep oceans took longer to become oxygenated. There is still a lot debate about how quickly the deep oceans became oxic.
One very relevant feature of the ocean’s oxygenation history to modern environmental concerns is Ocean Anoxic Events (OAEs). The geological record shows that several times in Earth’s history, the oceans have returned to a state of anoxia for intervals of a few hundred thousand years. These periods are associated with global warming and worryingly, with mass extinctions as this graph shows:
So how do these OAEs happen? Picture this: a major volcanic event such as the formation of a large igneous province occurs, and large amounts of carbon dioxide (CO2) are released into the atmosphere as a result. In response, surface temperatures rise and ocean circulation becomes sluggish as thermal stratification strengthens. Perhaps methane reserves in the deep ocean become destabilised and come to the surface, adding to the greenhouse effect of CO2 and further warming the planet. On land, chemical weathering becomes more prevalent (also due to higher temperatures), leading to higher nutrient availability in the oceans. Biological activity increases, consuming oxygen in the oceans faster than it can be replaced. A state of ocean anoxia is almost inevitable in these circumstances.
This is just one idea of how OAEs occur. Whilst there is geological evidence for many of the events associated with OAEs, in many cases we are not really sure which are causes and which are consequences – it’s a ‘chicken or egg’ situation.
What we know for sure is that atmospheric CO2 is on the rise due to anthropogenic activities. Runaway global warming due to this could cause another period of reduced oxygen in the oceans, if not total anoxia. There is some evidence that this might already be starting to happen, although the situation isn’t clear. Scientists only realised that ocean deoxygenation might be a consequence of global warming about 15 years ago, so it’s not something we’ve been monitoring for long. It does appear that wide expanses of the North Pacific have been declining in oxygen content for the past few decades, and the tropics may be experiencing systematic losses in oxygen as well. Furthermore, most climate models predict that dissolved oxygen levels will decrease by 1-7% in the future. On the other hand, there are many uncertainties in these models and studies from other parts of the ocean don’t show any clear trends at present. As with other modern climate change problems, the only way to prevent anthropogenic deoxygenation for sure is to cut CO2 emissions.
References: Keeling, R. F., Kortzinger, A. and Gruber, N. Ocean deoxygenation in a warming world. Ann. Rev. Mar. Sci. 2, 199-229 (2010).
Holland, H. D. The oxygenation of the atmosphere and oceans. Phil. Trans. R. Soc. B 361, 903-915 (2006).
Hough, M. L., et al. A major sulphur isotope event at c. 510 ma: A possible anoxia–extinction–volcanism connection during the early-middle Cambrian transition? Terra Nova 18(4), 257-263 (2006).