A new study from Oxford Earth Scientists explains the cause of ocean deoxygenation during the last ice age with important implications for how the ocean’s oxygen levels may change in response to climate change.
Photosynthesis by marine phytoplankton and algae living at the ocean’s surface “fix” the carbon dioxide (CO2) that is dissolved in seawater into organic matter, drawing it down from the atmosphere and releasing much of the oxygen that humans breathe. When these plankton—and other organisms that feed on them—die, they sink into the ocean depths where bacteria use oxygen to break down the decaying matter and release CO2 back into seawater. This transfer of carbon from the surface of the ocean to the deep is called the “biological pump”, and is one of the primary ways that microscopic organisms in the ocean control the amount of CO2 in the atmosphere, and hence climate. Knowing the “strength” of the biological pump (how much carbon it sequesters in the deep ocean), and what controls it, not just in the present-day ocean but also under different climate conditions, is fundamental to our understanding of the carbon cycle and climate change. As this consumption of oxygen in the ocean by bacteria is directly tied to the release of carbon, scientists have long used measurements of dissolved oxygen in the ocean as an indicator of the strength of the biological pump.
Now, in a new study published in Nature Geoscience, researchers have challenged long-held assumptions about the relationships between oxygen, biology and carbon storage, and provide one of the first explanations for why the deep ocean at the peak of the last ice age was depleted in oxygen.
Led by DPhil student Ellen Cliff and Professor Samar Khatiwala of Oxford, in collaboration with Professor Andreas Schmittner at Oregon State University, a computer model was used to simulate the physical circulation, chemistry and biology of the ocean under different climate conditions, so as to understand how oxygen is related to the strength of the biological pump. The amount of oxygen that water at the surface of the ocean can theoretically hold without losing it to or gaining it from the atmosphere—its equilibrium concentration—is essentially determined by the temperature of the water. The colder it is, the more oxygen water can absorb. (This is why the ocean is currently losing oxygen as it heats up due to global warming; a study last year by Prof. Khatiwala and other Oxford researchers showed the ocean was gaining heat at a rate the Guardian newspaper equated to an average of one atomic bomb explosion per second over the last 150 years.) As water is transported into the interior by the ocean’s circulation, consumption by bacteria produces an oxygen deficit that is more or less proportional to the amount of carbon dioxide respired. Measuring this deficit can in principle measure the strength of the biological pump. However, an important assumption here is that the water at the surface was in equilibrium.
Figure 1: This schematic shows the major processes that affect oxygen depletion and equilibration in the ocean. In surface waters where sunlight penetrates, photosynthesizers use CO2 from the seawater to build organic matter, making oxygen as a byproduct. This draws down CO2 from, and releases oxygen to, the atmosphere. When these creatures die they sink and are fed on by bacteria, using up oxygen and releasing CO2. As such deeper waters become more depleted in oxygen, which is shown by the coloured bar turning orange. When waters rise back to the surface, oxygen from the atmosphere replenishes these waters bring them closer to equilibration (the bar turns to blue). Around Antarctica where sea ice is present, these waters may not be brought back to oxygen equilibrium as the sea ice prevents gases from the atmosphere reaching the seawater. These waters sink back to depth with a deficit of oxygen, leading to further deep ocean oxygen depletion. Photo credit: Ellen Cliff
To investigate the extent to which this assumption holds, the team compared the actual surface oxygen concentration with its theoretical equilibrium value—the difference is known as “disequilibrium oxygen”—and tracked how it moves through the ocean using a novel computer algorithm. Disequilibrium oxygen can also lead to a deficit that may be mistaken for that produced by respiration.
Surprisingly, it was found that in the preindustrial ocean (pre-1850), roughly a third of the oxygen deficit was a result of disequilibrium rather than oxygen consumed by bacteria. This goes against the consensus that the effect of disequilibrium on oxygen is negligible, and one immediate consequence of the study is that previous work based on measuring the oxygen deficit may have significantly overestimated the strength of the biological pump.
So what causes this disequilibrium in the first place? Ocean circulation ultimately brings waters depleted in oxygen due to bacterial consumption back to the surface in regions such as the North Atlantic and the ocean around Antarctica. Here, these upwelled waters absorb oxygen from the atmosphere (reducing the disequilibrium). This study discovered that there is seldom enough time for these waters to reach theoretical equilibrium before they sink back down into the interior.
Remarkably, this effect was found to be even greater during the peak of the last ice age—the “Last Glacial Maximum” (LGM)— when the Earth was about 5°C cooler and the ocean would have been expected to have higher oxygen concentrations. However, measurements in seafloor sediments of chemical “proxies” of oxygen content, show lower oxygen in the deep ocean. Previous studies interpreted this oxygen “deficit” to imply more carbon storage in the LGM ocean and thus a stronger biological pump. This would also partially account for the lower atmospheric CO2 during the LGM, a phenomenon that has long eluded explanation. What this study instead showed was that overall the “rain” of organic matter into the deep ocean—the engine that drives the biological pump—was in fact slightly weaker during the LGM, and that the deficit in oxygen was instead caused by greater disequilibrium. This is attributed to the more widespread presence of sea ice in the ocean around Antarctica in the glacial period compared with the present-day ocean. Sea ice acts as a cap on the surface preventing the water from gaining oxygen from the atmosphere.
The study not only adds a new wrinkle to how scientists interpret chemical indicators of past ocean oxygen change, but may prove important to our understanding of the ocean’s biological pump and how it responds to future climate change.
The full results are published in Nature Geoscience and can be accessed here: https://www.nature.com/articles/s41561-020-00667-zhttps://www.nature.com/articles/s41561-020-00667-z
Featured image: View of the Southern Ocean off Antarctica showing present-day sea ice cover, courtesy of NASA.