An Ocean Short of Oxygen

As average global temperatures rise, larger parts of the world's oceans could become anoxic dead zones. This is shown by an analysis of the oxygen conditions during the past 20,000 years

Every summer, a few places in the north-east Pacific now see vast numbers of dead marine animals including fish, shrimp or molluscs being washed up on the beach by the waves. It is a financial catastrophe for the local fishing industry and an ecological one for the sea. These mass mortalities are caused by the animals suffocating because their water contains too little oxygen, or even none at all.

Although this phenomenon is still localised, that could change in the future, as revealed by a new publication in “Nature Geoscience”.

The two authors, Samuel Jaccard from the Geological Institute at ETH Zurich and Eric Galbraith from McGill University, Canada, have evaluated oxygen data from seafloor sediment cores. Based on this data, they have reconstructed how the oxygen content in the oceans throughout the world has changed in the past 20,000 years, focusing particularly on the Pacific and Indian Oceans.

When temperature rises, oxygen vanishes

Their analyses show that the marked average global temperature rise of around two degrees centigrade occurring between the peak and the end of the last ice age, i.e. 20,000 to 10,000 years ago, had a massive effect on the oxygen content of seawater. The oxygen content of many oceans fell dramatically as a result of the temperature increase, and oxygen-depleted marine zones expanded more than during the cold period.

Expansion of the dead zones

These dead zones are particularly critical because they are situated in areas of the ocean that are home to the largest quantity of life. This means a shrinkage of habitat especially for the large fish of the open ocean. Entire food chains are threatened with deprivation. Biodiversity in the oceans will probably decrease, which will also pose serious problems for the fishing industry.

Poor mixing

The oxygen in seawater originates mainly from gas exchange between the sea surface and the atmosphere. Algae and other green life-forms also produce the vital gas. However, oxygen is consumed when dead organic material sinks down towards the sea bed and is degraded in the process by various organisms, mostly microbes. Warmer temperatures in the uppermost layers of water disturb this delicate equilibrium, because for physical reasons warm water is able to absorb less oxygen than colder water.

Furthermore, more stable layers form when seawater warms up. Warm, lower-density water lies on top of cold, denser water. These layers scarcely intermix, reducing the efficiency of the gas exchange with the atmosphere and between the various bodies of water.

Reference: Jaccard SL & Galbraith ED. Large climate-driven changes of oceanic oxygen concentrations during the last deglaciation. Nature Geoscience (2011). Published online 18 December 2011. DOI: 10.1038/ngeo1352

Source: ETH Zürich