How can oceans become anoxic

The occurrence of anoxic events is linked to changing ocean currents, increase in greenhouse gases like carbon dioxide and a warming climate. These global events have happened many times and are recorded in the geologic record as a thick layer of black carbon-rich shale with no evidence of marine life.

THE core! Look at the contrast between the black anoxic sediment and the white fossil rich sediment! Credit: Paul Bown. Imagine the greenhouse world of the cretaceous — huge volcanoes were spewing carbon dioxide into the oceans and atmosphere, it was hot, there were lots of plants, and, in general, the warm wet climate increased the rate of rock weathering on the planet. In the oceans, phytoplankton flourished. The increased amount of weathering washed huge amounts of critical nutrients like phosphate and nitrate into the oceans.

Phytoplankton, small marine plants, depends on nutrients and carbon dioxide to grow, and now they had plenty of both. For the rest of the oceans, the distribution of oxygen is, therefore, governed by a delicate balance of supply from the surface via circulation and mixing and consumption by marine life through respiration.

Across the global oceans today, there are various pockets with low or no oxygen — including parts of the tropical oceans off California, Peru and Namibia and the subsurface waters of the Arabian Sea.

The oxygen levels in these zones are low enough to be lethal to most marine life. Low oxygen regions can also release nitrous oxide N2O — a potent greenhouse gas — into the atmosphere. And the risk of deoxygenation is not just limited to these specific zones. Reducing oxygen levels is occurring at all oxygen concentrations in all ocean basins and affects a growing number of coastal regions. While the oxygen dissolved in seawater only amounts to around 0. In addition, the respiration of organisms continuously consumes oxygen essentially everywhere in the ocean.

As the maps below indicate, deoxygenation varies considerably across the oceans. They show gains blue shading and losses red in oxygen in the ocean down to 1, metres upper map and beyond lower. The largest declines in oxygen have predominantly occurred along the equator and in the Arctic. Oxygen change in the ocean.

Data are taken from Schmidtko et al. Ocean oxygen loss driven by excessive organic matter and growth of algae and the threats it poses to ecosystem services are documented in many regions but likely under-reported in others. Oxygen loss driven by ocean warming, on the other hand, was only recognised more recently and is not as fully understood.

The two causes can also interact — warming-induced oxygen loss is tipping coastal areas into hypoxia and may contribute to the dramatic increase in reports of coastal hypoxia. Global distribution of deoxygenation in the coastal and global ocean from Breitburg et al. Photo: from Breitburg et al.

Deoxygenation is one of multiple human-induced stress factors affecting marine ecosystems. Because respiration consumes oxygen and releases carbon dioxide, ocean deoxygenation is often accompanied by ocean acidification, making it necessary to mitigate these stressors together.

Similarly, a warming ocean decreases oxygen supply but increases oxygen requirements of marine organisms, making it critical to understand the combined effects of these stressors.

Loss of oxygen from the ocean will have severe impacts on marine biodiversity and the functioning of ocean ecosystems.

Much remains unknown as to the long-term human health, social and economic consequences of ocean oxygen loss.

Oxygen is necessary to sustain the life of all the fishes and invertebrates, and deoxygenation is already causing measurable declines of fish species. As naturally oxygen-poor systems, these areas are particularly vulnerable to ocean deoxygenation, and such effects may ripple out and affect hundreds of millions of people.

Degraded habitats caused by climate change-driven hypoxia and increased algal blooms may lead to reduced catches for fisheries and possible collapse of regional stocks. If dromaeosaurs had learned to pilot industrial bottom trawlers on the continental shelf, they would have gone bankrupt pulling up empty nets. The source of the great smothering in the Cretaceous seems to have been a molten font burbling deep beneath an ancient sea that separated North from South America.

The lava from these eruptions makes up much of what today is known as the Caribbean large igneous province, a vast expanse of frozen lava that stretches from Ecuador in the Pacific to the Antilles bracing against the open Atlantic. In the United States, large igneous provinces might be more familiar to Manhattanites gazing across the Hudson at the towering basalt cliffs of the New Jersey Palisades which, along with volcanic rocks of the same age from Nova Scotia to Brazil, are tied to a catastrophic mass extinction million years ago , or to windsurfers in the black canyons of the Columbia River Gorge which was formed by a later, smaller eruptive event.

The worst mass extinction of all time, the end-Permian mass extinction million years ago, left behind a large igneous province so sweeping that today it blankets much of Siberia.

Last summer, scientists in the Gulf of Mexico watched with growing alarm as the largest dead zone in recorded history spread across the sea , from Texas to the mouth of the Mississippi. Similar low-oxygen seas are spreading around the world. Though not as exciting as Jurassic Park, summertime boating in the lifeless Gulf is just about as close as you can get to experiencing the Late Cretaceous planet of OAE 2. Where the Mississippi meets the Gulf of Mexico south of Louisiana, this plant food from the heartland proves to be as good as advertised, fertilizing huge blooms of algae that, when they die, decompose and rob the seas of oxygen.

In , fertilizer from soy and corn farms in Ohio fueled an algae bloom on Lake Erie so large and noxious that it shut down drinking water for the city of Toledo. Erie vacationers have grown accustomed to the annual appearance of toxic-slime season.

So what was driving the global dead zones of the Late Cretaceous? That leads back to the molten forge burbling insidiously under the Caribbean. One of two things seems to have been happening. On the one hand, this strange volcanism could have been seeding the metastasizing algae blooms directly, by injecting a blast of trace metals, like iron, into the seawater.

This would have fertilized the ancient oceans much like some brash geoengineers have proposed doing today to sequester carbon in the ocean. On the other hand, the volcanism might have fueled these runaway plankton blooms more obliquely. By injecting huge amounts of carbon dioxide into the oceans and atmosphere, they drove global warming and more intense weather, as inevitably happens when you inject too much CO2 into the atmosphere.

Indeed, carbon-dioxide-driven global warming is a feature of many of the worst mass extinctions in Earth history.