And they form quickly.
The ocean system is very sensitive to changes in pH and temperature. Not only do they alter and damage ecosystems such as coral reefs, they can also lead to what scientists call “dead zones”. A new study has found a link between sudden ocean warming at the end of the last ice age and hypoxic (low-oxygen) conditions that lead to large marine dead zones.
Researchers from Oregon State University collected marine sediment cores from the North Pacific, and recorded trace sediment quantities of organic material produced by plankton.
What researchers found were that large-scale warming events, 14,700 and 11,500 years ago, occurred very quickly and resulted in low-oxygen regions. When oceans warm and microscopic diatoms bloom, oxygen is absorbed by these diatoms. Since diatoms have large shells they easily sink to the ocean floor, absorbing even more oxygen on their way down. Since the oceans are currently in a state of warming with the top ocean layer heating up at a rate of 0.2 degrees Fahrenheit per decade, there are concerns that low-oxygen zones will develop once again.
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"Our study reveals a strong link between ocean warming, loss of oxygen, and an ecological shift to favor diatom production," said lead author Summer Praetorius, currently a postdoctoral researcher at Carnegie Institution for Science. "During each warming event, the transition to hypoxia occurred abruptly and persisted for about 1,000 years, suggesting a feedback that sustained or amplified hypoxia."
Just because oceans are warming does not mean a dead zone is the end result — there has to be ample nutrients and iron available for diatoms to form, and the higher North Pacific is lacking iron. But there are ways for iron to be extracted. If there is even a small loss of oxygen, it causes a chemical reaction that releases iron found within the sediments of the continental margin, which provides an essential nutrient for diatom growth.
What is alarming is that there is a region in the North Pacific just south of Alaska, nicknamed “the Blob” by meteorologists and oceanographers, that has been warming over the last two years. Researchers say it is on a scale similar to past events. This year, the Blob has a record breaking bloom of diatoms.
Praetorius notes, "While it's too soon to know how this event ties into the long-term climate patterns that will emerge in the future, the current conditions seem eerily reminiscent of the past conditions that gave way to extended periods of hypoxia."
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