A flood of nutrients may have created an oxygen-starved ocean about 250 million years ago, preventing life from bouncing back for a few million years after a mass extinction wiped out 90 percent of marine species, a new study indicates.

The enriched, yet oxygen-starved ocean would have been similar to today's dead zones that appear in the modern ocean often as a result of agricultural runoff, as in the Gulf of Mexico.

The Permian-Triassic extinction, which hit about 250 million years ago, is believed to have been the result of widespread volcanic eruptions in what is now Siberia, which poured carbon dioxide into the atmosphere. Although the dates are inexact so far, it seems that life took an unusually long time to recover — possibly as much as 5 million years. [Oceans in Peril: Primed for Mass Extinction?]

Too much of a good thing

Chemical evidence from limestone deposited on the ocean floor during this time indicates that too much of a particular kind of life — tiny photosynthetic organisms, like certain bacteria and possibly algae — may have kept other marine species from recovering and diversifying.

"There was actually a lot of life in the ocean, but the life was not the typical life you would expect to find in oceans today," said lead researcher Katja Meyer, a postdoctoral researcher at Stanford University. The troublemakers appear to have been bacteria that can thrive without oxygen, including some that produce the toxic gas hydrogen sulfide. It's not clear whether or not algae — which need oxygen — were present, Meyer said.

Here's how it may have happened: The elevated carbon dioxide resulted in acid rain, which weathered the land (eroding sediments), releasing nutrients such as phosphorus, which were carried into the oceans with runoff. The extra nutrients fed these tiny organisms, causing them to flourish in the sunny surface waters. But when they died and sank to the seafloor, their decomposition sucked oxygen out of the water, creating what is called an anoxic, or oxygen-free, environment. The oceans also became sulfurous.

At other points in Earth's history, life has recovered more quickly from major setbacks. For instance, it took most animal groups hundreds of thousands of years to rebound after the Cretaceous-Tertiary extinction wiped out the dinosaurs, according to Lee Kump, a geoscientist at Pennsylvania State University who was not directly involved with the research, although he did provide feedback to the researchers and is Meyer's former adviser.  

"Scientists have argued about causes, either it was just in the nature of evolution, when it gets set back so abruptly and so intensely… Or it could signal more persistent, inimical oceanic conditions that delayed recovery," Kump said.

The evidence

Meyer and colleagues looked at the ratios of carbon isotopes — atoms with different molecular weights — for a clue as to what happened. By looking at limestone deposited beneath shallow and deep water at the time, they found a telltale difference between the ratios of a light carbon isotope, carbon 12 and the heavier carbon 13.

Photosynthesis is the process of taking in carbon dioxide and with the energy from sunlight, turning that into sugars needed to grow. It seems that photosynthetic life prefers the lighter version of carbon, carbon 12, leaving the heavier carbon 13 isotopes behind. So, in shallow waters, where microscopic organisms were flourishing, little carbon 12 was deposited, compared with carbon 13.

In deeper waters, there was more opportunity for decomposition to occur, which means the once-living tissues that had taken up carbon 12 released their contents into the water. That meant limestone deposited in deeper waters had more carbon 12 to the limestone and shifting the ratio. The gradient they found between deep and shallow water deposits was twice as large as today's, indicating that a significant increase in photosynthesis was occurring.

The recovery

Fortunately for larger marine organisms, these toxic oxygen-free conditions weren't sustainable. After the volcanoes stopped pouring carbon dioxide into the atmosphere, levels would have declined, decreasing the greenhouse gas's effects on climate and weathering of land.

As a result, fewer nutrients would find their way into runoff. At the same time, organic matter and nutrients were being taken out of circulation as they were deposited on the seafloor, allowing biological productivity to return to normal, Meyer said.

Life on land at the time — insects, amphibians, the ancestors of mammals and reptiles — was also decimated by the Permian-Triassic. However, this study's results are relevant only to marine life, according to Meyer.

The study appeared online in the journal Earth and Planetary Science Letters.

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