Scientific Method / Science & Exploration

Too hot to handle: Triassic temps may have killed tropical species

The equatorial oceans were over 40°C, and largely devoid of fish.

These mountains in China preserve the ocean floor of the Permian-Triassic period.

When ecologists talk about climate change, they tend to recognize there will be winners and losers. While it could drive some species to extinction, others will migrate readily to follow their shifting habitats or adapt to the changing conditions. For the most part though, nobody's expecting we'll end up with ecosystems that are largely barren.

But a new study of the aftermath of a mass extinction event suggests temperatures once got so hot that they left our planet's equatorial regions a place of "lethally hot temperatures," where the few survivors were mostly stunted invertebrates.

As a whole, life on Earth didn't have a lot going well for it at the start of the Triassic. The previous geological period, the Permian, ended with the massive eruptions that generated the Siberian Traps and triggered the biggest mass extinction event on record: the Great Dying. The volcanic activity and subsequent ecosystem changes pumped massive amounts of carbon dioxide into the atmosphere, leaving the few survivors to face intense greenhouse warming and oceans where it was difficult to obtain oxygen.

Just how intense was the warming? The authors of a paper attempt to find out by looking at the ratio of oxygen isotopes preserved in fossils from southern China. At the time, this land mass was near the equator on the east side of Pangea, covered in the shallow seas of the Tethys Ocean.

Based on their findings, the Great Dying was accompanied by a huge increase in water temperatures in the area, which rose from 21° to 36°C over the course of 800,000 years. After a period of decline, they surged again, this time reaching at least 38°C, and possibly exceeding 40°C—that's over 100° Fahrenheit. And this was ocean water; temperatures on land were likely to have been even more extreme.

That, the authors point out, is probably hot enough to kill many plants ("few plants can survive temperatures persistently above 40°C"). This is also near the limits of what animals can survive. Things are actually worse in the ocean, where increased temperatures both put strain on the organism directly and lower the amount of dissolved oxygen available.

So, the authors turned to databases of fossil finds from that era. They found the equatorial regions were relative wastelands, even as other areas of the planet were starting to recover. Fish fossils are common in many latitudes, but "very rare" near the equator. Their relatives, the tetrapods (four-limbed vertebrates) were "generally absent between 30°N and 40°S in the Early Triassic." A group of red algae were completely missing. There are few coal deposits from this era, too, which suggests major disruptions of plant life.

What was around? Some marine invertebrates. But these were so unusually small compared to their normal sizes that scientists previously described what they called a "Lilliput effect." The authors here show this effect is only present in the equatorial regions. At higher latitudes, these species are normally sized.

The authors conclude these high temperatures complicated life's recovery from the Great Dying by making a large chunk of the planet nearly inhabitable. "Extreme global warming," they write, "may progressively force taxa to vacate the tropics and move to higher latitudes or become extinct."

Science, 2012. DOI: 10.1126/science.1224126  (About DOIs).

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