Rocks hold clues to ancient die-offs

BALTIMORE, Md. — Some 200 million years ago, a massive die-off of species worldwide occurred. Such an event is known as a mass extinction. And by looking at very old rocks, researchers are now finding what appears to have led to those die-offs: a quickly warming climate.

At the time, volcanoes had been spewing huge quantities of carbon dioxide and other greenhouse gases into the air. These gases trapped extra heat in the atmosphere just as they do today. This led to a rather rapid warming near Earth’s surface. Many species couldn’t adapt to the speedy change in climate and simply died off.  

So what? Well, driving cars, running factories, heating homes and other human activities today release many tons of greenhouse gases into the air every second. Earth’s current low-grade fever may be on track to one day simulate those ancient conditions. The difference: This time people — not volcanoes — are playing a role in driving the change. And it’s this current warming that has made understanding the impacts of that ancient hot spell so important.

For their new study, the researchers focused on the warming of ancient seas.

Rocky clues to ancient seas

The researchers measured the sea’s warming by studying multi-layered rocks from Great Britain. They are called stromatolites (Strow-MAT-eh-lytes). Secretions from microbes helped form these rocks. And each layer of the rocks offers a snapshot of the microbes that had been alive as it formed.

One layer shows signs of algae. These belonged to a group known as prasinophytes (Praa-SIN-oh-fytes). These algae thrive only when other species are in trouble. As such, “it’s known as a disaster species,” explains Victoria Petryshyn. She’s a geologist who works at the European Institute of Marine Sciences in Plouzané, France. She also works at the University of California in Los Angeles (UCLA).  

Geologists know that a large number of species died suddenly around 200 million years ago. This helped them link the prasinophyte layer to that mass extinction.

Better still, the rock’s chemistry let Petryshyn and her team figure out how warm the water was when the rock formed. One layer formed in water that was about 34° Celsius (93° Fahrenheit). A later layer from the same general time period formed at approximately 37 °C (99 °F). That’s about 10 °C higher than the ocean now gets in the summer in Oahu, Hawaii!

Such temps would have made the air above the ancient sea “very toasty and very humid,” Petryshyn observes.

Her team described its results on November 1, here, at the Geological Society of America’s annual meeting.

How they calculated the heat

The scientists measured the sea’s temperature by weighing the rock. Its most common mineral is calcium carbonate. As its formula (CaCO_3.) shows, this rock is made up of calcium, carbon and oxygen. However, “if you look at molecules of calcium carbonate, they are not all identical,” observes Aradhna Tripati. She, too, is a geologist at UCLA.

The various calcium carbonate molecules “actually weigh slightly different amounts,” she says. And that’s because the molecules contain different isotopes — forms of the elements — that serve as their building blocks.  You might think of isotopes as different flavors of an element, Tripati says. They differ in the number of neutrons that each atom has in its core (or nucleus).

The dense core of each atom contains a mix of those neutrons together with other particles known as protons. Each atom of a particular element will have the same number of protons in its core. Only the number of neutrons can vary. The more neutrons an element has, the heavier it will be. The heavier molecules of CaCO₃ are made from heavier forms of oxygen and carbon atoms.

Lower temperatures cause more of the heavy carbon and heavy oxygen atoms to clump, or bind together, in this type of rock.  So stromatolite layers that form in colder water should have more mass and heavier CaCO₃ molecules. Or so the scientists thought. Now the team probed the ancient rocks, looking for confirmation of that.  

Their first step: They drilled into the rock at different spots. This produced dust that was “about as fine as baby powder,” says team member Robert Gammariello. He’s a student in Tripati’s lab at UCLA.

He and Petryshyn put a tiny bit of powder from each drilled hole into capsules. Then they weighed them. From there, they could calculate the share of heavy to light calcium carbonate molecules in each sample.  Next the team drew upon earlier work that had been done with modern carbonate rocks. The chemical make-up of those rocks is similar to the ancient ones. Scientists knew what temperature those rocks had formed at. So, they had used it to develop an equation that would link temperature to isotope clumping. And that equation matched well with what physics says about how heat (energy) affects matter.

Now the team used their equation with the ancient rock. They knew the ratio of clumped isotopes. So they used math to calculate what the temperatures were when the rock layers formed. And their findings matched well with what climate scientists had predicted with computer models. These had used other data to make temperature calculations for the same period. Petryshyn says that the similar findings from two different approaches help show the computer models were correct.

Foretelling the future

“This is very exciting work,” says Margaret Fraiser. She is a geologist at the University of Wisconsin-Milwaukee. Using the clumped isotopes to understand climate change is important and hadn’t been done before, she says. “The more we understand the deep time record of climate change,” she explains, “the more we can use it…to help humankind better predict the future.”

Seawater temperature is linked to air temperature. And air temperature goes up by about 2 °C every time carbon dioxide in the atmosphere doubles. Based on that, Tripati and her team calculate that carbon dioxide levels were around 2,200 parts per million (ppm) when the mass extinction took place some 200 million years ago. 

Carbon dioxide levels today are high, but not that high. They’re only about 400 ppm. But greenhouse gas emissions have not stopped rising. If they don’t drop substantially, temperatures will keep rising, too.

The evidence from 200 million years ago suggests that’s cause to worry, Petryshyn and Tripati told Science News for Students. Just a 2 °C hike in warming would be enough to cause dramatic changes. And, notes Tripati, “That’s the type of warming we might readily get in the next 50 to 100 years.”

Power Words

(for more about Power Words, click here)

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

atmosphere   The envelope of gases surrounding Earth or another planet.

calcium  A chemical element which is common in minerals of the Earth’s crust. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

calcium carbonate  The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO₃ (meaning it contains one calcium atom, one carbon atom and three oxygen atoms).

carbon  The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide (or CO₂)   A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

climate   The weather conditions prevailing in an area in general or over a long period.

climate change  Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

concentration   (in chemistry) A measurement of how much of one substance has been dissolved into another.

element  (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

equation   In mathematics, the statement that two quantities are equal. In geometry, equations are often used to determine the shape of a curve or surface.

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.

greenhouse gas   A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

isotopes  Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers of neutrons in their nucleus. As a result, they also differ in mass.

mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

microbe  Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O₂), but water is made of two hydrogen atoms and one oxygen atom (H₂O).

neutron A subatomic particle carrying no electric charge that is one of the basic pieces of matter. Neutrons belong to the family of particles known as hadrons.

nucleus  Plural is nuclei. (in chemistry and physics) The central core of an atom, containing most of its mass. It contains subatomic particles known as neutrons and protons.

parts per million (billion or trillion)  Frequently abbreviated as ppm (or ppb or ppt), it is a measure of the number of units of some material that it mixed into another. The units should be the same (or equivalent) for both materials. The term is used to describing extremely small concentrations of one chemical dissolved in another. For example, a solution of 300 parts per billion of sodium in water would mean that there are 300 sodium atoms for every billion water molecules.

prasinophytes    A group of algae that tend to thrive when other species are in trouble. As such, biologists sometimes refer to them as “disaster species.”

proton A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

ratio  The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).

stromatolite     A type of layered rock that forms when cyanobacteria in water create huge communities. Their sticky surfaces trap sediments floating in the water. That accumulating sediment reacts to calcium carbonate in the water. This creates limestone, which builds up very, very slowly. A century-old stromatolite may grow a mere 5 centimeters (2 inches) in size.

thermodynamics  The branch of physics that studies the effects of heat, temperature and work on matter in a system.