Chlorophyll by-product reveals Earth’s carbon history

Carbon dioxide plays a key role in the carbon cycle, one of our planet’s most fundamental geochemical processes. To study the dynamics of the carbon cycle throughout Earth’s history, scientists try to pin down atmospheric CO₂ concentrations in eons past. Such estimates go back some 400 million years, but many in the field continue to debate their accuracy and internal consistency.

“This has been a huge challenge for the paleoclimate community,” says Caitlyn Witkowski, a graduate student in the lab of Jaap Sinninghe Damsté at the Royal Netherlands Institute for Sea Research. She and her colleagues have now reconstructed CO₂ levels going back 500 million years with the help of phytane, a byproduct of chlorophyll (Sci. Adv. 2018, DOI: 10.1126/sciadv.aat4556).

When studying Earth’s past CO₂ levels, scientists use several indirect measures, or proxies. Each proxy has limitations. Some researchers rely on molecules found in planktonlike organisms that evolved relatively recently, but such proxies provide information about limited periods in Earth’s history. Other nonmolecular methods are not very accurate.

To establish phytane as an accurate single proxy, Witkowski and her colleagues analyzed 306 samples of ocean sediments and oils from different time periods and geographical locations. They extracted phytane from the samples, and then determined how much carbon-12 and carbon-13 the phytane contained. Photosynthesizing organisms preferentially use CO₂ containing the lighter carbon, because it requires less energy to process. So when CO₂ levels in the atmosphere are high, the phytane contains relatively high quantities of carbon-12, but when CO₂ levels are lower, the amount of carbon-13 increases.

Comparing amounts of the two isotopes in marine phytane, they were able to quantify CO₂ levels reaching back to the Cambrian period, when complex animals emerged on the planet. The estimates the researchers obtained closely matched the trends observed with already-existing proxies, except during two time periods when their calculated levels were much higher. Both corresponded to periods of high temperatures on Earth, when CO₂ levels would be expected to be high. “We ended up with a really nice long record that goes beyond any other proxies,” Witkowski says.

“The advantage of this work is that these molecules are everywhere, so it opens up the environments and time periods where one can create a record,” says Katherine Freeman, a geoscientist at Pennsylvania State University.

Other researchers have estimated past CO₂ levels by measuring carbon isotopes in a molecule called alkenone, produced by a specific class of phytoplankton. But these organisms have only been around 50 million years. Researchers have identified chlorophyll from as far back as 1.7 billion years ago. That means it may be possible to create a reliable record of atmospheric CO₂ stretching back significantly farther than the 500 million years the study covered.

“Certainly before I saw this paper, I would have said ‘no way’,” could records reach back that far, says Gavin Foster, a geochemist at the University of Southampton. “Now I am quietly confident.”