Fossil algae reveal 500 million years of climate change

Earth scientists can go far back in time to reconstruct the geological past and paleoclimate and better predict future climate conditions. Using the organic phytane molecule, a product of chlorophyll debris, scientists from the Netherlands Institute for Research on the Sea (NIOZ) and the University of Utrecht have managed to develop a new indicator (proxy) of the old CO.₂ levels. This new organic proxy provides not only the most continuous recording of CO₂ never reached, it also covers an unprecedented half-billion years. The data confirm the idea that goes up in CO₂ levels that took millions of years now occur in a century. These results are published in Progress of science November 28th.

As a CO₂ is increasing today, it is essential to understand the impact of these changes. To better predict the future, we need to understand the long-term changes in CO₂ on geological history. Direct measurements of past CO₂ for example, bubbles in ice cores containing old gases are available. However, ice cores have a limited life of one million years. To go back in time, scientists from the Earth have developed various indirect measures of CO₂ from proxies, for example from algae, leaves, old soils and chemicals stored in old sediments to restore the environmental conditions of the past.

Phytane, a new way of traveling in time

A new proxy, using a chlorophyll degradation product, allows geochemists to derive a continuous record of CO₂ levels in deep time. NIOZ scientists have recently developed phytane as a promising new organic substitute that reveals half a billion years of CO₂ levels in the oceans, from the Cambrian to the recent period.

Using the new proxy, they have been able to create the most continuous record ever made of old levels of carbon dioxide. "We have developed and validated a new way of traveling back in time: going back in time and to more places," says Caitlyn Witkowski, a scientist at NIOZ. "With phytane, we now have the longest CO₂-record with a single marine proxy. This new data is invaluable for modelers who can now make more accurate predictions for the future. "

Witkowski and his colleagues have selected more than 300 samples of marine sediments from deep water cores and hydrocarbons from around the world, reflecting the majority of geological periods of the last 500 million years.

Fossil molecules

Past chemical reactions can be "stored" in fossil molecules and can therefore reflect various old environmental conditions. Geochemists can infer these conditions, such as seawater temperature, pH, salinity, and CO₂ levels. Organic materials, such as phytane, reflect the pressure of CO₂ in seawater or in the atmosphere (pCO₂).

Although all organic matter can reflect CO₂, phytane is special. Phytane is the pigment responsible for our green world. Anything that uses photosynthesis to absorb sunlight, including plants, algae and some species of bacteria, contains chlorophyll, of which phytane is a constituent. Plants and algae absorb CO₂ and produce oxygen.

As chlorophyll is present throughout the world, phytane is also present everywhere and is a major constituent of decomposed and fossilized biombad. "Phytane does not change chemically over time, even if it's millions of years old," Witkowski says.

Isotopic carbon fractionation

CO₂ from the past is estimated from organic matter, such as phytane, by the phenomenon of carbon isotopic fractionation during photosynthesis. When taking CO₂Plants and algae prefer the isotope of light carbon (12C) to the isotope of heavy carbon (13C). They only use the heavy carbon isotope when the CO₂ the levels in the water or the surrounding atmosphere are low. The proportion between these two isotopes thus reflects the level of carbon dioxide in the environment at the time of growth.

This also explains why Witkowski did not use terrestrial plants as a source of research, using only phytane from marine sources (fossilized). The plant world is divided into so-called C3 and C4 plants, each with its own unique light / heavy carbon ratio. Phytoplankton all have very similar ratios compared to their plant counterparts. Witkowski: "By choosing only marine sources, we could limit the uncertainty of the phytane source in the dataset."

"In our data, we are seeing high levels of carbon dioxide, reaching 1000 ppm over the current 410 ppm.In this regard, current levels are not unique, but the speed of these changes has never been seen before, changes that usually take millions of years have now pbaded in a century.₂Data can help us understand the future of our planet. "In future research, phytane can be used to go even further back in time than the Phanerozoic, the oldest found in samples dating back two billion years.

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More information:
"Phytoplankton molecular fossils reveal a secular trend to PCO2 over Phanerozoic" Progress of science (2018). advance.sciencemag.org/content/4/11/eaat4556