Decomposing algae have not said their last word yet!

Name of proxy

Phytane, a compound resulting from the degradation of Chlorophyll-a (Chl a), a green pigment in plants and algae that is involved in photosynthesis

Type of record

Atmospheric carbon dioxide concentrations

Paleoenvironment

Marine sediments and oils

Period of time investigated

Phanerozoic (last 540 million years)

How it works

Before we can start predicting the potential impact of human activities on climate change, we first need to study the behavior of atmospheric CO₂concentration (pCO₂) in the past. This represents a challenge given that continuous atmospheric measurements started only about 60 years ago.

To reconstruct past pCO₂, a single well-constrained, ubiquitous proxy would be ideal. Unfortunately, it does not exist and we have to combine the estimates from many different types of proxies, each with their own advantages and weaknesses.

Different potential compounds that can give information on the pCO₂ over the last hundreds of millions of years have been explored. The idea is based on the fact that algae consume CO₂ and use that carbon to build organic compounds. But carbon atoms are not all the same. They can have different masses depending on the composition of their nucleus. We therefore differentiate between heavy carbon (¹³C) and light carbon (¹²C). Algae rather use the lighter, more common carbon (¹²C), but when less CO₂ is available, they must use both ¹²C and ¹³C– or go hungry.

Therefore, it is considered that the more abundant is the CO₂, the smaller is the ¹³C/¹²C ratio of the algae. Likewise, when CO₂ is lower, the ¹³C/¹²C ratio in algae is higher. Long after an algae died, the ¹³C/¹²C ratio of its organic compounds will remain preserved in ancient sediments and oils and reflect the environment in which they were first produced.

Consequently, this ratio is compared to the ratio in CO₂ consumed by the algae in order to calculate the fractionation factor (Ɛ_p) due to CO₂-fixation. The fractionation factor is a measure of the discrimination occurring between heavy and light carbon. It can therefore be used to estimate the abundance of CO₂ at the time the organic compounds were first produced.

Key Findings

This pCO₂ estimation method via Ɛ_p is generally applied to organic compounds that come from specific species of algae. However, this has limitations – a single species is limited by its evolutionary history and its global abundance.

Here instead, we decided to explore the possibilities of using a general biomarker – an organic compound that all algae have and that contributes to the ocean record. We have explored naturally occurring CO₂ vents, pockets of bubbling CO₂ caused by volcanic activity, to test several different general biomarkers using this Ɛ_p method.

Collecting water samples, plankton nets, and sediment at a CO₂ vent in Japan

One of the biomarkers that seems to work well are phytol and phytane, which are byproducts of Chl a – a green pigment in plants and algae involved in photosynthesis. Phytane was used in specific case studies (e.g. Bice et al., 2006; Damste et al., 2008) but has not been tested extensively. We are now calibrating this potential proxy, by comparing it with other well-established pCO₂ proxies over different timescales, and forming a compilation that extends the entire Phanerozoic (the past 540 million years!).

This proxy could significantly increase how far we can reconstruct pCO₂ – Chl a and its products have been found in samples over 2 billion years old and is found everywhere (both spatially and temporally) throughout the geologic record.

Edited by Caroline Jacques and Célia Sapart