What about lithium?
The weathering of the rocks of the terrestrial crust, which are mainly silicates like granite, is a slow but very important phenomenon for the mitigation of the Earth climate. Indeed, a great quantity of CO2 is captured during the leaching of silicates by rain and wind, and because CO2 is the main greenhouse gas, this process considerably influences the global climate. It is then interesting to know about the variations of silicates weathering during geological times, to better reconstruct past climates.
That is when lithium takes action.
Lithium is a small chemical element contained in silicates, and the lithium isotopic ratio (see box) changes during rock weathering. Thus, rock weathering increases the quantity of 6Li in the rivers, and when the rivers arrive at the sea they modify the lithium isotopic ratio of the oceans. If we know the lithium isotopic ratio in the oceans, we can deduce if the terrestrial rocks weathering upstream was intense or not so intense, and subsequently if a lot of CO2 was captured or not.
Foraminifera are archives.
However, it may be easy to analyse lithium in present seawater, but how to measure it
What is an isotope?
A chemical element often has two stable states, that is two versions, which have the same number of protons, but a different number of neutrons.
For lithium, we find 7Li and 6Li. 7Li is naturally more abundant than 6Li. We call "lithium isotopic ratio" the proportion of 7Li compared to 6Li.
in seawater, 20.000 years ago? The best option is to use the natural archives that are fossil organisms, like fossil foraminifera for example. When foraminifera build their shell, they accidentally capture the chemical elements contained in seawater... like lithium. We can then measure the isotopic ratios in 20.000 years old foraminifera shells, to know what were the isotopic ratios in seawater at that time.
The only condition to be able to use lithium in foraminifera shell as a marker of the seawater one, is that no biological effect occurs during the incorporation of lithium in the shell. But, lithium is not used in any known physiological process, not during respiration, nor during nutrition, etc. Probabilities are therefore very weak. However, better to be sure and test it.
Culture experiments.
I have implemented culture experiments using benthic foraminifera, to test if variations of pH values or of quantity of dissolved carbon in the water have an impact on the lithium isotopic ratio in the shell. As expected, no effect was observed. Moreover, previous studies have demonstrated that temperature also has no observed effect on lithium incorporation in the foraminifera shell.
We can then considered the lithium isotopic ratio in foraminifera shell as a marker of the seawater ratio, and use these data to reconstruct the silicates weathering conditions in the past, as well as the associated climates.
Article published in January 2023 in the journal Minerals
At the foot of the volcano
Nishinoshima is a submarine volcano located off the coast of Tokyo, around 1000 km south of mainland Japan. A small part of the volcano is emerged: this is Nishinoshima Island. The volcano was dormant during 40 years, then it suddenly woke up in November 2013. Since then, it is still active, with regular lava flows along with gas and ash explosions.
It is hard to study faunas living around volcanos, especially when they are submarine, for accessibility and security reasons. By chance, after an observation mission implemented in 2015 by a volcanologists team, I got sediment coming from around Nishinoshima. Among the grains of andesite and dacite, which are fragments of volcanic rocks, I spotted foraminifera! Then I sorted them out and identified them, it is actually the first time that benthic faunas are studied in the region. Although the number of specimens was low, the foraminifera assemblages I observed were stunning by their species diversity.
These preliminary results participate in improving our understanding of how dynamic environments such as volcanos work, and which are the associated faunas. The collected data will be used as knowledge baseline for, for example, a future larger scale study around Nishinoshima.
Article published in May 2023 in the journal Paleontological Research
Regular reports about Nishinoshima activity on Global Volcanism Program