The thermal maximum at the transition between the Paleocene and Eocene 55.9 million years ago represents one of the most significant climate crises the Earth has ever experienced. A new study published in the journal Science Advances shows that this extreme climate warming was associated with increased erosion and weathering of continental rocks. These processes are thought to have taken away a significant amount of CO2 from the atmosphere which has caused the climate to stabilize.
The Earth has several mechanisms that influence climate variability in one direction or another. Surface processes such as mechanical erosion and chemical weathering of rocks are mechanisms that are now well known for their ability to limit the amount of CO2 in the atmosphere.
Alteration of silicate rocks
Of these surface processes, the most CO2-intensive are reactions that attack silicate minerals and convert them into clay. Rivers then carry the carbonaceous residues to the oceans, where marine organisms use them to grow and produce the calcite that forms their shells.
When these animals die and settle to the bottom, the carbon stored in their biomass and shells is buried in the sediment. Over very long periods – tens to hundreds of thousands of years – this chain of reactions effectively removes CO2 from the atmosphere and stores it on the ocean floor.
However, we still have insufficient knowledge of how quickly this process responds to large environmental changes such as the one we are experiencing today. One way to look at this issue is to look into the Earth’s past and find a natural analogy to what is happening today.
Extreme global warming 56 million years ago
Earth’s geological history has recorded several events associated with major disturbances in the carbon cycle. These crises are characterized by periods of extreme climate warming. This is the case of the Paleocene-Eocene Thermal Maximum (PETM), which occurred 55.9 million years ago. The origins of this crisis are still disputed, but it is known that at that time a large amount of carbon was released very rapidly into the Earth’s atmosphere (in less than 5 000 years), causing rapid global warming and, in particular, an 8 °C rise in water temperature.
These sudden environmental changes have had many consequences. These changes had led to ocean acidification and disruption of the hydrological cycle which led to mass extinctions in the deep oceans and major biotic changes at the surface.
This climate crisis has lasted for approximately 100 000 years and it took another 50 000 to 100 000 years to resolve. This phase of resilience is of particular interest to scientists trying to determine whether if the rate of erosion had been high enough could the recovery had taken less time.
Erosion was 2 to 3 times greater during the PETM
To investigate the importance of surface processes at that time, scientists used lithium isotopes to quantify the rate of erosion and chemical weathering of rocks. Lithium has two isotopes: 6Li and 7Li. Interestingly, the ratio of these two isotopes (δ7Li) can change during chemical reactions. In seawater, this ratio is strongly influenced by clay formation, so δ7Li is a good indicator of erosion and weathering rates.
Samples from the PETM period were analyzed. The δ7Li measurements show that the water cycle was significantly enhanced during this climate crisis. Precipitation was greater, increasing mechanical erosion and chemical weathering of continental rocks. The authors of the study have modeled that during the PETM the erosion rate was 2-3 times higher than before the crisis.
The massive addition of nutrients to the oceans promoted the formation of organic matter in parallel with the precipitation of carbonates. The rapid uptake of carbon sequestered in marine organisms and carbonate rocks was certainly the main driving parameter that allowed the climate to rebalance and end that extreme weather event.
These findings, published in Science Advances, can help us understand how the Earth as a whole can and will respond to climate change in the future. From this point of view, it is even possible to consider solutions that could help speed up this natural carbon sequestration process to combat the current ongoing climate change.