Global warming is not new to the history of our planet, and so, by studying previous periods of global warming, scientists hope to uncover secrets that can be used to combat global warming today. The Middle Eocene Climatic Optimum, which occurred some 40 million years ago, has attracted particular scrutiny because of its unique properties. researchers have found that clay formation during this period led to an unusually long period of global warming. This has important consequences for the climate today, by suggesting that enhanced weathering can be used to protect the climate. 

The Middle Eocene Climatic Optimum

The Middle Eocene Climatic Optimum (MECO) is a part of the Eocene epoch, a geological period that occurred between around 56 million and 33.9 million years ago. The Eocene epoch is so named because it is the dawn of modern fauna and the ancient Greek for dawn is ἠώς (ēṓs) and the ancient Greek for “new” or “modern” is καινός (kainós). The Eocene epoch is the second epoch of the Paleogene Period in the modern Cenozoic Era. The Eocene epoch had the warmest period of the Cenozoic, and it also experienced a shift into an icehouse climate and the rapid expansion of the Antarctic ice sheet. This cooling began around 49 million years ago, with carbon and oxygen isotopes showing this transition. We do not have a clear idea for why this cooling began, but one suggestion is that carbon dioxide concentrations in the atmosphere declined to 2,000 parts per million (ppm). This period of global cooling persisted until a sudden and temporary reversal in the Bartonian, an event known as the MECO. 

Source: International Subcommission on Paleogene Stratigraphy

The MECO, which occurred some 40 million years ago, lasted for around 400,000 years, and was a period of global warming, with rising atmospheric carbon dioxide concentrations and deep-ocean acidification. There is evidence from stable isotopic analysis of segments gotten from drilling in Southern Ocean drilling sites, of warming for around 600,000 years from 41.5 million years ago. A similar warming is observed in the Northern Hemisphere in the Scaglia Limestones of Italy. Oxygen isotope analysis indicates that there was a change in the ratio of heavier oxygen isotopes to lighter ones, showing global warming.  That this period of global warming occurred is due to the rise in carbon dioxide, is surmised from the fact that carbon isotope signatures have ruled out methane release. What we see is a sharp rise in atmospheric carbon dioxide with a maximum of 4,000 ppm, the highest of the Eocene. 

Unlike other previous eras of global warming in the previous phases of Eocene global warming, the MECO was unusually long. Previously, there had been warming phases on million-year timescales, such as the Early Eocene Climatic Optimum. Global warming over timescales of tens of thousands of years, such as the Palaeocene–Eocene Thermal Maximum (PETM), led to large doses of C-depleted carbon and deep-sea carbonate dissolution, thanks to the ocean acidification, resulting in shoaling of the carbonate compensation depth (CCD). These warming events are well researched. However events over an intermediate scale, such as the MECO, are less understood. Theory would suggest that temperature-dependent silicate weathering should have regulated this phase, reducing carbon dioxide during the phase. We can detect these chemicals when touring certain caves, such as going on a Katla cave tour to see the rock formation and the carbon dioxide levels. Understanding why this anticipated regulation did not occur as thought, may help us understand how we can shorten our own phase of global warming.

During the MECO, surface temperatures over the Tethys Ocean rose to 32–36 °C, and Tethyan seawater became more dysoxic. Carbonate accumulation in ocean depths of more than three kilometers occurred around the peak of the MECO, indicating that ocean acidification occurred in the deep ocean. 

The Importance of Clay Formation

With that thought in mind, a group of scientists, led by Alexander J. Krause, a postdoctoral researcher at University College London, investigated silicate weathering during the MECO. They published their results in the prestigious Nature journal, under the title, “Enhanced clay formation key in sustaining the Middle Eocene Climatic Optimum”. 

The researchers measured lithium isotope ratios, tracers for silicate weathering, taken from open-ocean carbonate-rich sediments. They found a positive δ7Li excursion, indicating a warming event, of around 3‰. Using box model simulations, they determined that this showed a “shift from congruent weathering with secondary mineral dissolution, to incongruent weathering, with secondary mineral formation”. Based on these observations, Krause and his fellow researchers surmised that prior to the MECO, there was greater soil shielding over the continents. Warming began as a result of continental volcanism, but continued thanks to clay formation, a process which sequestered carbonate-forming cations, preventing the carbonate-silicate cycle from ending the warming phase. With these insights, the researchers posit that clay mineral dynamics could have an as-yet unheralded importance in the carbon cycles of climatic events of 100,000 years or more. 

Using Weathering to Protect the Climate

Krause and his associates indicate that they study paleoclimates in order to benefit the present climate. The findings of their research suggest that by enhancing chemical weathering of rocks, by, for example, plowing finely crushed rock into fields, they can protect the climate. These crushed rock particles would erode quickly, binding atmospheric carbon dioxide, leading to the climate’s recuperation. 

All over the world, negative emissions technologies (NETs) are already being investigated. What Krause et al’s research suggests is that clay formation could hamper carbon dioxide absorption, given that clay retains the calcium and magnesium that should go to the ocean. Instead, carbon dioxide would flow into the oceans, without being bound there, and would escape into the atmosphere. The weathering would have limited impact on the climate. 

When the rock particles are completely dissolved due to weathering, this enhanced weathering would be wholly efficient. if all the weathered materials become clay, the impact would be nullified. 

In the PETM, we see enhanced rock erosion leading to the climate normalizing quickly, whereas in the MECO, there was clay formation. A confounding factor is determining how quickly rock dissolves based on various local factors, in order to see if enhanced weathering can be used to protect the climate.