Privileged Planet: hot period forty million years ago (Introduction)

by David Turell , Sunday, September 03, 2023, 17:06 (237 days ago) @ David Turell

Related to clay formation:

https://phys.org/news/2023-09-clay-formation-prolonged-global-event.html

"Global warming is not solely a modern-day occurrence but has been a prominent feature of Earth's geological history for millennia. One such event occurred approximately 40 million years ago, lasting ~400,000 years, known as the Middle Eocene Climatic Optimum (MECO).

"This event is considered unusually long when compared to climatic perturbations earlier in the Eocene, such as the Paleocene-Eocene Thermal Maximum approximately 56 million years ago, which lasted ~200,000 years. New research published in Nature Geoscience suggests the MECO may have been impacted by changes to rock weathering, specifically the weathering of silicate minerals, such as quartz, feldspar, mica, pyroxene and clays.

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"Chemical weathering of silicate rocks helps to counterbalance rising atmospheric carbon dioxide levels, as this gas dissolves in rainwater and the acidic product then weathers rocks, with new minerals formed and often producing calcium carbonate that is then stored on the seafloor. Therefore, drawing carbon dioxide out of the atmosphere helps to reduce the effects of greenhouse forcing and so decreases the planet's temperature.

"Dr. Alexander Krause from University College London, U.K., and colleagues conducted analyses on carbonate rock cores obtained from the seabed from ocean drilling projects in the equatorial and southern Atlantic Ocean and equatorial Pacific Ocean. They measured lithium isotope ratios (the relative abundances of the same element but with different atomic masses, 6Li and 7Li, herein referred to as δ7Li), which is considered an indicator of silicate weathering.

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"...the result is dissolution of carbonate outpacing sedimentation. The suggestion by Dr. Krause and colleagues is that more clay formation on land would retain calcium and magnesium, which are attracted to clays in soils, thus reducing the calcium reaching the ocean to form carbonates on the seafloor.

"The weathering regime for this scenario is plotted on a Dellinger boomerang to determine changes in weathering intensity. Prior to the MECO, erosion was relatively low on high-latitude floodplains being exposed as sea level decline, as well as small pockets in the tropics where near-surface air temperatures likely reached more than 30°C, leading to global net dissolution of secondary minerals.

"However, with time an increased hydrological cycle and volcanic activity incising bedrock with lava flows would have changed this to a pattern of primary mineral erosion, with formation of secondary clays. These clays harvested calcium and magnesium, disrupting the transport of the former to the ocean and the carbonate-silicate cycle (terrestrial silicate rock dissolution and marine carbonate formation). Finally, a tipping point would have been reached so that the cycle once again reversed, where clay dissolution outweighed clay formation. (my bold)

"Consequently, a global increase in atmospheric carbon dioxide and surface temperatures alongside the erosion would likely have sustained the MECO for longer and Dr. Krause's modeling suggests that this weathering regime from ~40 million years ago is not too dissimilar to that experienced today."

Comment: an example of the constant cycling of major systems on Earth. Our current environmental warming is simply part of the constant slow change. It is a reminder of the long term affects weathering and erosion. The Earth is constantly evolving. Our presence is a major factor.