60 - 0 mya Cenozoic Era

Soil Impacts Plants Grass-Herbivore Symbiosis Soil animals Ground-dwelling beetles

Cenozoic

British geologist John Phillips first coined the name ‘Cenozoic’ derived from the Greek meaning “recent life’ and this period leads up to our time on earth. This is an 'Era' as it covers several 'Periods'.

The start of a very bad day.Drawing by Shutterstock/Esteban De Armas

Extinction

The end of the Cretaceous Period and the beginning of the Cenozoic Era is famous for the extinction which wiped out the dinosaurs. It is often called the K-Pg event and now is widely accepted as being caused by an asteroid hitting earth. But it could only have hit the earth from one side. The impact was not all over earth, like previous mass extinctions, but mainly on one side. What impacts did it have on soil - presumably different on different sides of the earth from where the asteroid hit?

Impact Site

“The impact site, known as the Chicxulub crater, is centred on the Yucatán Peninsula in Mexico. The asteroid is thought to have been between 10 and 15 kilometres wide, but the velocity of its collision caused the creation of a much larger crater, 150 kilometres in diameter - the second-largest crater on the planet.”

The asteroid is believed to have hit Earth with the force of more than 100 trillion tons of TNT, generating a burst of thermal radiation that would have incinerated everything in the vicinity.

The Earth burned in spots as molten rock and super-hot ash fell out of the sky and onto flammable plant matter and are often found near the asteroid impact site of Chicxulub Crater

"Carbon embedded in the rocks was vaporised by the impact, eventually forming new carbon structures in the atmosphere" These new structures are called carbon 'cenospheres' and were previously only know to geologists investigating industrial activity These tiny airborne ‘beads’, along with waves of ash and soot blanketed the planet.

Asteroids hit Earth typically at high speeds of 16 to 32 km/sec. During the impact, the kinetic energy in the asteroid (or energy of motion) is converted to explosive energy, blowing debris of dust, soil, and rocks not only into the atmosphere, but out into space, where it fell back into the top of the atmosphere.

Rest of Earth

But what about the rest of the environment - especially the soil? Did the earth change? Here it seems to get ‘lighter’ coloured. Is that a one-off?

Early calculations in the 1980s (using in part ideas worked out by Carl Sagan and his colleagues) showed that the dust entered the high atmosphere and shrouded the Earth and blocked the sunlight for several months. Althought the time scale for that has changed.

"Simulations of the atmospheric injection of such a plume of micrometre-sized silicate dust suggest a long atmospheric lifetime of 15yr, contributing to a global-average surface temperature falling by as much as 15°C. Simulated changes in photosynthetic active solar radiation support a dust-induced photosynthetic shut-down for almost 2 yr post-impact". (Senel et al 2023)

Together dust and beads blocked the sun, causing a global winter that killed off more than 75% of the species on the planet—including, most famously, dinosaurs.

This would have killed many plants, disrupting the food chain, yet soils – and grasses, survived. We should now ask: How?

Radiation and Heat

The radiant energy from the meteorites would have heated the surface to boiling temperatures for some minutes, and would have been enough to kill many animals and plants on or near the surface. However, in regions on the other side of the earth, with heavy rainstorms or snowstorms, these organisms would have survived longer. Sea creatures would have been buffered from effects in the first hours, but plankton on the surface might have died out over the weeks of darkness, decreasing the food supply for small fish, which affected the bigger fish, and so on.

For several hours following the Chicxulub impact, the entire Earth was bathed with intense infrared radiation from ballistically reentering ejecta. The global heat pulse would have killed unsheltered organisms directly and ignited fires at places where adequate fuel was available. Sheltering underground, within natural cavities, or in water would have been a necessary but not always sufficient condition for survival.” (Robertson et al 2004)

Iridium Anomaly

In the soil layer that separates the Cretaceous Period from the Cenozoic Era, dating 66 Mya, they found an excess of the element iridium, which is common in meteorites. "The iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide." (Goderis et al 2023)

Plants and animals had to survive both the acute impact of the asteroid and then its long-term effects, but also just 10 million years later, there was an increase in carbon dioxide, caused by methane. This led to a rise in global temperatures. It meant that tropical conditions could be found in the arctic, producing unusual growing conditions.

Methane Burp

There is an hypothesis called ‘methane burp’ (or gas hydrate dissociation) for an explanation of the sudden onset of the Paleocene-Eocene Thermal Maximum (PETM) about 55 mya.

According to the hypothesis, the PETM was triggered when large deposits of methane hydrates in ocean sediments were warmed, releasing methane through the ocean and into the atmosphere. The methane then oxidized, forming carbon dioxide. The increase in the concentration of this CO2 led to atmospheric warming. This environmental methanogenesis is catalysed by a complex microbial community (Katayama et al 2023) basically consisting of fermenting bacteria and methanogenic archaea. (Conrad 2020)

Methanogenesis

This is happening all over the world now, as a result of disturbing oil and gas, thus contributing to global warming. Methane is stored all over the world and is often found next to coal oil and gas. So when these are materials mined, they are bound to release old stores of methane. The earth that holds the methane water is often frozen, so when there is drilling – or increases in temperature, these frozen hydrates cages evaporate and the methane escapes. This is what has been happening across Russia drilling and Tundra melting. Many drilling sites elsewhere in the world burn off or collect the 'natural gas'.

Methanogenesis in soil

In soils with the potential for methanogenesis, important microbial functions include hydrolysis of complex organic matter, various fermentation processes, methanogenic processes and processes involved in the sequential reduction of inorganic compounds. Hydrolytic and fermenting bacteria degrade complex organic matter into more simple organic compounds, and leaving methane stored in earth – particularly frozen water called methane hydrates. The methane is trapped in lattice; it melts away when the earth warms, releasing the methane.

Phylogenetic Fuse

We saw the appearance of angiosperms in the last period, yet the dominance of angiosperms in terrestrial biomes didn’t come about until about 66-56 mya (Palaeocene).. Why the large delay? It is called the “long phylogenetic fuse”, between the first divergence of angiosperms from their sister group (the stem node), and then when the group began to diversify in earnest (the crown node). The time-lag differed, with the shortest lags in temperate and arid biomes compared with tropical biomes. (Raminez-Barahona et al 2020). Does that help tell us how the soil in different areas faired after the extinction? More in plants

Green green grass

The extinction of the dinosaurs allowed mammals to dominate the land, leading to changes in vegetation and, subsequently, soil forms. The early Cenozoic - Paleogene period - saw the spread of grasses (Monocot angiosperms) and the development of vast grasslands. This led to the formation of Mollisols, which are characterized by thick, dark A horizons rich in organic material from grass roots.

Soil horizons became more distinct as the complexity of ecosystems increased. Mollisols developed rich A horizons, while in other regions, soils like Alfisols and Inceptisols began to show well-defined profiles with clear distinctions between the A, E, and B horizons.

The Paleogene period experienced significant climate changes, with periods of warming and cooling, which influenced soil formation. Warmer, wetter conditions favoured the development of deep, well-drained soils with pronounced B horizons, while cooler periods contributed to the development of soils with more organic-rich horizons.

Again the soil has played a major role in saving life on this planet. This Cenozoic Era adds the final touches to what we know as 'Soil'

We have the separate continents, the tropics and temperate lands, and the various horizons that make up soil

The 4-way coevolution of grass-herbivore-dung beetle-worm associations created a whole new ecosystem, grasslands, vital for the planet today

What could possibly go wrong, that may be worse than an asteroid or methane burp? Let's find out now

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