Tag: Mass extinctions

The snuffing out of up to 90 percent of all terrestrial and marine species at the end of the Permian (252 Ma) was the outcome of lethal climatic warming. It probably stemmed from a stupendous episode of flood basalt volcanism and intrusions in what is now Siberia that burned vast amounts of peat or coal in the basin that the flows filled (see: Coal and the end-Permian mass extinction; March 2011). The carbon dioxide so released created planetary hyperthermia and toxic acid rain. For at least five million years Earth was an almost sterile world, a notable absence being dense vegetation on the land surface – the Early Triassic is devoid of coal, whereas there is plenty of Late Permian age. Much the same slow recovery of life is found in meagre collections of land and marine animal fossils of that age. Yet, other mass extinctions were followed by recovery and species diversification at a much faster pace.

One conceivable explanation could be the near absence of vegetation whose photosynthesis and burial would otherwise draw down CO₂ and the same goes for its marine equivalent phytoplankton. But there is a powerful inorganic means of carbon sequestration: silicate weathering. The chemistry depends on carbon dioxide dissolved in water. For simple silicates it can be expressed as:

2CO₂ + H₂O + CaSiO₃ → Ca²⁺ + 2HCO₃^– + SiO₂.

The higher the ambient temperature, the faster such reactions proceed. Most silicates are more complex and many common ones, such as feldspars, include aluminium, so that another product of weathering is insoluble, fine-grained clay minerals. So various soluble metal ions (Ca, Mg, K, Na etc), dissolved bicarbonate ions, silica in various guises and clays eventually end up in the sea. Once there, it is possible for them to recombine, as for instance calcium and bicarbonate ions:

Ca²⁺ + 2HCO₃^-­→ CaCO₃ + CO₂ + H₂O

Despite some CO₂ gas being released, this reaction results in a net sequestration of carbon in calcium carbonate. Incidentally, the same kind of chemical reaction occurs in the soils produced by weathering. The carbonate may cement soils to form a hard crust of caliche or ‘calcrete’. Chemical weathering enhanced by a hot climate, it might seem, should reduce the greenhouse effect quickly: a feedback mechanism that normally stabilises climate. But that did not happen after the P-Tr extinction event, thereby stressing all remaining life forms. A group of scientists at the University of Waikato in New Zealand have developed a possible explanation for this potentially fatal hazard for life on Earth (Isson, T.T. et al. 2022. Marine siliceous ecosystem decline led to sustained anomalous Early Triassic warmth. Nature Communications, v. 13, article 3509; DOI: 10.1038/s41467-022-31128-3). It focuses on the silica (SiO₂) released by chemical weathering, which enters the ocean in the form of a colloid: Si(OH)4, a form of silicic acid known as ‘reactive silica’. Under ‘normal’ conditions, this is removed by organisms, such as diatoms and radiolaria, and is constantly recycled on a time scale of about 400 years, some contributing to deep-ocean oozes in the form of chert. But, like all other marine organisms, they too were victims of the P-Tr mass extinction.

Reactive silica colloids in seawater also participate in inorganic chemical reactions, combining with dissolved metal ions to form complex hydrated aluminosilicates, i.e. more clay minerals. The reactions change the alkalinity of seawater. As a result dissolved HCO₃^–ions transform to CO₂ gas and water. Despite the complexity of the chemistry that interweaves the carbon and silicon cycles, there is a simple conclusion. If the abundance of silica-secreting marine organisms falls drastically while continental weathering continues to deliver silica, clay-mineral formation on the ocean floor results in release of CO₂ that reverses the effect of enhanced weathering and thus maintains hyperthermal conditions. The other outcome is that less chert and flint granules form Terry Isson and colleagues examined the varying proportion of chert in cores through Lower Triassic marine sediments. A ‘chert gap’characterises the 4 to 6 Ma following the P-Tr boundary event. This can be explained in part by extinction of silica-secreting organisms and by inorganic reactions converting the reactive silica that enhanced weathering delivered to the oceans to clay minerals. This supports the idea that the inorganic part of the silica cycle maintained greenhouse conditions in the absence of organic ‘competition’ for reactive silica. Many other biogeochemical cycles link biological and chemical processes that combine to affect climate: involving phosphorus, nitrogen and iron, to name but three.