Tag: Phanerozoic

Ups and downs of the Phanerozoic biosphere: a mathematical approach

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Palaeontologists became aware that the fossil record was marked by bursts of extinctions as data about fossil diversity through time steadily grew. That it did was a matter of dogged collection wherever field work was possible, identifying different species, classifying them, publishing the data and cataloguing them, each with a leavening of dispute. Only when those databases had become increasingly global and been collated in digital form did it become possible to chart the course of evolution in a comprehensive and comprehensible manner. First, that was simply as time series then in relation to other kinds of geological record, such as those of broad sea-level and climatic changes, environmental geochemistry, tectonics and relatively short-lived events such as large igneous provinces (LIPs) and asteroid impacts.

The fluctuation of life on Earth during Phanerozoic Eon: the last 542 Ma. Credit: Wikimedia Commons

‘Everyone knows’ the ‘Big Five’ mass extinctions: at the ends of the Ordovician (a loss of 85% of species at ~444 Ma); Devonian (70% of species at ~369 Ma); Permian (81% of species at 252 Ma); Triassic (70% of species at 201 Ma); and Cretaceous (75% of species at 66 Ma) Periods. The magnitudes relate to marine life, the terrestrial fossil record being more incomplete and less reliable. And there have been many lesser bursts of extinction and evolutionary radiations that followed to replenish and further diversify life.  Extinction peaks signify sudden environmental change for which several plausible ‘causes’ have been suggested: asteroid impact, LIPs, ocean anoxia and a few others involving astronomical factors.  Biodiversity during the Phanerozoic was always in a state of flux, but with progressive increases after each major crisis to modern times. There are many physical or abiotic processes that may have influenced biodiversity. So there is much uncertainty in the interplay of cause and effect, in much the same way as fluctuations in the world economy since the Industrial Revolution are difficult to fathom, even though financial data are much more precise and limited. Fossil and environmental data are plagued by heterogeneity, irregular sampling and the uncertainties in geochronology and the largely geochemical proxies deployed to monitor past environmental change. Another factor is the plain fact that, as well as abiotic processes, life itself affects environments; again economics – e.g. the interplay of supply, demand and price – offers stark lessons about the reductionism that still dominates much scientific discourse.

A multinational group led by mathematician Ivan Sudakow of the Open University have analysed carbon and oxygen data from the last 539 Ma – the Phanerozoic Eon – to model how the link between the carbon cycle and Earth’s climate relates to the fossil record. Rather than trying to grapple with a reductionist approach, i.e. ‘cause and effect’, they have attempted to show how the ‘backdrop’ to evolution varied through time: how the Earth System was organised during the half-billion years of ‘visible life’ (Sudakow, I. Et al. 2026. Transitions between persistent climate–carbon regimes coincide with elevated Phanerozoic biosphere vulnerability. Nature Communications v. 17, article 7559; DOI: 10.1038/s41467-026-75655-9). They omit the 100 Ma-long Ediacaran Period (635 to 539 Ma) when tangible life was abundant, if primitive.  A measure of biotic vulnerability derived from the fossil record is central to their comparative analysis. It conflates the turnover of genera with biotic diversity. The authors suggest that a high turnover at a time of low diversity indicates high vulnerability. The Ediacaran fauna is nowhere near as diverse as those following the Cambrian Explosion and simply vanished from the record at the end of the Neoproterozoic, hence its omission.

Biotic vulnerability through the five ‘mega-climate’ states of the Phanerozoic, and the ‘Big Five’ mass extinction events. Credit: Sudakow et al. 2026; Fig. 5

The methodology, data plots and language used by Sudakow et al. are pretty obscure for a general geological audience and thus for me! Be that as it may, their complex analysis leads them to suggest plausibly that climate and the carbon cycle remained in persistent states during five long periods: Cambrian–Upper Ordovician (539–447 Ma); Upper Ordovician–lowermost Carboniferous (447–358 Ma); Carboniferous–Middle Triassic (358–239 Ma). Middle Triassic–early Palaeogene (239–57 Ma); Post-Palaeogene (57–0 Ma). Biotic vulnerability fluctuated within them, the largest mass extinctions occurring when it rose to peaks towards the start and/or end of each persistent climate-carbon state. But their work grossly simplifies the vast complexity of interwoven biotic and abiotic processes. It leaves out global changes in sea-level, the varying supply of elements released to the oceans by continental weathering – nutrients (N, K, P and Fe), toxins (e.g. Hg and S) and skeleton-building elements (e.g. Si, Ca and Mg) – and fluctuations in dissolved oxygen that reflect varying redox conditions; to name but four. Each of them would impact on life in different ways, and each can be parameterised in detail by geochemical data from sedimentary rocks

So, a useful test for the authors’ hypothesis would be to perform the same kind of analysis using, for instance, well-documented sea-level changes since the Early Cambrian, which would have shifted the availability and location of ecological niches. Likewise, the course of super-continent accretion and break up. There are also Phanerozoic time series for the oxygenation of seawater, especially for deep-water sediments, which show irregular pulses of anoxia, some of which are implicated in extinctions such as the end-Permian. And there are more data that are relevant to the co-evolution of Earth and life. I would not be surprised if such studies produced different scenarios from those arising from investigations of just surface average temperature (SAT) and the carbon cycle. No doubt others will follow where Sudakow et al. have led. Perhaps that team has plans to diversify its approach.

A cautionary philosophical question, however, is, ‘How far can rationalisation, even using mathematically sophisticated methods, hope to unravel geological history and past fluctuations in the biosphere?’

The compendium of peer-reviewer comments and suggestions helped me to follow the logic of this somewhat arcane paper. Readers might also benefit from reading them.

See also: Shifts between stable climate-carbon states coincide with increased Phanerozoic biosphere vulnerability. Scienmag, 5 August 2026; Study of climate–carbon behaviour through the ages gives vital lessons on the earth’s capacity when under threat. Open University News, 30 July 2026

Surface temperature self-regulated by the Earth System during the Phanerozoic

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During the past 539 Ma (the Phanerozoic Eon) Earth’s geological history saw the explosion of rapidly evolving life in the oceans and on the land. The pace of that evolution swung up and down through a complex sequence of extinctions and adaptive radiations. They resulted from many intertwined inorganic changes: tectonics; impacts; igneous events; global climate change; atmosphere and sea-water composition. Although palaeoclimatic knowledge has become ever more detailed over the last few decades, its most important record, the varying temperature of the land surface and oceans, is lacking in precision. The timing of climatic events is not the issue, but the magnitude of changes in global mean surface temperature. The latter is largely down to the main tool in assessing past temperatures: the isotopic composition of oxygen (δ18O) in  marine fossils. In particular, the record for the Lower Palaeozoic has remained stubbornly odd. In the Cambrian and Ordovician Periods it implies that low-latitude seawater temperatures reached levels of 40 to 50 °C, that seem literally life threatening: phytoplankton at the base of modern marine ecosystems die at water temperatures above 35°C. Yet the fossil record is teeming throughout the Lower Palaeozoic at all latitudes. Some manner of imprecision in the oxygen-isotope method gives the impression of wild fluctuations and a dramatic overall cooling of the planet through the Phanerozoic: the temperature record as it stands seems implausible.

The carbonate-silicate cycle within the longer-term carbon cycle. Source: Wikimedia Commons

A group of palaeoclimatologists from China, the UK, Australia and the US have combined a variety of geochemical proxies, sedimentary records and climate modelling to correct the marine-carbonate δ18O record (Zheng, D. and 12 others 2026. Tight regulation of Earth’s long-term temperature over Phanerozoic time.  Nature Communications, in press 4 May 2026; DOI: 10.1038/s41467-026-72672-6). Their approach is based on a chemical index of alteration (CIA), i.e. a measure of the degree of chemical weathering of the source for sedimentary rocks. The CIA compares their content of immobile aluminium oxide (Al2O3) with calcium, sodium and potassium oxides that are more easily moved in solution. Analyses of recent river sediments show a positive correlation between CIA and local temperature, so CIA in ancient sedimentary rocks is a potential proxy for the ambient temperature of the region from which those sediments were derived. The CIA also depends on other factors, such as the intensity of physical erosion and transport. However, allowing for these factors in modern environments does not affect the correlation with ambient temperature: the method remains robust. The geochemical data from sedimentary rocks required to use CIA as an independent check on O-isotope derived temperature are available in abundance from all continents for most of the Phanerozoic.

The study by Zheng et al. suggests that throughout the Phanerozoic global mean temperature remained consistently within the 10 to 30°C range. Thus Palaeozoic ocean temperatures were comparable with those of the succeeding Mesozoic and Cenozoic Eras. The team concludes that various negative feedback processes inherent in the Earth System have been able to regulate its surface temperature through the Phanerozoic. The most important of these is climate-dependent silicate weathering in which acidic rain – produced by CO2 dissolved from the atmosphere – breaks down silicates to yield dissolved bicarbonate ions that combine with calcium and magnesium ions to precipitate carbonates. Such a process draws down the main greenhouse gas from the atmosphere. There are other aspects of the carbon cycle that also draw down atmospheric CO2 and reduce the greenhouse effect, such as burial of organic debris. Tectonics also shapes climate by modulating both silicate weathering and CO­2 emissions from volcanic activity.

It should be emphasised that anthropogenic global warming is proceeding at a far higher rate than natural negative feedback processes. We simply cannot rely on silicate weathering to reverse whatever climatic outcome results from what the current global economy does so very quickly. Yet the findings by Zheng et al. do seem likely to force a change in thinking about climate change on a geological timescale.

See also: Earth’s long-term temperature kept tight control. Scienmag; 4 May 2026