End-Ordovician mass extinction, faunal diversification, glaciation and true polar wander

Enormous events occurred between 460 and 435 Ma around the mid-point of the Palaeozoic Era and spanning the Ordovician-Silurian (O-S) boundary. At around 443 Ma the second-most severe mass extinction in Earth’s history occurred, which eliminated 50 to 60% of all marine genera and almost 85% of species: not much less than the Great Dying at the end of the Permian Period. The event was accompanied by one of the greatest biological diversifications known to palaeontology, which largely replaced the global biota initiated by the Cambrian Explosion. Centred on the Saharan region of northern Africa, Late Ordovician glacial deposits also occur in western South America and North America. At that time all the current southern continents and India were assembled in the Gondwana supercontinent, with continental masses that became North America, the Baltic region, Siberia and South China not far off: all the components that eventually collided to form Pangaea from the Late Silurian to the Carboniferous.

The mass extinction has troubled geologists for quite a while. There are few signs of major volcanism having been involved, although some geochemists have suggested that very high mercury concentrations in some Late Ordovician marine sediments bear witness to large, albeit invisible, igneous events. No large impact crater is known from those times, although there is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in earlier Ordovician sedimentary rocks, around the Baltic Sea. Another suggestion, poorly supported by evidence, is destruction of the atmospheric ozone layer by a gamma-ray burst from some distant but stupendous supernova. A better supported idea is that the oceans around the time of the event lacked oxygen. Such anoxia can encourage solution of toxic metals and hydrogen sulfide gas. Unlike other mass extinctions, this one was long-drawn out with several pulses.

The glacial epoch also seems implicated somehow in the mass die-off, being the only one known to coincide with a mass extinction. It included spells of frigidity that exceeded those of the last Pleistocene glacial maximum, with the main ice cap having a volume of from 50 to 250 million cubic kilometres. The greatest of these, around 445 Ma, involved a 5°C fall in global sea-surface temperatures and a large negative spike in δ13C in carbon-rich sediments, both of which lasted for about a million years. The complex events around that time coincided with the highest ever extinction and speciation rates, the number of marine species being halved in a short space of time: a possible explanation for the δ13 C anomaly. Yet estimates of atmospheric CO2 concentration in the Late Ordovician suggests it was perhaps 8–16 times higher than today; Earth should have been a warm planet then. One probable contributor to extreme glacial conditions has been suggested to be that the South Pole at that time was well within Gondwana and thus isolated from the warming effect of the ocean. So, severe glaciation and a paradoxical combination of mass extinction with considerable biological diversification present quite an enigma.

A group of scientists based in Beijing, China set out to check the palaeogeographic position of South China between 460 and 435 Ma and evaluate those in  O-S sediments at locations on 6 present continents (Jing, X., Yang, Z., Mitchell, R.N. et al. 2022. Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction. Nature Communications, v. 13, article 7941; DOI: 10.1038/s41467-022-35609-3). Their key tool is determining the position of the magnetic poles present at various times in the past from core samples drilled at different levels in these sedimentary sequences. The team aimed to test a hypothesis that in O-S times not only the entire lithosphere but the entire mantle moved relative to the Earth’s axis of rotation, the ‘slippage’ probably being at the Core-mantle boundary [thanks to Steve Rozario for pointing this out]. Such a ‘true polar wander’ spanning 20° over a mere  2 Ma has been detected during the Cretaceous, another case of a 90° shift over 15 Ma may have occurred at the time when Snowball Earth conditions first appeared in the Neoproterozoic around the time when the Rodinia supercontinent broke up and a similar event was proposed in 1994 for C-O times albeit based on sparse and roughly dated palaeomagnetic pole positions.

Xianqing Jing and colleagues report a wholesale 50° rotation of the lithosphere between 450 and 440 Ma that would have involved speeds of about 55 cm per year. It involved the Gondwana supercontinent and other continental masses still isolated from it moving synchronously in the same direction, as shown in the figure. From 460 to 450 Ma the geographic South Pole lay at the centre of the present Sahara. At 445 Ma its position had shifted to central Gondwana during the glacial period. By 440 Gondwana had moved further northwards so that the South Pole then lay at Gondwana’s southernmost extremity.

Palaeogeographic reconstructions charting true polar wander and the synchronised movement of all continental masses between 460 and 440 Ma. Note the changes in the trajectories of lines of latitude on the Mollweide projections. The grey band either side of the palaeo-Equator marks intense chemical weathering in the humid tropics. Credit Jing et al. Fig 5.

As well as a possible key to the brief but extreme glacial episode this astonishing journey by a vast area of lithosphere may help account for the mass extinction with rapid speciation and diversification associated with the O-S boundary. While the South Pole was traversing Gondwana as the supercontinent shifted the ‘satellite’ continental masses remained in or close to the humid tropics, exposed to silicate weathering and erosion. That is a means for extracting CO2 from the atmosphere and launching global cooling, eventually to result in glaciation over a huge tract of Gondwana around 445 Ma. Gondwana then moved rapidly into more clement climatic zones and was deglaciated a few million years later. The rapid movement of the most faunally diverse continental-shelf seas through different climate zones would have condemned earlier species to extinction simultaneous adaptation to changed conditions could have encouraged the appearance of new species and ecosystems. This does not require the catastrophic mechanisms largely established for the other mass extinction events. It seems that during the stupendous, en masse slippage of the Earth’s lithosphere plate tectonic processes still continued, yet it must have had a dynamic effect throughout the underlying mantle.

Yet the fascinating story does have a weak point. What if the position of the magnetic poles shifted during O-S times from their assumed rough coincidence with the geographic poles? In other words, did the self-exciting dynamo in the liquid outer core undergo a large and lengthy wobble? How the outer core’s circulation behaves depends on its depth to the solid core, yet the inner core seems only to have begun solidifying just before the onset of the Cambrian, about 100 Ma before the O-S events. It grew rapidly during the Palaeozoic, so the thickness of the outer core was continuously increasing. Fluid dynamic suggests that the form of its circulation may also have undergone changes, thereby affecting the shape and position of the geomagnetic field: perhaps even shifting its poles away from the geographic poles …

The Earth System in action: land plants affected composition of continental crust

The essence of the Earth System is that all processes upon, above and beneath the surface interact in a bewildering set of connections. Matter and energy in all their forms are continually being exchanged, deployed and moved through complex cycles: involving rocks and sediments; water in its various forms; gases in the atmosphere; magmas; moving tectonic plates and much else besides. The central and massively dominant role of plate tectonics connects surface processes with those of our planet’s interior: the lithosphere, mantle and, arguably, the core. Interactions between the Earth System’s components impose changes in the dynamics and chemical processes through which it operates. Living processes have been a part of this for at least 3.5 billion years ago, in part through their role in the carbon cycle and thus the Earth’s climatic evolution. During the Silurian Period life became a pervasive component of the continental surface, first in the form of plants, to be followed by animals during the Devonian Period. Those novel changes have remained in place since about 430 Ma ago, plants being the dominant base of continental ecosystems and food chains.

Schematic diagram showing changes in river systems and their alluvium before and after the development of land plants. (Credit: Based on Spencer et al. 2022, Fig 4)

Land plants exude a variety of chemicals from their roots that break down rock to yield nutrient elements. So they play a dominant role in the formation of soil and are an important means of rock weathering and the production of clay minerals from igneous and metamorphic minerals. Plant root systems bind near-surface sediments thus increasing their resistance to erosion by wind and water, and to mass movement under gravity. This binding and plant canopies efficiently reduce dust transport, slow water flow on slopes and decrease the sediment load of flowing water. Plants and their roots also stabilise channels systems. There is much evidence that before the Devonian most rivers comprised continually migrating braided channels in which mainly coarse sands and gravels were rapidly deposited while silts and muds in suspension were shifted to the sea. Thereafter flow became dominated by larger and fewer channels meandering across wide tracts on which fine sediment could accumulate as alluvium on flood plains when channels broke their banks. Land plants more efficiently extract CO2 from the atmosphere through photosynthesis and the new regime of floodplains could store dead plant debris in the muds and also in thick peat deposits. As a result, greenhouse warming had dwindled by the Carboniferous, encouraging global cooling and glaciation. 

Judging the wider influence of the ‘greening of the land’ on other parts of the Earth system, particularly those that depend on internal  magmatic processes, relies on detecting geochemical changes in minerals formed as direct outcomes of plate tectonics. Christopher Spencer of Queen’s University in Kingston, Canada and co-workers at the Universities of Southampton, Cambridge and Aberdeen in the UK, and the China University of Geosciences in Wuhan set out to find and assess such a geochemical signal (Spencer, C., Davies, N., Gernon, T. et al. 2022. Composition of continental crust altered by the emergence of land plants. Nature Geoscience, v. 15 online publication; DOI: 10.1038/s41561-022-00995-2). Achieving that required analyses of a common mineral formed when magmas crystallise: one that can be precisely dated, contains diverse trace elements and whose chemistry remains little changed by later geological events. Readers of Earth-logs might have guessed that would be zircon (ZrSiO). Being chemically unreactive and hard, small zircon grains resist weathering and the abrasion of transport to become common minor minerals in sediments. Thousands of detrital zircon grains teased out from sediments have been dated and analysed in the last few decades. They span almost the entirety of geological history. Spencer et al. compiled a database of over 5,000 zircon analyses from igneous rocks formed at subduction zones over the last 720 Ma, from 183 publications by a variety of laboratories.

The approach considered two measures: the varying percentages of mudrocks in continental sedimentary sequences since 600 Ma ago; aspects of the hafnium- (Hf) and oxygen-isotope proportions measured in the zircons using mass spectrometry and their changes over the same time. Before ~430 Ma the proportion of mudrocks in continental sedimentary sequences is consistently much lower than it is in post post-Silurian, suggesting a link with the rise of continental plant cover (see second paragraph). The deviation of the 176Hf/177Hf ratio in an igneous mineral from that of chondritic meteorites (the mineral’s εHf value) is a guide to the source of the magma, negative values indicating a crustal source, whereas positive values suggest a mantle origin. The relative proportions of two oxygen isotopes 18O and 16O  in zircons, expressed as δ18O, indicates the proportion of products of weathering, such as clay minerals, involved in magma production – 18O selectively moves from groundwater to clay minerals when they form, increasing their δ18O.

While the two geochemical parameters express very different geological processes, the authors noticed that before ~430 Ma the two showed low correlation between their values in zircons. Yet, surprisingly, the parameters showed a considerable and consistent increase in their correlation in younger zircons, directly paralleling the ‘step change’ in the proportions of mudstones after the Silurian. Complex as their arguments are, based on several statistical tests, Spencer et al. conclude that the geologically sudden change in zircon geochemistry ultimately stems from land plants’ stabilisation of river systems. As a result more clay minerals formed by protracted weathering, increasing the δ18O in soils when they were eroded and transported. When the resulting marine mudrocks were subducted they transferred their oxygen-isotope proportions to magmas when they were partially melted.

That bolsters the case for dramatic geological consequences of the ‘greening of the land’. But did its effect on arc magmatism fundamentally change the bulk composition of post-Silurian additions to the continental crust? To be convinced of that I would like to see if other geochemical parameters in subduction-related magmas changed after 430 Ma. Many other elements and isotopes in broadly granitic rocks have been monitored since the emergence of high-precision rock-analysing technologies around 50 years ago. There has been no mention, to my knowledge, that the late-Silurian involved a magmatic game-changer to match that which occurred in the Archaean, also revealed by hafnium and oxygen isotopes in much more ancient zircons.   

See also: https://www.sci.news/othersciences/geoscience/land-plants-continental-crust-composition-11151.htmlhttps://www.eurekalert.org/news-releases/963296