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.   

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Plants first to succumb to the end-Permian event

We have become accustomed to thinking that up to 90% of organisms were snuffed out by the catastrophe at the Permian-Triassic boundary 252 Ma ago. Those are the figures for marine organisms, whose record in sediments is the most complete. It has also been estimated to have lasted a mere 60 ka, and the recovery in the Early Triassic to have taken as long as 10 Ma. There are hints of three separate pulses of extinction related to: initial gas emission from the Siberian Traps; coal fires; and release of methane from sea-floor gas hydrates at the peak of global warming. Various terrestrial sequences record the collapse of dense woodlands, so that the Early Triassic is devoid of coals that are widespread in the preceding Late Permian. A new detailed study of terrestrial sediments in the Sydney Basin of eastern Australia reveals something new (Fielding, C.R. and 10 others 2019. Age and pattern of the southern high-latitude continental end-Permian extinction constrained by multiproxy analysis. Nature Communications, v. 10, online publications: DOI: 10.1038/s41467-018-07934-z).

The distinctive, tongue-like form of Glossopteris leaves that dominate the coal-bearing Permian strata of the southern coninents. Their occurrence in South America, Africa, India, Australia, New Zealand, and Antarctica prompted Alfred Wegener to suggest that these modern continents had been united in Pangaea by Permian times: a key to continental drift. (Credit: Getty Images)

Christopher Fielding or the University of Nebraska-Lincoln and colleagues focused on pollens, geochemistry and detailed dating of the sedimentary succession across the P-Tr boundary exposed on the New South Wales coast. The stratigraphy is intricately documented by a 1 km deep well core that penetrates a more or less unbroken fluviatile and deltaic sequence that contains eleven beds of volcanic ash. The igneous layers are key to calibrating age throughout the sequence (259.10 ± 0.17 to 247.87 ± 0.11 Ma using zircon U-Pb methods). The pollens change abruptly from those of a Permian flora, dominated by tongue-like glossopterid plants, to a different association that includes conifers. The change coincides with a geochemical ‘spike’ in the abundance of nickel and a brief change in the degree of alteration of detrital fledspars to clay minerals. The first implicates the delivery of massive amounts of nickel to the atmosphere, probably by the eruption of the Siberian Traps , which contain major economic nickel deposits. The second feature suggests a brief period of warmer and more humid climatic conditions. A third geochemical change is the onset of oscillations in the abundance of 13C that are thought to record major changes in plant life across the planet. These features would have been an easily predicted association with the 252 Ma mass extinction were it not for the fact that the radiometric dating places them about 400 thousand years before the well-known changes in global animal life. Detailed dating of the Siberian Traps links the collapse of Glossopteris and coal formation to the earliest extrusion of flood basalts, which suggests that the animal extinctions were driven by cumulative effects of the later outpourings

Related article: Chris Fielding comments on the paper at Nature Research/Ecology and Evolution

Read more on Palaeobiology and Stratigraphy

Greening and changing the land

English: Liverwort Liverworts are small plants...
A very British liverwort mat. Image via Wikipedia

Evidence for the earliest colonisation of the continents by plants is in the form of spores and body fragments from terrestrial sediments of Middle Ordovician age (~470 Ma) (Rubinstein, C.  et al. 2010. Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana). New Phytologist, v. 188, p. 365-369)suggest that the first vegetation cover involved simple ground-hugging plants that lacked stems of roots, very like the liverworts that I struggle to deter from my gravel drive. Vinegar is the only solution, preferably boiling, but that does not harm their spores and inevitably they re-emerge. Rearranging the gravel, of a pale pink limestone, is one of a very few means of keeping fit that I can bear, and I suppose the liverworts spice that up a little: but I do detest them. Part of their irritation is that they form an impermeable coating to what once was a passable if minor aquifer that channelled rainfall that would otherwise repeat the house-flooding that greeted me within a day of my moving in. So it was with some solemnity that I read a paper on how these damnable organisms transformed the Ordovician continental surface and the geomorphological processes that shaped it (Gibling, M.R. & Davies, N.S 2012. Palaeozoic landscapes shaped by plant evolution. Nature Geocience, v. 5, p. 99-105).

Sedimentologists have shown that rivers of earlier times formed wide tracts of ephemeral braided channels that transported and reworked sands and gravels that were not hampered by any vegetable binding agent. Floods merely accelerated the braiding and spread coarse sediment across valley floors, repeated spates washing out almost of the fines to take them ultimately to the continental shelves: there are few if any relics of Cambrian and older muddy floodplains. Moreover, untrammelled by vegetation any remaining fine material would be picked up by wind, even in humid climates, to meet the same marine fate. Overbank deposits of silts and clays, unsurprisingly, demand banks over or through which floodwater  escapes from defined channels and is then delayed by low gradients away from the main flow, so to deposit the fines carried by its sluggish speed. Except in arid terrains where braided channels are still the rule, in succeeding geological time evidence grows for nowadays familiar channels, meanders with point bars and eroded opposite banks, levées and floodplains on every conceivable scale. Apparently, they became conspicuous in Silurian times and then forming 30% of all fluvial sediments by the Devonian.

Meanwhile, plants were diversifying though evolution of vascular systems that transport sap up supporting structures that emerged in parallel eventually to form trunks and branches. The consequent rise in volume and in area exposed to sunlight and photosynthesis of a plant’s tissues increased the potential to draw CO2 from the air, witnessed by changes in carbon isotopes that show carbon burial rising shortly after the mid-Ordovician from far lower values in earlier times. (Incidentally, it seems likely that such meagre colonisers as early liverworts thrived sufficiently to contribute to the cooling in the Upper Ordovician that led to sporadic glacial episodes).  Preservation of wood in peats – liverworts are not implicated in any kind of fossil-fuel production – helped to maximise carbon burial by the end of the Palaeozoic Era. But trees make logs and, carried by rivers, logjams. By the Upper Carboniferous effects of damming become common in fluvial sediments, which seemed to serve the formation of islands within wide river channels.

By the present day, vegetation has come to dominate all but the most arid river systems. Even in central Australia sturdy gums able only to get water from below ephemeral river beds end up defining the flow regime and stabilising it on low relief plains that would otherwise be ravaged by sheet floods every rainy season. The authors support stratigraphic observations through the use of scaled down models of channels in vegetated areas by the cunning use of alfalfa seeded to sprout during simulated dry conditions then resuming channel flow in a flume tank.

Gilboa Fossils - Gilboa, New York
Fossils tree stumps from Gilboa, New York (Photo credit: Dougtone)

The earliest substantial trees, represented by wood fragments rarely assignable to any particular structure, occur in the Middle Devonian (385-400 Ma). Although some groups can be differentiated, how their encompassing woodland ecosystems looked has been a mystery until recently . Being ‘priitive’ it has been assumed to be very simple, unlike the well-documented forests of the Carboniferous coal swamps. But, once in a while, a site of exceptional preservation is unearthed, one such being a palaeosol that clearly formed on the floor of a Middle Devonian woodland exposed by quarrying in New York state, USA (Stein, W.E. et al. 2012. Surprisingly complex community found in the mid-Devonian fossil forest at Gilboa. Nature, v. 483, p. 78-81). Once backfill accumulated during the quarry’s active life was removed it became possible to plot the arrangement of roots systems of the last trees to live at the site before inundation and preservation.  Together with other plant material found in the ancient soil, the growing sites have been reconstructed to assess the full ecosystem involved. It was a great deal more complex than previously thought possible, with a series of tiers formed by three large tree types: tall, lollipop-like Eospermatopteris; smaller lycopsid-like trees and subsurface propagators related to gymnosperms that sprouted to form an understorey that may have climbed the larger trees in the manner of vines. Its setting was akin to that of modern mangrove swamps – by the sea – subject to sea-level change that inundated, killed and preserved the coastal woodland.