‘Mud, mud, glorious mud’

Earth is a water world, which is one reason why we are here. But when it comes to sedimentary rocks, mud is Number 1. Earth’s oceans and seas hide vast amounts of mud that have accumulated on their floors since Pangaea began to split apart about 200 Ma ago during the Early Jurassic. Half the sedimentary record on the continents since 4 billion years ago is made of mudstones. They are the ultimate products of the weathering of crystalline igneous rocks, whose main minerals – feldspars, pyroxenes, amphiboles, olivines and micas, with the exception of quartz – are all prone to breakdown by the action of the weakly acidic properties of rainwater and the CO2 dissolved in it. Aside from more resistant quartz grains, the main solid products of weathering are clay minerals (hydrated aluminosilicates) and iron oxides and hydroxides. Except for silicon, aluminium and ferric iron, most metals end up in solution and ultimately the oceans.  As well as being a natural product of weathering, mud is today generated by several large industries, and humans have been dabbling in natural muds since the invention of pottery some 25 thousand years ago.  On 21 August 2020 the journal Science devoted 18 pages to a Special Issue on mud, with seven reviews (Malakoff, D. 2020. Mud. Science, v. 369, p. 894-895; DOI: 10.1126/science.369.6506.894).

Mud carnival in Brazil (Credit: africanews.com)

The rate at which mud accumulates as sediment depends on the rate at which erosion takes place, as well as on weathering. Once arable farming had spread widely, deforestation and tilling the soil sparked an increase in soil erosion and therefore in the transportation and deposition of muddy sediment. The spurt becomes noticeable in the sedimentary record of river deltas, such as that of the Nile, about 5000 years ago. But human influences have also had negative effects, particularly through dams. Harnessing stream flow to power mills and forges generally required dams and leats. During medieval times water power exploded in Europe and has since spread exponentially through every continent except Antarctica, with a similar growth in the capacity of reservoirs. As well as damming drainage these efforts also capture mud and other sediments. A study of drainage basins in north-east USA, along which mill dams quickly spread following European colonisation in the 17th century, revealed their major effects on valley geomorphology and hydrology (see: Watermills and meanders; March 2008). Up to 5 metres of sediment build-up changed stream flow to an extent that this now almost vanished industry has stoked-up the chances of major flooding downstream and a host of other environmental changes. The authors of the study are acknowledged in one Mud article (Voosen, P. 2020. A muddy legacy. Science, v. 369, p. 898-901; DOI: 10.1126/science.369.6506.898) because they have since demonstrated that the effects in Pennsylania are reversible if the ‘legacy’ sediment is removed. The same cannot be expected for truly vast reservoirs once they eventually fill with muds to become useless. While big dams continue to function, alluvium downstream is being starved of fresh mud that over millennia made it highly and continuously productive for arable farming, as in the case of Egypt, the lower Colorado River delta and the lower Yangtze flood plain below China’s Three Gorges Dam.

Mud poses extreme risk when set in motion. Unlike sand, clay deposits saturated with water are thixotropic – when static they appear solid and stable but as soon as they begin to move en masse they behave as a viscous fluid. Once mudflows slow they solidify again, burying and trapping whatever and whomever they have carried off. This is a major threat from the storage of industrially created muds in tailings ponds, exemplified by a disaster at a Brazilian mine in 2019, first at the site itself and then as the mud entered a river system and eventually reached the sea. Warren Cornwall explains how these failures happen and may be prevented (Cornwall, W. 2020. A dam big problem.  Science, v. 369, p. 906-909; DOI: 10.1126/science.369.6506.906). Another article in the Mud special issue considers waste from aluminium plants (Service, R.F. 2020. Red alert. Science, v. 369, p. 910-911; DOI: 10.1126/science.369.6506.910). The main ore for aluminium is bauxite, which is the product of extreme chemical weathering in the tropics. The metal is smelted from aluminium hydroxides formed when silica is leached out of clay minerals, but this has to be separated from clay minerals and iron oxides that form a high proportion of commercial bauxites, and which are disposed of in tailings dams. The retaining dam of such a waste pond in Hungary gave way in 2010, the thixotropic red clay burying a town downstream to kill 10 people. This mud was highly alkaline and inflicted severe burns on 150 survivors. Service also points out a more positive aspect of clay-rich mud: it can absorb CO2 bubbled through it to form various, non-toxic carbonates and help draw down the greenhouse gas.

Muddy sediments are chemically complex, partly because their very low permeability hinders oxygenated water from entering them: they maintain highly reducing conditions. Because of this, oxidising bacteria are excluded, so that much of the organic matter deposited in the muds remains as carbonaceous particles. They store carbon extracted from the atmosphere by surface plankton whose remains sink to the ocean floor. Consequently, many mudrocks are potential source rocks for petroleum. Although they do not support oxygen-demanding animals, they are colonised by bacteria of many different kinds. Some – methanogens – break down organic molecules to produce methane. The metabolism of others depends on sulfate ions in the trapped water, which they reduce to sulfide ions and thus hydrogen sulfide gas: most muds stink. Some of the H2S reacts with metal ions, to precipitate sulfide minerals, the most common being pyrite (FeS2). In fact a significant proportion of the world’s copper, zinc and lead resources reside in sulfide-rich mudstones: essential to the economies of Zambia and the Democratic Republic of Congo. But there are some strange features of mud-loving bacteria that are only just emerging. The latest is the discovery of bacteria that build chains up to 5 cm long that conduct electricity (Pennisi, E. 2020. The mud is electric. Science, v. 369, p. 902-905; DOI: 10.1126/science.369.6506.902). The bacterial ‘nanowires’ sprout from minute pyrite grains, and transfer electrons released by oxidation of organic compounds, effectively to catalyse sulfide-producing reduction reactions. NB Oxygen is not necessary for oxidation as its chemistry involves the loss of electrons, while reduction involves a gain of electrons, expressed by the acronym OILRIG (oxidation is loss, reduction is gain). It seems such electrical bacteria are part of a hitherto unsuspected chemical ecosystem that helps hold the mud together as well as participating in a host of geochemical cycles. They may spur an entirely new field of nano-technology, extending, bizarrely, to an ability to generate electricity from moisture in the air.

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Are Martian clays magmatic in origin?

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Artist’s Concept of Curiosity’s touchdown(credit: Wikipedia)

The remote detection of spectral features in the infrared that suggest abundant clay minerals on the surface of Mars is the basis for a widely-held view that Mars may once have had moist climatic conditions that encouraged life to form (see The Martian ‘sexy beast’ in September 2012  EPN). The presence of clays, along with suggestive landforms, has also been used to speculate that Mars once harboured long-lived lakes and perhaps even a huge ocean on its northern hemisphere, between 3.7 to 4.1 Ga. It was the clays that pitched the recently arrived Curiosity (aka Mars Exploration)Rover at the Gale crater and its central Aeolis Mons. The latter, also known as Mount Sharp, preserves about 5 km of layered rocks, the lowest of which are clay-rich and hypothesised to be sediments laid down in a lake that filled the crater. Provided Curiosity operates according to plan, we will know soon enough whether or not the layered rocks of Mount Sharp are indeed sediments, but a soon-to-be-published article suggests another explanation than weathering for the production of abundant clay minerals on Mars (Meunier, A. et al. 2012. Magmatic precipitation as a possible origin of Noachian clays on Mars. Nature Geoscience, published online 9 September 2012; DOI: 10.1038/NGEO1572).

Focusing the 100-millimeter Mastcam [detail]
Layered rocks on the flanks of Mount Sharp in Gale crater from Curiosity’s Mastcam (NASA Goddard via Flickr)
The French-US team provides evidence from terrestrial lavas that abundant iron- and magnesium-rich clays, known as smectites, may form at a late stage during crystallization of magma. If magma contains water – and most magmas do – as more and more anhydrous silicates crystallise during cooling water builds up in the remaining liquid. Once silicate crystallisation is complete there remains a watery fluid capable of reacting with some of the silicates to form clay minerals; a process often referred to as pneumatolysis. How much clay is formed depends on the initial water content of the magma. Pneumatolysisoperates on hot lava, whereas weathering occurs at ambient temperature provided the climate is able to support liquid water at the surface. Mars is currently far too cold for that, and ideas of a wet surface environment earlier in the planet’s history demand an explanation for a much warmer climate. Clay minerals do not appear to be present in Mars’s younger rocks, so Meunier and colleagues suggest that as the planet’s mantle evolved early water-rich magmas were gradually replaced by ones with less water: interior Mars was gradually de-gassed and its magmas lost the ability to alter minerals that crystallised from them.

Now, clay minerals are extremely resistant to change except through high-temperature metamorphism. Once formed they can be blown around – Mars has probably always been a very windy place – to end up in aeolian sediments that are plentiful on Mars.  Also, if occasionally water flowed on the surface perhaps by subsurface water venting suddenly, fine-grained pneumatolytic clays would easily be picked up, concentrated as flow speed lessened and deposited in waterlain sedimentary layers.  A dilemma that faces the Curiosity science team is what significance to assign to clays in sediment layers, when they no longer provide unequivocal evidence of weathering.  But will the resistant layers on Mount Sharp turn out to be pneumatolytically altered lava flows?
Note added 28 September 2012: The first scientific triumph of the Curiosity Rover is imagery of sediments in what had been suggested to be an alluvial fan washed into Gale crater. They show gravels with rounded pebbles.