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DOUGLAS MAGNO / AFP VIA GETTY IMAGES
By David Malakoff, Nirja Desai, Xing Liu
Glop. Mud. Ooze. Cohesive sediment. Call it what you want, mud – a mixture of fine sediment and water – is one of the most common and important substances on Earth. Not quite a solid, not quite a liquid, mud covers the bottom of our lakes, rivers and seas. It helps form massive floodplains, river deltas, and mud flats that store large amounts of carbon and nutrients, and support vibrant communities of people, flora, and fauna. But mud is also a killer: landslides bury thousands of people every year.
Earth has been a muddy planet for 4 billion years, since water became abundant. But the way it forms and moves has changed dramatically. About 500 million Years ago, the arrival of terrestrial plants accelerated the decomposition of rock into fine particles, slowed runoff and stabilized sediment, allowing thick layers of mud to accumulate in river valleys. The tectonic changes that gave rise to the mountains, as well as the climate changes that increased rainfall, accelerated erosion and helped cover the seabed with mud several hundred meters thick. Over time, many mud deposits have hardened into mudrock, the most abundant rock in geological history, making up about half of all sedimentary formations.
Now humans are a dominant force in the mud world. From around 5,000 years ago, erosion rates exploded in many parts of the world when our ancestors began clearing forests and planting crops. Even more rivers and valleys filled with sediment, changing the landscapes beyond all recognition. In some places, dams and dikes have trapped this mud, preventing fresh sediment from feeding the floodplains, deltas and mudflats and causing them to shrink (see graphic below). And industrial processes have started to produce massive amounts of new forms of sludge – waste from mines and factories – that are loaded with toxic compounds and often stored behind dams that can fail, setting off deadly torrents.
Despite its omnipresence, mud still conceals mysteries. Biologists, for example, are just beginning to grasp the vast menagerie of organisms that live in mud and unravel the remarkable adaptations that allow them to cope with particular challenges, like lack of oxygen. And biogeochemists are still grappling with the huge role sludge plays in the carbon cycle, and therefore influencing the global climate. Problems like this, as the cliché says, are still clear as mud.
Mud in motion
Humans are reshaping the world’s mud supply, altering where and how quickly it accumulates. Thousands of years ago, our ancestors drove erosion rates up by starting to clear forests and plant crops, mud lakes and rivers. Today, deforestation and urbanization are pushing some rivers to carry more sediment, even as dams and efforts to curb erosion block the supply of sediment to other waterways. These changes, along with changes in precipitation brought on by climate change, lead to sometimes dramatic transformations in river deltas, coastal mudflats and the amount of mud that eventually builds up on the seabed.
(Graphic) N. Desai /SCIENCE AND X. Liu /SCIENCE; (DATA) JINREN NI
(Graphic) N. Desai /SCIENCE AND X. Liu /SCIENCE; (DATA) MISSION OF TOPOGRAPHY RADAR HYDROSHEDS / SHUTTLE NASA; MURRAY ET AL., NATURE, 565, 222 (2019)
(GRAPHIC) N. DESAI and X. LIU /SCIENCE; (DATA) ROTHACKER ET AL., SCI. REPORTS, 8, 247 (2018); AT ET AL., SCI. BULLETIN, 65, 1, 62 (2020); NIEHUIS ET AL., NATURE, 577, 514 (2020); MURRAY ET AL., NATURE, 565, 222 (2019)
History of David Malakoff; graphics by Nirja Desai and Xing Liu.
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