Unearthing mankind’s profound impact on mud and carbon cycling

  • A group of William & Mary students engage in field work with VIMS researchers on a mud flat. Photo by Gail Schwieterman
    A group of William & Mary students engage in field work with VIMS researchers on a mud flat. Photo by Gail Schwieterman    
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Most people avoid mud, but not a global team of scientists who recently detailed humans’ immense impact on this often-overlooked, carbon-rich aggregate. According to their study, featured on the cover of Nature Geoscience, humans have significantly altered the movement of mud across the landscape and in the oceans, causing yet-to-be quantified impacts on the storage and cycling of organic carbon.

Steve Kuehl, a professor at William & Mary’s Virginia Institute of Marine Science, utilized his expertise in geological oceanography as a co-author on the study.

“In our paper, we delve into understanding how human perturbations, like deforestation, construction of dams and levees, groundwater and hydrocarbon withdrawal, bottom trawling, and human-induced climate warming in polar regions, are affecting how the organic carbon complex in mud is being either preserved as a sediment deposit or remineralized as CO2 and recycled into the atmosphere,” said Kuehl. “If we are going to accurately predict future global COtrends, this is an issue we need to understand.”

The study represents an overview and assessment of the impacts of human activities on the transfer, storage and recycling of organic carbon in mud deposits from a global perspective.

Mud is derived from the chemical and physical alteration of rocks, and the resulting fine-grained minerals are magnets for organic carbon. Huge quantities of mud and mud-associated organic carbon move across the landscape as it is transferred from land toward the sea, and human activities have fundamentally changed the transport pathways and natural sequestration of organic carbon. Overall, the concentration of organic carbon increases as mineral size decreases, making fine-grained mud a much greater source of organic carbon than larger-grained sediments such as sand.

This figure shows the landscape processes controlling fluxes of mud and the impact of humans along source-to-sink gradients. The numbers represent units of Tg yr-1, where one Tg is a million metric tons. Source: Nat. Geosci., ISSN 1252-0908To assist in quantifying mud’s impact on global carbon cycling, the scientists identified the major pathways of mud movement and human-induced changes in organic carbon burial or release. For example, agricultural expansion has led to a tremendous release and transport of soil organic carbon, whereas reservoirs created by damming rivers may trap and remove some of this material. Humans also alter the character of the organic matter that controls its reactivity, that is the tendency for different organic components to be recycled to the atmosphere as CO2.

“We need to have a better understanding of the different fractions that make up the total pool of organic carbon that spans a range of reactivity,” said Kuehl. “Some types of carbon, such as those from petrogenic (rock) sources, are more difficult to break down. However, different environmental conditions can prime or increase the reactivity of that carbon. For example, priming occurs when rock sediment from river run off is introduced to plankton in the sea. It’s kind of like adding gasoline to wood.”

Myriad human activities contribute to a changing planet

The way mud moves around our planet has changed drastically due to the actions of humans. The Great Acceleration, a period beginning in the mid 20th century and continuing today, marks a dramatic surge in human activity coinciding with immense global changes.

The authors summarize that human activity has increased net changes in organic carbon derived from mud from mountain glaciers, land erosion, dams and lake reservoirs, river export, permafrost thaw, ice-sheet erosion, and coastal margins. They are uncertain about the changes for tidal flats and flood plains and suggest possible decreases for coastal wetlands. Mechanistic and budgetary study of these factors is encouraged in the study.

The researchers note that mud transport and sequestration processes remained relatively stable throughout much of the Holocene Epoch, which began approximately 11,700 years ago. However, because the timescales involved contain complex and transient events, it is difficult to determine whether relatively recent human-caused mobilization of mud results in more or less sequestration of organic carbon.

“These are complex environments, and we need to account for many factors when determining the impact of mud movements in relation to climate change,” said Kuehl. “In my area of expertise, river deltas, organic carbon in mud can be sequestered relatively efficiently, like in situations off the Mississippi Delta where sediment is shunted out into the deep Gulf of Mexico. Or, it can be recycled into the atmosphere very efficiently, like in the Amazon where the water exiting the river is constantly churning and exposing the sediment to oxygen. This churning even happens to an extent here in Chesapeake Bay tributaries, for example in the York River, where there is a mobile layer of sediment that is constantly shifting.”

The creation of reliable models will be needed to more accurately consider source-to-sink carbon cycling, and VIMS scientists and students are working to establish the scientific evidence needed for them. One little-studied aspect is the role of varying carbon pools. These have different reactivity, which affects their ability to be recycled or preserved.

“One of my students, Evan Flynn, is collaborating with researchers at Woods Hole’s NOSAMS facility. She took samples from her field work in the Ayeyarwady Delta in Myanmar and ran it through a device known as a mud burner, which separates the carbon fractions based on increasing temperature ranges during combustion, one measure of reactivity, and then determines the radiocarbon age of the different fractions,” said Kuehl. “These types of studies help in determining the carbon sources and their susceptibility to oxidation. They will be essential for developing accurate models of the global carbon cycle associated with mud.”

The complex relationship between humans and mud will continue as we advance technologically, alter our environments and look for ways to mitigate our impact on climate change. Studies such as this one will be essential to informing those efforts.

The full manuscript is available online at Nature Geoscience.