Controlling Mississippi River Causes Faster Carbon Transport to Sea

A geoscientist from Tulane University has co-authored a new study that demonstrates that human efforts to control the Mississippi River might have had an unexpected positive impact: more quick transport of carbon to the ocean.

Published in the AGU Advances journal, the study explains the research by a team of scientists who sought to learn more about the destiny of organic carbon that is transported in huge amounts by the Mississippi River.

Organic carbon is predominantly acquired from soils, vegetable matter and rocks across the Mississippi River’s drainage basin that covers around 40% of the United States.

We estimate that over the past century, the amount of organic carbon lost to the atmosphere during Mississippi River transport to the Gulf of Mexico has been reduced by at least 2.5 billion pounds (over one million metric tonnes) per year.

Torbjörn Törnqvist, Study Co-Author and Vokes Geology Professor, Department of Earth and Environmental Sciences, Tulane University

The scientists employed an innovative method by quantifying the age of the carbon deposited in Mississippi Delta sediments and the deposition time of the sediment itself. It was discovered that approximately 1000 years ago, the carbon was normally over 5,000 years older compared to the sediment in which it occurred.

It is below 3,000 years older in the historic time.

This shows that during prehistoric times, organic carbon took a much longer time to make it down to the mouth of the river, and a lot of it was lost along the way. The main reason that this has dramatically changed is that we have built levees, which prevents flooding and the dispersal of organic matter onto the vast floodplain where much of it would degrade and return back into the atmosphere.

Torbjörn Törnqvist, Study Co-Author and Vokes Geology Professor, Department of Earth and Environmental Sciences, Tulane University

Moreover, the question is whether this implies that carbon is currently buried at higher rates in the Gulf of Mexico compared to the past. Törnqvist took part in an oceanographic cruise off the Louisiana coast in February 2020, with the aim of gaining better insights into what happens with organic carbon from the Mississippi River as soon as it enters the ocean.

Findings from this expedition are anticipated to be obtained in the years to come.

The new study has possible implications on the global carbon cycle, by taking the quick changes to other huge rivers into account—such as those present in the tropics—that carry considerable amounts of carbon.

The study's lead author is Zhixiong Shen, a former research assistant professor in Törnqvist’s group.

They are working as an associate professor at Coastal Carolina University. The other co-authors involved in the study are from the University of South Florida and the University of Salzburg in Austria. This work was financially supported by the U.S. National Science Foundation.

Journal Reference:

Shen, Z., et al. (2021) Engineered Continental‐Scale Rivers Can Drive Changes in the Carbon Cycle. AGU Advances. doi.org/10.1029/2020AV000273.