Engineered Continental‐Scale Rivers Can Drive Changes in the Carbon Cycle

Abstract

Floodplains of large alluvial rivers modulate the composition of riverine organic carbon (OC) and control OC oxidative loss, constituting a critical component in the global river‐atmosphere‐ocean carbon cycle. Therefore, anthropogenic management disconnecting rivers from their floodplains is expected to reduce the oxidative loss and to change the quality and quantity of riverine OC exported to the ocean. Here, we test this idea by combining two chronometers— 14 C age spectra of OC and optically stimulated luminescence ages of quartz—to interrogate sediments of the Lower Mississippi River (LMR) system to constrain the anthropogenic effects on carbon cycling in a continental‐scale sediment routing system. The 14 C age of the LMR OC has been reduced from >5,000 yr in prehistoric sediments to <3,000 yr in historic and modern sediments with significantly narrowed age spectrum width, following centuries of embanking the LMR. Bank stabilization reduced the river‐floodplain sediment exchange by ∼90%, effectively cutting off older floodplain OC from the river and reducing OC residence time in the severely truncated floodplain system, and expedited the downstream transmission of OC. The reduced residence time will have decreased riverine OC loss and enhanced younger OC delivery to marine sediments. We estimate that the oxidative loss of the LMR OC has been reduced by ≥ 1.1 Tg C/yr or 40%. Extrapolation to other large rivers that have undergone anthropogenic changes similar to the LMR illustrates that this process likely represents a carbon sink that can significantly increase if currently free‐flowing large tropical rivers are embanked in the future. Rivers play a key role in the global carbon cycle by releasing carbon dioxide to the atmosphere and controlling carbon transmission from land to ocean. How the carbon cycle has been affected by engineering activities that have fundamentally changed natural riverine processes is unclear. Using the Lower Mississippi River (LMR) as an example, we demonstrate that bank stabilization, which allows water and sediment to shoot through the river system directly into the ocean without interactions with the floodplain, can reduce river‐floodplain organic carbon exchange by 90%. Consequently, carbon transport is expedited through such river systems, which decreases the amount of organic matter that is converted to CO 2 and, rather, increases its delivery to seafloor sediments. The loss of carbon from the LMR has been reduced by ≥ 40% or 1.1 Tg C per year due to river embankment, suggesting a significant human‐caused shift in river‐atmosphere‐ocean carbon cycling that may occur in other large rivers as well. Embankment and bank stabilization cuts off old floodplain organic carbon (OC) sources and expedites OC transmission in the Lower Mississippi River (LMR) Reduced river‐floodplain exchange decreases the oxidative loss and increases the OC flux and export to the ocean by ≥ 40% in the LMR Embanking continental‐scale rivers likely represents a significant change in global river‐atmosphere‐ocean carbon cycling