Extreme floods increase riverine CO2 outgassing - A case study on the unprecedented 2019 Mississippi River flood

Abstract:

In 2019, the Mississippi River experienced the largest flood that surpassed by far the 1927, 1973, and 2011 mega floods. River water at Baton Rouge in south Louisiana, located 357 kilometers upstream of the river’s mouth at the Gulf of Mexico, stayed above flood stage consecutively 212 days from the 6^th of January through the 5^th of August. The extremely long-lasting flood discharged 632 km³ freshwater, 33% more than the annual average discharge in the past three decades (i.e., 474 km³). This unprecedented event provided an opportunity to test the hypotheses that summer floods not only increase lateral but also vertical transport of riverine carbon, and that increase in vertical transport, primarily carbon dioxide (CO₂) outgassing from river surface, occurs mainly due to higher terrestrial carbon inputs and warmer temperatures. To test the hypothesis, we conducted intensive river sampling and in-situ measurements on partial pressure of CO₂ (pCO₂) and other water physical, chemical and biological parameters in 5-day intervals during the entire flood period. Water samples were analyzed for the concentration of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and their carbon–13 isotopes. Compared to reported average pCO₂ from the past (1500 μatm±743), we found significantly elevated pCO₂ in the river water during the flood period (2214 μatm±812; max: > 7000 μatm), which was positively correlated with water temperature (r²=0.82). During the flood period with sustained high pCO₂, the lower Mississippi River released 230 Gg C from its last 357-km river reach, which is 18% greater than the annual CO₂ emission (195 Gg C) reported for the past three years. A total of 3.71 Tg DOC and 14.3 Tg DIC was laterally transported into the Gulf of Mexico during the flood period, which were close to or greater than the previously reported average annual DOC (3.95 Tg) and DIC (12.25 Tg) exports. The findings support the initial hypotheses and suggest that future climate change may intensify carbon cycling in world’s rivers.