Carbon dioxide degassing and lateral dissolved carbon export during the unprecedented 2019 Mississippi river mega flood – Implications for large river carbon transport under future climate

Abstract

In 2019, the Mississippi River experienced the longest and largest flood that surpassed by far the 1927, 1973, and 2011 mega floods. This unprecedented event provided an unique opportunity to test the hypotheses that summer floods increase not only lateral dissolved carbon transport but also carbon dioxide (CO2) degassing, and that the increase in CO2 outgassing from river surface occurs mainly due to higher terrestrial carbon inputs and warmer temperatures. To test the hypotheses, we conducted intensive river sampling and in-situ measurements on partial pressure of CO2 (pCO2) and other water physical, chemical and biological parameters in 6-day intervals throughout the entire flood period (January 6th to August 5th, 2019). 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 measured at the same place from the recent past (1500 μatm ± 743), we found significantly elevated pCO2 in the river water during the flood period (2217 μatm ± 805; max = 3359 μatm), which was positively correlated with water temperature (ρ = 0.83). During the flood 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 2016–2018. A total of 3.73 Tg C DOC and 14.14 Tg C DIC was laterally transported into the Gulf of Mexico during the flood period, which were close to or greater than the previously reported annual averages. The findings support the initial hypotheses and strongly suggest that future climate change will intensify carbon cycling in world’s rivers. Our study adds firsthand results on land-sea carbon transport from by far the largest Mississippi River flood and helps constrain uncertainties in carbon budgeting of global rivers.