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[Display omitted] • The 2019 mega Mississippi River flood transported 3.73 Tg C DOC and 14.14 Tg C DIC. • Average CO 2 degassing rate of 3.67 g C m−2 d−1 during the extreme flood was very high. • Increased floodplain connectivity and temperature affected dissolved carbon transport. • Exclusion of floods can lead to large uncertainties in river carbon budget estimates. 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 (CO 2) degassing, and that the increase in CO 2 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 CO 2 (p CO 2) 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 p CO 2 measured at the same place from the recent past (1500 μatm ± 743), we found significantly elevated p CO 2 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 p CO 2 , the lower Mississippi River released 230 Gg C from its last 357-km river reach, which is 18 % greater than the annual CO 2 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • The 2019 mega Mississippi River flood transported 3.73 Tg C DOC and 14.14 Tg C DIC. • Average CO 2 degassing rate of 3.67 g C m−2 d−1 during the extreme flood was very high. • Increased floodplain connectivity and temperature affected dissolved carbon transport. • Exclusion of floods can lead to large uncertainties in river carbon budget estimates. 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 (CO 2) degassing, and that the increase in CO 2 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 CO 2 (p CO 2) 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 p CO 2 measured at the same place from the recent past (1500 μatm ± 743), we found significantly elevated p CO 2 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 p CO 2 , the lower Mississippi River released 230 Gg C from its last 357-km river reach, which is 18 % greater than the annual CO 2 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. [ABSTRACT FROM AUTHOR]

• Mississippi River exported 3.95 Tg DOC and 12.25 Tg DIC annually between 2015 and 2018. • The Mississippi River basin had an annual flux rate of 1.4 g DOC and 4.2 g DIC m−2. • The river was regularly CO 2 -saturated (1500 ± 743 µatm), outgassing 654 g C m−2 yr−1. • DOC concentrations and p CO 2 were 10 and 24% higher than past studies, respectively. • Continuous increase in temperature and discharge will increase river carbon export. Air temperatures, precipitation, and river discharge have been steadily increasing in many river basins over the last century. This is especially the case with North America's largest river basin, the 3.2-million-km2 Mississippi River Basin, which comprises over 41% of the total land surface area in the contiguous USA. Yet we lack an understanding of how these changes will influence the export of terrigenous carbon through river systems into coastal waters and the atmosphere. This study investigated concentrations and mass fluxes of DOC, DIC, and CO 2 during 2015–2018 in the Lower Mississippi River. We found an annual riverine export of 3.95 Tg C and 12.25 Tg C for DOC and DIC into the Gulf of Mexico, with 75 and 67% of their respective loads discharged during the wet season from January through July. The riverine mass exports represent a flux rate of 1.4 g DOC–C m−2 and 4.2 g DIC–C −2 from the Mississippi River Basin, which are considerably higher than those reported for other river basins. Surface water of the Lower Mississippi River was constantly supersaturated by dissolved CO 2 , with its partial pressure (p CO 2) ranging from 526 to 3961 µatm and an average outgassing rate of 654 g C m−2 yr−1. Export of DOC and p CO 2 measurements were substantially greater than estimates reported over ten years prior, accompanied by significant increases in river discharge and air temperatures. These findings strongly suggest that DOC and CO 2 fluxes from the Mississippi River Basin into coastal waters and the atmosphere will very likely increase in the future due to increasing biological productivity and subsequent carbon export to rivers under increasing discharge and air temperatures. [ABSTRACT FROM AUTHOR]