The mangrove <scp>CO₂</scp> pump: Tidally driven pore‐water exchange

Abstract

Abstract Atmospheric carbon dioxide (CO 2 ) is fixed by mangrove vegetation and stored in its biomass and sediments. Part of the sediment carbon can be exported to coastal waters via tidally driven pore‐water exchange. Here, we quantify pore water‐derived dissolved CO 2 export using in situ, high‐resolution observations of 222 Rn and CO 2 over a spring‐neap tidal cycle in a mangrove‐fringed estuary (Coffs Creek, Australia). 222 Rn‐derived pore‐water exchange rates were 11.5–34.9 cm d −1 (23.0 ± 6.7) over 30 tidal cycles. Pore‐water exchange released CO 2 from intertidal sediment at rates of 61–213 (136 ± 43) mmol m −2 d −1 . This is equivalent to ~ 94% of the total CO 2 input into the estuary and approximately two times of the water‐atmosphere CO 2 emission. These observations reveal that tidal pumping is a major regulator of both mangrove pore‐water exchange and associated dissolved CO 2 export to the ocean. Combining our estimates with literature data, a first‐order global pore water‐derived dissolved CO 2 export from mangroves was estimated to be 83 ± 50 Tg C yr −1 . This is higher than an earlier estimates of global mangrove CO 2 emissions to the atmosphere (34.1 ± 5.4 Tg C yr −1 ) and carbon burial in sediments (18.4–34.4 Tg C yr −1 ), implying that pore water‐derived CO 2 escapes to the atmosphere within and beyond mangrove waters. Overall, CO 2 ‐rich pore water seems to be a widespread, important pathway of CO 2 into mangrove‐dominated estuaries and should be considered in mangrove carbon assessments in the context of global climate change and blue carbon.