Alex Cabral, Yvonne Yau, Gloria Reithmaier, Luiz Cotovicz, João Barreira, Göran Broström, Bárbara Viana, Alessandra Fonseca, Isaac Santos

Mangrove soils are highly enriched in organic carbon. Tidal pumping drives seawater and oxygen into mangrove sediments during flood tide and releases carbon-rich porewater during ebb tides. Here, we resolve semi-diurnal (flood/ebb tides), diel (day/night) and weekly (neap/spring tides) drivers of porewater-derived CO2 fluxes in two mangroves and update global estimates of CO2 emissions. Tidal pumping controlled pCO2 variability within the mangrove creeks. The highest values of pCO2 (2,585-6,856 µatm) and 222Rn (2,315-6,159 dpm m-3) and lowest values of pH (6.8-7.1) and dissolved oxygen (1.7-3.7 mg L-1) at low tides were due to enhanced porewater export. 222Rn and pCO2 in mangrove porewater were respectively 4-15 and 38-41 times greater than surface waters. pCO2 increased by 50±30% from high to low tide, 9±22% from day to night and 57±5% from neap to spring tide with clear changes on hourly, diel, and weekly time scales. Both porewater-derived CO2 and water-air outgassing increased with tidal amplitudes (r2 = 0.34, p < 0.05). Combining our new estimates with literature data, global porewater-derived (16 sites) and water-atmosphere (52 sites) CO2 fluxes in mangroves would be upscale to 45±12 and 41±10 Tg C y-1, respectively. These fluxes account for 25% of net primary production and 238% of sediment carbon burial rates in global mangroves. Overall, our local observations and global compilation suggest that porewater-derived CO2 exchange is a major but often unaccounted source of CO2 in mangroves – which can be emitted to the atmosphere or laterally exported to the ocean – and should be included in carbon budgets to solve global imbalances.