Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes

The study addresses how intertidal wetlands (mangroves and saltmarshes) affect coastal carbonate chemistry and the balance between alkalinity (TA) and dissolved inorganic carbon (DIC) export (outwelling), with implications for coastal acidification and blue carbon sequestration. Coastal ocean pH is influenced by anthropogenic CO2 uptake and natural processes (e.g., upwelling, groundwater, riverine and wetland inputs). Intertidal wetlands store large carbon stocks and export carbon laterally, altering carbonate chemistry. Within sediments, aerobic and anaerobic respiration, as well as carbonate dissolution, generate DIC and TA at different ratios, and tidal pumping and bio-irrigation transport these products to surface waters. Because the TA:DIC ratio governs buffering capacity and pH response, quantifying TA and DIC outwelling from mangroves and saltmarshes is critical to evaluate their roles in coastal acidification and long-term carbon sequestration.

Prior work highlights the ocean’s buffering via alkalinity and the complexity of coastal carbonate systems compared to the open ocean. Processes like aerobic respiration (producing mainly DIC) and anaerobic pathways (denitrification, sulfate, Mn and Fe reduction) produce TA and DIC in different proportions. TA generated via anaerobic respiration represents a permanent TA increase only when reduced products are removed (e.g., pyrite formation or N2 production). Metabolic carbonate dissolution can also supply TA. Earlier compilations of marine buffer factors emphasized coral reefs and open ocean, with less attention to intertidal wetlands. The outwelling hypothesis has regained attention in blue carbon science, with studies showing wetlands as sources of inorganic and organic carbon to coastal waters and shelves, potentially enhancing CO2 outgassing and influencing pH. However, global budgets and the magnitude of wetland-driven carbonate inputs relative to riverine sources and other oceanic TA sources remained uncertain.

The authors compiled global datasets of TA and DIC in porewaters and surface waters from mangrove- and saltmarsh-dominated systems worldwide, including time series and spatial surveys (38 mangrove and 8 saltmarsh creeks/estuaries for TA/DIC observations). They analyzed TA:DIC relationships (slopes and ratios) normalized to site salinity, compared observed slopes to those expected from canonical sedimentary processes (e.g., aerobic respiration, denitrification, sulfate reduction, carbonate dissolution, Mn and Fe reduction), and examined controls (salinity, dissolved oxygen, DOC). They used tidal time series to quantify low- versus high-tide differences in TA and DIC as indicators of porewater inputs, alongside natural porewater tracers (e.g., radon) to assess porewater influence. Mixing models in selected estuaries (six mangrove-dominated systems) were used to partition TA and DIC inputs. They computed the buffer factor βH to quantify resistance to pH changes in porewaters and surface waters. For outwelling fluxes, the team compiled published TA and DIC outwelling rates (26 mangrove and 14 saltmarsh sites for DIC; 17 mangrove and 6 saltmarsh sites for TA), noted tidal/seasonal/event variability, and assessed geographic patterns. Global upscaling used area-weighted medians to estimate total DIC and TA exports from mangroves (140,000 km²) and saltmarshes (55,000 km²), and compared these to known global TA and DIC sources (e.g., rivers, groundwater, denitrification, silicate weathering). Carbon budget components (NPP, burial, CO2 outgassing, POC/DOC/DIC/TA outwelling) were synthesized to evaluate the fate of fixed carbon and remaining imbalances.

- Porewaters are enriched in TA (153–34,500 µmol kg−1) and DIC (844–28,200 µmol kg−1), about 2–3 times higher than surface waters (5–11,500 and 37–9390 µmol kg−1, respectively). - TA:DIC slopes (median ± SE) were 0.82 ± 0.07 in porewater and 0.75 ± 0.04 in surface water, consistent with mixed aerobic and anaerobic processes and some carbonate dissolution/denitrification influence. - Averaged per site, porewater TA:DIC ratios (0.90 ± 0.04) were slightly lower than surface water ratios (1.03 ± 0.01), and >70% of wetlands exported more DIC than TA, implying a tendency to acidify adjacent waters. - At 61% of sites with time series, both TA and DIC were higher at low tide, indicating porewater-driven outwelling; 76% showed larger DIC than TA tidal differences. Surface water pH decreased by 0.3 ± 0.1 units at low tide relative to high tide. - βH (buffer factor) was fivefold higher in porewaters than surface waters and 2–5 times higher than in oceanic waters, indicating high porewater buffering despite enhanced acidification potential from DIC exports. - Outwelling fluxes: global area-weighted DIC outwelling (median ± SE) was 81 ± 47 mmol m−2 d−1 (mangroves, n=26; average 155; range −97 to 1051) and 57 ± 104 mmol m−2 d−1 (saltmarshes, n=14; average 242; range −2 to 1200). TA outwelling was 81 ± 55 mmol m−2 d−1 (mangroves, n=17; average 134; range −1 to 951) and 25 ± 11 mmol m−2 d−1 (saltmarshes, n=6; average 26; range −2 to 69). - TA:DIC outwelling ratio median ± SE was ~0.8 ± 0.2 (range 0.1–4 in mangroves; 0.6–1 in saltmarshes), confirming DIC-dominated exports at most sites (71%). - In updated carbon budgets, DIC outwelling was the dominant fate of fixed carbon, exceeding sediment burial, CO2 outgassing, and POC/DOC outwelling. Yet, 28% (mangroves) and 47% (saltmarshes) of NPP remain unaccounted for. - Global upscaling: intertidal wetlands export 5.3 ± 3.2 Tmol y−1 DIC and 4.6 ± 2.8 Tmol y−1 TA. TA outwelling from wetlands equals ~7% of total ocean TA sources (71 Tmol y−1), exceeding individual sources like submarine groundwater discharge (1.0 Tmol y−1), denitrification (1.5), submarine silicate weathering (2.8), and organic matter burial (3.0). - Geographic/temporal variability: DIC outwelling from mangroves was ~2× higher in wet vs. dry seasons; macrotidal Chinese saltmarshes exhibited DIC outwelling up to 20× higher than microtidal U.S. saltmarshes. However, globally, environmental drivers showed minor/undetectable influences given current data.

Findings show that intertidal wetlands frequently export proportionally more DIC than TA, which tends to lower pH in adjacent coastal waters, particularly during low tide when porewater inputs are strongest. Although porewaters exhibit high buffering capacity (high βH), the relative excess of DIC export leads to local acidification, with magnitude modulated by seawater mixing, oxygenation, freshwater inputs, organic matter loading, and seasonality. The strong linkage between TA:DIC ratios and pH underscores how wetland-derived carbon alters coastal carbonate chemistry and the capacity of coastal waters to absorb anthropogenic CO2. Despite the acidification tendency, TA outwelling provides a long-lived oceanic carbon sink and contributes materially to the global alkalinity balance, highlighting wetlands’ dual role in both local acid-base dynamics and long-term carbon sequestration. Incorporating TA exports alongside DIC outwelling into coastal and global carbon budgets reframes the significance of mangroves and saltmarshes as hotspots of inorganic carbon processing, with implications for managing blue carbon strategies and predicting coastal ocean acidification trajectories.

This study synthesizes global observations to show that mangroves and saltmarshes commonly export more DIC than TA, often acidifying nearby coastal waters, while simultaneously delivering substantial TA that constitutes a long-term carbon sink. DIC outwelling emerges as a dominant fate of wetland NPP, surpassing sediment burial and atmospheric CO2 exchange, yet significant fractions of fixed carbon remain unaccounted for, likely including direct sediment CO2 emissions. At the global scale, intertidal wetlands contribute appreciably to ocean alkalinity (≈7% of total TA sources) despite their small area, and their aggregate DIC export is on the order of one-sixth of riverine DIC inputs. Future work should expand geographic coverage (South America, Africa, Europe, Southeast Asia), enhance seasonal and event-driven sampling, refine intertidal area estimates and outwelling methodologies, and trace the spatial footprint of wetland-driven acidification into continental shelf waters to better integrate these fluxes into global marine carbon and alkalinity budgets.

The dataset is geographically biased toward the USA, China, and Australia, with sparse coverage in several regions. Many outwelling estimates are based on short deployments (often ~two tidal cycles) with limited seasonal resolution; only 17 of 40 sites had seasonal data. Intertidal wetland area contributing to outwelling is uncertain, and different methods yield variable flux estimates. Environmental drivers showed weak global relationships, potentially due to data limitations. Site-specific factors (climate, geomorphology, hydrology, tidal amplitude) and episodic events introduce high variability, constraining the generalizability and precision of global upscaling.