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

The ocean is a significant sink for anthropogenic carbon dioxide (CO2) emissions. Increased CO2 dissolution in the ocean causes ocean acidification, threatening marine organisms and ecosystems. Coastal ecosystems are particularly vulnerable, jeopardizing their services. Coastal water acidification is more complex than in the open ocean, with greater spatial and temporal variability. Besides anthropogenic CO2, local carbon sources like upwelling, groundwater, rivers, and wetlands impact the coastal carbonate system. Intertidal wetlands (mangroves and saltmarshes) are biogeochemical hotspots storing large amounts of carbon in sediments and laterally exporting it to the coastal ocean (outwelling), altering carbonate chemistry and seawater pH. These wetlands produce organic carbon, partially mineralized to release inorganic carbon as carbonate alkalinity and dissolved inorganic carbon (DIC). Exported total alkalinity (TA) represents a long-term carbon sink, buffering the coastal ocean against acidification, while exported DIC can enhance acidification. Different diagenetic processes produce TA and/or DIC in wetland sediments. Aerobic respiration produces mostly DIC, while anaerobic respiration produces both DIC and TA. TA produced during anaerobic respiration contributes to a permanent increase if coupled with the removal of reduced compounds. Porewater acidification from organic matter degradation can drive metabolic carbonate dissolution, producing TA. Tidal pumping and bio-irrigation transport DIC and TA-enriched porewater to surface waters and the coastal ocean. The TA:DIC ratio is a major property of carbonate chemistry, determining seawater's buffering capacity against external acid inputs. This study investigates whether mangroves and saltmarshes buffer coastal waters and re-examines their potential to sequester atmospheric CO2 by compiling TA and DIC contents in porewater and surface water from worldwide mangrove and saltmarsh systems.

Extensive research has established the significant role of mangroves and salt marshes in carbon sequestration. Studies have highlighted the substantial carbon storage in their sediments (e.g., 54 Tg C y−1) and the importance of lateral carbon export (outwelling) in influencing coastal carbonate chemistry and pH. Previous work has explored the production of total alkalinity (TA) and dissolved inorganic carbon (DIC) through various diagenetic processes in wetland sediments, including aerobic and anaerobic respiration, metabolic carbonate dissolution, and the role of tidal pumping and bioirrigation in transporting these components to coastal waters. The importance of the TA:DIC ratio in determining the buffering capacity of seawater and its implications for ocean acidification has also been well-documented. However, existing studies have often focused on individual processes or specific locations, leaving a gap in understanding the global-scale patterns and impacts of mangrove and saltmarsh carbon outwelling on coastal carbonate chemistry and carbon budgets. This study synthesizes data from a diverse range of global sites to address this knowledge gap.

This study compiled data on total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations in porewater and surface water from 45 mangrove and 16 saltmarsh sites worldwide. The data were obtained from time series and spatial surveys, and the sources of the datasets are listed in Supplementary Table 1. The study employed several analytical techniques, including analysis of porewater and surface water samples for TA and DIC concentrations and analysis of TA:DIC ratios. Multiple processes produce TA and DIC within sediments at different ratios, resulting in specific TA:DIC slopes. The study analyzed the TA:DIC regressions per site, normalized to the median salinity, to understand the combinations of TA and DIC production during aerobic respiration, sulfate reduction, denitrification and metabolic calcium carbonate dissolution. The relationships between TA:DIC ratios, salinity, dissolved oxygen, and radon were also explored to assess the contributions of porewater inputs and mixing with seawater. Tidal dynamics were analyzed using time series data to assess the influence of tidal pumping on TA and DIC export. Mixing models (Supplementary Figs. 4 and 5) were used to estimate TA and DIC inputs from estuarine waters. Furthermore, the study compiled TA and DIC outwelling fluxes from mangroves and saltmarshes to update the carbon budgets of these ecosystems, accounting for carbon burial, aquatic CO2 outgassing, and particulate and dissolved organic carbon outwelling. The influence of various environmental factors, such as temperature, precipitation, tidal amplitude, sediment characteristics and carbon accumulation rates on outwelling fluxes was examined. The global area-weighted DIC and TA outwelling were calculated using median values and upscaled to the global area of mangroves and saltmarshes. The study also considered the buffer factor βH to assess the buffering capacity of porewater and surface water against pH changes.

Analysis of porewater and surface water samples revealed that TA and DIC concentrations in porewaters were significantly higher than in surface waters. TA:DIC ratios were slightly higher in saltmarshes compared to mangroves, suggesting a combination of processes such as aerobic respiration, sulfate reduction, denitrification and metabolic calcium carbonate dissolution. In most intertidal wetlands, however, DIC production exceeded TA production. At 61% of the sites, surface water TA and DIC concentrations were higher at low tide than at high tide, indicating tidally-driven porewater export. Systems with greater tidal amplitudes had higher tidal TA and DIC ranges. Mixing models indicated higher DIC than TA inputs in most sites, potentially enhancing local acidification. Surface water pH decreased during low tides compared to high tides. The buffer factor βH was significantly higher in porewater than in surface waters or oceanic waters, highlighting the high buffer capacity of mangrove and saltmarsh porewaters. Analysis of outwelling fluxes showed that most sites exported more DIC than TA, with TA:DIC outwelling ratios ranging from 0.1 to 4 in mangroves and 0.6 to 1 in saltmarshes. Globally, DIC outwelling was the dominant carbon fate, exceeding carbon burial, aquatic CO2 outgassing, and particulate and dissolved organic carbon outwelling. However, a substantial portion of carbon fixed by net primary production remained unaccounted for. TA outwelling rates were higher than carbon burial rates, indicating TA outwelling as a significant carbon sequestration mechanism. Upscaling median values to the global area of mangroves and saltmarshes, revealed substantial DIC and TA export to the coastal ocean, comparable to riverine inputs. TA outwelling from coastal wetlands constituted a significant portion of the total TA sources into the ocean.

The findings highlight the significant role of mangroves and saltmarshes in coastal carbonate chemistry and carbon cycling. The dominance of DIC outwelling over TA outwelling in most sites suggests a potential for localized coastal acidification. The high buffer capacity of porewater minimizes this acidification potential, although the impact is highly site-specific. The unaccounted for carbon in the budgets suggests that additional research is needed to better quantify carbon fates, such as CO2 outgassing from exposed sediments and potentially other less-understood processes. The identification of TA outwelling as a significant carbon sequestration mechanism changes the understanding of blue carbon ecosystems, shifting perspectives on their contributions to the global carbon cycle. These results underscore the need for incorporating wetland contributions into global marine carbon budgets.

This study demonstrates that mangroves and saltmarshes are substantial sources of both DIC and TA to coastal waters. While DIC outwelling can lead to localized acidification, the high buffer capacity of porewater and significant TA outwelling mitigate this effect and contribute to carbon sequestration. The large amount of unaccounted-for carbon highlights the necessity of further research to fully understand carbon cycling in these ecosystems. Future research should focus on refining global carbon budgets, investigating seasonal variations across diverse regions, and examining the large-scale impact of wetland-driven acidification on marine biogeochemistry and ecosystems. Incorporating these findings into global marine carbon models is essential to accurately assess the role of coastal wetlands in climate change mitigation.

The study's global-scale analysis relies on existing data, which may have inherent limitations in terms of sampling frequency, spatial coverage and methodological consistency across sites. The uncertainties associated with outwelling estimations and the spatial heterogeneity of wetland characteristics could influence the accuracy of the global-scale estimates. Furthermore, the study's focus on inorganic carbon cycling might overlook other crucial biogeochemical processes and feedback mechanisms that influence coastal carbonate chemistry and carbon budgets. The lack of comprehensive seasonal data for many sites limits the ability to assess the temporal variability of these processes.