Respiration in salt marsh sediments enriches pore water with dissolved inorganic carbon (DIC) that tidal pumping exports laterally to the coastal ocean, but the controls on this flux and its magnitude remain poorly constrained. Tamborski et al. (2021) quantified pore water exchange and DIC export at a microtidal Cape Cod marsh (Sage Lot Pond) with radium-isotope methods and posed four linked unresolved questions: (1) why pore water DIC concentrations differ strongly between adjacent marsh units with comparable exchange rates; (2) the magnitude, frequency, and biogeochemical consequences of storm-driven flushing of interior marsh DIC; (3) whether the observed positive correlation between mean sea level and pore water exchange generalizes across sediment types, tidal ranges, and water table depths, and whether sea-level rise will consistently increase or decrease lateral carbon fluxes; and (4) the 'true' magnitude of lateral DIC fluxes at regional and global scales. Global meta-analyses and syntheses now provide first-order answers to (4) and a statistical answer to the direction question in (3), while the mechanistic cores of (1)-(3) remain open. The problem - a mechanistic, climate-sensitive quantification of the marsh DIC outwelling term of the coastal carbon budget - remains open.

Background

Intertidal salt marshes are highly productive coastal wetlands that accumulate organic carbon in waterlogged, anoxic soils; this storage is the basis of the 'blue carbon' concept, under which tidal wetlands are conserved and restored for climate mitigation. A substantial part of the carbon fixed by marsh plants is not buried: microbial and root respiration in marsh sediments generates dissolved inorganic carbon (DIC - the sum of dissolved CO2, bicarbonate, and carbonate ions) in the sediment pore water, and the rise and fall of tides pumps this enriched pore water out of the marsh platform into tidal creeks and the coastal ocean. This recirculation is called pore water exchange (also tidal pumping or, in aggregate with fresh groundwater flow, submarine groundwater discharge), and the associated lateral carbon delivery is termed 'outwelling', after Odum's classical hypothesis that exported marsh material fuels coastal productivity. Along eastern North America, lateral carbon outwelling from tidal wetlands is estimated to exceed carbon burial by roughly a factor of five, with about half carried as DIC (Najjar et al. 2018).

The key measurement difficulty is that pore water exchange integrates over complex, spatially variable subsurface flow paths through marsh peat (the organic-rich marsh soil), creek banks, and the underlying sandy aquifer mixing zone (the 'subterranean estuary'). At the study site of the source - Sage Lot Pond, a fringing microtidal marsh (mean tidal range 0.4 m) on Cape Cod, Massachusetts, USA - this exchange had previously been constrained from tidal-creek radium mass balances (Wang et al. 2016; Chu et al. 2018). Tamborski et al. (2021) added a complementary sediment-side method, the 224^{224}Ra:228^{228}Th disequilibrium approach: the radium isotope 224^{224}Ra is far more soluble than its particle-bound radioactive parent 228^{228}Th, so a deficit of 224^{224}Ra in the upper centimeters of sediment measures how quickly pore water is flushed through them; the deficit integrates over about 10 days. Combined with seasonal tidal-creek time series interpreted through three-end-member mixing models that use the radium isotopes 224^{224}Ra and 226^{226}Ra to separate coastal-ocean, marsh-pore-water, and brackish-groundwater contributions, they showed that most of the DIC flux (1.7-5.4 g C m⁻² d⁻¹) issues not from the sediment surface but from the low-marsh sediment horizon intersected by mean low tide; that area-normalized DIC flux varies by a factor of about four between two adjacent drainage basins (marsh hydrologic units) in the same estuary; and that interannual variability in pore water exchange correlated with monthly mean sea level (r2=0.60r^2 = 0.60, 2015-2019), suggesting that continued sea-level rise may increase lateral carbon export at this site - the opposite of the decrease predicted by models of other marshes cited in that paper.

The same study reported the observation that motivates two of the open questions: a pore water profile taken one day after a moderate storm (35.4 mm rain, July 2019) in the interior of marsh site 2 showed about half the DIC concentration (9.0 mM) of a creek-side station sampled days earlier (17.8 mM), implying that storm waves and rainfall can rapidly flush DIC that had accumulated in slowly exchanged interior sediments - but such events had never been systematically sampled. The authors concluded by questioning what the 'true' magnitude of lateral DIC fluxes is at regional and global scales, given how few marsh units had ever been measured. Subsequent syntheses have confirmed that porewater-derived carbon export from salt marshes is a first-order term in coastal carbon budgets - comparable to or larger than sediment carbon burial, the quantity on which blue-carbon accounting currently rests (Santos et al. 2021; Chen et al. 2024) - which makes the four unresolved issues identified by Tamborski et al. (2021) the main obstacles to a predictive, mechanistically grounded marsh-to-ocean carbon budget.

Problem Statement

The problem, as posed by Tamborski et al. (2021) and still standing in part, is to close the pore water exchange-driven DIC outwelling term of the salt marsh carbon budget - jointly determining what controls it, how it responds to physical forcing, and how large it is - through four linked questions: (1) Controls on inter-unit variability: why do pore water DIC concentrations differ strongly between marsh hydrologic units that exhibit comparable pore water exchange rates - at the source site, deep pore water DIC at one unit was nearly twice that of the adjacent unit, contributing to a roughly fourfold difference in area-normalized DIC flux - and what are the relative roles of peat thickness, geomorphology (e.g., abundance of pannes - shallow unvegetated depressions - and pools, and position in the tidal frame, i.e., the elevation range spanned by the tides), and upland hydrology in setting the concentration available for export? (2) Event-driven dynamics: what are the magnitude, frequency, and biogeochemical consequences of pore water exchange and DIC export during and after extreme hydrologic events such as storms, given the preliminary observation of rapid flushing of interior sediment DIC? (3) Sea-level sensitivity: does the positive correlation between mean sea level and pore water exchange observed at the microtidal Sage Lot Pond site generalize to marshes with different sediment types, tidal ranges, and water table depths, and will long-term sea-level rise consistently increase or decrease lateral carbon fluxes across diverse marsh settings - and if the sign is setting-dependent, which marsh attributes determine it? (4) Scaling: what is the true magnitude of lateral DIC fluxes at regional and global scales, with what uncertainty, given the still-small number of studied sites? These four questions jointly support one scientific objective: a mechanistically grounded, climate-sensitive quantification of lateral inorganic carbon export from intertidal salt marshes that can be embedded in regional and global coastal carbon budgets. A complete answer need not come from a single study, but it must make the four components mutually consistent - for example, the controls identified under (1) and (3) must explain the spatial variance that syntheses under (4) propagate. All four questions are jointly required for a complete answer, and because they are linked but separately testable, a demonstrated, well-supported resolution of any single question is progress on the problem even while the others remain open.

References

  1. Tamborski, J. J., Eagle, M., Kurylyk, B. L., Kroeger, K. D., Wang, Z. A., Henderson, P., & Charette, M. A. (2021). Pore water exchange-driven inorganic carbon export from intertidal salt marshes. Limnology & Oceanography, 66(5), 1774-1792. https://doi.org/10.1002/lno.11721
  2. Wang, Z. A., Kroeger, K. D., Ganju, N. K., Gonneea, M. E., & Chu, S. N. (2016). Intertidal salt marshes as an important source of inorganic carbon to the coastal ocean. Limnology & Oceanography, 61(5), 1916-1931. https://doi.org/10.1002/lno.10347
  3. Chu, S. N., Wang, Z. A., Gonneea, M. E., Kroeger, K. D., & Ganju, N. K. (2018). Deciphering the dynamics of inorganic carbon export from intertidal salt marshes using high-frequency measurements. Marine Chemistry, 206, 7-18. https://doi.org/10.1016/j.marchem.2018.08.005
  4. Najjar, R. G., Herrmann, M., Alexander, R., Boyer, E. W., Burdige, D. J., Butman, D., Cai, W.-J., Canuel, E. A., Chen, R. F., Friedrichs, M. A. M., Feagin, R. A., Griffith, P. C., Hinson, A. L., Holmquist, J. R., Hu, X., Kemp, W. M., Kroeger, K. D., Mannino, A., McCallister, S. L., McGillis, W. R., Mulholland, M. R., Pilskaln, C. H., Salisbury, J., Signorini, S. R., St-Laurent, P., Tian, H., Tzortziou, M., Vlahos, P., Wang, Z. A., & Zimmerman, R. C. (2018). Carbon budget of tidal wetlands, estuaries, and shelf waters of eastern North America. Global Biogeochemical Cycles, 32(3), 389-416. https://doi.org/10.1002/2017GB005790
  5. Guimond, J., & Tamborski, J. (2021). Salt marsh hydrogeology: A review. Water, 13(4), 543. https://doi.org/10.3390/w13040543
  6. Santos, I. R., Burdige, D. J., Jennerjahn, T. C., Bouillon, S., Cabral, A., Serrano, O., Wernberg, T., Filbee-Dexter, K., Guimond, J. A., & Tamborski, J. J. (2021). The renaissance of Odum's outwelling hypothesis in 'Blue Carbon' science. Estuarine, Coastal and Shelf Science, 255, 107361. https://doi.org/10.1016/j.ecss.2021.107361
  7. Yau, Y. Y. Y., Xin, P., Chen, X., Zhan, L., Call, M., Conrad, S. R., Sanders, C. J., Li, L., Du, J., & Santos, I. R. (2022). Alkalinity export to the ocean is a major carbon sequestration mechanism in a macrotidal saltmarsh. Limnology & Oceanography, 67(S2). https://doi.org/10.1002/lno.12155
  8. Zhu, P., Chen, X., Zhang, Y., Zhang, Q., Wu, X., Zhao, H., Qi, L., Shao, X., & Li, L. (2022). Porewater-derived blue carbon outwelling and greenhouse gas emissions in a subtropical multi-species saltmarsh. Frontiers in Marine Science, 9, 884951. https://doi.org/10.3389/fmars.2022.884951
  9. Menendez, A., Tzortziou, M., Neale, P., Megonigal, P., Powers, L., Schmitt-Kopplin, P., & Gonsior, M. (2022). Strong dynamics in tidal marsh DOC export in response to natural cycles and episodic events from continuous monitoring. Journal of Geophysical Research: Biogeosciences, 127(7), e2022JG006863. https://doi.org/10.1029/2022JG006863
  10. Chen, X., Santos, I. R., Zhan, L., Yau, Y. Y., Zhang, Y., & Li, L. (2024). Large porewater exchange reshapes saltmarsh carbon and greenhouse gas budgets on local and global scales. Science China Earth Sciences, 67(7), 2195-2209. https://doi.org/10.1007/s11430-023-1305-6
  11. Ouyang, X., Maher, D. T., & Santos, I. R. (2024). Climate change decreases groundwater carbon discharges in global tidal wetlands. One Earth, 7(8), 1442-1455. https://doi.org/10.1016/j.oneear.2024.07.009
  12. Tamborski, J. J., Eagle, M. J., Thorpe, M. T., Charette, M. A., Kurylyk, B. L., Rahman, S., Kroeger, K. D., O'Keefe Suttles, J., Mann, A. G., Brooks, T. W., & Wang, Z. A. (2025). Evidence of mineral alteration in a salt marsh subterranean estuary: Implications for carbon and trace element cycling. Journal of Geophysical Research: Biogeosciences, 130, e2025JG008758. https://doi.org/10.1029/2025JG008758
  13. Eagle, M. J., Kroeger, K. D., Pohlman, J. W., Tamborski, J. J., Wang, Z. A., Brooks, T. W., O'Keefe Suttles, J., & Mann, A. (2025). The δ13C signature of dissolved organic and inorganic carbon reveals complex carbon transformations within a salt marsh. Journal of Geophysical Research: Biogeosciences, 130(6), e2025JG008898. https://doi.org/10.1029/2025JG008898
  14. Zhao, M., Zhang, Y., Wilson, A. M., Moore, W. S., Smith, E., Lang, S. Q., Sheng, C., Zheng, C., Li, H., Xu, B., Han, G., & Xiao, K. (2026). Large blue carbon export through submarine groundwater discharge: Insights from a marsh-creek basin system. Global Biogeochemical Cycles, 40, e2025GB008781. https://doi.org/10.1029/2025GB008781
  15. He, S., Maiti, K., Mize, S. V., Snedden, G., Groseclose, G., Myers, R. D., Ward, E. J., Driscoll, F. S., Hunter, G., Merino, S., Camus, E. G., Diz, A., Kroeger, K. D., Eagle, M. J., Shangguan, Q., & Stagg, C. L. (2026). Multi-year continuous lateral fluxes of dissolved carbon from a microtidal saltmarsh. Journal of Geophysical Research: Biogeosciences, 131, e2026JG009760. https://doi.org/10.1029/2026JG009760
  16. Zhang, M., Cai, X., Weston, N. B., Wu, Z., Giblin, A. E., Hunt, C. W., Tsao, S. S., Zhang, N. H., & Raymond, P. A. (2026). Spatiotemporal modulation of alkalinity and DIC outwelling from saltmarsh porewater in New England's largest marsh complex. Journal of Geophysical Research: Biogeosciences, 131, e2026JG009711. https://doi.org/10.1029/2026JG009711
  17. Yu, S., Li, X., Liu, J., Brunner, P., Yao, R., Yuan, B., Cai, X., & Yu, X. (2026). Effects of creek topology on salinization of coastal marsh due to storm surges. Water Resources Research, 62, e2025WR041838. https://doi.org/10.1029/2025WR041838