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

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m−2 d−1 in mangroves and 57 ± 104 mmol m−2 d−1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.


Use of TA:DIC ratios
The ratio of CO₃²⁻ to HCO3 -is a major property of carbonate chemistry and determines the buffering capacity of seawater.This is reflected in the Revelle factor (and other buffer factors) that depends on the ratio of CO₃²⁻ to HCO3 -, which is proportional to TA:DIC (Egleston, et al. 1 ).Consequently, TA:DIC ratios drive pH changes and influence the capacity of seawater to take up anthropogenic CO2, affecting ocean acidification and carbon sequestration.
The change in pH with increasing TA is relatively minor (e.g., pH increases from 7.41 to 7.48 when TA increases from 2000 to 10000 µmol/kg at a fixed TA:DIC = 1) compared to the changes in pH associated with changing TA:DIC ratios (e.g., pH increases from 6.50 to 7.41 when TA:DIC ratio increases from 0.8 to 1 when DIC = 2000 µmol/kg, Figure S1 and 2).The linear regressions of TA:DIC ratios show a significantly positive trend using either pH or H + concentrations (Figure 2 and Figure S3).The scatter around TA:DIC ~ 1 is due to the minimum buffer capacity at this point, where a given increase in CO2 will cause a larger decrease in pH compared to when TA:DIC > 1 (Wang,et al. 2 ).Tamborski, et al. 3 Wang, et al. 2 Song, et al. 4 Chu, et al.Pérez-Lloréns, et al. 10 Chen, et al. 12 Yau, et al.Ray, et al. 16 Akhand, et al. 17 Akhand, et al. 18 Akhand, et al. 19 Akhand, et al. 20    Table S5 | Net primary production (NPP) and major carbon fates of mangrove and saltmarsh production presented in MgC/ha/y as median ± SE (average).The unaccounted carbon fate was calculated as total NPP minus the sum of the major carbon fates.Using averages for DIC outwelling would close carbon budgets, as evident from slightly negative unaccounted carbon fates, but averages are unlikely to be representative of the skewed dataset with clear outliers.The direct CO2 flux to the atmosphere and indirect production via DOC and POC respiration exceeds carbon burial in saltmarsh and mangrove sediments.The CO2 return to the atmosphere is clearly important from a carbon budget perspective but does not contribute to carbon sequestration.Carbon burial and alkalinity outwelling are the two key pathways storing atmospheric CO2, while CO2 flux to the atmosphere is a recycling term linked to primary production and respiration.

Fig. S4 |
Fig. S4 | Pristine mangrove estuaries are sources for TA and DIC.a, TA and (b) DIC plotted against salinity, and conservative mixing lines, measured during spatial surveys.Conservative mixing lines were estimated from TA and DIC concentrations at the lowest and highest salinities at each site.

Fig. S5 |
Fig. S5 | Four out of six pristine mangrove-dominated estuaries had larger estuarine DIC than TA inputs.The standard estuarine mixing model was used to calculate estuarine TA and DIC sources/sinks in surface water along six pristine mangrove estuaries with a clear salinity gradient.Deviations between TA and DIC concentrations, measured during spatial surveys, and conservative mixing lines were calculated as a percentage and averaged for each estuary.Positive excess values indicate a source within the estuary, whereas negative values indicate a sink.

Fig. S6 |
Fig. S6 | Seasonal changes of TA:DIC ratios in coastal wetlands and anthropogenic impacts.TA:DIC ratios measured during different seasons in porewater (PW) and surface water (SW) at (a) mangroves and (b) saltmarshes.c, TA:DIC ratios in pristine versus anthropogenically impacted sites.Outliers were excluded from the graph.

Fig. S7 |
Fig. S7 | Most intertidal wetlands had higher DIC than TA outwelling rates.a, Regression between TA and DIC outwelling and (b) TA:DIC outwelling ratios per site.

Fig. S8 |
Fig. S8 | Most studies measuring TA outwelling rates from intertidal wetlands were conducted in the USA and Australia.Rates are scaled to the interintertidal wetland area.The location of some sites was adjusted slightly to allow visualization.The precise coordinates are available on TableS3.

Fig. S12 .
Fig. S12.pH as a function of increasing alkalinity at fixed TA:DIC ratios.

Table S1 |
Information about sites with TA and DIC observations, including site ID, location, sampling type, i.e., and in porewater (PW) or in surface water measured during time series (TS) or spatial surveys (SV), number of samples, study length, season, and reference.Total sample numbers were 171 in mangrove porewaters, 1945 in mangrove surface waters, 243 in saltmarsh porewaters, and 940 in saltmarsh surface waters.

Table S2 |
TA:DIC ratios, TA:DIC slopes, TAn:DICn slopes per site in porewater (PW) and surface water (SW), and tidal range during study periods.

Table S3 |
TA and DIC outwelling rates from mangroves and saltmarshes, including method, location, season, and reference.

Table S4 |
57 and DIC outwelling rates per site (averaged single observations from TableS3).Average annual temperature and average annual precipitation were gathered from corresponding publications or nearest weather stations.Tidal range, sediment accumulation rate (SAR), and carbon accumulation rate (CAR) were retrieved from global datasets57.

Table S6 |
Global alkalinity balance of the ocean.