finding 15.2 : key-message-15-2

Soil carbon accumulation rate (i.e., sediment burial) in North American tidal wetlands is currently 9 ± 5 Tg C per year (high confidence, likely), and estuarine carbon burial is 5 ± 3 Tg C per year (low confidence, likely).

This finding is from chapter 15 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Carbon burial, which accounts for all carbon accumulated in coastal sediments over an annual time period, has been documented for Key Finding 2, with geological approaches in multiple studies. Accumulation of carbon stock over a period of time using a marker horizon is relevant to specific periods of time by the method used (e.g., recent years, marker horizons, and radioisotope tracers of different decay rates). The data reported here refer to isotopes of cesium (137Cs) and lead (210Pb) dates alone, thus representing long-term average annual accretion rates for the past 50 years (since 1963). Rates of burial (Ouyang and Lee 2014; n = 125 samples) provide a range for comparison with other reviews that do account for mangrove subhabitats. No significant differences in carbon burial are detected for habitat types by salinity or vegetation type when comparing with Chmura et al. (2003) or with Breithaupt et al. (2014). Estuarine carbon burial is estimated for CONUS using the model of Herrmann et al. (2015) and scaled to all of North America using estimates of estuarine area.

New information and remaining uncertainties: Carbon burial rate is a bulk measure of multiple processes, both old and new carbon inputs as well as both autochthonous and allochthonous sources. As such, carbon burial through those processes has varied drivers, with different dominating processes across the landscape. Overestimation is possible when accretion of mineral sediment brings lower carbon densities than equilibrium conditions. Underestimates are possible when accretion is reported at historic rates and not adjusted for current rates of sea level rise. Mapped areas are a likely underestimate because they do not include freshwater tidal marshes in Canada or Alaska. Further, high uncertainties are associated with wide ranges of rates through different dating approaches. Estuarine carbon burial rate uncertainties stem from errors in the model of Herrmann et al. (2015) and, more importantly, the scaling of CONUS results to all of North America. Particularly problematic is the lack of rigorous mapping of estuarine extent outside of CONUS.

Assessment of confidence based on evidence: Because mapping limitations and 50-year averages of tidal wetland carbon accumulation are inferred rather than being the current rates under accelerated sea level rise, these estimates likely are lower than the actual rates of burial. Thus, while these data represent measured rates, this analysis relies on a fairly small range of locations and a small subset of available published data. Estuarine burial rates are not confident because Canada and Mexico have limited data applicable to the modeling strategy of Herrmann et al. (2015).

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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