A meta-analysis of vertical accretion data in North American Coastal Marshes

Monday, 15 December 2014
James Robert Holmquist1, Lauren N Brown2 and Glen M MacDonald1, (1)University of California Los Angeles, Los Angeles, CA, United States, (2)Univ California Los Angeles, Los Angeles, CA, United States
North America’s coastal marshes are uniquely vulnerable to sea-level rise (SLR), and coastal subsidence (Torio and Chmura, 2013). While many studies have measured rates of vertical accretion in marshes, none have synthesized accretion rates to report basic summary statistics or make geographical comparisons. In this meta-analysis we synthesized accretion data, and SLR estimates when available, from 75 different sources reporting 704 measures from artificial plots, Cs-137 dates, Pb-210 dates, pollen horizons in sediment cores, and C-14 dates. Accretion generally decreased over longer time spans, likely due to a combination of shallow autocompaction, below ground carbon loss, and the recent acceleration of SLR. Artificial plots had an average time span of ~4 years and had the highest average accretion rate of all methods (7.8±9.9 mm/yr). Cs-137 and Pb-210 dates represented ~30 and ~100 years of accretion respectively, and had relatively moderate rates of accretion, 5.6±3.8 and 4.0±2.5 mm/yr respectively. C-14 dates had the lowest accretion rates (average = 2.5±2.5 mm/yr), and represented the longest records ranging from 206-9000 years. A subset of 15 studies (n=130) reported Cs-137 and tide gauge data in order to calculate accretion relative to eustatic SLR and coastal subsidence/uplift. A cursory analysis of these studies indicates that the subsiding regions of the Gulf Coast are the most vulnerable to SLR, while tectonically uplifting regions of the West Coast are the most resilient. The mapped deficits are overly optimistic as Cs-137 accretion rates do not fully represent net elevation change. Sediment elevation table and modeling data indicate that shallow autocompaction rates range from 1.2-27.8 mm/yr and net elevation gain ranges from -23.4 to 24.9 mm/yr in North America. Despite some methodological inconsistencies, this database indicates that the current relative SLR of ~10 mm/yr is contributing to massive submergence of Gulf Coast marshes. Models predict similar SLR estimates for the rest of the worlds’ coastlines over the next century, indicating the potential for massive coastal marsh loss in North America even where marshes are currently resilient to SLR.

Torio, D. D., & Chmura, G. L. (2013). Assessing Coastal Squeeze of Tidal Wetlands. Journal of Coastal Research, 29(5), 1049-1061.