The rate and fate of coastal carbon burial Matthew L. Kirwan, Virginia Institute of Marine Science Main Points Marsh size Marsh accretion 1. SLR drives wetland carbon burial in vertical and lateral dimensions 2. Accelerated SLR enhances carbon burial until a threshold representing the very survival of the wetland carbon sink 3. Human impacts to wetland migration as important as climatic drivers Goodwin Island, York River, VA
1. Sea level rise drives wetland carbon burial (conceptual, model, and field evidence) Conceptually: Wetlands are depositional environments, whereas terrestrial forests and grasslands generally are not Sea level rise allows soil volume to expand through time Given sufficient SLR, little saturation effects McLeod et al., 2011
1. Sea level rise drives wetland carbon burial (conceptual, model, and field evidence) Numerical models of soil accretion: Many different models and sets of assumptions Flooding tends to enhance biomass and mineral sediment deposition rates: dz/dt = f(1/z) Kirwan et al. 2010, Geophys. Research Letters Accelerated SLR enhanced flooding accelerated accretion rates (soil volume expansion) Until marsh is so flooded that vegetation dies (threshold effect) Kirwan, Guntenspergen, D Alpaos, Temmerman, Morris, Mudd,, 2010 GRL
Organic matter accumulation (g/m2/yr) 1. Sea level rise drives wetland carbon burial (conceptual, model, and field evidence) Point-based carbon model, response to SLR and warming Model approach: Point based, soil cohort model simulating Spartina alterniflora growth, sedimentation, and organic matter production and decay. Response to IPCC sea level rise and warming. Fixed root:shoot ratio. plant death Kirwan and Mudd, 2012 Nature Key finding: accelerating sea level rise leads to sustained increase in carbon burial rates, much greater than effect of warming, until marsh drowns Mechanisms: Increased flooding duration enhances carbon production, soil accretion, and burial decreases decomposition
1. Sea level rise drives wetland carbon burial Herbert et al., in review 219 C Accumulation Rates from Ouyang & Lee (2014) + CRMS sites RSLR from tide gauges and interpolated LA subsidence rates (Jankowski et al. 2017) C accumulation associated with large increase in vertical accretion, only small increase in soil C density Effect of SLR >> local factors (temperature, tide range)
Modeled threshold SLR rate (mm/yr) 2. Threshold SLR rates for vertical marsh survival 5 differnt marsh models Large range of threshold rates Threshold rate of SLR increases with suspended sediment concentration and tidal range 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 Suspended sediment concentration (mg/l) Kirwan, Guntenspergen, D Alpaos, Temmerman, Morris, Mudd,, 2010 GRL
Modeled threshold SLR rate (mm/yr) 2. Threshold SLR rates for vertical marsh survival 5 differnt marsh models Large range of threshold rates Threshold rate of SLR increases with suspended sediment concentration and tidal range Overlap with IPCC rates, so drowning of some marshes 80 70 60 50 40 30 20 10 0 Range of IPCC SLRR projections at 2100 0 10 20 30 40 50 60 70 80 90 100 Suspended sediment concentration (mg/l) Kirwan, Guntenspergen, D Alpaos, Temmerman, Morris, Mudd,, 2010 GRL
Lateral changes in marsh size and carbon burial Edge erosion or progradation Upland Migration Marsh accretion Goodwin Island, York River, VA
Lateral changes in marsh size and carbon burial dx s /dt = k e W k a w s p 1 C Edge Erosion Mariotti and Carr, 2014 f(wind, bay depth, fetch) dz/dt = (a m + a o )/ρ m Vertical Accretion Kirwan and Mudd, 2012 f(sediment, depth, biomass) dx l /dt= R/m Upland Migration f(slr, slope) Carbon production and decomposition in soil cohort model Carbon exchange between bay and marsh platform, marsh migration tends to preserve forest carbon Initial marsh elevation and carbon profile in equilibrium with historical SLR rate Predict marsh size and carbon burial after 150 yr simulations Kirwan, Walters et al., 2016 GRL
Lateral changes in marsh size and carbon burial
Modeling Marsh Response to Sea Level Rise Lateral changes in marsh size and carbon burial Edge Erosion Upland Migration
Effect of Slope and SLR on Marsh Size For gentle slopes, marsh expansion rates increase until drowning For steep slopes or anthropogenic barriers, accelerated SLR yields inevitable contraction No migration case Drowning (Kirwan et al., 2016 GRL) Kirwan et al., 2016 GRL
Implications for carbon burial not as intuitive: does decreasing marsh size mean reduced carbon burial? Preliminary model results
Change in marsh carbon across transect Change in Marsh Organic (upland Carbon slope = -.001) No Underlying Forest SLR (mm yr -1 ) 15 10 5 Drowning Expansion x 10 4 1.5 X 10 4 1 0.5 0-0.5 dc(kg) Marsh carbon burial generally positive and accelerates with SLRR until threshold value Expansion: Enhanced C from migration Enhanced C from vertical accretion Enhanced C from eroded C 1 Contraction 10 50 100 SSC (mg L -1 ) Herbert et al., in prep -1-1.5 Drowning and Contracting: enhanced C accumulation in surviving marsh partially offsets eroded C
Conclusions and final thoughts on wetland migration Models and field evidence: some loss of existing marsh inevitable under future SLR. So maintenance and possible expansion depends on migration. Entire continental U.S.: 1m SLR = 11,000 km 2 new intertidal area; Existing marshland= 16,000 km 2 (Morris et al., 2012) Chesapeake Region: ~100,000 acres uplands converted to wetland since 1850 (offsetting historical erosion) (Schieder et al., in review) For moderate SLR: Natural marsh expands, dyked marsh contracts (Kirwan et al., 2016). Photo: Matt Kirwan Marsh (and carbon) response to SLR depends on human responseinteraction b/w SLR and human impacts as important as SLRR itself (Kirwan and Megonigal, 2013)
Acknowledgements Ellen Herbert, David Walters, Lisamarie Windham-Myers David Nicks, Mark Bessen Commission for Environmental Cooperation NASA Carbon Monitoring System USGS LandCarbon NSF Coastal SEES