Long-term Variability in Inorganic Sediment Contributions to the Great Marsh, Massachusetts

Abstract

FitzGerald, D.; Ryerson, O.; Hughes, Z.J.; Black, S.; Georgiou, I.; Hein, C., and Novak, A., 2020. Long-term variability in inorganic sediment contribution to the Great Marsh, Massachusetts. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 490–494. Coconut Creek (Florida), ISSN 0749-0208.In New England, the marsh platform sits above mean high water and is flooded about 8 times a month. This high marsh, dominated by Spartina patens, has a vertical accretion rate of ∼2.5 mm/yr, significantly lower than the 6-7 mm/yr accretion rate of low marsh, dominated by Spartina alterniflora. Accelerating sea-level rise will eventually cause the high marsh to transition to low marsh; the rate of this change is controlled, in part, by the contribution of inorganic sediment. Temporal and spatial variability of inorganic marsh sedimentation during the past ∼ 2.5 ka was examined at the Great Marsh, Massachusetts, using twenty cores, each ∼180-cm long, along five transects. Transects were aligned perpendicular to bays and major channels at different compass quadrants to assess influences of wind and tidal flow during storms. Cores were sampled every 20 cm to determine percent organic matter (loss on ignition) and grain size distribution (using a laser particle size analyzer). Data show a weak correlation for grain size versus distance from the nearest channel or bay. Additionally, no consistent vertical trends in grain size were observed, either within individual sampling sites nor among coring transects. Instead, we find coarsening upwards trends at some sites, likely due to increased channelization and tidal velocity as the marsh matured. Elsewhere, sediments fine upwards in response to deepening bays and channels. Finally, transects exhibiting variable grain size trends are likely influenced by ice rafted sediment. These results demonstrate the complexity of sedimentation on the marsh platform and the challenge in accurately predicting patterns of sediment transport using a single suspended grain size in hydrodynamic models in these systems.