TEMPORAL AND SPATIAL VARIABILITY IN INORGANIC SEDIMENT CONTRIBUTIONS TO THE GREAT MARSH, MASSACHUSETTS

Salt marshes provide crucial ecosystem services, yet are now threatened due to accelerating sea-level rise. In New England, the marsh platform is mostly “high marsh,” which sits close to the mean high water level and floods a few times a month. This environment is characterized by Spartina patens and typically exhibits vertical accretion rates of ~2.5 mm/yr, significantly slower than the 6-7 mm/yr accretion rates commonly seen in low marsh (dominated by Spartina alterniflora). Even if these marshes are able to persist by transitioning to low marsh, ecosystems will change significantly and the protection they provide to inland areas during storms will vastly diminish. Addition of inorganic sediment to the marsh surface is an important factor contributing to vertical accretion and marsh stability. To study the temporal and spatial variability of inorganic marsh sedimentation at the Great Marsh in Massachusetts during the past ~2.5 ka, we examined 20, ~180-cm long sediment cores along five transects. Transects were aligned perpendicular to bays and major channels at different compass quadrants to capture influences of wind and tidal flow during storms. Cores were sampled every 20 cm; percent organic matter was based on loss on ignition and inorganic grain size distributions were determined using a laser particle size analyzer. The data show a strong correlation for both grain size (R² =0.94) and grain sorting (R²=0.58) with distance from the nearest channel or bay. These trends demonstrate that wave energy and current velocity decrease toward the marsh interior, thereby reducing sediment transport competency. However, we observe no consistent vertical trends in grain size either within individual sampling sites nor among coring transects. Rather, 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 having variable grain size trends are likely influenced by the introduction of sediment by ice rafting. These results demonstrate the complexity of sedimentation on the marsh platform and the challenge in accurately predicting patterns of sediment transport and marsh resilience using only a single suspended grain size in marsh hydrodynamic modeling.