Controls on sediment accretion and blue carbon burial in salt marshes: Insights from the Oregon coast

Through ecogeomorphic feedbacks with vegetation, fluvial sediment supply and relative sea level rise interact to control the rates of sediment accretion and organic carbon burial in salt marshes. Better understanding of these interactions is needed to both predict the resilience of salt marshes to environmental stressors, and determine the role of estuaries in influencing the source-to-sink sediment routing system. Here, we use sediment cores collected across wetland elevation zones (mudflats, low marsh, high marsh, and scrub shrub) of four Oregon estuaries to reconstruct the past century of sediment and carbon accumulation. Our study sites are ideal natural laboratories to study the competing effects of sediment supply (6 to 340 x 10³ t y^-1) and rate of sea level change (-1.4 to 1.9 mm y^-1) as both differ between estuaries, but secondary characteristics (e.g., tidal range, water temperature, and vegetation density) do not vary. Results thus far show that sediment accumulation rates, measured by excess ²¹⁰Pb, ranged from 0.9 to 4.4 mm y^-1 (n = 38) and carbon burial rates ranged from 18 to 160 g C_org m^-2 y^-1. Carbon burial rates were closely linked to vertical accretion, highlighting the importance of determining factors affecting sediment accretion. Mean high marsh sediment accretion for each estuary was similar to relative sea level rise when rates were > 1 mm y^-1; however, mean values were surprisingly much greater (3.1 and 1.2 mm y^-1) than relative sea level when rates were neutral (0 mm y^-1) or negative (-1.4 mm y^-1). Within each estuary, accretion rates also varied with elevation, with some salt marshes displaying highest accretion rates at mid-marsh while others accreted most quickly at high marsh elevations. Thus, preliminary results suggest site-specific factors may control sediment accretion and carbon burial. To investigate this further, we are analyzing historic aerial photography as this can relate changes in horizontal accretion with sea level rise and sediment supply. Integrating results from this analysis with what we have learned thus far, we will discuss how such changes may impact the survival and carbon burial capacity of salt marshes.