Controls On Sediment Accretion And Blue Carbon Burial In Tidal Saline Wetlands: Insights From The Oregon Coast, Usa

Oregon estuaries provide important opportunities to assess controls on tidal saline wetland carbon burial and sediment accretion as both rates of relative sea level rise (RSLR; -1.4 0.9 to 2.8 0.8 mm yr(-1)) and fluvial suspended sediment load relative to estuary area (0.23 to 17 x 10(3) t km(-2) yr(-1)) vary along the coast. We hypothesized that vertical accretion, measured using excess Pb-210 in least-disturbed wetlands within seven Oregon estuaries, would vary with either RSLR or sediment load relative to estuary area, and carbon burial would correlate strongly to sediment accretion. Mean rates of high marsh accretion (0.8 0.2 to 4.1 0.2 mm yr(-1)) indicate that Oregon tidal wetlands have mainly kept pace with twentieth-century RSLR with the exception that the accretionary balance in the central coast is negative, suggesting drowning. Experiencing the fastest rates of RSLR, central-coast estuaries may foreshadow the fates of other Oregon estuaries under future accelerated sea level rise. Comparison of mass accumulation rates with sediment loads, however, indicates low trapping efficiency and therefore no fluvial sediment limitation. Thus, nonlinear feedback between RSLR and sediment accretion may enhance wetland resistance to drowning. Among wetlands keeping pace with or exceeding RSLR, sediment accretion displays no significant relationship with elevation but rather appears controlled by both the rate of RSLR and relative sediment load, highlighting the importance of incorporating both factors into future studies of tidal saline wetlands. Carbon burial rates, controlled by sediment accretion, will likely increase with future accelerated sea level rise.Plain Language Summary Salt marshes and brackish scrub-shrub and forested tidal wetlands, collectively known as tidal saline wetlands, provide valuable services such as habitat for wildlife, protection from coastal flooding, and carbon sequestration. Despite their importance, tidal saline wetlands are threatened by sea level rise and scientists must better understand what controls their growth so as to make predictions of their future survival. Additionally, sediment accumulation within tidal saline wetlands is an important control on the amount of carbon sequestered in coastal areas; thus, understanding how these systems will evolve in the coming decades may shed light on the global carbon cycle and climate change. New results from the Oregon coast indicate that both the local pace of sea level rise and the amount of sediment delivered by rivers control tidal saline wetland growth. Because sea level rise is relatively slow in Oregon and Pacific Northwest rivers deliver relatively high amounts of sediment to the coast, Oregon tidal saline wetlands may be resistant to future drowning. However, of the seven estuaries studied, two showed evidence of drowning over the last century; the reasons for this are unclear but may relate to the local pace of sea level rise or other processes not accounted for in this study.Key PointsOregon tidal saline wetlands kept pace with twentieth-century sea level rise, except in the central coast, the reasons for which are unclear In estuaries with a positive accretionary balance, both sea level rise and sediment load relative to estuary area control accretion Blue carbon burial rates, provided herein, are the first such measurements for Oregon tidal saline wetlands