Sediment Accumulation in Salt Marshes Across the Southeastern United States

The southeast coast of the United States is home to an invaluable landscape of salt marshes. Marshes are positioned as transition zones between salty and brackish coastal waters and the terrestrial environments of the coastal plain. They are of broad ecological significance, serving as primary production zones, marine life nurseries and natural filters, absorbing and regulating nutrient exchange between terrestrial and marine environments. They are also of great economic importance, functioning as flood controls and erosion prevention/reduction systems for the coast. Marshes grow, shrink and migrate in response to sediment accretion and erosion. The factors influencing sedimentation include influx of sediment from rivers and streams, sediment supply from storm events, in situ sediment production, oceanic influx, and sea-level rise. Rates of sedimentation are expected to vary across a single marsh, between individual marshes, and from one system of marshes to another. This study examines three southeastern U.S. salt marsh systems – Outer Banks of North Carolina, Georgetown area of South Carolina and Savannah area of Georgia, to evaluate how sedimentation in these three systems responds to fluvial and tidal influences. The estuarine systems where these marshes are located vary significantly in river input and tidal range, allowing for examination of relationships with these and other factors, e.g., development and erosion. Sediment accumulation rates were measured using 210Pb and 137Cs geochronology. There was great variability from marsh to marsh, and system to system. In individual marshes, shoreline cores generally had the highest accumulation rates, but this varied significantly depending on marsh morphology and the proximity of tidal creeks. Winyah Bay yielded the highest average accumulation rates, while accumulation rates in Ossabaw Sound were often more similar to those at Pea Island, NC, than to those in Winyah Bay.

美国东南海岸分布着宝贵的盐沼景观资源。盐沼作为咸水、微咸水沿岸水域与海岸平原陆地环境之间的过渡带，具有广泛的生态意义——既是初级生产区、海洋生物育苗场，也是天然过滤器，能够吸收并调节陆地与海洋环境间的养分交换。其经济价值同样显著，可作为海岸防洪设施与侵蚀防控系统。盐沼会随沉积物堆积与侵蚀发生扩张、收缩或迁移，影响沉积作用的因素包括河流输入的沉积物、风暴事件带来的沉积物补给、原位沉积物生成、海洋输入及海平面上升。沉积速率在单个盐沼内部、不同盐沼之间以及盐沼系统之间均存在差异。本研究选取美国东南部三个盐沼系统（北卡罗来纳州外滩、南卡罗来纳州乔治敦地区、佐治亚州萨凡纳地区），评估这些系统的沉积作用如何响应河流与潮汐影响。这些盐沼所在的河口系统在河流输入量与潮差方面差异显著，为探究其与开发、侵蚀等因素的关系提供了条件。研究通过210Pb与137Cs地质年代学（geochronology）测定沉积速率，发现不同盐沼及系统间的速率差异显著。在单个盐沼中，岸线岩芯的积累速率通常最高，但这一规律会因盐沼形态及与潮汐溪流的邻近程度而存在显著变化。Winyah湾的平均积累速率最高，而Ossabaw Sound的积累速率常与北卡罗来纳州Pea岛更为相似，而非Winyah湾。