Sediment accumulation along a glacially impacted mountainous coastline: north‐east Gulf of Alaska

Abstract

Tectonically active coastal regions of the world recently have been suggested to supply the bulk of sediment from land to the oceans. Seabed sampling on the continental shelf and in coastal embayments of the north‐east Gulf of Alaska (Alsek River to Prince William Sound) was performed to examine the temporal and spatial variability of sediment accumulation in a mountainous coastal setting. Cores of varying lengths (30–300 cm) were collected at 84 stations to provide information on sedimentary processes using radiochemical (210Pb and 137Cs) techniques. Four types of 210Pb activity profiles were observed, dominantly reflecting steady‐state sediment accumulation. However, nonsteady‐state profiles also were measured, resulting in part from episodic deposition near glacier‐fed rivers and on the Copper River Delta. Sediment accumulation rates in the eastern half of the study area are highest at midshelf depths (≈100 m) (≥10 mm yr−1) and near rivers draining the Bering Glacier (≈20 mm yr−1). On the Copper River Delta, sediment accumulation rates are highest for the delta front (> 20 mm yr−1) and decrease westward along the sediment dispersal route. Total annual sediment accumulation is 90–140×106 tons yr−1 on the shelf in the study area. Annual sediment accumulation for the total marine environment in the study area (including Icy and Yakutat Bays) exceeds 250×106 tons yr−1, potentially making this region the largest sink for sediment in North America. Spatial patterns in sediment accumulation on the shelf are similar between centennial and Holocene time‐scales, reflecting the dominance of the Copper River and Bering and Malaspina glaciers as sediment sources. Temporal variability in accumulation rates between centennial and Holocene time‐scales exists for portions of the study area near fiords and demonstrates the considerable changes that occur in sediment supply during glacial advances and retreats.