Sources of sediment to the Neuse River estuary, North Carolina

The Neuse River estuary is part of the large Albemarle/Pamlico estuarine/lagoonal system (APES) of North Carolina. Exchange between the APES and the open ocean is restricted to inlets in the Outer Banks barrier islands. Freshwater discharge of the Neuse River is low in relation to the size of its estuary, so that the estuary is normally brackish over most of the area which accumulates fine-grained sediments. Mud (silt + clay) accumulation rates are low ( 6 mm year⁻¹).From the mud distribution and the dynamics of suspended sediment in the estuary it has been inferred that the Neuse retains fines efficiently, with probable episodic (high discharge) losses seaward, to Pamlico Sound.As part of a study of particle transport, deposition, and accumulation in the Neuse estuary, we have collected radiochemical data (10 cores) and chemical data (seven cores) on sediment cores from the main channel of the estuary between New Bern, North Carolina and the estuary mouth. K/Al increases in the lower estuary, consistent with earlier reports of increasing illite in the clay fraction. Landward transport of sediment from Pamlico Sound is the most plausible explanation for the increasing K/Al (illite). A marine sediment source may, therefore, be important for the lower estuary.The distribution of sediment inventories (quantities per cm² of sediment column) of particle-reactive radionuclides is consistent with the hypothesis of landward sediment transport. Inventories of excess ²¹⁰Pb, ¹³⁷Cs, and ^239,240Pu are coherent through the estuary and show the effects of particle redistribution processes on regionally uniform inputs. To obtain excess (anthropogenic) Zn and Cu, we use metal/Al normalization to estimate natural backgrounds. Anthropogenic metal inputs are concentrated at the head of the estuary, and sediment inventories of excess Zn and Cu generally decrease seaward. Normalization of the excess Zn and excess Cu inventories to excess ²¹⁰Pb inventories corrects the raw metal inventories for effects of lithology and sediment redistribution. The normalized excess Zn inventory decreases smoothly seaward, while the normalized excess Cu inventory shows an unexplained mid-estuary maximum. Low normalized inventories of anthropogenic metals at the estuary mouth indicate that little modern riverine sediment is stored there.Increasing K/Al (illite) and decreasing anthropogenic Zn and Cu in the lower estuary are thus both consistent with a predominantly marine sediment source. We conclude that landward transport of muds from Pamlico Sound has contributed significantly to the sediment balance of the lower Neuse estuary.