Sediment and organic carbon focusing in the Shelikof Strait, Alaska

Abstract

Profiles of excess 210Pb and 137Cs in sediment cores were used to determine sediment accumulation rates in Outer Cook Inlet and the Shelikof Strait, Alaska. In Outer Cook Inlet and the northern Shelikof Strait, sediment accumulation rates averaged 0.16 cm yr − 1 and ranged from 0.10 to 0.21 cm yr − 1 . In contrast, sediment accumulation rates were > 4 times higher in the central and southern Shelikof Strait where the average was 0.68 cm yr − 1 with a range of 0.46 to 0.94 cm yr − 1 . In addition to higher rates of sedimentation, higher activities of excess 210Pb and higher levels of silt + clay, total organic carbon, Pb and Mn also were found in surface sediments from the central and southern Shelikof Strait relative to Outer Cook Inlet and the northern Shelikof Strait. These trends are directly related to the progressive sorting and focusing of sediments along the transport pathway from Cook Inlet to the Shelikof Strait. The flux of excess 210Pb and organic carbon to the sediments increased by > 2- and 4.8-fold, respectively, from Outer Cook Inlet and the northern Shelikof Strait to the central and southern Shelikof Strait. Based on δ 13C values for surface sediments, the amount of carbon derived from marine sources increased from ∼30% in Outer Cook Inlet and the northern Shelikof Strait to ∼60% in the central and southern Shelikof Strait. The good correlation between the accumulation rate of sediment organic carbon and excess 210Pb fluxes ( r = 0.93) suggests that scavenging of excess 210Pb by organic carbon is an important process in this region. Modern sediment budgets for Outer Cook Inlet and the Shelikof Strait indicate that > 70% of the total sediment accumulation of 61 × 10 6 tons yr − 1 is deposited in the central and southern Shelikof Strait. These sediment budgets are in good agreement with the calculated value of 70 × 10 6 tons yr − 1 of riverine and erosional sediment transported into Outer Cook Inlet and the Shelikof Strait. Collectively, the data support focusing of fine-grained sediments and organic carbon to the central and southern Shelikof Strait.