The sediment budget of an urban coastal lagoon (Jamaica Bay, NY) determined using ²³⁴Th and ²¹⁰Pb

The sediment budget of Jamaica Bay (New York, USA) has been determined using the natural particle-reactive radionuclides ²³⁴Th and ²¹⁰Pb. Inventories of excess thorium-234 (²³⁴Th_xs, half-life = 24.1 d) were measured in bottom sediments of the Bay during four cruises from September 2004 to July 2006. The mean bay-wide inventory for the four sampling periods ranged from 3.5 to 5.0 dpm cm⁻², four to six times that expected from ²³⁴Th production in the overlying water column. The presence of dissolved ²³⁴Th and a high specific activity of ²³⁴Th_xs on particles at the bay inlet (∼30 dpm g⁻¹) indicated that both dissolved and particulate ²³⁴Th could be imported into the bay from the ocean. Based on these observations, a mass balance of ²³⁴Th yields an annual input of ∼39 ± 14 × 10¹⁰ g sediment into the bay. Mass accumulation rates determined from profiles of excess ²¹⁰Pb (half-life = 22.3 y) in sediment cores require annual sediment import of 7.4 ± 4.5 × 10¹⁰ g. Both radionuclides indicate that there is considerable marine-derived sediment import to Jamaica Bay, consistent with earlier work using ²¹⁰Pb. Such sediment input may be important in sustaining longer-term accretion rates of salt marshes in the bay.