Accumulation records of radionuclides and trace metals in two contrasting Delaware salt marshes

Abstract

Single cores from two Delaware salt marshes were used to reconstruct the historical deposition of trace metals (Cu, Pb, and Zn) and radionuclide tracers (U, 210Pb, and 137Cs) during the 20th century. Although the sedimentation rates of both cores were similar to the local sea-level rise (∼0.3 cm/yr), the two marshes have different depositional conditions. The Wolfe Glade salt marsh has maintained dense vegetation leading to almost complete preservation of the airborne-unsupported 210Pb, while the Great Marsh has a large fluvial soil input leading to some loss of unsupported 210Pb. The Wolfe Glade salt marsh accumulates more authigenic 238U from seawater but loses more 137Cs to seawater in association with larger organic decomposition relative to the Great Marsh. The reconstructed history shows that the atmospheric deposition of Cu, Pb, and Zn was maximum during the earlier 20th century, with a secondary peak during the 1970s for Pb. The anthropogenic stable Pb isotopic signature ( 206Pb/ 207Pb=1.187) at this depth confirms that the source of Pb for this maximum is a nongasoline source, discounting postdepositional remobilization. This study suggests a possible strong regional pollution for Pb, along with Cu and Zn, in eastern Delaware during the earlier 20th century.