Dating Recent (200 Years) Events in Sediments from Lakes, Estuaries, and Deep Ocean Environments Using Lead-210

Abstract

Sediments from lakes, continental margins and deep oceans record events caused by man and by natural processes and, by using radioactive tracers, their chronological record can be determined. The rhythmic pattern of sedimentation rates and benthic mixing processes has been determined using 210Pb at lowland lakes of Washington State, at the Puget Sound Estuary and at two deep ocean sites - one 60 km off San Francisco at 1000 m and the other 350 km off New York City at 4000 m. The Pb input as a function of time is shown, for example, to increase from background to some 30 times background at Lake Washington (Seattle), and 13 times background at Sinclair Inlet (near a naval shipyard) of Puget Sound. The dates for maximum inputs of other trace metals into Sinclair Inlet were established; their relative enrichments above background were Cu..8.0 Zn..2.7, Ni..1.9, Cr..2.2. Man's effect on the Lake Washington watershed due to deforestation and urban development has resulted in three changes in sedimentation rates since 1850. The 210Pb profiles in cores collected at the mouth of the Hudson Canyon (4000 m deep) show wide variations which may be due to sediment redistribution by deep currents and by biological mixing. An episodic event in the canyon may be registered synchronously, 100 years ago, at two stations; one at 2800 m and one at 4000 m deep. By measuring 239,240Pu in deep sea cores together with 210Pb, the past 30 years since the bomb testing can be identified since it is generally assumed that both 210Pb and 239,240Pu are associated with particulate matter. The deposition inventory for 210Pb and 239,240Pu shows that redistribution processes are active at the deep ocean stations and that 210Pb serves as a time dependent tracer for both advective transport and biological mixing of sediments.