Response of lead cycling in the surface Sargasso Sea to changes in tropospheric input

Abstract

Lead and its stable isotopes have been analyzed in surface water samples (0–600 m) and trapped particles collected at the Bermuda Atlantic Time Series Station (U.S. Joint Global Ocean Flux Study) in April and November 1989. These results are compared with wet atmospheric lead deposition as determined from precipitation continuously collected in Bermuda since August 1988 as pan of the Atmosphere-Ocean Chemistry Experiment program. Despite an expected seasonal variability, lead concentrations in surface waters have clearly decreased by 30 to 40% since 1979 in response to a corresponding decline by a factor of 5 to 8 of the tropospheric deposition. This result is corroborated by stable lead isotope measurements with 206Pb/207Pb ratios which are significantly less radiogenic (1.18–1.20) in the first 500 m of the 1989 profile than those measured in 1984 (1.20–1.21). This isotopic shift reflects changes of lead ore supply in the United States as well as a relative increase of the Eurafrican contribution to lead input in the northwest Atlantic that is likely due to the reduction of lead emissions from gasoline consumption in North America. This fast response of lead to interannual variations of the tropospheric input into surface waters is related to its efficient bioreactivity, as demonstrated with a sediment trap deployed at 150 m in April 1989. Sediment trap results show the rapid penetration of lead into the first 200 m associated with large particles during a period of high plankton activity. Retrospective isentropic air mass trajectories in three dimensions coupled with the precipitation events collected in Bermuda in 1988–1989 show that 30 to 40% of the annual lead deposition originates from the trade easterly meteorological regime. This input is clearly evidenced with lead isotopic signals observed in surface waters (0–100 m) in April and November 1989. We show that lead accumulated in the seasonal mixed layer (0–50 m) reflects this atmospheric input. Taking into account the isotopic signal measured in this mixed layer (1.178±0.001) as well as the respective contribution of the temperate westerlies and trade easterlies to atmospheric lead deposition to the Sargasso Sea, we calculate that the isotopic signature from the Eurafrican regions is 1.155 (±0.004). Based on these results, we calculate that the actual 206Pb/207Pb average ratio in surface waters of the Sargasso Sea is 1.188 (±0.004).

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