Lead concentrations in the northeast Pacific: evidence for global anthropogenic perturbations

Abstract

Lead concentrations were determined by isotope dilution mass spectrometry in 34 surface- and deep-water samples collected in the northeast Pacific between Hawaii and California and off the California coast using a deep-water sampler protected against fouling by contamination from the ship and hydrowire. Measured concentrations lie more than one order of magnitude below previously published open ocean values and they show that in most cases 90% or more of the total lead is in a dissolved form. Lead concentrations are about 10-fold higher in surface and thermocline waters than in deep waters; values drop as low as 1 ng/kg (5 pmol/kg) below 3500 m depth. Lead profiles thus appear different compared to those of most trace metals, which show enrichment in deep waters compared to surface concentrations. Lead concentrations in surface waters increase from 5 to 15 ng/kg (25 to 75 pmol/kg) along a transect starting from a location 200 km off the California coast and continuing towards the center of the North Pacific Gyre. This increase is congruent with that observed for ^(210)Pb concentrations in the same waters sampled at the same time. Lead is supplied to the open North Pacific largely from the atmosphere, at a rate of about 60 ng/cm^2 yr, which exceeds the prehistoric oceanic output flux of authigenic lead recorded in pelagic sediments about tenfold. This excess originates from emissions from smelters and combustion of leaded gasoline, overwhelming natural lead inputs that entered the ocean during prehistoric time probably largely through rivers. Vertical lead concentration profiles below the surface mixed layer are probably not in steady state. There, concentrations must be increasing in response to the increase of anthropogenic inputs because the estimated lead residence times are more than 20 years in the thermocline and about 80 years in deep waters. Based on an estimated 10-fold input and concentration increase since the mid-eighteenth century in the surface mixed layer in the central northeast Pacific, it is calculated that the 10 ng/kg average concentration between 100 and 900 m is 2 to 5 times larger, and the 1.8 ng/kg average concentration between 900 and 5000 m depth is about 2 times larger than it was in the mid-eighteenth century. Profiles of lead concentrations in the North Atlantic are expected to be shifted generally to larger values by a factor of 2 to 3 compared to those in the North Pacific because of the effects of greater industrial lead contamination, while lead concentration profiles in the South Pacific are expected to be shifted generally to lower values compared to the North Pacific by about this same factor because of the effects of lesser industrial lead contamination.

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