I. Ancient Arctic Ice Does Not Contain Large Excesses of Natural Lead. II. Chronological Variations in Lead and Barium Concentrations and Lead Isotopic Compositions in Sediments of Four Southern California Off-Shore Basins

Citation

Ng, Chihang Amy (1982) I. Ancient Arctic Ice Does Not Contain Large Excesses of Natural Lead. II. Chronological Variations in Lead and Barium Concentrations and Lead Isotopic Compositions in Sediments of Four Southern California Off-Shore Basins. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/mn3s-dd97. https://resolver.caltech.edu/CaltechETD:etd-08232006-143453

Abstract

Part I This study settled the dispute and proved beyond doubt that excess lead today is 300-fold instead of the 5-fold, as proposed by other investigators, greater than prehistoric time. And, virtually all of the present-day excess of lead above natural levels was shown to be caused by industrial lead emissions to the atmosphere.

Concentrations of lead and potassium were determined in a series of ice samples taken in sequence from the outside to the interior of several >2000 year old ice cores drilled from the Greenland and Antarctic ice sheets. Concentrations of lead and potassium were observed to decrease continuously in going from the exterior to the interior of the cores, which indicated that surface contamination had penetrated to the interior of the core, making it impossible to determine the original concentration of lead in the ice. Concentrations measured at the centers of these cores must therefore represent upper limits of lead concentration originally present in the ice. There are l.6 ng Pb/kg ice in 5000 year old Greenland ice and l.4 ng Pb/kg in 2000 year old Antarctic ice.

These data verified the earlier findings reported by Murozumi et al. (1969) of <1 ng Pb/kg ice in 800 B.C. Greenland ice near Camp Tuto and also at New Byrd Station. Our findings also support their observation of a continual progressive increase of lead concentration with time even before 1900 A.D. These new data refuted the high concentration values of 45 ng Pb/kg ice and 70 ng Pb/kg ice reported by Herron et al. (1977) and Cragin et al. (1975) in pre-1900 Greenland ice and their claims of no concentration change with time before 1900. These new data also refute the lead concentration values of 26 ng Pb/kg ice reported by Boutron and Lorius (1979) in snow strata of Dome C, Antarctica for the period 1914 to 1974 and their claims that this high lead concentration is natural and has been present since ancient times.

Potassium concentrations at the center of the ice core are 2 x 10³ ng K/kg ice in 5000 year old Greenland ice and 9 x 10² ng K/kg ice in the 2000 year old Antarctic ice. The decrease in potassium concentrations within the central third of the core was, unlike that for lead, relatively small, indicating the potassium contamination effects were not large within the central portions of the cores.

Part II This study documented the chronological changes of lead and barium fluxes from the Los Angeles Urban Complex to adjacent San Nicolas, Santa Barbara, Santa Monica, and San Pedro coastal basins; and identified industry as the sources of the excess lead and barium.

The natural deposition fluxes of acid leachable lead in Santa Barbara, Santa Monica, and San Pedro basins were about 0.7, 0.1 and 0.2 µg Pb/cm² respectively. A model, using clay as a major transport vehicle for soluble lead, was proposed to explain the large lead flux within the Santa Barbara basin compared to the other basins. Sedimentation fluxes of industrial and natural leachable lead within these three basins today are 3- to l0-fold greater, being about 2.1, 1.1 and 1.8 µg Pb/cm² yr respectively. Directly deposited large sewage particles account for 0%, 66%, and 75% of the industrial lead deposition fluxes respectively. Isotopic compositions of the excess leachable lead change in accordance with corresponding known changes of isotopic compositions of industrial lead in the Los Angeles atmosphere. Lead remaining in the acid leached sediment residues originates from igneous and clay minerals, exhibing no changes in concentration or isotopic composition since pre-industrial times.

Deposition fluxes of total barium among the three basins were proportional to mass deposition fluxes before 1950. Afterwards, there are barium concentration maxima with time in both Santa Monica and San Pedro basin sediments that are attributable to industrial sewage rather than to erosion from barium-rich sedimentary evaporite strata exposed locally along the shore. A slight increase of barium concentration in present-day Santa Barbara basin sediments may reflect dispersal of barium rich drilling mud from local drilling operations.

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