Browsing Radiocarbon, Volume 25, Number 2 (1983) by Subjects
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Anthropogenic Changes in Organic Carbon and Trace Metal Input to Lake WashingtonAn example of how man's contaminants are introduced, deposited, and retained in sediments giving a chronological record of events has been developed for Lake Washington, Seattle. Significant amounts of both inorganic and organic compounds in the environment originate from fossil fuel sources, such as power plants and motor vehicles. Many organic compounds are introduced also from contemporary biogenic materials. Through the combined carbon isotope analysis technique (CCIA), we can distinguish between fossil and contemporary carbon sources classes (using 14C), as well as sources within each class (using 13C). To establish the chronology of the organic carbon pollutant input to the lake sediment, the ages of the layers were determined using 210pb dating techniques. Sediment profiles of trace metals and a fallout radionuclide plutonium were also obtained and compared with the carbon isotope profiles. The results show that the total organic carbon (TOC) concentration correspond to 93% modern carbon before 1905. This 14C concentration in TOC decreased to ∼60% modern in the 1930's and now is between 95 and 80% modern. The lipid fraction is ca 30% modern and the total aromatic hydrocarbon fraction reached a minimum of 5% modern in 1954. The large decrease in 14C of TOC around 1930 is believed to be due to coal dust or fly ash. The trace metal concentration also increased substantially at this time. The pattern observed in the sediment thus reflects the change in the local energy consumption pattern from a predominately coal to an oil-based economy. From the plutonium profile we infer that mixing occurs for 3 or 4 years before the sediment layers are compacted.
The Radon Problem in 14C DatingDue to traces of radium and uranium in most 14C samples, radon appears as a radioactive contamination in the CO2 prepared by combustion. This contamination must be removed by an active purification prodecure or by storing the CO2 prior to measurement. No effective electronic discrimination against radon and its daughter elements can be performed. The necessary storage time until radon has decayed varies widely, especially for marine shells. The latter material, collected from Norway and Svalbard, has been a main object for the present investigation. In a few cases, a measureable amount of radon may be left even after eight weeks. The behavior of radon and its daughter elements in a CO2 proportional counter has been studied.