doi:10.1016/j.gca.2004.10.008 High-resolution historical records from Pettaquamscutt River basin sediments: 2. Pb isotopes reveal a potential new stratigraphic marker ANA LÚCIA LIMA, 1, *BRIDGET A. BERGQUIST, 2 EDWARD A. BOYLE, 2 MATTHEW K. REUER, 2,† FRANCIS O. DUDAS, 2 CHRISTOPHER M. REDDY, 1 and TIMOTHY I. EGLINTON 1 1 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (Received January 27, 2004; accepted in revised form October 14, 2004) Abstract—A high-resolution record of Pb deposition in Rhode Island over the past 250 yr was constructed using a sediment core from the anoxic Pettaquamscutt River basin. The sedimentary Pb concentration record shows the well-described maximum associated with leaded gasoline usage in the United States. Diminished Pb variability during recorded periods of local industrial activity (1735 to 1847) supports the greater importance of regional atmospheric lead transport vs. local inputs. The Pb isotopic composition at this site shows a clear maximum in anthropogenic 206 Pb/ 207 Pb in the mid-1800s. Similar peaks have also been observed in sediments from Chesapeake Bay and the Great Lakes, suggesting a common source. Possible causes for this event include mining and smelting of Pb ores in the Upper Mississippi Valley district, which accounted for almost all Pb production in the United States in that period. The timing of this event can provide an important stratigraphic marker for sediments deposited in the past 200 yr in the Northeastern United States. The downcore profile of anthropogenic 206 Pb/ 207 Pb provides a classic example of how changes in the mixture of ores for production of tetraethyl lead caused a regional-scale shift in the sedimentary record, and suggests that coal could have played a secondary role in Pb emissions after 1920. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Anthropogenic lead has been introduced into the environ- ment since refinement of lead-bearing sulfide ores and produc- tion of silver by cupellation were developed 5000 yr BP (Settle and Patterson, 1980). Natural processes like volcanic eruptions and rock weathering release Pb to the environment (Nriagu, 1978), but are insignificant compared to high-temper- ature processes such as utilization of leaded gasoline additives, nonferrous metal smelting, coal combustion, steel and iron manufacturing, and cement production (Nriagu and Pacyna, 1988). Because the main mechanism of Pb dispersion in the environment is atmospheric transport, anthropogenic Pb con- tamination is widely distributed and has been detected in polar ice caps (Hong et al., 1994; Rosman et al., 1997), peat bogs (Shotyk et al., 1998; Weiss et al., 1999), remote ponds (Shira- hata et al., 1980), corals (Shen and Boyle, 1987) and aquatic sediments (Graney et al., 1995). Since crustal material is, in general, more radiogenic than coals and ores, studies com- monly rely on the difference in isotopic composition among sources to discern the influence of natural and anthropogenic inputs to a site. While variations in Pb isotopic ratios are frequently used to apportion sources, the concentration profile of anthropogenic Pb has been used as a relative marker to corroborate sediment chronologies. Determination of accumu- lation rates and subsequent dating of sediments is vital in reconstructing historical records. Depending on the timescale of interest, the natural radionuclides 14 C (half-life = 5730 yr), 210 Pb (22.3 yr), 228 Th (1.9 yr) or 234 Th (24 d) can be used in determining sedimentation rates in freshwater and marine en- vironments (Koide et al., 1973; Robbins and Edgington, 1975; Anderson et al., 1988; Spliethoff and Hemond, 1996; Shotyk et al., 1998; Fuller et al., 1999). Lead-210 chronology is the most widely used technique for dating recent (100 –200 yr) sedi- ments for contamination studies, although it becomes imprecise after 100 yr. To validate 210 Pb dates, independent chrono- logical evidence is used whenever possible. Radionuclides de- rived from nuclear weapons testing (e.g., 137 Cs, 239 + 240 Pu) are the most reliable markers since their widespread deposition follows the well-documented history of atmospheric fallout (Anderson et al., 1988; Spliethoff and Hemond, 1996), but other chronological markers can also be employed. For exam- ple, the increase in abundance of ragweed (Ambrosia) pollen grains is interpreted to define the period when deforestation, agricultural development, or intense urbanization took place (Bruland et al., 1975). However, since this transition occurred at different times in different regions, the depth-age relation- ship is only valid within a narrow geographic area. In contrast, the appearance of certain organic contaminants (e.g., polychlo- rinated byphenyls [PCBs]) and the peak in Pb utilization oc- curred simultaneously in most of the United States and both can be used as relative markers in sediments from a variety of regions (Latimer and Quinn, 1996; Van Metre et al., 2000). Unfortunately, as with 137 Cs (peak in 1963), these chemical markers are only useful for sediments deposited in the last 60 yr (Latimer and Quinn, 1996). There is, therefore, a hiatus in chronological markers for records that span between 100 to 200 yr. In this paper, we report a large mid-19th-century peak in 206 Pb/ 207 Pb ratio in anoxic sediments from the Pettaquamscutt River, Rhode Island, and note that a similar feature was ob- served in sediments from the Great Lakes (Graney et al., 1995), Chesapeake Bay (Marcantonio et al., 2002), and possibly in * Author to whom correspondence should be addressed (alima@ whoi.edu). † Present address: Environmental Science Department, Colorado Col- lege, Colorado Springs, CO 80904 Geochimica et Cosmochimica Acta, Vol. 69, No. 7, pp. 1813–1824, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 1813