Heavy Stable Isotope Investigations in Environmental Science and Archeology

A mercury isotope technique was developed using a Neptune Multi Collector Inductively Coupled Mass Spectrometer (MC-ICPMS) in low resolution mode. The amount of sample used was approximately 10 nanograms of Hg and the external precision obtained for δ198Hg/202Hg by sixty replicate analysis of Almaden cinnabar is 0.55‰. The mercury extraction chemistry developed in this study for various kinds of environmental samples (peat, vegetation, soil and shale) has a very high yield (96% -100%) and there is no observed isotope fractionation induced during the extraction chemistry. This study also demonstrates that drying and heating of samples can significantly fractionate mercury isotopes. An ombrotrophic peat bog from Penido Vello, NW Spain has been analyzed for mercury isotopes and oxygen isotopes. The isotopic composition of mercury in the top of the core dominated by anthropogenic mercury is very similar to the isotopic composition of mercury from the bottom of the core dominated by pre-industrial mercury. This indicates that the Hg (0) emitted from the various sources probably homogenized quickly in the atmospheric pool and original isotopic source signatures were not preserved during deposition. A different explanation could be that since coal combustion, the main source of anthropogenic mercury releases all the mercury that has accumulated for thousands of years, essentially most of the modern anthropogenic mercury is representative of pre-industrial mercury, thereby showing no difference in isotopic composition. A third explanation could be that the analytical precision of the technique probably is insufficient to resolve meaningful isotopic variations that occur in mercury contributed from different sources. In the oxygen isotope record of the Penido Vello peat core, Medieval Warm Peiod (MWP), Little Ice Age (LIA) and Recent Warming (RW) are more pronounced whereas Roman Warming Period (RWP) and Dark Ages Cold Period (DACP) are less pronounced. Mass Independent Fractionation (MIF) of mercury isotopes produced by both Nuclear Volume Effect (NVE) and Magnetic Isotope Effect (MIE) could be an important tool in constraining models of mercury sources and pathways in the environment. Deviations from mass-dependent fractionation were calculated as the difference between the measured δ199Hg and δ201Hg (δAHgmeasured) of a sample and the δ199Hg and δ201Hg (δAHgMD) obtained from the linear scaling defined by the even-mass isotopes. The NVE induces a MIF in the odd-A Hg isotopes in the ratio, Δ201Hg/ Δ199Hg≈ 0.5, with the odd mass isotopes enriched in the reduced species such as Hg (0) and methylmercury and depleted in oxidized species such as Hg (II). The sediment samples and the Patagonia peat analyzed in this study are enriched in the odd-A Hg isotopes in approximately correct ratios for NVE. The Penido Vello peat core and Spanish moss samples are depleted in the odd-A Hg isotopes and exhibit Δ201Hg/ Δ199Hg≈ 1 which is consistent with MIE. Galena artifacts recovered at Late Archaic – Mississippian archaeological burial and habitation sites in southern and central United States have been provenanced by combining both lead and sulfur isotopes. The lead isotope database for the Mid Continental mineral deposits allows us to determine, with a high degree of certainty, the origin of galena artifacts found. Most of the seventeen archaeological burial/habitation sites contain galena artifacts which have lead isotopic characteristics that imply Central Missouri as a single source. Eight of the nine galena artifacts analyzed for sulfur show identical sulfur and lead isotope correlation with the Central Missouri lead district. From chemical and isotopic analysis, it seems that only certain areas especially the northern and northeastern North America have galena artifacts sourced from Upper Mississippi Valley lead district. There is no evidence, except for one artifact from Poverty Point site in Louisiana that southern and eastern USA had any galena originating from Upper Mississippi Valley. Further the evidence doesn't show of any significant temporal change in galena sources over a time span of thousand years in the southern and eastern USA.