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[1] The recent discovery of mass-independent fractionation of mercury isotopes allows new constraints to be placed on the mercury cycle. Here we report new Hg isotopic analyses of zooplankton and fish from different trophic levels of a freshwater lake (Lake Jackson, Florida) bearing systematic mass-independent fractionation of mercury isotopes. Fish muscle tissues show a progressive enrichment in the odd-mass mercury isotopes having odd atomic mass numbers (199 and 201) with increasing trophic level. Trophic level was determined based on nitrogen isotopic composition as well as fish stomach content. Zooplankton in the lake contain mercury with D Hg values increase by $1% from $+0.4% in zooplankton, juvenile bluegill, and several other small fishes to D 199 Hg = +1.36% for the Florida gar, which is the top predator fish in the lake. Previous observations of odd-mass-number isotope enrichment of mercury have been explained by photoreduction and demethylation of methyl mercury in the water column or as isotope effects related to microbial methylation. However, our data and the data of Jackson et al. (2008) are also consistent with in vivo production of mass-independent fractionation. Considering the alternatives, mass-independent fractionation by metabolic processes is the most straightforward explanation for the mercury isotope data. There are two known mechanisms for mass-independent fractionation of mercury, i.e., the nuclear volume effect and the magnetic isotope effect. While the data are insufficient to serve as proof, the magnitude of the mass-independent effect and the nearly equal enrichment of 199 Hg and 201 Hg seem most suggestive of a magnetic isotope effect.

Field and geochronological studies in the southern Appalachians reveal a space-time relationship of thrust and other large faults to their relative positions in the orogen, and their times of formation in relation to thermal-metamorphic peaks. Alleghanian thrusts of the Valley and Ridge-Cumberland Plateau are of the décollement type, resulting from compression of the foreland during the waning stages of mountain building. At least one pre-metamorphic thrust is known in the Blue Ridge. Other pre-, syn-and late metamorphic thrusts have been recognized in the Blue Ridge, Chauga belt and Inner Piedmont, related to Taconic (450–480 Ma) metamorphism. Several later thrusts in the Blue Ridge and the Gold Hill–Silver Hill fault in the Piedmont have been dated as Devonian. Syn- to post-metamorphic faults in the eastern Piedmont are Hercynian. These were developed during or following the Hercynian metamorphism, which overprints earlier events. Several large faults, notably the Brevard, record episodic movement histories spanning much of the Palaeozoic. The Brevard Fault had a pre- to syn-metamorphic (Taconic) movement history. Homogenization of Sr isotopes occurred in blastomylonites in the Brevard Fault after regional metamorphism, dated at 356 ± 20Ma. Both these early events involve ductile behaviour. Mylonites near the base of the Blue Ridge thrust sheet developed by 367 ± 20 Ma, yet elsewhere the allochthon locally overrides rocks as young as Carboniferous, indicating later (Alleghanian?) transport of the allochthon. Phyllonites in the Goat Rock–Bartlett Ferry fault zone are dated at c . 380±20Ma. One or more later events (Hercynian?) occur in the brittle realm. The Brevard Fault probably served as the root zone for early-to mid-Palaeozoic thrusts in the Blue Ridge. Later brittle deformation caused it to ramp over the rear of the Blue Ridge thrust sheet. There is a space-time transgression of thrusting in the southern Appalachians, beginning with early pre-, syn- and late metamorphic (Taconic) thrusting in the metamorphic core. Devonian (Acadian) thrusting and high angle faulting also affected the metamorphic core. Later (Hercynian–Alleghanian) faults are restricted to the flanks of the orogen. This space-time relationship of faulting to thermal peaks and position in the orogen should be observable in other well-exposed orogenic belts and other portions of the Appalachian–Caledonide system.

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