A comprehensive, joint Soviet-American study of the chemistry of Lake Baikal, the world's deepest (1,632 m) lake, was carried out in July 1988. In this paper, we report the major, minor, and preliminary trace element concentrations for three profiles obtained at or near the deepest and central part of the three major basins of the lake. With the exception of Ba, the distributions of major and minor elements were homogeneous, displaying no variations greater than analytical uncertainties. Average concentrations in pmol kg-I (1 SD) are titration alkalinity = 1,093(6), SOd2-= 57.4(1.3), Cl = 12.3(0.7), Ca = 402(7), Mg = 126(l), Na = 155(4), and K = 24.1( 1 .O); and in nmol kg-' are Sr = 1,350(30), Li = 296(12), Ba = 74.7(2.6), Rb = 7.10(0.23), and U = 1.77(0.12). Excluding K and Cl, these values compare favorably with previously published results. Although some hydrothermal activity is known to occur within the lake, it does not appear to significantly affect major ion cycling. The residence times of the major ions are 330 yr or the same as that of water in the basin and so are controlled predominantly by their riverine fluxes. There is not yet enough information to assess whether hydrothermal processes affect minor element cycles. Ba concentrations decrease with depth, showing abrupt decreases near the bottom at two stations. It appears to undergo some form of uptake at the sediments, but further study is required to discern the processes governing Ba distribution. Lake Baikal, located in an intracontinental rift zone in the central region of southern I Present address: Centre National D'Etudes Spatiales,

Steady-state and time-resolved absorption and fluorescence measurements were used to probe the mechanism(s) of excited singlet state quenching by stable nitroxyl radicals in a series of fluorescamine-derivatized nitroxides. Rate constants for intramolecular quenching (k q) acquired from fluorescence lifetime measurements were very high, ranging from ∼0.3 × 1010 s-1 to ≥5 × 1010 s-1, and showed little dependence on solvent polarity. The k q's did not track the values of the Dexter and Förster spectral overlap integrals, thus indicating that energy transfer, through either mechanism, cannot account for the quenching. Quenching by electron transfer also appears unlikely, owing to the dependence of k q on solvent polarity and the complete lack of radical ion generation in highly polar solvents. Time-resolved absorption measurements revealed the presence of a very weak transient centered at ∼470 nm in the diamagnetic reference compounds that was not observed in the paramagnetic nitroxides; this transient was tentatively assigned to the excited triplet state of the fluorescamine moiety. The rapid singlet state quenching in this series of compounds thus appears to arise from enhanced internal conversion.

The work presented here consists of a literature review and calculations to estimate the importance of photochemistry to carbon cycling in the oceans, followed by a photophysical study of a series of stable nitroxide radical probes that have been used for the quantitative detection of individual carbon-centered radicals and reducing species in natural waters. Two appendices follow. The first contains preliminary experiments utilizing one of the nitroxide probes in an investigation of hydroxyl radical production rates and steadystate concentrations in seawater. The second consists of an investigation of the singlet lifetimes of humic acids (HA), in order to aid in understanding their photochemical cycling and influence on other compounds.The impact of photochemical reactions on global oceanic carbon cycling was calculated from literature values. The results indicate that between 1 and 13% of all dissolved organic carbon in the oceans is oxidized photochemically. This is a significant flux term, much larger than that of riverine input for example.A photophysical study of nitroxide radical probes was undertaken. For all of the compounds studied, steady-state absorption and fluorescence spectra were identical to those of the parent fluorophores. A decrease in fluorescence lifetime and quantum yield of tens-to hundreds-fold was observed for the paramagnetic compounds relative to their diamagnetic counterparts. Very rapid fluorescence quenching rates (3 to 80 x 1010 s-1 ) were calculated for the fluorescamine moiety of the paramagnetic nitroxide compounds in a variety of solvents. Calculated energy minimized geometries were very similar for all compounds which implies that geometric differences are not responsible for the variations found in fluorescence lifetimes and quantum yields between compounds. Calculated Firster and Dexter overlap integrals do not support deexcitation by these mechanisms. Time-resolved absorption measurements resulted in no evidence for transient species due to either intersystem crossing to the triplet state or charge transfer. Of the mechanisms considered, direct internal conversion to the ground state, is most likely given our results.An investigation of the utility of 3-(aminomethyl)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy free radical (3-amp) for detection and quantification of hydroxyl radicals in natural waters found that the addition of primary probe compounds resulted in the generation of secondary carbon-centered radicals that were successfully trapped by 3-amp. Competition kinetics experiments with dimethyl sulfoxide resulted in a natural scavenger rate constant that matched previous literature results for coastal seawater. As expected, the addition of formate resulted in decreases, and the addition of nitrite in increases, in the hydroxyl radical trapping rate by this method. The resulting quantum yield values were about an order of magnitude higher than previous literature results. However, probablyI IYIIIIYYI Wilkl 4 due to the use of different latitudes at which to estimat...