Industrialization and urbanization around San Francisco Bay as well as mining and agriculture in the watersheds of the Sacramento and San Joaquin rivers have profoundly modified sedimentation patterns throughout the estuary. We provide some constraints on the onset of these erosional disturbances with Be-10 data for three sediment cores: two from Richardson Bay, a small embayment near the mouth of San Francisco Bay, and one from San Pablo Bay, mid-way between the river delta and the mouth. Comparison of pre-disturbance sediment accumulation determined from three C-14-dated mollusk shells in one Richardson Bay core with more recent conditions determined from the distribution of Pb-210, and Th-234 [Fuller, C.C., van Geen, A., Baskaran, M., Anima, R.J., 1999. Sediment chronology in San Francisco Bay, California, defined by Pb-210, Th-234, Cs-137, and Pu-239,Pu-240] shows that the accumulation rate increased by an order of magnitude at this particular site. All three cores from San Francisco Bay show subsurface maxima in Be-10 concentrations ranging in magnitude from 170 to 520 x 10(6) atoms/g. The transient nature of the increased Be-10 input suggests that deforestation and agricultural development caused basin-wide erosion of surface soils enriched in Be-10, probably before the turn of the century. (C) 1999 Elsevier Science B.V. All rights reserved.

We report measurements of the isotopic composition of lithium in basalts using a multicollector magnetic sector plasma-source mass spectrometer (MC-ICP-MS). This is the first application of this analytical technique to Li isotope determination. External precision of multiple replicate and duplicate measurements for a variety of sample types averages +/-1.1 parts per thousand (2 sigma population). The method allows for the rapid (similar to 8 min/sample) analysis of small samples (similar to 40 ng Li) relative to commonly used thermal ionization methods. The technique has been applied to a suite of samples from Kilauea Iki lava lake, Hawaii. The samples range from olivine-rich cumulitic lava to SiO2- and K2O-enriched differentiated liquids, and have delta(7)Li (per mil deviation of sample Li-7/Li-6 relative to the L-SVEC standard) of +3.0 to +4.8. The data indicate a lack of per mil-level Li isotope fractionation as a result of crystal-liquid fractionation at temperatures greater than 1050 degrees C, This conclusion has been tacitly assumed but never demonstrated, and is important to the interpretation of Li isotope results from such geochemically complex environments as island arcs. Copyright (C) 1999 Elsevier Science Ltd.

An evaluation of early solar system chronometry by the Pb-Pb and U-Pb methods is provided. Specifically, three consequential factors are examined: procedure of age calculation, extent of terrestrial Pb contamination, and initial Pb isotopic composition. On a Pb-Pb diagram, high temperature inclusions of the Allende meteorite are tightly organized into a well-defined line (inside a potentially dispersive mixing field), which is consistent with the inclusions containing initial Pb that is more primitive than that of Canon Diablo troilite (PAT). Consequences of the possible existence of a pre-PAT Pb to the evolution history of the solar nebula are discussed.

Trace element concentrations and B isotope compositions were determined for lavas from the Eastern Volcanic Front (EVF), the Central Kamchatka Depression (CKD), and the Sredinny Range (SR) in the Kamchatka are. Trace element ratios and delta B-11 values of the EVF lavas show across-arc variations that are consistent with those observed in the adjacent Kurile are: B/Nb, Pb/Nb, and delta B-11 are highest at the volcanic front (14 to 22, 2.0 to 2.4, and +4.9 to +5.6 parts per thousand, respectively) and systematically decrease with increasing slab depth (to similar to3, similar to0.9, and similar to2 parts per thousand, respectively) whereas Rb/Nb, Ba/Nb, K/Nb, Be/Nb, and Li/Zr do not vary significantly across the are. The values for the SR (B/Nb = 1.7 to 2.0, Pb/Nb = 0.9 to 1.2, and delta B-11 = -3.7 to -1.1 parts per thousand) located far behind the EVF are consistent with these across-are trends. However, the CKD lavas exhibit elevated B/Nb (6 to 13) and delta B-11 values (-0.6 to +3.6 parts per thousand) although the slab depths are greater than at the EVF, Despite variations in B/Nb and delta B-11, the values for all these three volcanic zones of Kamchatka can be explained in terms of simple mixing between a depleted mantle wedge and slab-derived aqueous fluids with a relatively homogeneous delta B-11 (+6.2 +/- 0.5 parts per thousand). This mixing trend resembles those observed in Kurile, Izu, and Mariana, suggesting that fluids derived from altered oceanic crust and/or serpentinized peridotite in the subducting slab predominate in the slab-mantle interaction beneath these western Pacific arcs. Given this mixing relationship, the high B/Nb and delta B-11 values observed in the CKD indicate particularly large fluid influx beneath this region, which may partly result from subduction of the altered crust of the Emperor seamount chain in addition to the altered Mid-Ocean Ridge Basalt (MORB) crust. The compositions of the SR lavas in this study are consistent with melting of a weakly fluid-metasomatized mantle. However, the origin of magma in this volcanic zone is complicated, judging from the variable chemical characteristics reported for the SR lavas, Copyright (C) 2001 Elsevier Science Ltd.

Complementary, double- and single-resonance solid-state (H-1 and C-13) nuclear magnetic resonance (NMR) experiments were performed on a solvent extracted and demineralized sample of Murchison meteorite organic macromolecule. These NMR data provide a consistent picture of a complex organic solid composed of a wide range of organic (aromatic and aliphatic) functional groups, including numerous oxygen-containing functional groups. The fraction of aromatic carbon within the Murchison organic residue (constrained by three independent experiments) lies between 0.61 and 0.66. The close similarity in cross-polarized and single-pulse spectra suggests that both methods detect the same distribution of carbon. With the exception of interstellar diamond (readily detected in slow magic angle spinning single-pulse NMR experiments), there is no evidence in the solid-state NMR data for a significant abundance of large laterally condensed aromatic molecules in the Murchison organic insoluble residue. Given the most optimistic estimation, such carbon would not exceed 10%, and more likely is a fraction of this maximum estimate. The fraction of aromatic carbon directly bonded to hydrogen is low (similar to30%), indicating that the aromatic molecules in the Murchison organic residue are highly substituted. The bulk hydrogen content, H/C, derived from NMR data, ranges from a low of 0.53 +/- 0.06 and a high of 0.63 +/- 0.06. The hydrogen content (H/C) determined via elemental analysis is 0.53. The range of oxygen-containing organic functionality in the Murchison is substantial. Depending on whether various oxygen-containing organic functional groups exist as free acids and hydroxyls or are linked as esters and ethers results in a wide range in O/C (0.22 to 0.37). The lowest values are more consistent with elemental analyses, requiring that oxygen-containing functional groups in the Murchison macromolecule are highly linked. The combined H-1 and C-13 NMR data reveal a high proportion of methine carbon, which requires that carbon chains within the Murchison organic macromolecule are highly branched. Copyright (C) 2002 Elsevier Science Ltd.