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The proliferation of dams has impacted over 67% of the world's largest rivers, degrading ecosystems, severing river-watershed connectivity, and reducing water, sediment, and carbon supply from rivers to the coastal ocean by 20 to 100%. Many studies have focused on the downstream impacts of dams, but few have quantified the amounts and quality of organic matter stored within impoundments. This study examined organic carbon (OC) sources and accumulation in a reservoir in northern California to characterize the organic matter retained in terrestrial systems by these man-made structures and relate the amounts and quality of OC to mining, damming and flood events in the watershed. Englebright Lake was chosen for the study site because high sedimentation rates, resulting from 19th century hydraulic gold mining, offered high-resolution records of historic (1940-2002) change, detailed records of land-use and flood events provided a history of the watershed, and the construction of an upstream dam in 1970 offered the opportunity to analyze its hydrologic impacts. Sediments from Englebright Lake as well as soil, vegetation and aquatic samples from the lake and its watershed were analyzed as part of this study. Measured plutonium radioisotope profiles were used to calculate sediment and OC accumulation rates, and organic matter was characterized using total organic carbon (TOC) and nitrogen (TN) content, stable carbon and nitrogen isotopes (delta 13C and delta15N), and fatty acid, sterol, and lignin biomarkers. OC signatures from end-member samples were used to describe aquatic (plankton and algae) and terrigenous (soil and plant) sources and interpret sediment OC profiles in Englebright Lake. Mass accumulation rates of OC ranged from 0.8 to 216 kgoc m-2 yr-1, with the highest mass accumulation rates corresponding to flood events. During the 60-year record measured in this study, 036 Tg OC were buried in Englebright Lake, 64% of which was from terrigenous sources (based on data from a two end-member delta13C mixing model). Flood events in 1964 and 1997 had a significant impact on OC accumulation, and terrigenous biomarkers, including long chain fatty acids (LCFA), lignin phenols (Sigma8), and plant sterols (stigmasterol, campesterol, sitosterol), increased 2 to 5 times in delta front cores during these events. Aquatic biomarkers in bottomset deposits responded to the upstream dam construction, and concentrations of aquatic sterols (27-nor-24-cholesta-5,22-dien-3beta-ol and cholesta5,22-dien-3beta-ol) increased after 1970, while diacid concentrations (a terrigenous biomarker) decreased following the construction of the dam. Records of hydraulic mining impacts were observed as a thick layer of OC-poor (TOC < 0.1%) sediments accumulating between 1950 and 1980 in topset deposits. Overall, the OC sequestered in Englebright Lake suggests that the magnitude of the carbon sink provided by dams and impoundments is substantial, and should be considered in global carbon cycle budgets as well as in plans for future dam construction or removal projects.

Analyses of surface sediment samples from the Chesapeake Bay and sediment cores from the James and the Potomac River showed that pentacyclic triterpenoids of the hopanoid skeleton were ubiquitously present in all samples. The hopanoids have been identified and quantified by gas chromatographic retention data obtained on SE-52 stationary phase, and mass spectral comparisons with the branched/cyclic fraction of a Lorraine coal extract as well as published data from authentic standards. Hopanoid acids are of extended 17(beta)(H), 21(beta)(H)-structure, ranging from C(,31) to C(,33). The 17(beta)(H), 21(beta)(H)-bishomohopanoic acid (C(,32)) is always the major acid found in the samples. All acids were present as a single epimer (22R). The 17(alpha)(H), 21(beta)(H)-hopane series is predominant in all the samples, with lesser amounts of the 17(beta)(H), 21(beta)(H)-hopane series and some hopenes also present. The extended 17(alpha)(H), 21(beta)(H)-hopanes (> C(,31)) are found as mixtures of the 22R and 22S diastereomers. This indicates that there is a significant input of fossil hopanes into the Chesapeake Bay. Generally, high concentrations were found at river-mouth stations and in the northern Bay areas associated with industrial activities and intense urban development. These results are consistent with an anthropogenic source for the aromatic hydrocarbons present in the samples. Fossil hopanes appear to derive from a variety of sources including coal, crude oil, refined motor oil, asphalt particles and street dust. A comparison of hopanoid distributions in Bay sediments with possible source materials suggests that motor oil, asphalt particles and street dust are potentially important sources of fossil hopanes to the Bay. There is evidence that the input of hopanoids to surface soils is related to highway usage. These source materials and the associated fossil hopanes are reaching the Bay via natural and urban runoff, either directly or via river transport. Final accumulation in Bay sediments is evident from the elevated concentrations of fossil hopanes at river-mouth stations. These accumulations indicate that rivers are important sources of fossil hopanes to the Bay. An anomaly in the S/R ratio of the 17(alpha)(H), 21(beta)(H)-homohopane (C(,31)) in many sediment samples from the Bay is interpreted as evidence of a microbially induced isomerization of 17(beta)(H), 21(beta)(H)-C(,31) hopane (R) to 17(alpha)(H), 21(beta)(H)-C(,31) hopane (R). . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.

The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like ultramafic to carbonatite intrusion formed from multiple magma pulses. This produced a main pipe of pyroxenite, carbonatite and foskorite (olivineapatite-magnetite-calcite) and many smaller syenite plugs around the pipe. The complex is the oldest known carbonatite in southern Africa and the only one known to host economic copper deposits. There are no known detailed studies on multi-mineral oxygen and hydrogen isotopes or multi-rock carbonate carbon isotope data for the Phalaborwa Complex. These isotope systems are useful for identifying mineral disequilibrium and alteration, analyzing contamination and constraining the mantle source. Previous work has shown that the nearby Bushveld Complex and Karoo lavas have higher-than-average-mantle oxygen isotope values, and constraining the Phalaborwa Complex magma δ18O values could help place it within the context of major magmatism events on the Kaapvaal Craton. The range in mineral δ18O values for all rock types in the Phalaborwa Complex is 2.24 to 18.3‰, but 90% of diopside and apatite have values within a 1.5‰ range. The δD values for Phalaborwa Complex phlogopite range from -112 to -52‰. The δ18O values of baddeleyite (2.99 ± 0.13‰, 1σ, n=2), olivine (6.12 ± 0.08‰, 1σ, n=2), diopside (7.37 ± 0.40‰, 1σ, n=24), magnetite (3.46 ± 0.75‰, 1σ, n=8), apatite (6.40 ± 0.59‰, 1σ, n=13) and aegirine (6.39 ± 0.65‰, 1σ, n=10) are thought to be magmatic, with most δ18O versus δ18O plots for different mineral pairs signifying equilibrium at high temperatures. Alkali feldspar (9.30 ± 1.15‰, 1σ, n=10) and micas (8.56 ± 3.16‰, 1σ, n=24) have more variable δ 18O values and phlogopite has probably undergone subsolidus exchange or alteration. Magma δ18O values estimated from constituent minerals for pyroxenites (~6.8 to 8.3‰), foskorite (~7.1 &amp; 7.2‰) and carbonatite (~6.5 to 9.0‰) are generally higher than normal mantle magmas. The δ18O value of syenite magma estimated from aegirine is ~8.8 to 10.3‰, and in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). The phlogopite δD values are consistent with typical mantle magma water δD. Despite evidence for fluid-rock interaction in the carbonatite-foskorite rocks, carbonatite δ13C-δ18O range overlaps with the unaltered, primary igneous carbonatite field (δ18O 8.13 to 12.00‰, δ13C -3.19 to -5.60‰). Local basement rock has average bulk δ18O values of 8.6 ± 0.7‰ (1σ, n=14). This is too low to be a realistic contaminant capable of raising the δ18O values in the mafic rocks, since approximately 66% contamination would be required. The higher δ18O values in the whole-rock syenites are consistent with an origin by partial melting of metasomatized country rock. The high-δ18O values of Phalaborwa Complex pyroxenite-foskorite-carbonatite magmas are from a high-δ18O source, and are similar to those of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma) and Karoo picrites (183 Ma). Hence, it is concluded that the stable isotope data is consistent with a geographically extensive and long-lived high-δ18O mantle source beneath southern Africa.

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