A 0.5-m-long core of sediment from Coburn Mountain Pond, Maine, U.S.A., has provided a virtually undisturbed and continuous record of sediment deposition for the last 200 yr in this small, essentially pristine tam. Organic carbon concentrations, C/N ratios, carbon isotopic compositions and extractable hydrocarbon molecular compositions indicate subtle differences in the sources of organic matter between sediments deposited before and after ca. 1950 but are otherwise constant. In contrast, total concentrations and molecular distributions of n-fatty acids and n-alkanols vary considerably. The fatty acid and alkanol geolipid fractions are both sensitive to early diagenetic alteration, yet the two fractions react differently. Concentrations and compositional parameters such as long/short, even/odd and branched/unbranched ratios fluctuate with sediment depth, and reflect amounts of microbial reworking which have varied repeatedly over the depositional history, n-Alkane distributions are relatively unaffected by the short-term changes and therefore better preserve the history ofbiolipid sources to the sediments.
Organic-carbon-rich "black shales" and adjacent organic-carbon-poor rocks from three different Cretaceous settings encountered during ODP Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich marlstone turbidites, large amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian/Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deep-water setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deep-water anoxia may have been involved.
The role of organic matter in the formation of metal ores has been investigated in the Nonesuch Formation at White Pine, Michigan. Organic matter in mineralized strata was characterized by isotopic, molecular and spectrophotometric procedures and compared to organic matter in non-mineralized rocks. Samples from the White Pine copper mine contain little organic carbon in comparison to samples from non-mineralized sections of the Nonesuch Formation. Residual organic matter in mine samples is depleted in solvent-extractable matter, is relatively aromatic in character, and appears to be thermally overmature, yet no difference in carbon isotopic contents exists between kerogcn isolated from mineralized and non-mineralized rocks. Comparisons suggest that oxidation of organic matter was involved in the reduction of oxidized metalliferous brines to precipitate copper sulfides and native copper and silver in this setting.
Organic-carbon-rich "black shales" from three different Cretaceous episodes sampled during Ocean Drilling Program Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich turbiditic marlstones, major amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian-Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated dcepwater setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which decpwater anoxia may have been involved.
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