δ13C Values Of Organic Carbon Research Articles

Paleoenvironment in Dae-Am San High Moor in the Korean Peninsula

We discuss paleoenvironmental changes at the Dae-Am San high moor, located near the Demilitarized Zone at 38°N. This area has been reported to be the only high moor in the Korean peninsula. The 14C age of the bottom sediment (75–80 cm in depth) at this site is about 1900 BP. Since the radiocarbon ages for the intervals at 50–55 cm and 75–80 cm were almost the same, we conclude that the deep layers (55–80 cm) in the high moor were all part of the original soil. Low organic C and N contents in the deeper layers support this inference. The 50–55-cm layer consists of sandy material with very low organic content, suggesting erosion from the surrounding area. The surface layer (0–5 cm) was measured as 190 BP, and the middle layer (30–35 cm) was 870 BP. The bulk sedimentation rate was estimated to be about 0.4 mm yr−1 for the 0–30-cm interval. The δ13C value of organic carbon in the sediments fluctuated with depth. The δ13C profile of the Dae-Am San high moor may be explained by climatic changes which occurred during the Little Ice Age and Medieval Warm Period.

Variscan and post-Variscan lead-zinc mineralization, Rhenish Massif, Germany: evidence for sulfide precipitation via thermochemical sulfate reduction

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydrocarbons contained in minerals. Application of this technique allows the identification of low-molecular-weight n-alkanes and some aromatic hydrocarbons in sulfides and gangue minerals from epigenetic Variscan and post-Variscan lead–zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguished by the composition of associated hydrocarbons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, n-alkanes in the C2–C9 range and aromatic hydrocarbons (benzene, toluene) are dominant in post-Variscan mineralization. The absence of high-molecular-weight hydrocarbons in ore minerals suggests highly mature gas associated with hydrothermal activity, during which hydrothermal fluids caused an increase in thermal maturation of organic matter and the generation of low-molecular-weight hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably governed by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 °C) generated predominantly methane, whereas basement brines (T < 175 °C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydrocarbons in sulfide precipitation via thermochemical sulfate reduction is indicated by geochemical characteristics of organic matter associated with the Plombieres Pb–Zn deposit, in eastern Belgium. Intense alteration phenomena were observed in near-ore kerogens, compared with unaltered kerogens far from the ore body, as well as by a very high maturity (5.40% Ro), a systematic depletion in 12C towards the vein-type mineralization, high atomic S/C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest that hydrothermal solutions caused a drastic increase in the thermal maturation of organic matter within the adjacent wall rock. Increased thermal maturation resulted in increased δ13C-values of organic carbon due to the preferential release of 12C. The change in the organic matter to a H-depleted and S-enriched bulk composition in association with sulfide ores strongly suggests that thermochemical sulfate reduction was responsible for organic degradation. Thus, thermochemical sulfate reduction probably triggered base metal sulfide precipitation in Variscan and post-Variscan ore deposits of the Rhenish Massif. Finally, based on data from this study and previous investigations, new genetic models are presented for both Variscan and post-Variscan mineralization in the Rhenish Massif.

Organic-matter source variation and the expression of a late Middle Ordovician carbon isotope excursion

Research Article| November 01, 1999 Organic-matter source variation and the expression of a late Middle Ordovician carbon isotope excursion Richard D. Pancost; Richard D. Pancost 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Katherine H. Freeman; Katherine H. Freeman 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Mark E. Patzkowsky Mark E. Patzkowsky 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (11): 1015–1018. https://doi.org/10.1130/0091-7613(1999)027<1015:OMSVAT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Richard D. Pancost, Katherine H. Freeman, Mark E. Patzkowsky; Organic-matter source variation and the expression of a late Middle Ordovician carbon isotope excursion. Geology 1999;; 27 (11): 1015–1018. doi: https://doi.org/10.1130/0091-7613(1999)027<1015:OMSVAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The carbon isotopic composition of organic matter is a powerful tool in the study of organic carbon burial and pCO2 through the geologic record. However, the organic-matter source can affect total organic carbon δ13C values and may complicate interpretation of environmental conditions. We examined the influence of organic-matter source variations on a late Middle Ordovician positive δ13C excursion in strata of the United States midcontinent. In a core from Iowa, the magnitude of the excursion recorded by carbonate carbon is ∼2.5‰, whereas the excursion recorded by total organic carbon is nearly 8‰. Although this difference could arise from a profound decrease in surface-water CO2 concentrations, association of the organic-walled microfossil Gloeocapsomorpha prisca with 13C-enriched total organic carbon (TOC) in this interval suggests that the excursion could also record a shift from normal-marine to G. prisca–dominated organic matter. Carbon isotope analyses of individual biomarkers indicate that both of these explanations are required to explain the data. Compounds inferred to be of G. prisca origin are enriched in 13C by 7‰ relative to compounds derived from other algal sources. Furthermore, compounds exclusively from either G. prisca or other sources exhibit a positive shift of only 3.5‰. Thus, the large excursion recorded by TOC reflects, in part, a change in the proportional contribution of G. prisca to TOC. That all compounds exhibit a positive carbon isotopic shift indicates that, independent of contributions from G. prisca, a positive organic δ13C excursion occurred and records a change in environmental conditions such as CO2(aq) concentrations. The 3.5‰ shift is similar in magnitude to that observed for TOC in correlative units from Pennsylvania, suggesting that this event has broad geographic significance. This work illustrates that organic-matter source variations as well as environmental conditions govern the TOC δ13C values and shows how compound-specific isotope analyses can resolve these multiple controls. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Organic carbon isotopic record across the Triassic-Jurassic boundary in Austria and its bearing on the cause of the mass extinction

Carbon isotope analyses of total organic carbon across the Triassic-Jurassic boundary in the classic section of Kendelbach, Austria, show that a positive excursion in δ13C values of organic carbon in the boundary marl correlates with a negative excursion in δ13C values in carbonates. In this section, the change in isotope values is inferred to be the result of diagenesis with no evidence of any primary change that could relate to bioproductivity. This and other evidence do not support the claim that there was a fall in productivity associated with the end-Triassic mass extinction, possibly caused by bolide impact.