Organic carbon proxies in black shales; molybdenum

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doi: 10.1016/j.chemgeo.2003.12.005
Author(s): Wilde, Pat; Lyons, Timothy W.; Quinby-Hunt, Mary S.
Author Affiliation(s): Primary:
Pangloss Foundation, Berkeley, CA, United States
University of Kentucky, United States
University of Missouri at Columbia, United States
Lawrence Berkeley Laboratory, United States
Volume Title: Geochemistry of organic-rich shales; new perspectives
Volume Author(s): Schultz, Richard B., editor; Rimmer, Susan N.
Source: Geochemistry of organic-rich shales; new perspectives, edited by Richard B. Schultz and Susan N. Rimmer. Chemical Geology, 206(3-4), p.167-176. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541 CODEN: CHGEAD
Note: In English. 51 refs.; illus., incl. 2 tables
Summary: Isotopic and elemental proxies are useful for discerning the original compositions of ancient rocks subject to later diagenetic/thermal alteration, low-rank metamorphism, outcrop weathering, etc. Recent work in the Cariaco Basin [Chem. Geol. 195 (2003) 131] has shown a high correlation between total organic carbon (TOC) content and Mo normalized to Al in these modern euxinic sediments: microlaminated, dark olive gray, silty clay (0-11.6 kyr BP), %TOC = 1486*(Mo/Al)+2.8, n = 13, r2 = 0.52, mean rate of deposition=36 cm/kyr; distinctly microlaminated, dark olive gray, clayey mud (11.6-14.5 kyr BP), %TOC = 1622*(Mo/Al)+0.22, n = 15, r2 = 0.89, mean rate of deposition = 79 cm/kyr. Here, we use these relationships to estimate the original TOC contents of ancient black shales with overall characteristics similar to those of the modern Cariaco sediments. These "Group IV" black shales as defined by Quinby-Hunt and Wilde [Econ. Geol. 91 (1996) 4] are characterized by relatively high concentrations of V, Mo and Co but low Mn contents. The Cariaco regressions and those from the Carboniferous of Iowa and the Devonian of New York were used to estimate the "original" TOC contents for Lower Ordovician black shales of the Baltica and Avalonia plates, where Corg values were not taken. For individual samples, the Carboniferous regression produced TOC values approximately double that derived from the regression equation of the Cariaco Basin lower anoxic zone. Such variations among the results from the four regressions suggest that there is no universal proxy for TOC using Mo/Al. These calculated TOC values, however, are consistent with the higher levels of primary production predicted from the paleogeographic settings of these intervals. In general, the Mo proxy for original TOC content, while approximate, works for oxygen-deficient sites of deposition where other proxies for C loss, such as Corg/Spy ratios in normal (oxic) marine shales, do not apply. Estimates of original TOC from Mo content in samples spanning the geologic record, combined with paleogeography and paleoecology, should be useful in estimating pathways of C synthesis and remineralization in ancient oceans and initial hydrocarbon potential of petroleum source rocks. Abstract Copyright (2004) Elsevier, B.V.
Year of Publication: 2004
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Appalachians; Atlantic Ocean; Avalonia; Baltica; Basins; Black shale; Carbon; Carbonates; Carboniferous; Cariaco Basin; Caribbean Sea; Chemical composition; Chemical ratios; Clastic rocks; Geochemistry; Iowa; Isotopes; Leg 165; Metals; Mineral composition; Molybdenum; New York; North America; North Atlantic; ODP Site 1002; Ocean Drilling Program; Organic carbon; Organic compounds; Paleoecology; Paleoenvironment; Paleogeography; Paleozoic; Petroleum; Petroleum exploration; Sea-level changes; Sedimentary basins; Sedimentary rocks; Source rocks; Sulfur; Total organic carbon; United States
Coordinates: N104222 N104222 W0651011 W0651011
Record ID: 2005026335
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands