Pyrite and organic matter in Phanerozoic normal marine shales

Details Pyrite and organic matter in Phanerozoic normal marine shales Pyrite and organic matter in Phanerozoic normal marine shales

Sep 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1967r&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 9, pp.1967-1976

Physics

18

Scientific paper

Data have been obtained, from our own chemical analyses and from the literature, for the concentrations of organic carbon (C) and pyrite sulfur (S) in over 600 samples of Cambrian to recent normal marine shales. ( Normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and/or indicators of bioturbation). All samples were selected to minimize 1. (1) loss of C and/or S due to weathering at the outcrop (by emphasizing the use of drill core material) 2. (2) analytical errors and non-diagenetic effects (by avoiding sediments low in C or S content such as sandstones) 3. (3) Fe limitation of pyrite formation (by avoiding limestones, cherts, and euxinic shales) 4. (4) metamorphic loss of C, relative to S (by avoiding rocks obviously subjected to, even low-grade, metamorphism). Our results indicate that (1) there is generally a good positive linear correlation between organic C and pyrite S for normal marine shales of all ages and (2) the mean C / S ratio for normal marine shales has varied over time. Devonian to Tertiary shales exhibit mean C / S weight ratios (1.8 ± 0.5) somewhat lower than Quaternary sediments (mean C / S = 2.8). This we believe is due to preferential C loss (relative to S) during biogenic methanogenesis plus diagenetic heating. Distinctly lower mean C/S values for the Cambrian and Ordovician (0.5 ± 0.1) cannot be explained solely in terms of diagenetic C loss and, instead, must represent low original ratios. The low original ratios, we suggest, were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time, because the major source of such material, vascular land plants, had not yet evolved. This, along with a possibly lower degree of bioturbation, contributed to enhanced pyrite formation and preservation and, thus, lower early Paleozoic C / S ratios.

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