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Title:Ordovician paleoceanography: Stable isotope and carbon-sulfur-iron evidence from the Caradocian Trenton Group, Mohawk Valley, New York
Author(s):Railsback, Loren Bruce
Doctoral Committee Chair(s):Anderson, Thomas F.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Physical Oceanography
Abstract:Oxygen isotope evidence from Caradocian Trenton Group brachiopods in the Mohawk Valley of New York suggests that Caradocian oceans were salinity-stratified and dominated at depth by warm, saline waters. $\delta\sp{18}$O values of these brachiopods, which lived on the eastern continental slope of the North American craton over an apparent depth range of two to three kilometers, cannot be readily explained in terms of known burial diagenetic trends in the Mohawk Valley and presumably reflect depositional conditions. $\delta\sp{18}$O values decrease with depth over the upper half of the Trenton paleoslope and range from $-$4.8 to $-$8.1 (PDB). These data, interpreted using a graphic modelling scheme constrained by both calcite isotopic compositions and seawater densities, suggest that both temperature and salinity increased with depth in the Caradocian oceans from intermediate to abyssal depths, and that warm saline deep waters (WSDW) dominated the latter. Modelling of isotopic data also suggests that the average oxygen isotopic composition of Caradocian seawater was no greater than about $-$4.0% (SMOW) and imply low-latitude earth-surface temperatures of less than 40$\sp\circ$C.
Comparison with characteristics of modern water masses suggests that WSDW originated in low-latitude evaporative marginal seas, and that overlying cooler, less saline waters originated in polar regions. This circulation system, which is much different from modern deep ocean circulation, may have been caused by warm equable climate, high sea level, and fortuitous location of continents during the Ordovician. The reduced delivery of dissolved oxygen to the deep oceans by this circulation system may have been a factor in the accumulation of abundant Caradocian black shales.
Examination of the C-S-Fe chemistry of Taconic Foreland Basin shales confirms sedimentologic and paleontologic observations that Trenton Group strata, and especially the Utica Shale, were deposited in oxygen-deficient to anoxic environments. S/C ratios in Trenton Group samples are anomalously low (except where carbon loss was greatest during burial diagenesis) and apparently were caused by iron limitation of pyritization. Degree of pyritization is not a good indicator of either iron limitation or oxygenation during Trenton Group deposition, because concentrations of truly reactive iron (for pyritization) were unusually low. In contrast, contemporaneous Snake Hill shales deposited in the eastern Taconic Foreland Basin have S/C ratios typical of Ordovician shales, in part because of their higher detrital iron contents. Utica Shale pyrites are more enriched in $\sp{34}$S (relative to contemporaneous seawater) than are typical organic-rich sediments; variation in $\delta\sp{34}$S values of these pyrites can be explained in terms of varying sedimentation rates and organic carbon contents.
Issue Date:1989
Rights Information:Copyright 1989 Railsback, Loren Bruce
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9010993
OCLC Identifier:(UMI)AAI9010993

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