Evaluation of high-frequency paleoenvironmental variation using an optimized cyclostratigraphic framework: Example for C-S-Fe analysis of Devonian-Mississippian black shales (Central Appalachian Basin, U.S.A.)
Liu JS, Algeo TJ, Jaminski J, Kuhn T, Joachimski M (2019)
Publication Type: Journal article
Publication year: 2019
Journal
Book Volume: 525
Pages Range: 303-320
DOI: 10.1016/j.chemgeo.2019.07.019
Abstract
Stratigraphic successions are rarely sampled at sufficiently dense spacing to permit evaluation of paleoenvironmental variation at the shortest fundamental frequency operative in the depositional system of interest, often because that frequency remains undetermined. Here, we provide an example of a sampling strategy optimized for recovery of high-frequency paleoenvironmental signals, based on (1) full-core analysis using a continuous survey tool such as X-radiography, followed by (2) fine sampling of selected small-scale cycles that record significant short-term paleoenvironmental variation. We show that this strategy results in recovery of high-frequency paleoenvironmental signals that are completely missed by low-density sampling protocols, permitting key insights into depositional and early diagenetic processes that may otherwise by overlooked. The present case study is based on C-S-Fe systematics of the Cleveland and Sunbury shales, which belong to the Upper Devonian-lowermost Carboniferous black shale succession of the Central Appalachian Basin in eastern North America. In these units, the fundamental scale of paleoenvironmental variation is recorded by pervasive decimeter-scale (~5- to 20-cm-thick) compositional cycles that are readily visible in outcrop, in which high-organic layers weather out as resistant ledges. We identified these cycles in drillcores using X-radiography, which proved to be an effective proxy for compositional variation owing to control of bulk density primarily by total organic carbon (TOC) content. We then sampled selected dm-scale cycles (n = 21) at a centimeter spacing, generating short, high-resolution, multiproxy chemostratigraphic records including TOC, sulfur and Fe concentrations, Fepy/FeHR (a redox proxy), δ34Spyrite, and organic maceral content. This analysis yielded the novel insight that all proxies related to authigenic iron sulfides (e.g., Stotal and Spyrite, Fetotal and Fepyrite, Fepy/FeHR, and δ34Spyrite) covary negatively with TOC within the dm-scale cycles. We interpret the unusual pattern of negative TOC-pyrite covariation in the Cleveland and Sunbury shales to have resulted from differential distributions of labile organic matter (OM), which drove H2S production via microbial sulfate reduction (MSR), and reactive Fe, which controlled pyrite formation. Labile OM was concentrated in high-TOC layers whereas reactive Fe was concentrated in low-TOC layers (possibly as Fe-oxyhydroxide coatings on silt grains), as a consequence of which part of the H2S generated in the high-TOC layers diffused upward into the low-TOC layers before becoming fixed as pyrite. This process is evidenced by several contrasts between high-TOC and low-TOC layers: (1) higher Spyrite concentrations and Fepy/FeHR values in the latter, (2) concentration of pyrite in the lower part of low-TOC layers, immediately above high-TOC layers, and (3) higher δ34Spyrite values in the latter, reflecting late diagenetic formation of the H2S that diffused upward into the low-TOC layers. To our knowledge, this process has not been documented previously in ancient marine shales, probably owing to insufficiently high-resolution analysis in earlier studies.
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APA:
Liu, J.S., Algeo, T.J., Jaminski, J., Kuhn, T., & Joachimski, M. (2019). Evaluation of high-frequency paleoenvironmental variation using an optimized cyclostratigraphic framework: Example for C-S-Fe analysis of Devonian-Mississippian black shales (Central Appalachian Basin, U.S.A.). Chemical Geology, 525, 303-320. https://doi.org/10.1016/j.chemgeo.2019.07.019
MLA:
Liu, Jiang Si, et al. "Evaluation of high-frequency paleoenvironmental variation using an optimized cyclostratigraphic framework: Example for C-S-Fe analysis of Devonian-Mississippian black shales (Central Appalachian Basin, U.S.A.)." Chemical Geology 525 (2019): 303-320.
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