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Paper No. 4
Presentation Time: 1:55 PM

GEOGRAPHIC CONTROL OF CAMBRIAN-ORDOVICIAN OCEAN CHEMISTRY: CONSTRAINTS FROM SHALE GEOCHRONOLOGY AND GEOCHEMISTRY


TRIPATHY, Gyana R., AIRIE Program Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, HANNAH, Judith L., AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA, and CEED Centre of Excellence, University of Oslo, 0316 Oslo, P.O Box 1048, Norway, STEIN, Holly J., AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482 USA, Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, 0316, Norway and YANG, Gang, AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, gyana.tripathy@colostate.edu

Changes in seawater chemistry influence marine biology; understanding evolutionary history of life on the Earth, therefore, requires rigorous understanding of controlling factors for the oceanic redox state. Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Re-Os geochronology and biostratigraphy for the same shales place paleoenvironments definitively in time. Plate reconstructions place the analyzed shale section in its proper global geographic position.

Here we bring Rock-Eval, major and trace elements and Re-Os isotopic data together to examine the environmental record at the Cambrian-Ordovician Global Stratotype Section and Point (GSSP) at Green Point in western Newfoundland. The Green Point shales are oil mature and contain Type-II organic material of marine origin. A Re-Os isochron for these shales provides the first depositional age for the GSSP boundary at 484 ± 16 Ma (2σ; Model 3 age; MSWD = 21; n = 13), with an initial 187Os/188Os ratio of 0.74 ± 0.05.

Factor analysis of the geochemical dataset for Green Point shales shows association of most trace elements with TOC and S contents, ensuring an authigenic origin for those trace elements and hence, their validity for evaluating paleoredox state. Relatively high enrichment factors for redox-sensitive elements (e.g., Re, U and Mo) compared to average shale, but lower enrichment factors compared to modern Black sea sediments, suggest deposition in anoxic waters. Comparison of global Cambrian-Ordovician shale geochemistry datasets, each placed in its proper global position at 485 Ma, shows that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica whereas depositional basins with colder waters (e.g. Avalonia and Gondwana) show geochemically less reducing conditions. These outcomes underscore the important role of paleogeography in regulating ocean conditions and marine life.

Session No. 73

T66. New or Improved Proxy Methodology for Enhanced Resolution and Accuracy of Climatic and Paleoenvironmental Interpretations in Sedimentary Records
Sunday, 27 October 2013: 1:00 PM-5:00 PM
Room 501 (Colorado Convention Center)
Geological Society of America Abstracts with Programs. Vol. 45, No. 7, p.199
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