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PATTERN AND TIMING OF THE LATE DEVONIAN BIOTIC CRISIS IN WESTERN CANADA: INSIGHTS FROM CARBON ISOTOPES AND ASTRONOMICAL CALIBRATION OF MAGNETIC SUSCEPTIBILITY DATA

By
Michael T. Whalen
Michael T. Whalen
Department of Geosciences, University of Alaska, Fairbanks, Alaska 99775-5780 USA
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David De Vleeschouwer
David De Vleeschouwer
MARUM-Center for Marine Environmental Sciences, Leobenerstrasse, 28359 Bremen, Germany, and Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
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Joshua H. Payne
Joshua H. Payne
Shell Exploration and Production Co., 150-A North Dairy Ashford, Houston, Texas 77079 USA
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James E. (JED) Day
James E. (JED) Day
Department of Geography–Geology, Illinois State University, Normal, Illinois 61790-4400 USA
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D. Jeffrey Over
D. Jeffrey Over
Department of Geological Sciences, State University of New York–Geneseo, Geneseo, New York 14454 USA
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Philippe Claeys
Philippe Claeys
Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
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Published:
January 01, 2017

ABSTRACT

Carbon stable isotope data from western Canada, in combination with biostratigraphic control and astrochronologic constraints from magnetic susceptibility data, provide insight into the pace and timing of the Frasnian–Famennian (F–F; Late Devonian) biotic crisis. In much of the world, this event is characterized by two organic–rich shales, which display geochemical anomalies that indicate low-oxygen conditions and carbon burial. These events, commonly referred to as the Lower and Upper Kellwasser events (LKE and UKE), have been linked to the expansion of deeply rooted terrestrial forests and associated changes in soil development, chemical weathering, and Late Devonian climate. The δ13C data generated from organic matter record a 3 to 4‰ positive excursion during each event. These data and other geochemical proxy data reported elsewhere corroborate hypotheses about enhanced biological productivity, driven by terrigenous input under exceptionally warm climatic conditions. In this hypothesis,a boom in primary production leads to successive development of anoxic bottom water conditions, decreased biotic diversity, and net transfer of carbon from the atmosphere to the ocean floor.

Despite the importance of the F–F events, a precise chronology for the events is lacking due to limited biostratigraphic resolution. Each of the F–F events falls within one conodont zone, with durations estimated on the order of 0.5 to 1.0 Myr. The LKE occurs very high in Frasnian Zone (FZ) 12, while the UKE begins within FZ 13B, just below the F–F boundary. A previous analysis of high-resolution magnetic susceptibility data from the studied sections in western Canada identified 16.5 eccentricity cycles, each lasting 405 kyr, within the Frasnian strata and one in the earliest Famennian. The present study reports δ13C anomalies associated with the LKE and UKE in the same sections. The LKE and UKE intervals comprise 7 to 8 and 13 to 13.5 m of stratigraphic section, respectively. Based on our analysis, this implies that they represent only one 405-kyr eccentricity cycle or less.We estimate that the duration of the LKE was a bit more than half of a long eccentricity cycle (~200–250 kyr), while the UKE was more protracted, lasting a full long eccentricity cycle (~405 kyr). The onset of both events is separated by one-and-a-half 405-kyr eccentricity cycles, indicating that they occurred about 500 to 600 kyr apart. This work demonstrates the utility of magnetic susceptibility, or other long time-series proxy data, used in conjunction with astronomical calibration to provide insight into the pacing of significant events in geologic time.

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Contents

Society for Sedimentary Geology

NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY

Society for Sedimentary Geology
Volume
107
ISBN electronic:
9781565763456
Publication date:
January 01, 2017

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