T185 Greenhouse to Icehouse Transition: Global Events of the Devonian, Carboniferous, and Early Permian (Posters)

Tuesday, 6 November 2018
9:00 AM - 6:30 PM
Halls J-K

STABLE CARBON ISOTOPE SIGNATURE OF THE LATE DEVONIAN (FRASNIAN) ALAMO IMPACT AT DEVILS GATE, NEVADA: GEOLOGICALLY INSTANTANEOUS TRANSFER OF 13C-ENRICHED CARBON FROM THE LITHOSPHERE TO THE DISSOLVED INORGANIC CARBON POOL

Paper number: T185-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

  • Olivia Koster Presenting Author
    University of North Carolina Wilmington, Wilmington, NC
  • Todd LaMaskin
    University of North Carolina Wilmington, Wilmington, NC
  • Chad S. Lane
    University of North Carolina Wilmington, Wilmington, NC
  • Andrea Hawkes
    University of North Carolina Wilmington, Wilmington, NC

Abstract

The km-scale Alamo bolide impact occurred on the shallow continental shelf of western North America during the punctata conodont Zone and has been invoked as the cause of a large, global ~3–7‰ negative carbon isotope excursion. However, the Alamo impact is not associated with any apparent negative effects on shallow benthic ecosystems. Here, we present a high-resolution (millennial-scale), whole rock stable inorganic carbon isotope (δ13Ccarb) record from in situ strata below and above the Late Devonian (Frasnian) Alamo impact in present-day central Nevada. The Alamo Impact layer at Devil’s Gate, Nevada is a 1.3 m succession of graded carbonate beds, distal to the impact zone, and situated conformably within relatively unaltered carbonate lithofacies. The succession is divided into four units including: (1) a basal 0.8 m normally graded intraformational cobble-pebble stromotoporoid breccia with a yellow dolomite matrix, (2) a 0.3 m succession of thin, swaley cross-bedded carbonate pebble breccia, (3) a thin (2–5 cm) bed of quartz-pebble conglomerate, and (4) a 10–15 cm succession of very-thin graded carbonate beds bearing cm-scale water-escape structures. Units 2, 3, and 4 bear distinctive hematite embayed and studded quartz. In the pre-impact section, δ13Ccarb values decline from 1.1 to 0.7 ‰ over 1.7 m and in the post-impact section, δ13Ccarb values are stable (average value 2.2 ± 0.2 ‰), increasing slightly over 1.3 m. Accordingly, we report a ~1.4 ‰ positive shift in δ13Ccarb across the Alamo impact layer at Devils Gate. A relatively low magnitude, positive δ13Ccarb excursion (~1.4 ‰) is consistent with biotic observations of a negligible response to the Alamo impact, and shows that the impact was not associated with a large-scale perturbation of the biogeochemical carbon cycle unlike other impact events. We suggest that the Alamo impact caused an input of 13C-enriched carbon to the long-term geochemical carbon cycle through the dissolution, dissociation, and vaporization of carbonate target rocks of the Late Cambrian–Late Devonian shelf due to acidification and superheating from the impact. Our new δ13Ccarb record of the Alamo impact archives an apparently distinctive circumstance of the instantaneous flux of 13C-enriched carbon through the long-term carbon cycle between the lithosphere and the ocean-atmosphere.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320594

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