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

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 (δ¹³C_carb) 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, δ¹³C_carb values decline from 1.1 to 0.7 ‰ over 1.7 m and in the post-impact section, δ¹³C_carb values are stable (average value 2.2 ± 0.2 ‰), increasing slightly over 1.3 m. Accordingly, we report a ~1.4 ‰ positive shift in δ¹³C_carb across the Alamo impact layer at Devils Gate. A relatively low magnitude, positive δ¹³C_carb 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 δ¹³C_carb 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.