A STRATIGRAPHIC MULTI-PROXY COVARIATION TEST FOR THE CAUSE OF THE LATE DEVONIAN PUNCTATA EVENT CARBON ISOTOPE EXCURSION, GUILMETTE FORMATION, EASTERN NEVADA, U.S.A

The Late Devonian “punctata event” represents one of the largest carbon isotope excursions in the Phanerozoic Eon, yet the cause of this carbon-cycle perturbation is unknown. To better understand the causes of the punctata event, and of carbon isotope excursions in general, we applied a multi-proxy approach using whole rock stable inorganic carbon isotopes (δ¹³C_carb), magnetic susceptibility (MS), and major- and trace-element composition on shallow-marine carbonates of the Guilmette Formation in eastern Nevada that span the falsiovalis through punctata (FZ 1–5) conodont zones (Early–Middle Frasnian). Our record shows at least ten shifts in δ¹³C_carb values depending the scale of observation used. In particular, a +4.5‰ shift occurs in the transitans (FZ 4) conodont zone followed by a –3‰ shift at the beginning of the punctata (FZ 5) conodont zone. A stepped +4‰ positive excursion in the punctata zone leads to the most positive values recorded, which then fall in a stepped fashion by 6‰ up to the stratigraphic level of the Alamo impact breccia (~mid-punctata zone). Since d¹³C_carb is typically considered to be a proxy for surface-water productivity with more positive values representing greater burial of ¹²C-enriched organic carbon and because surface-water productivity (and positive δ¹³C_carb values) can be driven by input of limiting nutrients, we hypothesize that increased δ¹³C_carb values will co-vary with increases in proxies that represent aeolian and siliciclastic influx. MS is widely interpreted as a proxy for an influx of aeolian-derived detritus and concentrations of Ti, Fe, and Zr are proxies for the influx of siliciclastic material. Our records show that δ¹³C_carb both co-varies and is anti-correlated with MS and major- and trace-element concentrations. Values of d¹³C_carb follow opposite trends as MS, Fe, Ti, and Zr in the lower half of the record, which is dominated by deeper-water, catch-up sequences, suggesting that during third-order transgressions, influx of limiting nutrients does not govern surface-water productivity. In contrast, MS, Fe, Ti, and Zr values co-vary with δ¹³C_carb in the upper-half of the record, dominated by shallower-water, keep-up sequences, suggesting a limiting nutrient control on δ¹³C_carb during third-order highstand and associated progradation.