THE LATE DEVONIAN (FRASNIAN) PUNCTATA EVENT IN THE GREAT BASIN, U.S.A.: A NEW HIGH-RESOLUTION δ13CCARB AND MAGNETIC SUSCEPTIBILITY RECORD

The Late Devonian (Frasnian) “punctata event” has been described as a multi-part δ¹³C excursion and represents one of the largest carbon-cycle disturbances of the Devonian Period. A four-part model for the punctata event that includes a sustained excursion of positive δ¹³C (parts III to IV) has been proposed as a reference for the punctata event; however, global correlations are equivocal and the model is based on a limited data set. Here, we present two very high-resolution records (millennial-scale) of whole rock stable inorganic carbon isotope (δ¹³C_carb) and magnetic susceptibility (MS) from the Guilmette and Devils Gate formations of the eastern Nevada continental shelf (ENCS) that each span the falsiovalis through punctata (FZ 1–5) conodont zones. Our records show that δ¹³C values fluctuated throughout the interval and specifically lack a sustained excursion of positive δ¹³C values. Instead, we document at least ten shifts in δ¹³C_carb values through the studied interval depending on our scale of observation. 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. This is followed by a stepped +4‰ positive excursion leading to the most positive values recorded in both of our sections. Values of δ¹³C_carb then fall in a stepped fashion by 6‰ up to the stratigraphic level of the Alamo impact breccia (~mid-punctata zone). Our results indicate that the proposed four-part model does not accurately represent δ¹³C variations during the punctata event which is more complicated than previously documented. MS values from the same samples follow opposite trends as δ¹³C_carb in the lower half of the record, dominated by deeper-water, catch-up sequences. In contrast, MS values follow the same trends as δ¹³C_carb in the upper-half of the record, dominated by shallower-water, keep-up sequences. These results suggest a coupled and de-coupled response to forcing mechanisms and argue against a simple limiting nutrient cause for observed δ¹³C variations. A spline fit of MS values is in extremely close agreement to values from the western Canada sedimentary basin which is interpreted to reflect the 405-Ka Milankovitch beat. These results corroborate existing studies suggesting that MS is an extremely powerful correlation tool in ancient carbonate successions.