Zinc and strontium isotope evidence for climate cooling and constraints on the Frasnian-Famennian (~372 Ma) mass extinction

The Frasnian-Famennian (F-F) mass extinction is one of the “Big Five” biotic crises during the Phanerozoic, whereas the cause of this catastrophic event still remains debated. Here we present for the first time high-resolution Zn and Sr isotope data obtained by a sequential leaching procedure for carbonate rocks across the F-F boundary from the Fuhe section in South China. Both δ66Zn and 87Sr/86Sr values rapidly increase in the Lower and Upper Kellwasser Horizons and Palmatolepis delicatula platys to P. minuta minuta conodont zones, and the positive isotope shifts are exactly located in the same horizons characterized by low ∆13C values (equivalent to low atmospheric PCO2 level, reflecting climate cooling) and sea-level fall stages. The strong temporal coupling of δ66Zn values raise with elevated 87Sr/86Sr ratios is most likely to have been caused by input of isotopically heavy Zn and radiogenic Sr from carbonate-dominated weathering, and/or uptake of isotopically light Zn into primary producers during climate cooling. Climate cooling could cause relative sea-level falls resulting in widespread exposure of carbonate platform, enhance carbonate influx proportion through differential weathering of carbonate and silicate rocks, and cause greater productivity by promoting oceanic thermohaline circulation and enhancing upwelling of bio-limiting nutrients. All of these processes would have contributed to the positive shifts of δ66Zn as observed. This proposed scenario is in accordance with the fact that the F-F mass extinction was almost limited to the shallow-water tropical stenothermal faunas, whereas the high latitude and deep-water faunas were less affected. Our data support and represent the composite effect of a series of events linked to climate cooling which may have induced this catastrophic event.