Delayed climate cooling in the Late Eocene caused by multiple impacts: high-resolution geochemical studies at Massignano, Italy

Abstract

High-resolution studies ( δ 13C, δ 18O, and elemental abundances) were done in rocks at and below the GSSP for the Eocene/Oligocene (E/O) boundary at Massignano, Italy. In addition to an earlier known Ir anomaly at 5.61 m, which is possibly linked to the Popigai impact event, we confirm the presence of two additional Ir anomalies in the intervals from 6.00 to 6.40 m and from 10.00 to 10.50 m, with maximum values of 259±32 ppt at 6.17 m, and 149±24 ppt at 10.28 m, respectively. The lower Ir anomaly might be derived from the Chesapeake Bay impact event, whereas for the other one no impact event is known. Similar δ 13C and δ 18O trends related to the two Ir anomalies indicate that the Ir anomaly at 10.28 m might be also derived from an impact into a continental shelf, similar to the Chesapeake Bay impact event. δ 18O values decrease in the high Ir layers to −1.16‰ and −1.17‰, respectively, which, together with the negative shifts in δ 13C in the Ir-rich levels, indicate a warm pulse superimposed on a general Late Eocene cooling trend that is characterized by δ 18O values ranging between −0.6‰ and −0.4‰. The release of methane hydrate after an impact in a continental shelf or seafloor, or impacts of 12C-rich comets during a 2.2-million-year-long comet shower, respectively, could produce these more negative carbon and oxygen excursions compared to the continuously decreasing trend over the whole Late Eocene Massignano section.