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

High-resolution studies (delta(13)C, delta(18)O, 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 delta(13)C and delta(18)O 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. delta(18)O values decrease in the high Ir layers to - 1.16parts per thousand and - 1.17parts per thousand, respectively, which, together with the negative shifts in delta(13)C in the Ir-rich levels, indicate a warm pulse superimposed on a general Late Eocene cooling trend that is characterized by delta(18)O values ranging between -0.6parts per thousand and - 0.4parts per thousand. The release of methane hydrate after an impact in a continental shelf or seafloor, or impacts of C-12-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. (C) 2004 Elsevier B.V. All rights reserved.