AN EXTRATERRESTRIAL COMPONENT AT THE PERMO-TRIASSIC BOUNDARY? CLUES FROM SIDEROPHILE ELEMENT ABUNDANCES

The Permian-Triassic (P-Tr) boundary is associated with the largest known mass extinction; the cause of this mass extinction is unknown, but suggestions have been made that a large impact event could have been responsible. Large-scale volcanic eruptions may also be involved. Evidence for an impact event at the P-Tr boundary is still controversial. Siderophile element anomalies were found at some P-Tr boundary locations, but their source is not clear and confirmation of their existence and extraterrestrial nature is still pending. However, recent dating attempts indicated that the environmental changes that resulted in the mass extinction were brought on within a short time, maybe as short as a few thousand years. Here we try to confirm the existence of the Ir anomaly that was found by Holser and colleagues in 1989 in samples from the Gartnerkofel P-Tr boundary at the Carnic Alps in Carinthia, southern Austria. We analyzed splits of the cores analyzed before, as well as samples from the Val Badia (western Dolomites, NE Italy) P-Tr boundary.

For the iridium analyses we used multiparameter gamma-gamma coincidence spectrometry, which allows the non-destructive determination of very low Ir concentrations (in the sub-ppb range). For the Gartnerkofel P-Tr boundary samples, we found Ir values of 65 - 155 ppt. These values are above background and confirm the anomalies found before; further analyses are in progress. The Val Badia section, which had not been analyzed for Ir contents before, gave even more encouraging results. The background samples have 21 - 65 ppt Ir, but one sample, coinciding exactly with a positive d13C (carbonate) peak, and an even more distinct d13C (alkanes peak), has 242 ppt Ir. A second (close) sample, also coinciding with a (less pronounced) d13C (alkanes peak) has 120 ppt Ir. These observations do not prove the extraterrestrial nature of the Ir, but it was possible to confirm the presence of an Ir anomaly. We are currently measuring the Os-isotopic composition and the platinum-group element abundances in the samples to obtain further clues regarding the origin of the siderophile element anomaly.