Description
The late Eocene period, spanning roughly 37.8-33.9 million years ago, witnessed an increase in the influx of extraterrestrial material to Earth. This is evidenced by 3He-enriched sediments and the presence of several impact craters, notably the Popigai (100 km diameter) and the Chesapeake Bay (40-85 km) craters, occurring 35.5 ±0.6 and 35.7 ±0.2 Ma, respectively. The Popigai layer is globally distributed in late Eocene marine sediments, while the Chesapeake Bay layer is mainly localized around the impact site. Detailed records of these events, including two Iridium-rich layers, are found in the Massignano section in Italy. The older Ir anomaly associated with other impact features represents the Popigai ejecta, while the younger Ir anomaly is traditionally linked with the Chesapeake Bay impact. However, recent study suggests that the Chesapeake Bay event occurred too recently to be associated with the observed young anomaly, as it is nearly a quarter of the age difference between the two Ir anomalies in the section. The Popigai ejecta has also been documented in another Italian locality, the Monte Vaccaro section. Significant enrichment of chondritic chromite grains has been observed in the same interval in both the Massignano and Monte Vaccaro sections. The Massignano section also contains abundant terrestrial chrome-spinel grains, complicating reconstructions of the micrometeorite flux. Conversely, the Monte Vaccaro section holds great potential for reconstructing the late Eocene flux due to its condensed nature and low content of terrestrial spinel grains.This study reports the results from 1557.7 kg of limestone collected at the Monte Vaccaro section. The sam-ples' small (32-63 μm) and large (63-355 μm) fractions were examined for extraterrestrial chromite grains, using the methods developed by the Astrogeobiology laboratory at Lund University Sweden (see papers 5 and 6). The analyzed samples contained 730 spinel grains in the large size fraction and 5690 grains in the small fraction. Among these, 10 and 53 grains represent equilibrated ordinary chondritic chromite (EC). The interval between 0.1 and 0.5 meters, just above the Popigai ejecta layer, exhibited an enrichment of chondritic chromite spinels as pre-viously observed in the Massignano section.
By combining the extraterrestrial flux distribution with previous data from [4], where the delay between the two impact events is estimated to be approximately 18 ± 11 thousand years (k.y.), the newly proposed stratigraphic interval precisely aligns with the EC peak. Essentially, our results support the new placement of the Chesapeake Bay impact layer, thus explaining the likely peak in extraterrestrial flux related to that impact event. Further anal-ysis of Iridium may provide stronger evidence to confirm this hypothesis.
| Period | 15 Aug 2024 |
|---|---|
| Event title | 87th Annual Meeting of the Meteoritical Society |
| Event type | Conference |
| Location | Brussels, BelgiumShow on map |
| Degree of Recognition | International |