Determining chondritic projectile size from marine osmium isotopes excursions: A comparison of the late Eocene and Cretaceous/Tertiary impact events

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Paquay, Francois۱; Ravizza, Greg۱; Dalai, Tarun۲; Peucker-Ehrenbrink, Bernhard۳
Holding Date 11 October 2008

Recognizing impact events in the geological record and estimating projectile size is critical to the understanding of the solar system dynamics over geological time. Current methods to estimate large chondritic projectile size are either based on well-defined iridium fluences or deduced from numerical modeling of a preserved impact crater. Iridium is the most commonly used Platinum Group Element in impact studies. Significant iridium enrichment in sediments sequences compared to averaged crustal rocks is commonly considered as the geochemical signature of extraterrestrial material. However, Ir fluences vary with local conditions, changing by a factor of 2-3 the size estimates. Similarly, impact crater are rarely preserved due to continuous reshaping of Earth’s crust or are overlaid by thick sediments layers preventing direct observations of the crater.
We present an independent approach to estimating projectile size using marine osmium isotopes excursions. Marine osmium isotopes are thought to be homogenized in the global ocean, and have short residence time, which allows capturing in deep-sea sediments sequences short-term variations such as the dissolution of extraterrestrial impact-derived Os in seawater. Knowing that chondritic meteorites are characterized by high Os concentrations (~400-800 ng/g) and low 187Os/188Os (~0.13), and assuming that a total vaporization of the projectile dissolves in seawater with subsequent removal of this input from the seawater to the sediment result in a lowered 187Os/188Os signal homogenized globally. Therefore only a single section can accurately record the meteoritic signature. As quantification of the impact-derived Os is possible using simple isotopic mass balance calculations, this leads to estimates of projectile size.
We applied this approach for the Late Eocene and the Cretaceous/Tertiary impact events. Here we show that the post-impact Os inventory calculated from the mass of impact-derived Os falls between 30 to 40 % of the Ir inventory for each event. This implies that Os dissolution is incomplete and that Os isotope-based estimates of projectile size that assume quantitative vaporization are biased to low values.
Additionally we show evidence that paired Os/Ir is a more effective tool to discriminate impact events that affects the global ocean chemistry against local particulate extraterrestrial enrichment resulting of cosmic dust as observed for the Late Eocene events.
Refinement of our calculations show that if 30% of impact derived-Os is dissolved in seawater there is a good agreement between Ir and Os based estimates but a significant difference is still noticed with numerical models of impact crater formation. In these latter studies, velocity of the incoming projectile and the angle of incidence are some of the variables that may alter the size estimate. We therefore urge further modeling studies to address specifically this discrepancy.