A large meteoritic event over Antarctica ca. 430 ka ago inferred from chondritic spherules from the Sør Rondane Mountains

Abstract

Large airbursts, the most frequent hazardous impact events, are estimated to occur orders of magnitude more frequently than crater-forming impacts. However, finding traces of these events is impeded by the difficulty of identifying them in the recent geological record. Here, we describe condensation spherules found on top of Walnumfjellet in the Sør Rondane Mountains, Antarctica. Affinities with similar spherules found in EPICA Dome C and Dome Fuji ice cores suggest that these particles were produced during a single-asteroid impact ca. 430 thousand years (ka) ago. The lack of a confirmed crater on the Antarctic ice sheet and geochemical and ¹⁸O-poor oxygen isotope signatures allow us to hypothesize that the impact particles result from a touchdown event, in which a projectile vapor jet interacts with the Antarctic ice sheet. Numerical models support a touchdown scenario. This study has implications for the identification and inventory of large cosmic events on Earth.

Fig. 1. Location of the sampling site in WN, Sør Rondane Mountains, Queen Maud Land, Antarctica.

(A) Landsat image of the sampling site on the summit of WN, where the particles studied here were recovered, along with the Princess Elisabeth Antarctica (PEA). Inset shows the locations of Dome Fuji (DF), Dome Concordia (DC), and BIT-58 for comparison (8, 9, 24). The ages of the various horizons can be found in Table 1. (B) The flat glacially eroded summit of WN on the border of the Antarctic plateau. ¹⁰Be exposure age of glacially eroded surfaces of WN range from 870 to 1740 ka (25). (C) The 30 × 30 × 10 cm sampling site on top of WN. Landsat 7 image courtesy of the Landsat Image Mosaic of Antarctica (LIMA) project. Photo credit: Matthias van Ginneken, University of Kent.

Fig. 2. Scanning electron backscattered images of WN particles.

(A) WN-IP#12 (SR), which consists in skeletal Fe-poor olivine (Fa < 10), Fe spinel, and minor interstitial glass. (B) WN-IP#6 (SP), which consists of large skeletal Fe-rich olivine (Fa > 10), and minor Fe spinel and interstitial glass. (C) and (D) represent polished section of WN-IP#12 and WN-IP#6, respectively. Insets in (C) and (D) are close-ups showing the morphology of olivine and Fe spinel crystals. Scale bars, 100 μm.

Fig. 3. Major element bulk compositions of the impact WN particles (WN-IP), normalized to CI-chondrite bulk composition (46).

Bulk compositions of impact particles from BIT-58 (average), L1 from EPICA Dome C ice core (average), and DF2641 from Dome Fuji ice core (range for individual analyses) are shown for comparison. Elements are arranged in order of increasing volatility from left to right (47).

Fig. 4. δ¹⁷O versus δ¹⁸O diagram (values in ‰ versus V-SMOW) for WN particles, DF2641 particles from the Dome Fuji ice core (9), and L1 particles from the EPICA Dome C ice core (48).

Antarctic inland ice values from (26) and tropospheric oxygen values from (32). Bulk isotopic compositions of chondritic meteorites are represented in rounded colored shaded areas, including carbonaceous chondrites [i.e., below the terrestrial fractionation line (TFL)], ordinary and Rumuruti chondrites (i.e., above the TFL), and enstatite chondrites (i.e., on the TFL) (–29). The blue/red shaded areas represent hypothetical mixing area between a carbonaceous chondritic impactor, Antarctic inland ice (blue), and atmospheric oxygen (red). Trend lines for SR and SP WN particles are in red and black, respectively, and broadly represent mixing lines pointing to increasing mixing with ice. A larger deviation from the TFL means less interaction with atmospheric oxygen. Inset: The effects of oxygen isotopic exchange of a chondritic impactor with Antarctic ice and atmospheric air.

Fig. 5. Temperature and density distribution in the impact plume after the contact.

(A) and (B) represent temperature and density after 10 s, respectively; (C) and (D) represent temperature and density after 36 s, respectively. Shortly after the impact of vapor jet into ice, temperature near the impact point (from −4 to 0 km along the X axis and up to 2-km altitude) remains extremely high. Mixing with vaporized ice (i.e., steam) occurs at this point. Thirty-six seconds after the contact, temperature drops below 3000 K; it is likely that impact spherules have condensed by that point (note, however, that the process of condensation and formation of particles is not included into the model). (B) and (D) show relative density (the ratio of the current density to the density of undisturbed atmosphere). Shock waves (dark gray areas) propagate outward from the impact point; density within the wake is below normal (vapor and air are hot). Blue/red contours show ice/projectile materials respectively. Intensive mixing takes place immediately after the impact (B); this mixture subsequently moves upward along the rarefied wake and reaches the upper troposphere during the first minute.