Late Eocene impact layers in the Southern Ocean: A geochemical and geochronological archive of the Popigai impact event

Traces of large extraterrestrial impacts preserved in distinct sedimentary layers can be used as event horizons for stratigraphic correlation and to assess relationships, e.g., to extinctions in the fossil record and to paleoenvironmental changes associated with the event [1]. During the Late Eocene, the two largest post-K/Pg impact structures - Popigai (Russia; ~100 km diameter) and Chesapeake Bay (USA; ~85 km diameter) - are thought to have formed as a result of nearly simultaneous impact events ca. 35–36 Mya [1–3] and may have caused significant biotic disruptions and altered climate conditions on a global scale [3,4]. A more detailed assessment of the exact timing of these events and impact-induced effects, however, requires high-resolution stratigraphic correlation and more accurate age data than currently available.

The two Late Eocene impacts have been stratigraphically associated with two distinct impact layers identified in contemporaneous deposits around the globe: (a) Chesapeake Bay with the North American tektite layer, and (b) Popigai with the clinopyroxene (cpx)-bearing spherule layer [2,3]. The occurrence of cpx-spherules at several deep-sea Southern Ocean sites drilled by the Ocean Drilling Program (ODP) was previously correlated to the Popigai impact [3]. However, a lack of geochemical data at some sites, as well as an absence of absolute age data, hitherto prevented rigorous verification that recovered spherules derived from the same or possibly multiple events.

Here, we report new geochemical data (major oxide compositions) of cpx-spherules and glassy microtektites collected from ODP Hole 689D (Maud Rise, South Atlantic) and from ODP Hole 738B (Kerguelen Plateau, Southern Indian Ocean). Site 738 previously had not been evaluated compositionally. The investigated samples from both sites display similar compositional ranges for Al₂O₃, MgO, FeO, CaO, Na₂O, K₂O and TiO₂ versus SiO₂ compared to reported literature data [3,5] for the cpx-spherule layer associated with the Popigai impact. Hence, our results confirm the provenance of the impacto-clastic layer at the studied Southern Ocean sites. Additionally, large glassy microtektites (200–700 µm in diameter) were recovered from these sites and are currently being prepared for high-precision ⁴⁰Ar-³⁹Ar analysis. This continuing direction of research will potentially yield the first radiometric ages for the impact layer and provide a new geochronological framework for assessing the timing and effects of Late Eocene impact events.

[1] Glass, B.P. and Simonson, B.M. (2013), Distal Impact Layers: Springer, 716 pp. [2] Koeberl, C. (2009), Geol. Soc. Am. Spec. Pap. 452: 17-26. [3] Liu, S. et al. (2009), Geol. Soc. Am. Spec. Pap. 452: 37-70. [4] Vonhof, H.B. et al. (2000), Geology 28: 687-690. [5] Glass, B.P. et al. (2004), Geochim. Cosmochim. Ac. 68: 3971-4006.