LATE EOCENE SPHERULE LAYERS: FROM IMPACT TO DEPOSITION

Distal impact ejecta have been found in late Eocene deposits around the globe. One of the two known spherule layers, the clinopyroxene-bearing (cpx) spherule layer, is linked to the Popigai structure in northern Siberia, the largest known impact in the Cenozoic. The other layer has been attributed to the 85-km-diameter Chesapeake Bay impact structure on the eastern coast of the USA, which is also linked to the North American tektite strewn field. Cpx spherule sites have so far been found within three lobes radiating from Popigai and follow ballistic trajectories suggesting the ejecta were concentrated into rays during the impact process and then spread in the atmosphere.

Using the two-phase fluid flow code KFIX-LPL, we modeled two scenarios of cpx spherule deposition through the atmosphere: (1) uniform distribution of 200-μm spherules entering the upper atmosphere and (2) injection of spherules in rays. For the simple scenario of uniform reentry, the falling spherules decelerate due to drag, compressing the upper atmosphere and accumulating in a band at ~70 km altitude. This causes heating of both the upper atmosphere (~4000 K) and the spherules (~1200-1500 K), the latter of which is radiated as thermal radiation (<2 kW/m² at the surface). If the spherules are injected as a ray, density gradients are created both across the edges of the ray between spherule-loaded and spherule–free atmosphere and above the ray between compressed and uncompressed atmosphere. This leads to lateral spreading of the ray. Horizontal spreading is enhanced for (1) higher spherule entry fluxes due to increased gradients across the edges of the rays and (2) smaller spherule sizes, which is consistent with the broad ejecta rays observed around Popigai versus the continuous distal ejecta layer around the larger Chicxulub crater. Our models predict that, if the proposed Popigai rays are real, decreased layer thickness and mean spherule size should be observed towards the edges of any ray transect. Atmospheric interactions must be considered for both the sedimentology and distribution of distal ejecta layers on Earth as well as on any planet with an atmosphere.