The impact theory rears its head again! From Science Dailey:
Conducting a wide range of exhaustive tests, the researchers conclusively identified a family of nanodiamonds, including the impact form of nanodiamonds called lonsdaleite, which is unique to cosmic impact. The researchers also found spherules that had collided at high velocities with other spherules during the chaos of impact. Such features, Kennett noted, could not have formed through anthropogenic, volcanic, or other natural terrestrial processes. “These materials form only through cosmic impact,” he said.
Kennett is refering to material found in a 13,000 year old layer of sediment on the floor of Lake Cuitzeo. The research is reported in PNAS and is open access.