Spherules and Impacts

We are assured by the US Geological Society that spherules in black mats have nothing to do with impacts at the Younger Dryas Boundary but a few days later we have this new paper in Nature (April 25th) – see http://phys.org/print254577289.html. It seems that asteroid/ comet impacts are okay as long as they happened billions of years ago. Jay Melosh focussed on spherules embedded in layers of rock that are said to have been vapourised during an impact collision with the Earth. It expanded as a giant vapour plume that rose into the atmosphere. Small droplets of molten rock condensed and solidified and fell back to the surface as a thin layer, where it was discovered, waiting for science to catch up.