http://www.thunderbolts.info/wp/2011/12 ... p-sea-ash/
Given that the sea floor is considered to be of variable age, i.e. youngest at the mid ocean ridges and oldest at the continental shelf margins* (as a general rule), it would seem reasonable to assume a recent origin for an evenly distributed layer (and subsequent superior layers) "over the entire ocean bottom".
If it is indeed of recent origin maybe its deposition is the event referred to in some of the ancient catastrophe myths where people, because of darkness and so on, were forced to migrate/live in caves/eat fungus (no sunlight required) to survive. The myths sometimes mention 40 years/days (geologically speaking, a very short period) as the duration of the catastrophe, which in turn would suggest that the higher deposition rates (10,000 metric tons per day) deduced by Pettersson are perhaps closer to the mark.
It is also odd that there is no sign of the sediment on dry land. One possible explanation could be that the dust only fell into, and was temporarily suspended in, water. Water which in turn had temporarily inundated what is now dry land and out of which the dust settled once it had receded to around its present level. Hmmm, that reminds me of some other old tale I've heard...
.* http://www.tectonics.caltech.edu/images ... or_age.pdf
P.S. Average particle size would give a clue as to how long the dust was able to remain in suspension if it did in fact encounter water on its way to its final resting place. It is also possible that a change in water chemistry had something to do with the precipitation of the dust.