Perceptive questions, David, and I'll do my best to respond to them.
Cordially, Dewey McLean
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In fact, multiple iridium spikes do occur at localities other than China.
Hm. These are still rather few -- though the wide geographic distribution could mean something --, so that, without knowing the local sedimentologies, I wonder if some bioturbation and groundwater percolation could have multiplied the peaks or at least smeared them out. It's also interesting that all 3 papers are quite old. Is there a reason why this topic has seemingly disappeared from the literature?