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James Lawrence Powell's 2020 Powell Books work — the principal accessible scholarly synthesis of the Younger Dryas Impact Hypothesis (YDIH) evidence accumulated since Firestone et al.'s 2007 PNAS paper. Powell, a geochemist and former president of the Franklin Institute and Reed College, synthesises the substantial accumulated evidence (platinum spikes, nanodiamonds, carbon spherules, widespread combustion markers, megafaunal extinctions, Clovis-culture collapse) for a major cosmic impact or atmospheric airburst event at approximately 12,900 BCE. The YDIH remains contested in mainstream geology; Powell's work is the principal recent advocacy synthesis.