Sedimentological analysis of the Ordovician-Silurian international boundary stratotype section at Dob's Linn, Scotland shows systematic changes in muddy turbidite microfacies which are interpreted in terms of a four phase glaciation. This comprised: (1) a pre-glacial cooling of the deep sea in the early Ashgill (Cautleyan), (2) initiation of intense thermohaline circulation in the late Rawtheyan, predating, (3) the start of the glacial maximum near the end of the Rawtheyan, and (4) rapid de-glaciation in the mid- to upper Hirnantian (lower part of the persculptus Biozone). It is proposed that the glacial maximum was triggered by a combination of negative greenhouse effect (‘Monterey Hypothesis’) and increasing surface albedo. Cessation of thermohaline circulation and the expansion of anoxia mark the rapid end of the glaciation in this deep-sea environment.

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