A review of Lower Palaeozoic black shale occurrences in the British Isles shows that carbon preservation in sediments, both in deep marine and shelf environments, occurs preferentially during phases of eustatic transgression (Caradoc and Lower Llandovery). Depletion of O2 in the water column during these times, which in some areas produced anoxic conditions, was probably the result of two oceanographic processes resulting from the sea level rise: (1) the development of a thermocline, which placed bottom waters in deep shelf areas below the level of recycling by wind-driven vertical circulation; (2) increased primary productivity, which caused an expansion of the oxygen minimum zone. This expansion, in ocean waters which were more prone to oxygen-starvation than those of today, resulted in prolonged accumulation of anoxic facies in the abyssal depths of the Iapetus Ocean (the Moffat Shales of the Southern Uplands). Phosphates, analogous to those forming today under oxygen-deficient waters off W-facing continental margins, were deposited in the Caradoc black shale facies of central Wales.

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