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Why do black shales indicate low-oxygen conditions in Ordovician deep seas?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

In the context of the Ordovician Period, the presence of widespread black shales in deepwater settings serves as evidence of anoxic, or oxygen-depleted, conditions.1

These conditions were primarily driven by sluggish ocean circulation. Unlike modern oceans, where deep water is generated by the sinking of cold, dense polar water, Ordovician bottom water originated in warm, saline tropical epicontinental seas. The relatively rapid evaporation in these warm environments increased the water's density due to high concentrations of dissolved salts, leading to stagnant circulation and resulting anoxia in deep marine environments.1

Furthermore, the accumulation of organic matter is a hallmark of black shales. In environments where oxygen levels are low, the decomposition of organic remains is significantly decelerated. This lack of oxygen prevents the breakdown of organic material, allowing it to be preserved within the sediment, which is a key factor in the development of black shales.23