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How did Ordovician ocean circulation contribute to deep-water anoxia?

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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.

During the Ordovician Period, deep-water anoxia—a state of oxygen depletion—was primarily driven by sluggish ocean circulation patterns. Unlike modern oceans, where deep water is oxygenated by the sinking of cold, dense polar water, Ordovician bottom water originated in warm, saline tropical epicontinental seas.1

In these warm, shallow environments, rapid evaporation increased the water's density due to high concentrations of dissolved salts, causing it to sink and circulate slowly. This lack of vigorous vertical mixing resulted in the widespread accumulation of black shales in deep-water settings, which serve as geological evidence of these oxygen-poor conditions.1

This circulation pattern only accelerated during extended periods of continental glaciation, when chilled surface waters descended more rapidly than warm, saline tropical waters, thereby increasing the strength of ocean currents and upwelling.1