Britannica Sources used in this answer

How did restricted ocean circulation contribute to oxygen-poor Cretaceous waters?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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

During the Cretaceous Period, restricted ocean circulation played a critical role in the development of oxygen-poor, or anoxic, conditions in deep marine environments. Because many developing ocean basins, such as the North Atlantic, were constricted, the global oceans experienced significantly less water circulation and mixing than they do today.123

This lack of vigorous circulation was compounded by minimal temperature differences between the poles and the Equator, which reduced the driving forces for ocean currents. Furthermore, the oceans became well-stratified into dense layers, preventing the vertical overturning and mixing necessary to replenish dissolved oxygen in deep waters. In these stagnant, oxygen-depleted settings, organic matter was preserved rather than decomposed, leading to the widespread deposition of [black shales].2435