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How may low-oxygen seas have contributed to the Early Jurassic extinction event?

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

During the Early Jurassic, an extinction event occurred at the Pliensbachian–Toarcian boundary (approximately 183 million years ago), which significantly interrupted the recovery of marine ecosystems following the earlier Triassic-Jurassic extinction.1

This extinction is believed to be linked to the onset of low-oxygen (hypoxic or anoxic) conditions in epicontinental seas. Evidence for these conditions is found in the presence of organic-rich [shales]. These environments were likely so oxygen-depleted that they prevented the survival of burrowing organisms and hindered the efficient breakdown of organic matter. As these restricted basins grew and connected with the open ocean, their lack of vigorous circulation further contributed to these low-oxygen levels, creating a hostile environment for many benthic (bottom-dwelling) invertebrates. This environmental stress resulted in significant biodiversity loss, with some regions experiencing the extinction of up to 84 percent of bivalve species.12