How could volcanic ash have disrupted photosynthesis during the Permian extinction?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

During the Permian extinction, the massive eruption of the Siberian flood basalts released vast quantities of volcanic ash into the atmosphere. This ash created dense clouds that darkened the skies, significantly restricting the amount of sunlight reaching the Earth's surface.12

Because photosynthesis relies on solar radiation to drive the process of carbon fixation, this reduction in available light inhibited the ability of plants to effectively convert carbon dioxide into organic matter. By lowering the efficiency of this process, the volcanic ash disrupted the biological cycle and reduced the rate at which carbon dioxide was extracted from the atmosphere, contributing to the environmental instability of the period.132