How could volcanic ash have disrupted photosynthesis near the end of the Permian Period?

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

Near the end of the Permian Period, the prolonged eruption of the Siberian flood basalts released massive quantities of volcanic ash into the atmosphere. These extensive clouds of ash likely darkened the skies, significantly restricting the amount of sunlight reaching the Earth's surface.123

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 process carbon dioxide. Consequently, the rate at which plants could extract carbon dioxide from the atmosphere was lowered, which may have contributed to broader disruptions in the carbon cycle and the subsequent mass extinction events.13