Question

In paleoclimatology, the study of annual tree growth rings—dendroclimatology—provides high-resolution proxy data for historical temperature fluctuations. While tree-ring width primarily reflects annual precipitation and temperature anomalies during the growing season, specialized anatomical features offer even more granular insights into extreme weather events. Among these, "light rings"—tree rings characterized by abnormally thin latewood cell walls—serve as reliable indicators of unseasonal, severe freezing events during the late summer months.

Light rings form when sudden cold spells prematurely halt the lignification process in tracheid cells, the structural elements of wood responsible for water transport. During a typical growing season, trees construct thick-walled latewood cells late in the summer to prepare for dormancy. However, if surface temperatures drop sharply prior to cell wall completion, xylem development ceases prematurely, leaving a distinct band of poorly lignified, pale cells visible under microscopic examination.

Dendroclimatologists have frequently correlated widespread regional occurrences of light rings in high-latitude conifers with sulfur-rich volcanic eruptions. Explosive eruptions inject massive volumes of sulfur dioxide into the stratosphere, forming sulfate aerosols that reflect incoming solar radiation and trigger abrupt global cooling. For instance, tree-ring records from subarctic North America reveal a high frequency of light rings in 536 CE, aligning precisely with atmospheric dust veil records preserved in polar ice cores. Consequently, light ring formation provides a precise chronological marker for identifying major volcanic forcing events in pre-instrumental climate history.

According to the passage, the premature halting of xylem development that produces light rings is directly caused by which of the following?