Question

Read the passage below:

[1] For centuries, historical climatologists attributed the global cooling event of 1453 CE primarily to localized shifts in North Atlantic oceanic circulation patterns. [2] However, recent chemical analysis of polar ice cores from Greenland revealed anomalous concentrations of fine-grained tephra and elevated sulfate aerosols strictly dated to the mid-fifteenth century. [3] Geochemical fingerprinting of these micro-tephra deposits demonstrated a distinct magmatic composition matching that of the Kuwae caldera in the Vanuatu arc, rather than European or Icelandic volcanic sources. [4] This empirical evidence confirmed that an ultra-plinian eruption in the South Pacific, rather than internal atmospheric variability in the Northern Hemisphere, served as the primary trigger for the decade-long volcanic winter that followed. [5] Consequently, researchers have begun re-evaluating historical agricultural yield logs across East Asia, finding that crop failures previously attributed to local mismanagement coincide precisely with the atmospheric fallout pattern of the Kuwae event.

Which sentence in the passage provides the specific geochemical finding that identifies the geographic origin of the mid-fifteenth-century volcanic deposits?