Final answer:
The younger dryas was triggered by both a massive outflow of freshwater into the North Atlantic Ocean and the subduction of the Juan de Fuca Plate.
Step-by-step explanation:
The correct answer is Both of the above. The younger dryas was presumably triggered by a massive outflow of freshwater into the North Atlantic Ocean and the subduction of the Juan de Fuca Plate. The massive outflow of freshwater into the North Atlantic Ocean disrupted the ocean circulation patterns, which in turn affected the climate and led to a cooling period known as the younger dryas. The subduction of the Juan de Fuca Plate also resulted in changes in ocean circulation patterns, contributing to the onset of the younger dryas.