Final answer:
Secretin is the hormone that triggers the pancreas to release bicarbonate-rich pancreatic secretions to neutralize acidic chyme in the duodenum, working together with cholecystokinin (CCK).
Step-by-step explanation:
When highly acidic chyme enters the duodenum, the hormone that stimulates the release of pancreatic secretions that contain very high bicarbonate ion content is secretin. Secretin's main function is to neutralize the acidity of chyme as it comes from the stomach into the intestine, to create a more favorable environment for the digestive enzymes to function. Additionally, cholecystokinin (CCK) is also involved in this process, promoting the release of enzyme-rich pancreatic juices and stimulating the gallbladder to release bile.