The Nature Conservancy unveiled an agreement yesterday under which it will raise $37 million to purchase and maintain the Palmyra Atoll, a privately owned cluster of pristine coral islets about 1,000 miles south of Hawaii. The conservancy wants to create a nature preserve for marine and climate research and allow limited, ecologically sound tourism on the atoll. Palmyra, which includes 680 acres of land on 52 islands and 15,512 acres of coral reefs, is rich in wildlife and one of the only places in the Pacific that has substantial freshwater resources but has never been permanently settled by humans. “Palmyra represents the most important unprotected marine-wilderness area left in the U.S. tropics,” said Jim Maragos, a coral reef biologist with the U.S. Fish and Wildlife Service.