The remote island of Nihoa has perhaps the most densely situated collection of archaeological structures existing in all the Hawaiian Islands, but it's not clear when it was first inhabited or when its major stone features were built.

Two archaeologists who recently spent a week on the island hope to establish a time line by using a new tool for archaeology: dating corals used in the construction of sites or placed on top as offerings.

"We hope there will be a whole new core of dates" that will help clear up the issue of when Polynesians were active in the Northwestern Hawaiian Islands, said Kekuewa Kikiloi, who collected corals for dating from several structures.

Chunks of coral found within the walls of structures on uninhabited Nihoa are expected to provide clues as to when they were first built. And pieces on the surface may suggest when the shrines were actively used.

The tiny island measures a half-mile from east to west and almost all of it is either steeply sloping or vertical. On its south-facing slopes, Polynesian visitors or residents built dozens of temples, shrines, agricultural terraces, living platforms and other structures.

Kehaulani Souza said the site reminded her of a Polynesian Machu Picchu, the stone Incan fortress city in Peru.

"It was so much to grasp all at one time, it was overwhelming," Souza said. "My breath was taken away because of all the structures from the bottom of the island all the way to the top."

Souza intends to research old documents and talk to elderly Hawaiians to try to identify the original Hawaiian names for Nihoa features that now have English names, such as East Palm Valley and Miller's Peak.

In addition to gathering materials for coral dating, Kikiloi measured and mapped archaeological sites. He said some of the more interesting sites had not been mapped, perhaps because of the danger of working around them.

Shrines atop Miller's Peak are on steep rocky promontories.

"It's pretty intense," Kikiloi said. "You're high up and right on the edge of the cliff."

The archaeologists said the island's temple designs are remarkably varied, including simple walled enclosures, stone platforms and structures with pebble paving.

The dominant features of many are significant upright stones — often collected from volcanic dikes on the island. Such uprights are reminiscent of some South Pacific island temples, but are occasionally found in the main Hawaiian Islands as well.

"There was a wide variation of things going on, and we're trying to figure out what is the timing of some of construction," Kikiloi said.

The traditional method for establishing dates of archaeological sites is to test the ratio of radioactive carbon-14 to standard carbon-12 in a sample of once-living material, such as wood. But Nihoa is a dense bird rookery, and the island's soil is quite acidic due to the excrement, known as guano. The acidity breaks down organic matter, so there isn't much wood to date, Kikiloi said.

The newer coral dating technique works in a slightly different way. Corals take up uranium from ocean water, and the uranium degrades at a known rate into thorium. The ratio of isotopes of uranium to isotopes of thorium provides a means for determining the age of the coral.

"Coral is abundant at most ceremonial sites. And you can get dates accurate to within 10 or 20 years — a lot more accurate than carbon-14 dating," Kikiloi said.

He and Souza were on the island for six days earlier this month along with biologists and Kaua'i science teacher David Boynton. The trip was sponsored by the U.S. Fish and Wildlife Service and the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve.

The group was dropped off and picked up by the NOAA vessel Hi'ialakai, which was conducting an educational tour of Nihoa, Mokumanamana and French Frigate Shoals with 10 Hawai'i educators.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.