Swampy tales yield clues about Hawaii’s climate past
Until about 14,500 years ago, Hawaiʻi was much cooler than today, with a glacial ice cap of more than 27 square miles sitting on top of Mauna Kea, and very likely an ice cap also on Mauna Loa. What happened to Hawaiʻi’s climate when Earth warmed and the ice sheets in the Northern Hemisphere started to retreat?
Two years ago, they hiked into Kaʻau Crater in the back of Pālolo Valley on Oʻahu to take a sediment core from the swamp within the crater walls. The expedition was a success. They extracted sediment core samples 5.5 meters deep.
Anomaly or pattern?
Radiocarbon dating revealed that this core goes about 14,000 years back in time and that its history overlaps the end stages of the last glacial period. Further analyses in Beilman’s lab showed highly unusual dense organic matter and nitrogen isotope values in a layer of the core dating back 5,200 years. Other sections of the core are now being analyzed by specialized labs on the mainland.
Whether the 5,200-year anomaly is the local manifestation of a massive tropical drought that occurred at the same time in many different places around the world is a question the researchers plan to tackle by extracting sediment cores from other swamps and lakes in Hawaiʻi.
One such other possibility is a bog near the Poamoho section of the Koʻolau summit trail. Reaching this swamp is a challenge, even without carrying all the heavy equipment needed to take core samples.
Timmermann and Beilman had tried several times in vain to get to the swamp. Then a graduate student from Beilman’s lab found a way down. One Sunday last August, Timmermann, IPRC postdoctoral fellow Malte Heinemann, Beilman and a group of Beilman’s students made another attempt. After a two-hour hike, they arrived at the Koʻolau summit trail, which they had to follow for another hour.
And then the real adventure began.
Fighting vertigo, they crept along the slippery, muddy, narrow trail, no more than a foot wide in places, with powerful wind gusts threatening to blow them off the ridge.
Finally, Heinemann recalled, “we saw the swamp; it looked so close, only a few hundred feet below us. But it took us nearly two hours to reach it. Each step, I thought might lead to nothingness, so I always held onto some kind of vegetation.”
Added Timmermann: “Once my hiking pole suddenly hit no ground. I looked and saw I was walking on a moss over-grown tree, next to a 100-foot-deep pit.”
Bushwhacking through thick vegetation, they reached a place where they could attach a rope ladder for the final descent. By the time they had found a good place to core into the sediment, it was early afternoon.
Taking core samples is tricky. An augur attached to the coring instrument cuts into the swamp’s surface, then the handle is twisted into the earth to a depth of about two feet. The section is pulled out, cut with a knife and gently wrapped in foil.
There was only time for two whole core sections, a total of about five feet.
“The top layer was wet,” said Heinemann. “Then came a good firm core, and then came a layer that was contaminated with little pebbles…a sign of a disturbance, a landslide. So we were a bit disappointed that we couldn’t get deeper.”
Before leaving, they installed a water logger to keep track of the water level in the swamp and a box to house instruments for collecting climate data.
What goes down…
Climbing back up the swinging rope ladder with heavy equipment to the summit trail was harder than the descent, especially for Beilman. He was carrying the core sections, which needed to remain horizontal in their foil wrappings.
The group finally reached the trailhead at dusk.
“We are planning more trips to this unique place in the hope that we can take longer cores,” said Timmermann. “We will send the samples to highly specialized labs in Ireland and at the University of California, Santa Cruz, to measure radiocarbon and hydrogen isotopes. We’re also thinking of collaborating with paleo-pollen experts from the University of Wisconsin.”
The sediments could be a useful addition to the sparse proxy data archive for Pacific climate in the distant past, he said.
—by Gisela E. Speidel, outreach specialist, International Pacific Research Center, and reprinted with permission from the International Pacific Research Center’s newsletter Climate.
Read the research report on a recent paper about ocean acidity and coral reefs coauthored by Timmerman.