Print
Email
by Erin Loury
When scientists go drilling, they are searching not for oil or gas, but for history – a record of Earth’s past climates written in layers of mud and rock pulled from the heart of the seafloor. “We’ve long understood that the best place to collect sedimentary records of how the planet has changed is in the ocean,” explains Robert Dunbar, the W.M. Keck Professor of Earth Sciences and Senior Fellow at the Woods Institute for the Environment at Stanford University. Discovering how Earth’s climate has transitioned between warm and cold periods over millions of years helps scientists better understand our current changing climate, and what the future may hold.
Dunbar recently returned from a 65-day expedition to East Antarctica aboard the scientific drilling ship JOIDES Resolution, marking his 28th trip to the polar continent. He and a team of international marine geochemists and climate scientists were interested in exploring the history of Wilkes Land, a region of East Antarctica south of Australia. The ice in this area is relatively unstable and susceptible to melting – just how much will have strong consequences for future sea level rise. “I think there is a risk that over the next century, enough of the coastal ice in parts of Antarctica will melt that sea level will rise by one to three meters,” Dunbar said.
Sediments accumulating on the sea floor year after year can provide a continuous record of Earth’s history, as they are protected from many of the erosive processes that weather terrestrial sediments. To reach this geologic goldmine, scientists at sea must first lower their drill pipe through 14,000 feet of water – like dangling a rope from the top of Mt. Whitney to reach sea level. Once the drill reaches the bottom of the ocean, it cuts a core about the width of a drainpipe deep into the sediment for hundreds of meters, even up to several kilometers of depth. The rock is so hard in places that the drill bit and the saw used to cut the cores are tipped with diamond.
Scientists aboard the ship work around the clock in 12-hour shifts to process these sediment cores once they arrive at the surface. The ship’s crew saws the cores into more wieldy 10 meter sections, striving to preserve every last bit of precious material. Scientists then pore over these long, skinny layer cakes in the laboratory, searching for clues of the changing past. Pebbles trapped in the core are the calling cards of wandering icebergs that scraped debris from the continent and dropped these pieces of rock in their wake. The quickly-evolving architecture of single-celled phytoplankton called diatoms help paleobiologists determine the age of the core, and estimate ancient levels of primary production. Fossil biomarker molecules from the remnants of feces even provide a glimpse of ancient marine mammal populations.
Dunbar said that the expedition yielded several valuable results, such as evidence confirming Antarctica’s warmer past. A core dating back 54 million years abounds with clues of a temperate Antarctic, including tree pollen from Antarctica’s past forests, muddy clay minerals formed by warm and wet weathering, and plankton called dinoflagellates that indicate ocean waters of 15–20 ˚C. “The best explanation for this, after thinking about many possibilities, is the high levels of carbon dioxide in the atmosphere,” Dunbar said. This reconstruction of the past provides a glimpse of Antarctica’s future at some point in a warmer, greenhouse world.
Another prize of the expedition are several 200-meter long sediment cores that span the Holocene epoch, from the present day to about 10,500 years ago. The sediment shows a consistent two centimeter layer deposited each year for the entire period, giving scientists an unusually high-resolution record of climate change and sea ice conditions in the recent geologic past. Dunbar said this valuable record will help scientists understand the natural climate variability caused by El Niño events and decadal oscillations, well before the influence of human activity. When Dunbar receives his samples from this core he will start by analyzing the very bottom, where the first sediments formed when the Antarctic ice began to retreat at the end of the last glacial period. “I want to know how fast this part of East Antarctica melted when the planet was warming up,” Dunbar said. “If we learn that sea level can rise at rates of five centimeters per year, and that it has done so every time Antarctica melted, then we certainly should include that risk as a possibility in our assessment of the centuries ahead.”
A third valuable contribution from the expedition is the observation of what Dunbar called “super greenhouse events.” At these points in time, one about 400,000 years ago, and another 1 million years ago, a huge volume of ice melted in Antarctica, Southern Ocean temperatures climbed several ˚C above what they are today, and global sea level rose anywhere from 20 to 50 feet. “We’ve known about a few of these events, but they haven’t been well explained. Now we’ve seen evidence in the new drill cores for even more of these unusual warm events,” Dunbar said. “Our best explanation now invokes some kind of climate “surprise” event: you exceed some threshold, and bang! Things happen really fast. We’d like to know what those things are because we may be on the verge of exceeding a similar threshold in our near-term greenhouse future.“
On top of his scientific activities aboard the ship, Dunbar also updated a blog full of stories, photos and videos about the project. This blog is part of the Exploratorium’s Ice Stories project, which spotlights scientific exploration of the Arctic and Antarctic. The project began during the International Polar Year (2007-2008). Dunbar’s blog gives a first hand account of how scientists use the past to predict the future, and the importance of doing so in our changing world.
“I think the biggest threat of climate change is in an altered hydrological cycle: where and when it rains, how much it evaporates, and changes in sea level,” Dunbar said. “We’re doing everything we can to discover the maximum possible rates of Antarctic ice melting. This information can help us predict what sea level changes might be possible in the next century or so.“
