60 days in Iceberg Alley, drilling for marine sediment to decipher Earth’s climate 3 million years ago

Competition is stiff for one of the 30 scientist berths on the JOIDES Resolution research vessel. I’m one of the lucky ones, granted the opportunity to work 12-hour days, seven days a week for 60 days as part of Expedition 382 “Iceberg Alley” in the Scotia Sea, just north of the Antarctic Peninsula.

I’m a paleooceanographer. My research focuses on how Earth’s oceans and climate operated in the past; I’m especially interested in how much and how fast the Antarctic ice sheets melted between 2.5 to 4 million years ago, the last time atmospheric carbon dioxide levels were about 400 parts per million, as they are today. This work depends on collecting sediment samples from the ocean floor that were deposited during that time. These sediment layers are like a library of the Antarctic’s past environment.

The JOIDES Resolution is the only ship in the world with the drilling tools to collect both soft sediment and hard rock from the ocean – material that we recover in long cylinders called cores. No wonder researchers from all over the world, at all career stages, are excited to have traveled from India, Japan, Korea, the Netherlands, Germany, Spain, Switzerland, Brazil, China, Germany, Australia, the United Kingdom and, of course, the United States to join the expedition.

Fieldwork 1,000 miles (1600 km) from port

Two months is actually a short amount of time in which to address scientific research questions, but there have been years of careful planning and detailed preparation in advance of this expedition. We scientists onboard make best use of our limited time by drilling at what we’ve already agreed should be the most informative locations.

[embedded content] An animation explains the drilling process.
When the ship arrives at the designated GPS location, the captain, the lab officer and the drilling engineer all check the position coordinates several times. With the ship’s thrusters keeping it precisely in place, workers lower coring equipment, including drill pipe, through an opening in the center of the ship. When the drill pipe reaches the coring depth – in our case ranging from 2,600 feet (800 meters) to 12,500 ft (3,800 m) – we lower a coring tool on a wireline down through the pipe.

Most of our cores are taken with an advanced hydraulic piston corer. In a process similar to using an elaborate cookie cutter, it punches through the ocean floor and collects a thin cylinder of the rock and sediment: our core sample. The wireline brings the 31-ft-long (9.5 m) core back to the ship. In the ship’s lab, we split the core lengthwise into an archive half – to be photographed and described – and a working half. This is the one we sample onboard for density, chemistry and magnetic properties.

Co-chief scientist Michael Weber and sedimentologists (core describers) Suzanne O’Connell and Thomas Ronge examine the archive half of a split core at the describing table. Stephanie Brachfeld/IODP, CC BY-ND
Today the Greenland and Antarctic ice sheets contain 99% of Earth’s fresh water. If all the Antarctic ice were to melt, average sea level would rise 200 feet (60 m). This won’t happen in your lifetime. But knowing how fast an event like this can occur – based on how fast ice has melted in the past – is critical to preparing for the sea level rise already accompanying Earth’s currently warming temperatures. Helping to understand that past change is one of the goals of our work on this expedition.

Establishing when it was that melting glaciers originally deposited the sediments we’re collecting is crucial and difficult. Only by dating this process can we figure out how fast the ice sheets disintegrated. There are two complementary approaches that researchers have traditionally used.

A microscopic fossil of diatom Actinocyclus actinochilus. Jonathan Warnock/Indiana University of Pennsylvania, CC BY-ND
Paleontologists look at tiny microfossils from organisms such as diatoms, radiolaria and dinocysts that are found in the sediment cores. Then they can match up the species they spot in the samples with the timeframes they were known to exist. For instance, a paleontologist might know from previous research that a particular species of diatom lived between 1.8 and 2.6 million years ago.

Sediment samples, called cubes, taken for future paleomagnetic research and marked styrofoam plugs identify where samples were taken for ‘moisture and density’ (MAD) measurements. Stephanie Brachfeld/IODP, CC BY-ND
A second method of dating depends on paleomagnetists measuring the strength and direction of the sediments’ magnetism. Over Earth’s history, the magnetic field has reversed, with magnetic north flipping to point south, at irregular intervals. Scientists know when the reversals occurred. In the period from 1.8 to 2.6 million years ago, for example, the magnetic field flipped four times.

The paleomagnetists look for reversals in the alignment of magnetic minerals in the sediment we collect, and if they find them, they can better identify when, within that 1.8 to 2.6-million-year time interval, the sediment was deposited. If reversals are not present, it might mean the sediment accumulated so fast that only one magnetic interval is represented, or that part of the sediment record is missing. To determine which possibility is more likely, they talk to the people describing the visual properties of the core to see if there are abrupt changes that might indicate a disruption in the sedimentary record.