Paleoclimatology A Record from the Deep
 

by Holli Riebeek · design by Robert Simmon · September 27, 2005
 

Clad in a hard hat and steel-toe boots, paleoceanographer Jerry McManus strides onto the deck of the JOIDES Resolution, staring through the steel rigging that supports the ship’s drilling equipment at the brilliant star-studded sky. Here, in the middle of the ocean, city lights do not dim the night sky, and the clear view is spectacular. McManus, an associate scientist at the Woods Hole Oceanographic Institution, has just completed another 12-hour shift in one of the ship’s six science labs, where he has been analyzing samples of the sea floor to glean bits of evidence about past climates.

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  Photograph of the ocean drilling ship JOIDES Resolution  

The JOIDES Resolution cruises the globe sampling sediments from the bottom of the world’s oceans. The ship is capable of drilling holes over 2,100 meters (6,890 feet) below the sea floor in water up to 8,000 meters (26,000 feet) deep. (Photograph courtesy JOI Alliance/IODP)

 

Even now, in the dead of the night, it is not quiet. The ship’s twelve powerful thrusters whine constantly as their 750-horsepower engines struggle against the ocean currents to keep the ship in one place while the drilling crew pulls long sections of mud from the sea floor. The science labs continue to bustle as another crew replaces those who are leaving for the night, and caterers and housekeepers move through the ship to support the science and drilling teams. McManus pauses to scan the surface of the ocean for signs of whales or other sea life. He saw a manta ray jumping once, but not tonight. He returns to his room in the forecastle deck. Two sets of bunk beds accommodate the four people who share the room, but he rarely sees his roommates. They are on other shifts in this all-too-brief voyage to coax climate secrets out of the ocean depths.

  Photograph of scientists over a core table

Researchers study a freshly recovered sediment core inside one of the science labs on board the Resolution. (Photograph courtesy JOI Alliance/IODP)

  Photograph of sea floor and ocean detirtus from the submersible Alvin

Year after year, a steady rain of dust, plants, and animal skeletons settles on the ocean floor. As new materials pile on top of old materials, layers of sediment form a vertical timeline extending millions of years into the past. McManus and his colleagues on the Resolution are drilling long cores of the ocean floor to read the timeline. The 470-foot-long research vessel is specially equipped to pull cores of mud from the sea floor. Much of equipment, and the ship itself, is adapted from tools the oil industry uses to drill at sea, and, as a result, the Resolution resembles an oil rig with its steel drill tower and deck-top cranes.

 

Over thousands and then millions of years, the sea floor becomes covered in a thick layer of sediment. Plants and animals living in the ocean above die and decay, contributing their skeletons along with dust, volcanic ash, and other inorganic material. Scientists use the accumulated material as a timeline to study past climate. (Photograph copyright Woods Hole Ocean and Climate Change Institute)

 

In the center of the ship, long sections of pipe snake down to the sea floor where a drill is fitted on the outside of the pipe. A solid piston inside the pipe moves up as the pipe plunges into the mud so that the pipe fills with mud as it sinks. The goal is to pull up a column of sediment without disturbing it. Stirring the sediment would destroy the timeline preserved in the layers. The pipe draws up 10-meter segments of earth at a time. A cone with a homing device rests over the drill hole so the pipe can be lowered into the same location to retrieve the next 10 meters until the drill hits the solid rock of the sea floor.

The Resolution is perhaps the most advanced scientific ocean drilling ship, and an international consortium of ocean researchers called the International Ocean Drilling Program is responsible for it. Though the technology is vastly different, the idea of a science-dedicated ocean exploring vessel isn’t too far off from the first explorations in the 1870s. On December 21, 1872, a three-masted, square-rigged wooden ship set sail from Portsmouth, England, to start a three-and-a-half-year voyage that would take the HMS Challenger from the North Atlantic to Antarctica and around the world. The ship’s crew and teams of physicists, biologists, and chemists from around the world sounded out the depths of the ocean, collected samples of plants and animals and ocean water, and recorded sea temperature at various depths. They published their results in a 50-volume report, each volume containing 29,500 pages. The voyage of the Challenger became the basis of modern oceanography.

Scientists on the Challenger dredged the ocean floor with large bags to collect plant and animal samples. They found that the ocean was covered in fine sediment that contained the fossils of sea animals. What was more, the fossils were different in cold areas verses warm areas. The finding thrilled paleoclimatologists, who wanted to use the fossils to determine how cold the oceans had been in the past. Scientists almost immediately began to devise systems of hollow pipes that could be used to bring a column of the sea floor to the surface.

  Sequence of photographs showing core drilling

Drilling on board the Resolution continues day and night. From top to bottom: drilling derrick, drill bit, re-entry cone, and a retrieved core. (Photographs courtesy JOI Alliance/IODP)

  Engraving of the HMS Challenger

These records from the deep yielded many important insights to the Earth’s past climates. Each layer within the core holds fossils of the tiny plants and animals that dominate the ocean, as well as grains of dust and minerals that can tell about wind and current patterns. Like land fossils, marine fossils offer clues about conditions in the ocean when the plant or animal lived. The cores are carefully labeled (“this way up” is a crucial designation for the vertical time lines) and divided into smaller sections for analysis.

 

The 19th-century voyage of the HMS Challenger set the standard for subsequent ocean research vessels. Among other discoveries, scientists on the Challenger realized that fossils retrieved from samples dredged from the sea floor revealed past climates. (Image courtesy NOAA Photo Library)

 

A Record from the Deep: Fossil Chemistry

 

The most valuable fossils found in sediment cores are from tiny animals with a calcium carbonate shell, called foraminifera. One species of foraminifera lives in the icy waters of the Arctic above Iceland and near Antarctica. When McManus and other scientists began to uncover a large number of fossils of polar foraminifera in cores collected off the coast of Great Britain as part of an ongoing research project, they knew that the waters there had once been much colder. Once the fossils had been dated, they told scientists when the ocean had been icy cold. By finding cold-water foraminifera of the same age elsewhere in the oceans, scientists can construct maps showing where cold water existed at various points in the Earth’s history.

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  Scanning electron micrograph of foraminiferal sand taken from an ocean sediment core

The microfossils themselves can speak volumes about the chemistry and temperature of the ocean. The calcium carbonate shells of foraminifera and coccoliths (their plant counterparts), and the silicon dioxide shells of radiolarians (animals) and diatoms (tiny plants) all contain oxygen. Oxygen in sea water comes in two important varieties for paleoclimate research: heavy and light. The ratio of these different types of oxygen in the shells can reveal how cold the ocean was and how much ice existed at the time the shell formed. In general, the shells contain more heavy oxygen when ocean waters are cold and ice covers the Earth. [For details, see The Oxygen Balance.]

Wind and Water Currents

A large deposit of microfossils of plants and animals can also tell scientists about ocean currents and wind patterns. Ocean plants and animals use the nutrients at the surface of the ocean, die, and then carry the nutrients with them as they sink to the sea floor. In some regions, strong ocean currents sweep nutrients up from the bottom to feed a thriving population. Called upwelling, the phenomenon drives plant and animal populations up until the nutrients are all used, and the microscopic plants and animals die. A small plant called a diatom takes particular advantage of upwelling. Ocean cores hint at patterns of upwelling when one contains a particularly thick layer of microfossils, especially diatoms, from the same time. Since upwelling currents are largely driven by the wind, these patterns also tell scientists something about wind and weather patterns.

 

Foraminifera skeletons found in sediment cores provide scientists a means to date cores. Fossils also contain information about ocean temperature, chemistry, currents, and surface winds. (Micrograph copyright Eric Condliffe, University of Leeds Electron Optics Image Laboratory)

scale for core photograph photgraph of a core cross-section

Dust in ocean cores can reveal weather and current patterns. Today, great plumes of Saharan dust snake their way across the Atlantic to the American continents. Dust blows into the Pacific from Asia’s vast interior deserts. When the dust shows up in ocean cores, scientists can analyze its chemistry to determine where it came from. By charting the distribution of the dust, scientists can see where the winds were blowing and how strong they were. The dust also gives scientists a glimpse into how dry and dusty the climate may have been at a particular time.

 

This image sequence shows the cross-section of a core drilled in the Mediterranean Sea. Sediment layers can be formed from dust, volcanic ash, river sediments, underwater mudslides, plant and animal skeletons, precipitated calcium carbonate, or salts left behind by an evaporated sea. (Image courtesy Integrated Ocean Drilling Program)

  Satellite image of African dust blowing over the Atlantic
 

Bits of continental dust can be swept into the ocean by rivers as well as the wind. The dust’s location on the ocean floor as well as the mineral content of the dust give scientists clues about where the dust came from and how it arrived at its final location on the ocean floor. For example, dust in the middle of the Pacific Ocean is more likely to have been carried on the wind than deposited by rivers.

Sediment made up of mineral grains from the continents can also tell about ocean currents. After the sediment is dumped in the ocean by the wind or rivers, it is swept on currents to its final resting place on the ocean floor. The distribution of the mineral grains can reveal how strong the currents were and where they flowed. Currents also carry icebergs from their point of origin toward the equator where the ice melts. Rocks and soil embedded in the icebergs sink to the ocean floor and later show up in ocean sediment cores. Their final location tells scientists where currents once flowed, and, indirectly, where water warm enough to melt the ice existed.

Ocean cores proved invaluable as scientists built a picture of the Ice Age Earth. They provided a record of a large part of the Earth stretching back millions of years, showing large patterns of climate change. In a project called Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) in the 1970s, sea cores allowed scientists to reconstruct the climate of the Earth in the last Ice Age 20,000 years ago. “It’s still one of the major successes of the deep sea,” says McManus. “Even today, it serves as the focus of new studies.”

 

Wind can transport dust grains thousands of miles from the interior of a continent out into the open ocean. Dust in sediment cores can indicate the direction of prevailing winds and conditions on land far from the core’s location. (NASA Image courtesy Jeff Schmaltz, MODIS Land Rapid Response Team)

  Sea surface temperature anomalies during the last glacial maximum derived from CLIMAP data

Though ocean sediment cores are immensely valuable for tracking climate over most of the Earth, they do not give scientists a picture of year-to-year change. For that, scientists look at information preserved in the ice sheets of Greenland and Antarctica. The next installment in this series will describe how scientists use ice cores to understand year-to-year variation in Earth’s past climate.

    References:
  • Bradley, R., 1999: Paleoclimatology, Academic Press, Harcourt Brace and Company, San Diego, California.
  • Imbrie, J. and K. P. Imbrie, 1979: Ice Ages, Enslow Publishers: Hillside, New Jersey.
  • McManus, Jerry, 2004: "Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes," Nature, vol 428, no 6985, pp 834-837.
back A Record from the Deep

 

The CLIMAP project developed maps of sea surface temperatures during the peak of the last Ice Age based on ocean and lake sediment cores. This map shows the difference in temperatures between then and now (blue areas were colder, red areas warmer). One interesting feature is the presence of much colder water in the North Atlantic—evidence that the Gulf Stream Current has shifted in the past. (NASA Image by Robert Simmon, based on CLIMAP 18K data archived at the NOAA National Geophysical Data Center)