Sediments

Sediments, pieces of worn rock and other natural debris, both on land and from the bottom of the oceans, tell scientists a story about what past climate conditions were like on Earth. On land, sediments can become trapped in snow and ice; in the oceans, they are trapped deep on the floor of the ocean. Sediments at the Earth's surface can be preserved over time by being deposited, undisturbed and cemented together over the years, and then subjected to great heat and pressure, when they are transformed into layers of rock.

Changes in the ways that air masses interact with one another over decades, centuries, millennia, or even longer can vary so significantly that the effects can become evident in marks left on sediments and rocks. According to a study conducted by members of the U.S. Geological

soiL

DEscRiPTioN

preferable climate

Alfisol

Moderately leached soil with a subsurface zone of clay accumulation and > = 35% base saturation.

Temperate and tropical regions

Andisol

soil formed in volcanic ash

All temperature regimes

Aridisol

caco3-containing soils of arid environments

Both hot and cold arid and semi-arid climates

Entisol

soils with little or no morphological development

A variety of climates

Gelisol

soils with permafrost within 6.5 feet (2m) of the surface

Alpine and Polar Regions where temperatures are continuously at or below freezing

Histosol

organic soils

Various climates

Inceptisol

soils with weakly developed subsurface horizons

A variety of climates except arid ones

Mollisol

Grassland soils with a high base status

Various climates

oxisol

Intensely weathered soils of tropical and subtropical environments

Zones with small seasonal variation and no soil freezing

spodosol

Acidic forest soils

Humid areas

ultisol

strongly leached soils with subsurface zone of clay and < 35% base saturation

Where precipitation exceeds evapotranspiration and water storage capacity; found in tropical areas

Vertisol

clayey soils with high shrink and swell capacity

Most major climate zones

Survey (USGS), sediments in the lakes in the upper Mississippi basin of the United States can provide a wealth of information on climate change for that area. They have documented both the Holocene paleoclimatic and paleoenvironmental changes. In their study, they looked at the various deposited sediments, the chemical characteristics of the lake beds, and the biological components of the area in order to reconstruct past conditions. They found that the best records came from lakes because their sediments generally contained identifiable annual increments of sediment.

One lake they visited and analyzed was Elk Lake in the Mississippi headwaters region of northwestern Minnesota, which had a nice sequence of varves associated with it. A varve is an annual layer of sediment. The Elk Lake varves they retrieved held an extraordinary amount of data about biological, chemical, and mineralogical components that they could relate not only to past climate but to environmental changes as well. Most of the sediments were lacustrine (formed in the bottom of the lake), but small amounts of silt and clay were blown into the lake by the wind and were mostly composed of quartz, making them identifiable from the lacustrine sediments. Both types of sediment layers were measurements of windiness. Diatoms are organisms that have heavy silica shells and require a certain level of turbulence to keep them in the photic zone (which also depends on windy conditions). Deposits of these were also analyzed in the sediments, indicating windy conditions. In addition, most of the sodium in sediments found in the deposits was blown in from soils surrounding the lake. Pollen was also blown into the lake.

Using all these proxy data, scientists were able to conclude that during the Holocene, after the last ice sheet retreated into Canada about 10,000 years ago, the area was first covered by a spruce forest and then replaced by a pine forest as soon as the climate began to warm. Then, about 8,500 years ago, climate continued to change and became drier. The pine forests were replaced by prairie vegetation (grass, sagebrush, and oak). During the next 4,500 years, the prairie ecosystem spread eastward 93 miles (150 km). Proxy information from soil samples taken in the area also confirms this. The sediments contained higher concentrations of sodium, indicating dry, drought conditions. Based on the proxy data collected and interpreted, experts at the USGS determined that this mid-Holocene drought affected a substantial area of North America and that active sand dune fields existed in Minnesota, the Dakotas, and Nebraska. One possible explanation for this is that a dry Pacific air mass controlled the climate of North America for a long period, a condition very similar to that experienced in the United States during the dust bowl of the 1930s, but on a much larger scale.

Worldwide, the remains of plants and animals, particles of matter, rock, dust, and clay accumulate at the bottoms of water bodies, such as oceans and lakes. Sediment deposits gradually build up over the years. In order to study the sediments, scientists must be able to retrieve them intact from their underwater environments. To preserve the soil profile, it must be removed intact. To accomplish this, a cylinder—also called a core—is extracted. The sediment core is then taken to a laboratory, and the contents are analyzed for the proxy evidence contained within them.

When ice cores are collected from deep ocean environments, the sediment cores within them may provide much useful information about past climate changes. Sediments can be infiltrated with fora-minifera (also called forams), extremely small marine animals. They are useful because they are good proxies for past ocean temperatures. When forams die, their shells sink to the ocean floor and become buried in the sediment. Because different types of forams live in different types of water temperatures, the species found in sediment cores can be used as proxies to infer the climate and temperature during the time they lived. In addition, when alive, forams take up two isotopes (two different kinds) of oxygen in order to grow their shells. The type used is determined by the temperature of the water. Because of this, the oxygen isotopes also act as proxies. By measuring the relative concentrations of the oxygen isotopes in the shells, scientists can verify and estimate past water temperatures, and hence climate conditions.

Cores are also retrieved from lake bottoms, a specialized field of science called limnology. All day every day, sediments accumulate. Embedded in these sediments are records of organisms that lived in and around the lake as well as proxy data that clue in scientists to the processes that occurred within the lake, the composition of the lake's water, the conditions of the watershed, and the atmospheric conditions that existed at the time of deposition.

The most commonly used proxies in paleolimnology are diatoms, which exist worldwide in almost every body of water, although each type is unique and identifiable. Some species prefer acidic waters and others alkaline waters. Some are found in nutrient-rich or nutrient-depleted environments. These differences help paleoclimatologists reconstruct past climate conditions and are desirable proxies because they are sensitive indicators of environmental conditions.

Plant remains are often found in the cores taken from lakes. These serve as extremely beneficial proxies because certain plants grow only in certain temperature ranges, precipitation ranges, and biomes. It is not uncommon to find leaves, wood, seeds, twigs, and pollen in the sediments at the bottom of a lake. Once the different types of vegetation are identified, scientists can determine what the climate was like in the area when the vegetation was alive.

If a core contains a high amount of silt and clay, this informs clima-tologists the climate was very cold at one time, with sparse vegetative cover and increased soil erosion. As water washed across the exposed soil, it carried it into lakes, where it eventually settled to the bottom. Conversely, if sediment cores have high organic content, it signifies a warm and humid climate that once supported ample vegetation.

Experts worldwide analyze sediment cores in order to understand the Earth's ancient climate. For example, Will Sager, at Texas A&M University, studies ocean sediments. He works with the Ocean Drilling Program (ODP), which uses a ship fitted with an oil well drilling rig to retrieve sediment cores from the oceans' bottoms. One of his major research objectives is to study variations in rock magnetic properties that can be used as proxies to determine climatic variations. The ship itself is almost as long as two football fields, with a derrick that towers 200 feet (61 m) above the waterline. A long pipe fitted to the rig can descend through 4 miles (6.4 km) of water to drill holes into the ocean floor and pull out ocean cores that are 36 feet (11 m) long. A seven-story laboratory hosts facilities for the examination of sediment and rock cores, studied by scientists from 21 countries—physicists, biologists, chemists, geologists, geographers, climatologists, and others. The core must be brought to the surface very carefully; if disturbed, millions of years of data could be destroyed. Many of the samples are analyzed on the ship in the on-board laboratory. Researchers analyze the particles and chemicals that have been trapped in the sediment for millions of years, specifically minerals such as silicates and dead microorganisms, plankton, and diatoms. As the sediments were deposited and buried, water was slowly squeezed out, and the sediments eventually turned to rock.

According to the USGS, geological oceanographers study thin slices of the retrieved cores. If adequate amounts of proxy data are available-minerals and microfossils—they can infer past temperatures of the ocean and how similar or dissimilar it was to today's ocean conditions. The ocean provides a unique environment for this because, unlike on land, erosion is not an issue, and a continual settlement of material is supplied to the ocean floor, providing sediment that shows continuous records of the past.

Oregon State University is also involved in analysis of deep-sea sediments. It maintain a repository of ocean core samples that scientists access in their quest to better understand past climate change. For example, Alan Mix, a professor at the College of Oceanic and Atmospheric Sciences (COAS), has spent considerable time analyzing fossil shells from the ocean floor and their carbon and oxygen isotopes in order to better understand upper ocean temperature change through time—an important aspect of global warming. Mix also analyzes forams and the seasonal cycle of how they settle to the ocean floor. The ways in which they accumulate in the sediments can tell scientists how climate has changed over time. According to Mix, "Many foraminifera—forams for short—live near the surface of the ocean. We can get a measure of near-surface temperatures, which says something about upwelling systems that bring cold water up from underneath, from deeper in the ocean. Other foraminifera live on the seafloor, and they allow us to track the organic matter that falls to the abyss."

Mix's main focus is on Earth's past ice ages because they provide useful information about global-scale climate change. He believes that it is the large-scale, long-lasting changes that involve the entire ocean that humans can expect to face with global warming over the next hundred years. Mix refers to the ice ages as "a natural laboratory of sorts to understand processes of climate change. You need to understand both temperature and salinity, and we think we have steadily improving tools to do that. It will usher in a new age of paleo-oceanography when we can really deal with both those variables."

According to Mix, "You can't just go randomly out there and plop a core down and expect to get something good and useful. Analytical tools include surveys using sound waves to make maps of the seafloor and what is beneath it. Although most coring is successful, sometimes researchers have to go back over the years to get it right."

Another interesting observation Mix has made from his research on the ocean is that, while all along scientists have believed the ocean was a passive component of climate change—that it simply reacted to changes in the atmosphere as a passive absorber of climate change— this is not true. Instead, the ocean is a dynamic player in ice age-scale climate changes and is much more sensitive to change than scientists originally thought.

Weathering rates from continents, deep ocean flow mapping, and oceanic thermohaline circulation changes are three areas being looked at today through the analysis of carbon-13 records. Scientists use the carbon-13 proxy to study the global carbon cycle. So far, carbon-13 research looks promising as a new proxy that will be able to shed light on past global warming.

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