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Dr. Randy M. Russell

Randy Russell

Sediments from Lakes Beds and Seafloors

Layers of sediments that slowly accumulate on the bottoms of lakes, seas, and oceans gradually enshrine a very long-term history of climate information. Sediment records can span hundreds of millions of years or longer, though the resolution of such records is typically only on the order of a century.

Rainfall and the runoff it produces washes soil into rivers, which flow into lakes and seas. As the moving river water, with its suspended sediments, reaches the calmer lake or ocean environment, the sediments fall to the bottom, gradually building up layer upon layer of deposits. Likewise, fine grains from distant dust storms and ash from far-off volcanoes can be blown over water, fall to the surface, and sink to the bottom to join the deposition process. Organic materials from creatures living within the water column also rain down upon the depths to join the muck at the bottom, particularly in deep-ocean environments.

Some types of deposition processes have seasonal cycles that leave annual layers in the sediments similar to tree ring patterns or ice core layers. For instance, sediments in rivers tend to arrive in greater quantities and to be of a different color during spring runoff than during the rest of the year when flow rates are lower. Sediment deposits strongly influenced by input from rivers thus can display annual layers.

Until they are well buried, the uppermost layers of a sedimentary deposit are subject to disturbance by several factors, which tends to diminish the value of this type of record. Earthquakes, deepwater currents, underwater landslides, and mixing by burrowing mollusks and worms can stir up the uppermost layers of a deposit, decreasing the resolution of the record. In coastal areas, waves and tides can destroy sediment deposits, and rising and falling sea levels can interrupt or make a jumbled mess of a deposition sequence. Near shore deposits often contain valuable data about precipitation rates due to the influence of river runoff on such deposits, but are unfortunately often destroyed by the numerous factors that attack shallow-water deposits.

Ocean sediment cores locations map
Light blue dots show locations from which ocean floor sediment cores have been extracted.
Credit: National Oceanic and Atmospheric Administration Paleoclimatology Program/Department of Commerce

Because subduction of the seafloor, driven by plate tectonics, gradually recycles all seafloor material, the oldest seafloor crust (and thus the oldest seafloor sediment deposit) is about 170 million years old. To locate older deposits, we must search sediments found on continents. Many continental deposits were formerly submerged, either in lakes or when shallow seas flooded the land. Lake and sea shoreline histories can provide information about precipitation and temperature trends in areas. Evaporative deposits, such as salt flats, can indicate periods of receding shorelines. Sediments deposited in lakes formed in the depressions left behind by receding glaciers provide important records of the climate of the last 20,000 years, including the waning years of last ice age. Some deposits on land, on the other had, provide records of conditions from as much as billions of years ago.

Some sediment deposits on land are not associated with lakes or seas. Deposits can be formed, for example, by wind-blown transport of materials. Fine-grained silt can be carried long distances by the wind and deposited in locales where the breeze lessens. Such deposits, called "loess", can be up to several hundred meters thick and in some places provide excellent climate records for the last 3 million years. Ash deposits from volcanic eruptions generally do not provide a continuous record of climate conditions, but, because of the powerful impact large eruptions can have on climate, do provide us insights into the occurrence of eruptive events that could influence climate. Glacial moraines, jumbles of boulders and rocks left behind by retreating glaciers when climates warm, also provide clues to dramatic eras of climate extremes (ice ages). Like volcanic ash deposits, sediments left by glaciers do not provide continuous climate records, and because they are often destroyed by later glacial advances, most provide information only about the most recent episodes of glaciation.

Sediments are deposited at different rates in different aquatic environments. Deposition rates vary from site to site, but typical values for the rate at which sediments accumulate are:

Deep ocean sediments generally have the poorest resolution, on the order of a thousand to five thousand years. Coastal ocean and sea sediments typically have resolutions of a century to a millennium, while lake cores can provide resolutions of a decade to a century.

Some lakes, especially deep lakes that contain little or no life-sustaining oxygen, can record annual records of climate in their sediments. The lack of oxygen in such lakes diminishes or eliminates the disturbance of the upper layers of sediment by worms and other burrowing animals. Paired layers of sediments, called "varves", are laid down in alternating light and dark layers reflecting seasonal differences in runoff sediment composition (light-hued mineral-rich sediments versus darker sediments rich in organic materials). These "varved" lake sediments provide a continuous climate proxy record with an annual resolution.

Ship collecting seafloor sediment cores
The ship JOIDES Resolution (left) has recovered thousands of sediment cores from the ocean floor using its prominent drilling rig. Scientists aboard the ship (right) clean and prepare one of the 9.5 meter-long cores soon after it was pulled up from the deep ocean.
Credit: Ocean Drilling Program

Sediment records are usually extracted in the form of long cores removed by drilling rigs similar to those used to drill for oil. For example, the cores extracted by the deep ocean drilling program conducted aboard the research ship JOIDES Resolutions are 9.5 meters long by a few centimeters in diameter when pulled from the drill rig. The cores are chopped up into 1.5 meter long sections soon after retrieval to make them easier to handle. Cores are sliced in half lengthwise; one half is used for scientific investigations, while the other half is often archived for future studies.

Segments of a sea sediment core
The long sediment cores extracted from the ocean floor have been cut into shorter segments and split lengthwise down the middle. The closeup (right) shows some of the details of a single core sample.
Credit: Ocean Drilling Program

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Last modified: 13 October 2010
Created: 11 October 2010