Past Climate Conditions

The stable isotope composition of organic matter and carbonates in sedimentary basins can be used to provide information on past environmental and climate conditions.

Different types of terrestrial and aquatic plants and animals have variable isotopic composition, depending on their specific photosynthetic pathways, positions they hold on the food chain (trophic level), or their feed sources. Carbon and nitrogen isotope composition (513C and 515N) of organic matter in sediment layers can be used to discern sources of organic matter.

Oxygen isotope composition (518O) of carbonate sources such as calcite (from ostracod and mollusk, or sedimentary minerals formed by precipitation or recrystallization during sedimentation), biogeonic silica, and organic matter can be used to reconstruct past conditions of temperature. Moreover, biological transfer functions for direct reconstruction of past temperatures are used with fossils of diatoms, chrysophytes (golden algae), chironomids (non-biting midges), and cladocera (water fleas), because presence of specific species in a given body of water and their abundance is strongly correlated with temperature.

Paleomagnetic information in sediment records is sometimes critical to establish chronologies, both to calibrate timescales for changes in source of minerals entering the sedimentary basin and to validate the integrity of the sediment record. The record of magnetism in sediments (variations in magnetic inclination and declination that is preserved in deposited sediments) is useful for interpreting past records of climate change. Temporal variations in the Earth's magnetic field are preserved in rocks and if magnetic particles are eroded from upland terrestrial environments and deposited in low-lying sedimentary basins they show small, but measurable, preferred orientation depending on the magnetic field at the time of sedimentation. This is a phenomenon referred to as depositional remnant magnetisms.

Several pre- and post-depositional factors can influence the utility of sediment records in reconstruction of past climatic conditions. The resolution of the sediment record can vary depending on rate of sediment delivery to the basin and type of the sedimentary basin considered.

For example, lake sediments provide a high-resolution record of past environmental conditions at the timescale of decades or a few centuries, while sediment records from oceans provide a lesser-resolution record of longer time periods. Lacustrine sediment records can provide a high-resolution record of paleo-climatic fluctuations, partly because rate of sediment deposition in lakes are up to two orders of magnitude faster than that in deep sea sediments.

Bioturbation (mixing due to biological activity) is another factor that affects integrity of sediment records and results in important changes to terrestrial sediment once it is deposited in a sedimentary basin. Pristine sediment records can be preserved when the currents at the bottom of a water column are weak and oxygen availability is too low to allow any mixing activity by living organisms. In the absence of bio-turbation, contamination of the sedimentary record due to mixing of newly-deposited sediments with old sediments is limited.

Because most of the sediment in floodplains, or at the bottom of river, lake, or ocean floors is made up of mineral particles that were transported from adjacent areas, sediment records provide essential archives of past conditions, not only in the specific basin from which they are collected, but also in the surrounding areas. By investigating the stratigraphy of sediment cores, it is possible to get useful information about mineralogical composition of contributing catchments, rates of sediment transport, and biological productivity. It is important to note that within a basin, the sediment record can be stratified vertically or laterally. Sediment records that include vertical successions of strata are representative of chronology or sequential passage of time, while sediment records that include lateral successions of strata are representative of changes in depositional environments during sedimentation or diagenesis.

When interpreting sediment records, most sediments are derived from multiple sources, each with its own paleoenvironmental record. Moreover, sediment layers do not record direct information about climate change; therefore, scientists must calibrate the information obtained from sediment records (for example, the isotopic composition of 518O for temperature) to known climatic data.

SEE ALSO: Climatic Data, Lake Records; Climatic Data, Oceanic Observations; Climatic Data, Proxy Records; Climatic Data, Sea Floor Records.

BIBLIOGRAPHY. R.W. Battarbee, "Paleolimnological Approaches to Climate Change, with Special Regard to the Biological Record," Quaternary Science Reviews (v.19, 2000); D.J. Burdige, Geochemistry of Marine Sediments (Princeton University Press, 2006); T.C. Johnson, "Sedimentation in Large Lakes," Annual Reviews in Earth and Planetary Sciences (v.12, 1984); P.A. Meyers and Elisabeth Lallier-Verges, "Lacustrine Sedimentary Organic Matter Records of Late Quaternary Paleoclimates," Journal of Paleolimnology (v.21, 1999).

ASMERET ASEFAW BERHE University of California, Berkeley Teamrat A. Ghezzehei Lawrence Berkeley National Laboratory

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