Climatic Data, Sediment Records

SEDIMENT LAYERS IN different types of depositional settings, which include floodplains, and the bottom of lakes, rivers, and oceans, can provide a useful archive of past climatic conditions over the last few decades to tens of thousands of years ago. Such sediment records are routinely used to reconstruct natural variability in past environmental conditions, as well as anthropogenic-driven changes in contemporary climate, including changes in effective moisture (the difference between precipitation and evaporation); temperature; atmospheric circulation patterns; wind regimes; sources, diversity, and productivity of organic matter; partial pressure of carbon dioxide (CO2) in the atmosphere; and paleomagnetic properties of the surrounding environment.

The sedimentary record of past climate change can be made up of biogenic, radiochemical, and magnetic components. Derivation of such information from sediments is possible because changes in climate conditions, such as rainfall and consequent changes in aeration and alkalinity of bottom waters, can influence the rate of sediment input, formation of secondary minerals, and rates of biological productivity in the sediment layer.

The organic matter in sediment layers is a complex mixture of authochtonous and allochtonous (produced in situ or deposited with erosion, respectively) organic compounds from littoral, planktonic, and [Page 248]bacterial sources. Sedimentary basins contain biologically rich and diverse fossil records that can be used to reconstruct surface lake-water temperature (which is strongly correlated with air temperature), water chemistry, and other climatic factors that affect abundance and productivity of floral species. The most common sources of authochtonous organic matter in lacustrine and marine environments are diatom algal floras (diatoms are the most common type of phytoplankton and a major group of eukaryotic algae).

With rates of sediment deposition up to two orders of magnitude faster than in the deep sea, lakes offer good climate records.

The ecological structure or community assemblages of indicator diatom species are highly correlated with the intensity and duration of some environmental conditions. For instance, the abundance and species composition of diatoms serve as proxies for past conditions in water-chemistry (including salinity and pH) and length of the growing season in the body of water.

On the other hand, allochtonous fossil records, including pollen and spores that are washed from surrounding catchments, give clues about past climatic conditions in the surrounding environment that could have produced the specific assemblage of plant communities. More precise information on factors influencing plant species abundance and productivity (of which climate is among the most important) in the surrounding environment canbe retrieved from macro-fossils, of large vegetation remains that are washed into the sedimentary basins. Fossils from insects such as chironomids also provide accurate records of lake-water temperature.