uUSGS-GD-Scientific Capabilities - MICROFOSSILS DIATOMS Technique




Diatoms are golden-brown algae that secrete a shell (frustule) of opaline (amorphous) silica. It was the intricate structure and beauty of the frustule that led amateur microscopists to study diatoms beginning early in the nineteenth century. They range in size from less than 1 to more than 1000 micrometers, but most range in size from 10 to 100 micrometers.

One of the characteristics that makes diatom useful in both biological and paleontological studies is that they are found almost anywhere there is sufficient light and moisture. Habitats include marine, brackish, and freshwater environments, as well as more unusual environments such as soil and near the mouths of caves. Diatoms have even been found in rime on antenna wires in the Arctic.

In the modern oceans and lakes, diatoms are important elements in the food chain. A high abundance of diatoms indicates that the waters are rich in nutrients, particularly nitrogen and phosphorous. Seasonal changes in light, temperature, and nutrient concentration result in a succession of dominant species in the diatom flora. In some regions, the high nutrient concentrations lead to the production of diatomaceous oozes. Over time, the most pure of these oozes lithify to become diatomites.

The susceptibility of opaline silica to dissolution at higher temperatures (>50 degrees C) or pH (>7) limits the age and geographic distribution of diatomaceous deposits. The earliest record of marine diatoms is from rocks of Early Jurassic age; freshwater diatoms did not appear until the Paleocene. Diatoms are well preserved and abundant in rocks of mid-Cretaceous age; the flora is already diverse, including numerous genera with differing morphologies. This Cretaceous diversity suggests that diatoms have a significantly longer evolutionary record, but that susceptibility of diatoms to dissolution has removed the earlier evidence of developmental history. Diatoms did not suffer the same level of extinction at the Cretaceous-Tertiary boundary as the other microfossil groups.

Although there are several periods of rapid evolutionary turnover during the Cenozoic, there is no evidence for mass extinction of diatoms. Most of the rapid evolutionary turnover is connected with the increasing development of a tropical-polar thermal gradient.


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URL http://geology.cr.usgs.gov/capabilities/paleoanal/microfos/diatoms/tech.html