CHAPTER EIGHT DIATOMS: FROM MICROPALEONTOLOGY TO ISOTOPE GEOCHEMISTRY

Abstract

Diatoms are unicellular organisms in which the cell is encapsulated in an amorphous silica box, called the frustule, composed of two intricate valves. The frustule is highly ornamented with pores (areolae), processes (labiate, strutted, internal or external, with or without extensions), spines, costae, horns, hyaline areas, and other distinguishing features. Diatom taxonomy is historically based on the shape and ornamentation of the frustules. Reconstruction of sea-surface temperatures (SSTs), sea-ice cover, water mass fluxes, fresh water input, productivity, and nutrient cycling represent only a small part of what it is potentially possible to do using diatoms, in both the fields of micropaleontogy and sediment geochemistry. These same parameters are also the most important in paleoclimate modeling, and can be used as boundary conditions or as an independent external check of model output. Diatoms are the main tool to achieve quantitative estimates of Antarctic sea-ice extent and concentration, although chemical content of ice cores may provide complementary information. Future developments of the diatom methods mainly concerns geochemistry. A great effort is being made to obtain reliable radiocarbon dates from the organic compounds intrinsic to diatom frustules in order to help in dating sediments devoid of foraminifera, to measure silicon isotopes and oxygen isotopes to quantify production and dissolution of diatoms.