δ18O Records

Diatom silica is a form of biogenic opal (SiO2.nH2O) that contains oxygen isotopes that can be used in lacustrine and marine palaeoenvironmental studies. Here we describe the various analytical considerations and methodologies to achieve good environmental signals. We describe diatom oxygen isotope systematics—in particular the mineral-water temperature fractionation, as well as silica maturation processes. Finally, we give examples of studies both in lakes and ocean environments. We conclude that diatom δ18O in sediments can be an extremely useful indicator of palaeoenvironmental change and tends to be most successful in areas where there are likely to have been large changes in the oxygen isotope composition of the host water. In lakes this can be due to changes in the precipitation/evaporation balance, or source of precipitation, and in oceans it can be due to variations in melt-water flux. The change in δ18Odiatom due to these factors is normally greater than temperature and methodological errors.