Oxygen isotopes of marine diatoms and relations to opal-A maturation

In order to develop the potential tool of diatom oxygen isotopes for paleoe nvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water, sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures. Freshly grown natural diatoms (phytoplankton samples and sediment trap mate rial) and cultured diatoms reveal similar oxygen isotope fractionation fact ors. The fresh diatoms have 3 to 10 parts per thousand lower isotope fracti onation factors than fossil (sedimentary) diatoms. A temperature-related ox ygen isotope fractionation could not be established for the laboratory cult ures (and the natural phytoplankton samples), and there is evidence that di atom growth rate until reaching the stationary growth state also controls t he measured silica-water oxygen isotope fractionation factor. It is possibl e, however, that slow diatom growth in sea surface water may well lead to a temperature-dependent silica-water oxygen isotope fractionation which is t he prerequisite for a use of diatom oxygen isotopes in palco-surface water studies. FTIR-spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for Si-O-Si/Si-OH correlates with the 3 to 10 parts per thousand delta O-18(silica) increase from fresh to f ossil diatoms. Open-system (flow-through) silica dissolution experiments su ggest that the diatom frustules are isotopically homogenous and that the in crease in O-18 is therefore not due to dissolution of isotopically light su rficial Si-OH groups. It is concluded that slow internal condensation react ions during silica maturation in surface sediments cause both an increase i n the intensity ratio of Si-O-Si/Si-OH and the O-18 content of framework ox ygen. These findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but r ather reflect variable O-18 contents of surface sediments. Copyright (C) 20 01 Elsevier Science Ltd.