Oxygen Isotope Calibration Study of Modern Diatoms in Freshwater Environments: Implications for Biogenic Silica as a Paleoclimate Proxy

Oxygen isotope values of biogenic silica from diatom frustules are a commonly used proxy in freshwater and marine environments, and provide a valuable archive of paleoclimatic information such as temperature and water cycle processes. Advances in analytical techniques have made oxygen isotope measurements of diatom silica more robust; however, to date, there are multiple published fractionation factors for biogenic silica, with no general consensus on which is 'correct.' Previous studies (e.g. Moschen et al, 2005) demonstrated that there is no difference in SiO2-H2O fractionation between different size fractions of diatoms and, therefore, no species-dependent effects. The SiO2-H2O fractionation factors observed in laboratory grown diatoms analyzed by Brandriss et al. (1998) and modern lacustrine diatoms (Moschen et al., 2005) are in close agreement (τ = -0.2‰/°C) and are defined by the equations 1000lnα SiO2-H2O = 15.56 (103 T-1) - 20.92 and 1000lnα SiO2-H2O = 20.5 (103 T-1) - 36.2, respectively. However, these studies are not in agreement with other published SiO2-H2O fractionation factors for biogenic silica in marine and freshwater environments. In order to effectively utilize diatom δ18O values as a climate proxy, it is necessary to understand how oxygen isotopes are fractionated during silica frustule formation and identify potential errors in δ18O values obtained through different analytic/purification processes. Here we present oxygen isotope data from modern diatom species collected from a wide variety of natural riverine and lacustrine environments in northern New Mexico, USA. Temperatures at collection sites ranged from 5.5°C to 37.8°C. Preliminary isotope data indicate a SiO2-H2O fractionation factor identical to Brandriss et al. (1998). Additional experiments were undertaken to examine the effect of differing chemical purification techniques (i.e. HNO3, H2O2, and NaOH) on modern diatoms to see if processing techniques might affect the δ18O values of modern samples. Visual inspection of diatom frustules with a scanning electron microscope before and after treatment with HNO3 indicates no physical alteration of the frustule structure. To discount the possibility of oxygen exchange between diatom SiO2 and HNO3, samples were treated with an 18O-enriched nitric acid (1000‰), and the resulting δ18O values were essentially unchanged. Organic content following treatment with HNO3 was measured with an elemental analyzer and diatoms were considered to be pure SiO2 once weight percent carbon dropped below 0.01%. When diatoms were treated with H2O2 alone, significant organic material (>5 weight percent carbon) remained. Oxygen isotope values were obtained using a laser-extraction, stepwise fluorination technique that provides an additional visual confirmation of diatom purity. When pure F2 was introduced to the laser chamber during prefluorination, any sample with greater than 0.5 weight % carbon reacted violently to produce CF4 and O2 gas, and resulted in anomalous δ18O diatom values.