Oxygen isotopes in diatom silica : a new understanding of silica-water oxygen isotope fractionation in diatom frustules and an application of diatom d180 values as a record of paleohydrologic variability in a middle-Pleistocene lacrustine core from the Valles Caldera, New Mexico

I present oxygen isotope data from modern diatom frustules that were collected from freshwater systems in the Jemez Mountains of northern New Mexico. The frustules display a constant silica-water fractionation over the measured temperature range (5.1 to 37.8°C) regardless of species. This relationship is in close agreement with other published silica-water fractionation factors for laboratory cultured diatom samples; however, it is as much as 8° lower than equilibrium quartz-water fractionations and 3 to 4° lower than observed silica water fractionations in diatomaceous silica collected from sediment traps and sediment cores. Post mortem loss of organic material results in an alteration or 'maturation' of diatom silica in which silica reequilibrates with a silica-water fractionation closer to the equilibrium quartz-water fractionation. Post-mortem alteration of silica d18O values occurs within 0.5 years in diatom frustules from a lacustrine setting in the Valles Caldera, New Mexico resulting in an increase of ~7.4° in the d18O values. The rapid post-mortem alteration of diatom d18O values explains much of the disparate data regarding silica-water fractionation for diatom silica and has profound implications for the use of diatom silica d18O values as a paleoclimate proxy. In the setting of open basin lacustrine environments where bottom temperatures are nearly constant at 4ºC, the maturation effect may facilitate reconstruction of variations in paleowater d18O values without the conflating influence of fluctuating water temperature and variable d18O water values that occur at the surface. Lastly, I present d18O values of diatom silica from a middle-Pleistocene lacustrine core (VC-3) from the Valles Caldera, northern New Mexico, which record considerable (>25°) variability in lake water d18O values during interglacial Marine Isotope Stage 11 (MIS 11) and glacial MIS 12. There is a strong correspondence between the d18O values of diatoms throughout the VC-3 core and mean annual temperatures recorded by MBT/CBT indices in the core. Average d18O values of diatom silica for MIS 11 and MIS 12 are 26.5° (± 5.3; n=62) and 18.5° (± 6.3; n= 27), respectively. At the MIS 12 termination at ~430 kya, there is abrupt (<2 ka) increase in the d18O values of the diatom silica from ~11 to ~34 °. Throughout the core the d18O values of the diatom silica are strongly correlated with mean annual temperature (MAT) reconstructed from MBT/CBT indices in the VC-3 core on orbital (precessional) and millennial time scales. Rapid changes in the d18O values during both MIS 11 and the millennial scale variations in MIS 12 termination indicate the extreme hydroclimatological sensitivity of this region, and present a strong argument linking long term aridity in the southwestern United States to decreases in summer precipitation.