Overview: This is a proposal to develop a second generation silicon isotope mass spectrometer. It is an admittedly unusual proposal for NSF Chemical Oceanography, and that issue is addressed directly in the main text.
The proposed instrument will support research on marine Si isotope biogeochemistry. The field of marine Si isotope biogeochemistry is about 20 years old having been pioneered at UCSB in the early 1990s. The field has grown to where over 18 laboratories from 8 countries are now measuring Si isotopes in marine systems. Si isotopes are a powerful tool for understanding the marine silica cycle and the contribution of silicifiers to marine biogeochemistry. Variations in isotopes of Si in biogenic silica and in dissolved silicic acid are used to constrain net silica production rates in the modern ocean and variations in the Si isotopic composition of diatom frustules and of deep-sea sponge spicules are used to reconstruct the history of diatom productivity in surface waters and changes in deep water silicic acid content over geologic time. The reconstructions of diatom productivity in both the modern and past ocean using Si isotopes require knowledge of the isotopic composition of the silicic acid supplied to surface waters through mixing. That need has inspired efforts to understand controls on δ30Si(OH)4 distribution in deep waters through programs like international GEOTRACES. Recent modeling studies have been surprisingly successful at simulating Si isotope distributions in the global ocean pointing to significant progress in our understanding of the controls on Si isotope distributions and their relation to ocean circulation and biogeochemistry.
In 2000 the P.I. was funded by NSF Chemical Oceanography to alter a MAT 252 IRMS mass spectrometer and Kiel III inlet system to enable their use for the analysis of Si isotopes. The project was successful and the prototype instrument has supported Si isotope research for the past 16 years via 6 NSF research awards to the PI and it has supported research by five other investigators from outside UCSB. Operation of the instrument is now highly problematic with no publishable data generated during 2017 to date. It is time for replacement.
Intellectual merit: The original prototype was a one-off instrument. For the second generation system the P.I. is partnering with Nu Instruments, Ltd to create a commercially available instrument based on the Nu Carb and/or Eurovector HT-PyrOH I° inlet system(s) and Nu Perspective isotope ratio mass spectrometer. The two main goals for the new instrument are 1) to increase sample throughput by a factor of four and 2) to lower sample size by an order of magnitude. To meet these goals both acid and thermal decomposition of silicon fluorides to generate the analyte SiF4 gas will be explored. The second generation system should be faster, as or more precise, and much easier for the novice compared to the original prototype and compared to alternatives like MC-ICP-MS.
Broader Impacts: The current prototype is the only instrument actively making measurements of isotopes of Si in marine materials in the country and it is failing. Success will restore routine measurements of isotopes of Si from marine systems in the United States. By partnering with Nu Instruments a commercial instrument will be produced that will be available to all at relatively low cost compared to competing technologies like MC-ICP-MS contributing to national and international capacity building in this field. Finally, achieving higher sample throughput would both lower costs and facilitate our ability to make the proposed UCSB instrument available to others further accelerating progress in the field.