Opal Isotope Laboratory

Situated in this laboratory is a Finnigan MAT 253 stable isotope ratio mass spectrometer equipped with a modified Kiel IV Device and a HekaTech EuroEA elemental analyzer.  Gas samples introduced into the mass spectrometer are ionized over a hot filament, accelerated down an evacuated metal tube, and passed through a magnetic field which separates them according the ratio of their mass to their charge.  Because isotopes of the same element have slightly different masses, ions bearing these different isotopes will fall into different detectors (known as Faraday cups), allowing the isotopic composition of the gas to be tabulated.

Our main element of interest, silicon (Si) has three main stable isotopes (with masses 28, 29, and 30 atomic mass units).  While these isotopes occur in the general proportion of 92.23%, 4.67%, and 3.10%, respectively, very minor variations in these abundances exist between materials.  These slight differences are due to isotopic fractionation during chemical reactions.  For example, when diatoms, a type of unicellular marine phytoplankton which carries out roughly half of the photosynthesis in the ocean and about 20% of that on Earth, creates its silica shell (more properly known as a frustule), they fractionate Si isotopes.  The frustule that is produced has a lower ³⁰Si to ²⁸Si than the dissolved silicon present in seawater that the diatoms took up and used to create the frustule.

By taking fossil diatoms from marine sediments, cleaning them up, and measuring their Si isotopic composition, we can create a picture of nutrient cycling in oceans of the past and try to understand how nutrient cycling links in with climate.  We can do something similar with the carbon isotopes and the nitrogen isotopes that are present in organic matter trapped within the fossil diatoms.