Calibrating the fractionation of stable oxygen and silicon isotopes in diatom silica through laboratory culture experiments

As more people today become concerned about global warming and the effect that humans are having on the climate, it becomes more important for people to understand what the climate and environment was like in the past, before humans began to take detailed records. In order better to understand the climate and environments of the past, scientists study sediment, which has accumulated on the bottom of lakes and oceans for thousands to millions of years. In many cases, these sediments are still accumulating. There are many features of the sediments, which provide information about past climates, one important component being fossilised diatoms. Diatoms are a group of algae / microscopic plants that live in a wide variety of aquatic environments, including lakes and oceans. Unlike other algae, diatoms have shells, called frustules, which are made of silica. The chemical abbreviation for silica is SiO2, meaning that it is comprised of silicon (Si) and oxygen (O2). One common form of silica is glass. Diatoms form their frustules from silica dissolved in the lake or marine waters in which they grow. The oxygen (O2) in the silica comes from the water itself (H2O). When diatoms die, their silica frustules sink and become fossilised within the sediments at the bottom of lakes and oceans. Over long periods of time, these sediments compress to become solid rock. Scientists study fossil diatom frustules in order to understand changes in past lake and ocean environments over time. One interesting feature of fossil diatom frustules is their chemical composition, which records the chemistry of the lake or ocean in which the diatom grew. Oxygen and silicon, the elements that combine to make silica, have several isotopes / meaning that their atoms can have different masses and still be chemically recognised as oxygen and silicon. The two most common oxygen isotopes are 16O and 18O. The most common silicon isotopes are 28Si, 29Si and 30Si. The number refers to the mass of the atom, so 18O and 30Si are slightly heavier than 16O and 28Si respectively. Because they are lighter, 16O and 28Si are more volatile than their heavier counterparts. Environmental changes can affect the relative proportion of heavy and light oxygen and silicon isotopes in water. For example, when water evaporates, more of the lighter, more volatile 16O is removed. As a consequence, the remaining water contains more of the heavier 18O. Similarly, when diatoms take silica from the water, they assimilate more of the lighter 28Si, increasing the ratio of 30Si:28Si in the remaining water. Because diatoms use the water and silica in their immediate environment, scientists believe that the relative amounts of oxygen and silicon isotopes in diatom silica reflect the conditions in which the diatoms lived. This can be used as a tool to infer the environmental conditions of the past. At present, very little is known about the precise relationship between isotope ratios in diatom silica and the conditions in which the diatoms grew. My research aims to investigate these relationships by growing diatoms in the laboratory and carefully controlling their growth conditions. I will test the effects of temperature, diatom species and nutrient silicon availability on the oxygen and silicon isotope ratios of diatom silica. The objective is to assess how well the isotope ratios of fossil diatom silica record present day conditions and therefore how useful they are to infer past environments. This will enable scientists to make more reliable assessments of past climate and environmental change.