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

随着今天越来越多的人开始关注全球变暖以及人类对气候的影响，在人类开始详细记录之前，了解过去的气候和环境变得更加重要。为了更好地了解过去的气候和环境，科学家们研究了沉积在湖泊和海洋底部数千至数百万年的沉积物。在许多情况下，这些沉积物仍在积累。沉积物有许多特征，它们提供了有关过去气候的信息，其中一个重要组成部分是硅藻化石。硅藻是一组藻类/微型植物，生活在包括湖泊和海洋在内的各种水生环境中。与其他藻类不同的是，硅藻有由二氧化硅制成的壳，称为锥体。二氧化硅的化学缩写是二氧化硅，意思是它由硅(Si)和氧(O2)组成。二氧化硅的一种常见形式是玻璃。硅藻是由溶解在它们生长的湖泊或海水中的二氧化硅形成的。二氧化硅中的氧气(O2)来自水本身(H2O)。当硅藻死亡时，它们的硅质锥体下沉，并在湖泊和海洋底部的沉积物中变成化石。经过很长一段时间，这些沉积物被压缩成坚硬的岩石。科学家们研究硅藻化石，以了解过去湖泊和海洋环境随时间的变化。硅藻化石的一个有趣特征是它们的化学成分，它记录了硅藻生长的湖泊或海洋的化学成分。氧和硅是合成二氧化硅的元素，它们有几种同位素，这意味着它们的原子可以有不同的质量，但在化学上仍然被识别为氧和硅。最常见的两种氧同位素是16O和18O。最常见的硅同位素有28Si、29Si和30Si。这个数字是指原子的质量，所以18O和30Si分别比16O和28Si略重。由于它们较轻，16O和28Si比较重的同类产品更易挥发。环境变化会影响水中重、轻氧和硅同位素的相对比例。例如，当水蒸发时，更多较轻、更易挥发的16O会被去除。因此，剩余的水中含有更多较重的18O。同样，当硅藻从水中吸收二氧化硅时，它们会吸收更多较轻的28Si，从而增加剩余水中30Si：28Si的比例。由于硅藻在其直接环境中使用水和硅，科学家认为硅藻硅中氧和硅同位素的相对数量反映了硅藻的生存条件。这可以作为一种工具来推断过去的环境条件。目前，硅藻二氧化硅中的同位素比例与硅藻生长条件之间的确切关系知之甚少。我的研究旨在通过在实验室中培养硅藻并仔细控制它们的生长条件来研究这些关系。我将测试温度、硅藻种类和营养硅有效性对硅藻二氧化硅氧、硅同位素比值的影响。目的是评估硅藻化石硅石的同位素比率记录当今条件的情况如何，因此它们对推断过去的环境有多大帮助。这将使科学家能够对过去的气候和环境变化做出更可靠的评估。

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.