Descent into the Icehouse

在过去6亿年左右的大部分时间里(约80%)，地球比今天更温暖、更潮湿，基本上没有冰--它处于被称为“温室气候状态”的状态。另一种模式，就像今天的冰盖覆盖两极--所谓的“冰屋状态”--对于气候系统来说是一种相对罕见的状态。在过去的6亿年里，这两个国家之间的过渡往往是快速的，每一次都对地球上的生命产生了戏剧性的影响。其中最近的一次基本气候转变开始于大约5000万年前，在约3400万年前在始新世/渐新世交界处达到顶峰，南极冰盖迅速增长。由于岩石记录的性质，最近的这一转变是温室到冰库转换过程中研究得最好、记录最充分的，但尽管如此，相关的过程仍然存在很大争议。最流行的假设是，它是由大气中二氧化碳浓度下降引起的，二氧化碳是一种重要的温室气体。尽管最近证实始新世/渐新世边界的最终快速转换与二氧化碳的急剧减少有关，但也有人认为二氧化碳可能不是整个转变的主要驱动力。相反，有一些潜在的候选者大体上分为两个阵营--要么这种气候转变纯粹是由地球内部的过程(如喜马拉雅山脉的抬升、海洋环流或火山释放二氧化碳)驱动的，要么它涉及地球表面的某些或所有方面，包括生物，这些方面可以用许多重要的方式引起和放大变化。由于化石燃料的燃烧，到本世纪末，大气中的二氧化碳浓度可能会达到始新世温室世界的典型数值。因此，更好地了解二氧化碳在推动地球气候的这些自然循环中的作用变得至关重要，因此，这项提议的主要目的是确定这种最新和戏剧性的气候状态变化的主要驱动因素。我们将使用多学科方法来实现这一点，这种方法既有新的数据收集，也有计算机建模。我们将生成的新数据将涉及对二氧化碳浓度的修正估计，以及全球范围内对海洋温度的估计，这些环境参数过去无法直接确定。我们将研究海洋生物、有孔虫和球虫的化石遗骸。这些生物在海底的泥浆中非常丰富，为过去的海洋气候数据提供了宝贵的档案，通过观察它们贝壳的化学成分或它们生物合成的有机化合物，我们可以确定海洋的温度或酸性程度。从这些参数中，我们还可以推断出他们环境中的二氧化碳含量。有了对气候系统如何演变到温室-冰库转变的更好理解，我们可以更好地调查导致这种变化的自然过程。鉴于气候系统的复杂性质，这最好是用各种复杂的计算机模拟方法来完成的。至关重要的是，只有用我们产生的新数据来指导这些计算机模拟，我们才能从无数潜在的过程中分离出触发这种气候根本性变化的过程，并更好地确定它们如何影响生命的进化。

For the majority (~80 %) of the last 600 million years or so the planet was considerably warmer and wetter than today, and largely ice free - it was in what is known as a 'greenhouse climate state'. The alternate mode, like today, where ice caps blanket both poles - a so called 'icehouse state', is a relatively rare condition for the climate system to be in. Over the last 600 million years the transitions between the two states tend to be rapid and each had dramatic consequences for life on Earth. The most recent of these fundamental climate transitions began around 50 million years ago and culminated at ~34 million years ago at the Eocene/Oligocene boundary with the rapid growth of the Antarctic Ice Sheet. Due to the nature of the rock record, this most recent transition is the best studied and most well documented of the greenhouse to icehouse switches, but nonetheless the processes responsible are still much debated. The most popular hypothesis is that it was caused by a decline in the atmospheric concentration of CO2 - an important greenhouse gas. Although it has been recently confirmed that the final rapid switch at the Eocene/Oligocene boundary was associated with a dramatic decline in CO2, it has also been suggested that CO2 may not have been the main driver of the overall transition. There are instead a number of potential candidates that fall broadly into two camps - either this climate transition was driven purely by processes internal to the Earth (such as uplift of the Himalaya, ocean circulation, or volcanic outgassing of CO2) or it involved some, or all, aspects of the Earth surface, including biology, that can serve to cause and amplify change in a number of important ways. Due to the burning of fossil fuels, atmospheric CO2 concentrations may reach values typical of the greenhouse world of the Eocene by the end of this century. It is therefore becoming imperative to better understand the role of CO2 in driving these natural cycles of Earths climate, and consequently, the principal aim of this proposal is to determine the main driver of this most recent and dramatic switch in climate state. We will achieve this using a multidisciplinary approach that has aspects of both new data collection and computer modelling. The new data we will generate will involve revised estimates of CO2 concentrations and globally widespread estimates of ocean temperature, environmental parameters that cannot be directly determined for the past. We will study the fossil remains of sea-dwelling microscopic organisms, the foraminifers and coccolithophorids. These organisms are very abundant in the mud on the floor of the oceans, providing an invaluable archive of past ocean climate data, and by looking at the chemical composition of their shells or the organic compounds they biosynthesise we can determine how warm or how acidic the ocean was. And from such parameters, we can also deduce how much CO2 was in their environment. Armed with this improved understanding of how the climate system evolved leading up to the greenhouse-icehouse transition we can better investigate the natural processes that caused the change. Given the complex nature of the climate system this is best done with a variety of sophisticated computer modelling approaches. Crucially, it is only by guiding these computer simulations with the new data we have generated that we can isolate which of the myriad of potential processes was responsible for triggering this fundamental shift in climate and better determine how they impacted the evolution of life.