A novel proxy for Arctic sea surface temperatures (PAST)

地球气候受到海洋、陆地和大气环境之间一系列动态相互作用的影响。海洋和大气之间的热交换特别重要，部分地由海表温度（SST）反映。因此，海温被用作评估过去地球气候变化的关键参数，也是预测未来变化的核心特征。过去海温的测定传统上是使用所谓的代理方法进行的，该方法应用于地球历史的地质档案，其中许多记录在世界海洋下的海洋沉积物中。在许多海温重建的替代方法中，基于海洋生物产生的脂类化学物质分布的方法是最发达和常用的。其基本原理是脂质分布是由宿主生物生长的温度控制的。在生命周期结束时，这些生物被沉积到海底沉积物中，在那里它们的化石残骸，包括它们的脂质，被保存了下来。因此，原始海温被保存在地质记录中，可以通过分析年代确定的沉积物岩心中的相关脂质来重建。许多基于脂质的海温代用品已经存在，并在过去40年左右的时间尺度上成功地用于重建温带-热带地区的海温。然而，现有的方法对北极和南极的典型低温（通常小于5摄氏度）不起作用，北极和南极约占世界海洋的25%。造成这种失败的原因是：(1)高纬度地区普遍缺乏源生物；(2)北极和南极海温低于传统脂质海温替代指标的可接受校准范围。这两个限制将在PAST中通过研究一种新的SST代理来解决，该代理基于一些所谓的高支化类异戊二烯（HBI）脂类（HBI iii和iv），这些脂类是由某些北极和南极常见的浮游植物产生的，其分布被假设由化学动力学控制，而不是生物功能，因此应该在所有温度下遵循严格的温度依赖性。这一假设虽然在生物系统中并不常见，但却得到了一些初步数据的支持，为进一步的研究提供了平台。过去可以采取两种相关的方法。第一个是通过分析北极和南极浮游植物样本来测试上述温度控制对HBI脂质分布的影响，这些样本也有温度数据。这些数据的发现将用于校准源环境中的温度-脂质分布关系。第二种方法是通过分析北极地表和短芯海洋沉积物来测试最近地质记录中相同的温度-脂质分布关系，这些沉积物代表了可以获得温度数据的时间框架（近几十年到几个世纪）。为了扩大新的海温代理的范围，还将分析来自温带地区（西英吉利海峡；温度范围= 8-16摄氏度）的水柱样本。一旦校准/测试，新的海温代理将使科学家第一次能够准确地重建地球历史上的北极海温，并确定其他地区的补充海温数据。这些数据对于改进诸如政府间气候变化专门委员会（IPCC）所使用的气候预报和预测模型至关重要。

Earth's climate is influenced by a suite of dynamic interactions between the marine, terrestrial and atmospheric environments. Heat exchange between the oceans and the atmosphere are particular important and is reflected, in part, by sea surface temperature (SST). As a result, SST is used as a key parameter for assessing changes to Earth's climate in the past and is also a central feature when predicting future change. The determination of past SST is traditionally carried out using so-called proxy methods applied to geological archives of Earth's history, many of which are recorded in marine sediments underlying the world's oceans. Of the many proxy methods for SST reconstruction, those based on the distributions of lipid chemicals made by marine organisms are some of the most developed and commonly used. The basic principle is that lipid distributions are controlled by the temperature in which their host organisms grow. At the end of their life cycle, such organisms are deposited into underlying ocean sediments where their fossilised remains, including their lipids, are preserved. As such, the original SST is preserved in the geological record and can be reconstructed by analysis of relevant lipids in well-dated sediment cores. A number of lipid-based SST proxies exist, and have been used with great success over the last four decades or so to reconstruct SST for temperate-tropical regions over timescales spanning Earth's history; however, no existing methods work for the characteristically low temperatures (typically <5 degrees Celcius) of the Arctic and Antarctic, which represent approximately 25% of the world's oceans. The reasons for this failure are (1) A general absence of the source organisms at high latitudes; (2) Arctic and Antarctic SST are below the accepted calibration ranges of traditional lipid-based SST proxies. Both limitations will be addressed in PAST by investigating a novel SST proxy based on some so-called highly branched isoprenoid (HBI) lipids (HBIs III&IV) made by certain phytoplankton common to the Arctic and Antarctic, and whose distribution is hypothesised to be controlled by chemical kinetics, rather than biological function, so should follow a strict temperature dependence over all temperatures. This hypothesis, although unusual for biological systems, is nonetheless supported by some preliminary data, which provide the platform for further investigation in PAST. There are two related approaches to be taken in PAST. The first is to test the aforementioned temperature control over HBI lipid distributions through the analysis of Arctic and Antarctic phytoplankton samples for which temperature data are also available. Findings from these data will be used to calibrate the temperature-lipid distribution relationship within the source environment. The second approach is to test the same temperature-lipid distribution relationship in the recent geological record through the analysis of surface and short core marine sediments from the Arctic that represent timeframes (recent decades to centuries) for which temperature data are also available. To broaden the scope of the new SST proxy, water column samples from a temperate location (Western English Channel; Temperature range = 8-16 degrees Celcius) will also be analysed. Once calibrated/tested, the new SST proxy will thus enable scientists, for the first time, to accurately reconstruct Arctic SSTs within Earth's history, and to determine complementary SST data for other regions. Such data are critical for the improvement of climate hindcasting and prediction models such as those used by the Intergovernmental Panel on Climate Change (IPCC).