Constraints from new geochemical proxies on temperature and sea level during critical climate transitions.

目前，极地地区是地球上任何地区变暖幅度最大的地区，这一特征在当前的任何气候模式中都没有得到很好的体现。冰盖和海平面对预估的温室变暖的潜在反应也是未知的。目前，全球海平面上升的速度远远超过预期，预计随着大气中二氧化碳含量的增加，海平面上升的速度还会加快。古气候记录有可能告诉我们气候变化的幅度和区域格局，以及导致这种变化的过程。尽管使用氧同位素（d18O；温度和海水d18O的代用物，后者反映冰盖的生长和融化）和最近的镁测温（对温度、海水Mg/Ca和碳酸盐离子浓度敏感）进行了几十年的研究，但整个地球历史上海平面和温度的演变知之甚少，而且极具争议性。我们计划采取的两种方法以前都没有使用过。首先，我们将使用一种新技术来测量有孔虫中同位素碳-13和氧-18之间的键的丰度，因为它完全由温度控制。生物分馏和海水化学对这一指标的影响似乎可以忽略不计。然而，测量非常耗时，因此我们将只在每个位置进行少量测量。我们将把这些温度与有孔虫d18O测量值结合起来估计海水d18O。此外，我们将使用这些温度和已发表的Mg/Ca记录计算海水Mg/Ca比。海水中Mg的浓度在大约1000万年的时间尺度上发生变化，Ca的浓度在大约100万年的时间尺度上发生变化，因此，尽管在我们研究的时期，海水Mg/Ca的比例可能与今天不同，但在我们研究的1500万年中，它们不太可能发生太大变化。我们可以将这些海水Mg/Ca比值应用于3个研究地点的高分辨率Mg/Ca记录，以便更准确地估计过去的温度变化。此外，我们可以通过引入另一组地球化学测量，即有孔虫的锂钙比（Li/Ca）和硼钙比（B/Ca），来估计碳酸盐离子浓度的变化对Mg/Ca的影响。通过引入多种地球化学指标，可以分离出有孔虫Mg/Ca记录的温度成分，建立更精确、准确的古温度重建。这些记录可以反过来与有孔虫d18O估计进行比较，以研究海水d18O的演化。然后，我们计划将这两组重建的温度和海水d18O测量值与大气二氧化碳水平的估计进行比较，以便研究气候和温室气体之间过去的关系。

Currently the polar regions are exhibit the greatest amount of warming of any region on the planet, a feature which is not well produced in any of the current generation of climate models. The potential response of ice sheets and sea level to projected greenhouse warming is also not known. Currently sea level is globally rising at a rate far greater than predicted, with rates predicted to accelerate as atmospheric CO2 levels increase. Paleoclimate records have the potential to inform us about the amplitude and regional pattern of climate change, and about the processes responsible for such change. Despite several decades of research using oxygen isotopes (d18O; a proxy of both temperature and seawater d18O, the latter of which reflects the growth and melting of ice sheets) and more recently, magnesium thermometry (sensitive to temperature, seawater Mg/Ca, and carbonate ion concentration), the evolution of sea level and temperature throughout Earth history is poorly known and highly controversial. The two approaches we plan to take have not been used before. First, we will use a new technique that measured the abundance of bonds between the isotopes carbon-13 and oxygen-18 in foraminifera, as it is controlled solely by temperature. Biological fractionation and seawater chemistry appear to have a negligible influence on this proxy. However, measurements are very time-consuming to make, so we will only make a few measurements at each location. We will combine these temperatures with foraminiferal d18O measurements to estimate seawater d18O. In addition, we will calculate seawater Mg/Ca ratios using these temperatures and published Mg/Ca records. Seawater Mg concentrations change over timescales of about 10 million years, and Ca concentrations over about 1 million years, so although during the period we are studying, seawater Mg/Ca ratios were probably different from today, it is unlikely that they changed very much over the 15 million years represented by our study. We can apply these seawater Mg/Ca ratios to high-resolution Mg/Ca records for the 3 study locations, in order to more accurately estimate past temperature variations. In addition, we can estimate the influence of changes in carbonate ion concentration on Mg/Ca by bringing in another set of geochemical measurements, the lithium to calcium (Li/Ca) and boron to calcium (B/Ca) ratio of foraminifera. By bringing in multiple geochemical proxies, we can isolate the temperature component of the foraminiferal Mg/Ca record and develop more precise and accurate reconstructions of past temperature. These records can in turn be compared with foraminiferal d18O estimates to study the evolution of seawater d18O. We plan to then compare both sets of reconstructed temperature and seawater d18O measurements with estimates of atmospheric carbon dioxide levels, in order to look at past relationships between climate and greenhouse gases.