Did the oceans cool during the Eocene-Oligocene greenhouse-icehouse transition?

地球历史上最重要的气候事件之一发生在大约3400万年前，当时南极东部第一次出现了厚厚的冰盖。我们不知道是什么引发了这一戏剧性的变化，但主要有两个想法。一种观点认为，南极洲的温度下降到足以让雪终年结冰并形成冰帽的程度。这样的降温可能是由于大气中吸热温室气体浓度的下降造成的。另一个想法来自于这样一个事实，即形成雪既需要寒冷的温度，也需要水分。也许南极洲已经足够冷了，但洋流的变化使南极的天气变得更潮湿，导致了冰雪？这项研究的目的是通过找出在南极东部冰盖首次形成时，即始新世地质时期结束时，海洋是否变得更冷来验证这些想法。被称为有孔虫的单细胞动物化石的化学成分可以用来告诉我们过去海洋的温度。我们从现代海洋中了解到，在温暖的海水中生长的有孔虫，其外壳中含有更多的镁[镁的数量在实验室中以与钙的比率(镁/钙)来测量]。对于同样生活在古代海洋中的某些有孔虫物种，这种关系已经被仔细地计算出来了。死亡后，孔洞与其他沉积物质一起坠落到海底，随着时间的推移，形成了厚厚的泥层。通过提取这些沉积物的岩心，挑选出有孔虫并测量它们的镁/钙比，我们可以构建一份海洋温度如何随着地质时间尺度的变化而变化的记录。在这项研究中，我们将使用坦桑尼亚东部的古代海底沉积物。这些都被抬升了，现在位于海平面以上。通过这些沉积物的岩心覆盖了始新世和上覆的渐新世(南极洲首次出现冰的时间)之间的边界区间。泥浆的沉积速度很快，这意味着这一段的泥沙厚度很大，而且可以详细地看到随时间的变化。岩心含有丰富的粘土材料，阻止了现代雨水通过沉积物，这可能会部分溶解孔洞壳并重新设定它们的化学组成。在这项研究中，我们将测量两种类型的孔洞的温度，一种生活在海面附近，另一种来自海底。如果海洋顶部和底部的温度都变冷了，但两者之间的温差保持不变，我们就可以判断这是一次全球变冷。然而，如果顶部到底部的温差随着时间的推移而减小，这将告诉我们，记录到的温度是由于随着冰盖的增长，海洋变得越来越浅。

One of the most important climatic events in the earth's history happened about 34 million years ago, when a thick ice sheet grew in East Antarctica for the first time. We don't know what triggered this dramatic change, but there are two main ideas. One view is that the temperature of Antarctica decreased until it was cold enough for snow to remain frozen all year round and form an ice cap. Such a cooling could have been caused by a decrease in the concentrations of heat-trapping greenhouse gases in the atmosphere. Another idea comes from the fact that both cold temperatures and moisture are needed to form snow. Maybe Antarctica was already cold enough but a change in ocean currents made the Antarctic weather wetter, leading to snow and ice ? This research aims to test these ideas by finding out if the oceans became colder at the time the East Antarctic ice sheet first formed, at the close of an interval of geological time called the Eocene. The chemistry of fossils of single-celled animals called foraminifera (forams), can be used to tell us the temperature of past oceans. We know from the modern ocean that where forams grow in warmer waters more magnesium is included in their shells [the amount of magnesium is measured in the lab as a ratio to calcium (Mg/Ca)]. This relationship has been carefully worked out for certain species of foram that also lived in ancient oceans. Upon death forams fall to the sea floor with other sedimentary material, building up thick layers of mud over time. By taking cores of this sediment, picking out the forams and measuring their Mg/Ca, we can construct a record of how the ocean temperature varied over geological timescales. For this study we will use ancient seafloor sediments from Eastern Tanzania. These have been uplifted and now lie above sea level. Cores through these sediments cover the boundary interval between the Eocene and the overlying Oligocene (the time when ice first appeared on Antarctica). The mud was lain down at a fast rate, which means that there is a large thickness of sediment for this interval and that changes through time can be seen in detail. The cores are rich in clay material which has stopped modern rainwater from passing through the sediments, which could partly dissolve foram shells and reset their chemistry. We will measure temperatures from two types of foram for this study, one which lived near the surface of the sea, and one from the seafloor. If the temperatures from the top and bottom of the sea both cool, but the temperature difference between the two remains the same, we can tell that it is a global cooling. However, if the top to bottom temperature difference decreases through time, this will tell us that the temperatures recorded are due to the sea becoming shallower as the ice caps grow.