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，当时南极洲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种戏剧性的变化，但有两个主要的想法。一种观点认为，南极洲的温度下降到足够冷的程度，雪可以全年冻结并形成冰盖。这种降温可能是由大气中吸热温室气体浓度的下降引起的。另一个想法来自这样一个事实，即形成雪需要低温和湿气。也许南极洲已经够冷了，但洋流的变化使南极的天气变得更潮湿，导致了冰雪？这项研究的目的是检验这些观点，方法是找出在始新世地质时期的末期，南极东部冰盖首次形成时，海洋是否变得更冷。单细胞动物有孔虫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，有孔虫生长在温暖的海水中，它们的外壳中含有更多的镁（在实验室中，镁的含量是通过与钙的比例来测量的）。对于同样生活在古代海洋中的某些有孔虫物种来说，这种关系已经被仔细研究过了。死后，有孔虫和其他沉积物质一起落到海底，随着时间的推移，形成厚厚的泥浆层。通过取这些沉积物的岩心，挑出有孔虫并测量它们的Mg/Ca，我们可以建立一个记录海洋温度在地质时间尺度上是如何变化的。在这项研究中，我们将使用来自坦桑尼亚东部的古代海底沉积物。它们被抬升，现在位于海平面以上。通过这些沉积物形成的岩心覆盖了始新世和上覆渐新世（冰首次出现在南极洲的时间）之间的分界线。泥浆沉积的速度很快，这意味着在这段时间内沉积物的厚度很大，而且随着时间的变化可以被详细地看到。岩心富含粘土物质，阻止了现代雨水穿过沉积物，而雨水可能会部分溶解有孔虫壳并重置它们的化学成分。在这项研究中，我们将测量两种有孔虫的温度，一种生活在海面附近，另一种生活在海底。如果海洋顶部和底部的温度都下降，但两者之间的温差保持不变，我们可以说这是全球变冷。然而，如果顶部和底部的温差随着时间的推移而减小，这将告诉我们，记录的温度是由于随着冰盖的增长，海洋变得更浅。

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.