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

在始新世-渐新世温室-冰室过渡期间海洋是否变冷？

基本信息

批准号：
NE/E007902/1
负责人：
Caroline Lear
金额：
$ 3.29万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FE007902%2F1
关键词：
Did oceans cool during Eocene

项目摘要

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.

地球历史上最重要的气候事件之一发生在大约3400万年前，当时南极洲东部的冰盖厚厚。我们不知道是什么触发了这种戏剧性的变化，但是有两个主要想法。一种观点是，南极的温度降低，直到足够冷，以使雪全年保持冷冻并形成冰盖。这样的冷却可能是由于大气中的热量温室气体的浓度降低所致。另一个想法来自这样一个事实，即形成雪需要寒冷的温度和水分。也许南极已经足够冷了，但是洋流的变化使南极天气潮湿，导致雪和冰？这项研究旨在通过发现东南冰盖首次形成时，在地质时代的一段时间被称为始新世的时间间隔时，该研究旨在测试这些想法。单细胞动物化石的化学称为有孔虫（孔），可用于告诉我们过去海洋的温度。我们从现代海洋中知道，在贝壳中包括更多镁在温暖的水中生长的地方（在实验室中测量镁的量作为钙与钙（mg/ca）的比率（mg/ca））。这种关系已经针对某些也生活在古老的海洋中的林木种类进行了精心制定。死亡后，孔有其他沉积物掉落到海底，随着时间的流逝，厚厚的泥浆层构成厚厚的泥土。通过拿起这种沉积物的核心，挑选孔孔并测量其Mg/ca，我们可以构建海洋温度在地质时尺度上如何变化的记录。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。这些已被抬高，现在位于海平面上方。通过这些沉积物的岩心覆盖了始新世与上覆渐新世之间的边界间隔（冰首次出现在南极洲上的时间）。泥浆以快速的速度放下，这意味着此间隔的沉积物厚度很大，并且可以详细地看到随着时间的变化。岩心富含粘土材料，该材料已阻止现代雨水穿过沉积物，这可能部分溶解了孔弹壳并重置其化学反应。我们将在这项研究中测量两种类型的Foram的温度，其中一项居住在海面附近，另一种生活在海底。如果从海底和底部的温度都冷却，但是两者之间的温度差保持不变，我们可以说它是一种全球冷却。但是，如果上下温度的差异随着时间的流逝而降低，这将告诉我们，记录的温度是由于冰盖随着冰盖的增长而变得较浅。