過往的海洋比我們認為得還要冷

過往的海洋比我們認為得還要冷

一組研究團隊發現目前為止用來估計過往海洋溫度的方法有所缺陷。他們的發現可能意味現在的氣候變遷是過去1億年以來前所未見的。

根據科學界常用的方法，在1億年以前海洋深處和極區海洋的表面溫度比現今的數值還要高了將近15度。但此方法如今受到考驗：這段時間以來海洋溫度可能實際上都保持著相對穩定的狀態，而令人更加憂心現今的氣候變遷程度。來自法國國家科學研究中心(CNRS)、索邦大學、史特拉斯堡大學的法國科學家，以及瑞士洛桑聯邦理工學院(EPFL)和洛桑大學的瑞士科學家近日刊登於《自然通訊》(Nature Communications)的研究得出了上述結論。

EPFL生物地球化學實驗室的主持人，同時也是洛桑大學的教授Anders Meibom表示：「如果我們是正確的，這項研究將會考驗數十年來的古氣候研究成果。」他明確指出：「海洋覆蓋了地球表面70%的面積，對於地球氣候來說具有非常重大的影響。如果我們想要完全瞭解海洋的運作方式，並且更加準確地預測現今氣候變遷的後果，就要先知道在地質時間中海洋溫度的變化程度。」

為什麼現行方法會有如此重大的缺陷？研究作者相信某些作用的影響一直都被忽略了。50多年以來科學家做出的預測都是基於研究有孔蟲的成果。有孔蟲是一種十分微小的海洋生物，從海床取出的沉積物岩芯中可以發現牠們的化石。依據存活當時所處的水溫，有孔蟲形成的鈣質外殼(稱作test)的氧-18含量會跟著變化。因此，利用從沉積物中找到的有孔蟲殼體，便可依據其氧-18含量來計算過去海洋溫度的變化歷程。根據之前做出的測量結果，海洋溫度在過去1億年來下降了大約15度。

但這些估算都是建立於一項原則：有孔蟲殼體化石保存在沉積物內部的期間，其氧-18含量是保持固定的。目前為止確實還沒有任何證據指出並非如此，不管是用肉眼或是顯微鏡都未看到有孔蟲化石產生變化。這篇最新研究的作者為了驗證自身的假設，他們在僅有氧-18的人工海洋中，讓這些小生物處在高溫環境之下。接著他們利用可以分析極小區域化學性質的儀器NanoSIMS(奈米級二次離子質譜儀)來觀察氧-18進入鈣質殼體的過程。結果證明有孔蟲殼體的氧-18含量實際上可以不留痕跡地發生變化，研究人員因此對其作為溫度計的可信度產生疑慮。「看起來保存十分完善的化石實際上並非如此。代表至今為主科學家對古海洋溫度做出的預測都是錯的。」研究主要作者Sylvain Bernard表示，他是CNRS位在巴黎的礦物學、材料物理學和宇宙化學研究所的研究人員。

對此法國和瑞士研究團隊來說，過往的測量結果並非反映出海洋溫度在過去1億年以來有逐漸下降的趨勢，僅代表有孔蟲殼體化石含有的氧-18本身的變化過程，而此變化似乎是由一種稱為再平衡(re-equilibration)的作用導致：沉積作用的過程中溫度會上升20至30℃，造成有孔蟲殼體跟周圍的水重新發生化學平衡。經過數千萬年之後，此作用會對古海溫的估算產生重大影響，特別是那些根據生存於冷水的有孔蟲所進行的研究。研究人員透過電腦模擬提出海洋深處和極區海洋表層的溫度之前都被估算得太高。

對Meibom來說，下一步十分明確：「為了重新審視古海洋溫度，我們需要仔細定量忽略已久的再平衡作用產生的影響。要達成這項目標，我們必須同時研究其他種類的海洋生物，才能徹底瞭解在地質時間尺度下，沉積物內部究竟發生了何種作用。」此文章的作者已經開始如火如荼地進行研究。

The oceans were colder than we thought

A team of researchers has discovered a flaw in the way past ocean temperatures have been estimated up to now. Their findings could mean that the current period of climate change is unparalleled over the last 100 million years.

According to the methodology widely used by the scientific community, the temperature of the ocean depths and that of the surface of the polar ocean 100 million years ago were around 15 degrees higher than current readings. This approach, however, is now being challenged: ocean temperatures may in fact have remained relatively stable throughout this period, which raises serious concerns about current levels of climate change. These are the conclusions of a study conducted by a team of French researchers from the French National Center for Scientific Research (CNRS), Sorbonne University and the University of Strasbourg, and Swiss researchers from the Swiss Federal Institute of Technology in Lausanne (EPFL) and the University of Lausanne. The study has just been published in Nature Communications.

“If we are right, our study challenges decades of paleoclimate research,” says Anders Meibom, the head of EPFL’s Laboratory for Biological Geochemistry and a professor at the University of Lausanne. Meibom is categorical: “Oceans cover 70% of our planet. They play a key role in Earth’s climate. Knowing the extent to which their temperatures have varied over geological time is crucial if we are to gain a fuller understanding of how they behave and to predict the consequences of current climate change more accurately.”

How could the existing methodology be so flawed? The study’s authors believe that the influence of certain processes was overlooked. For over 50 years, the scientific community based its estimates on what they learned from foraminifera, which are the fossils of tiny marine organisms found in sediment cores taken from the ocean floor. The foraminifera form calcareous shells called tests in which the content of oxygen-18 depends on the temperature of the water in which they live. Changes in the ocean’s temperature over time were therefore calculated on the basis of the oxygen-18 content of the fossil foraminifera tests found in the sediment. According to these measurements, the ocean’s temperature has fallen by 15 degrees over the past 100 million years.

Yet all these estimates are based on the principle that the oxygen-18 content of the foraminifera tests remained constant while the fossils were lodged in the sediment. Indeed, until now, nothing indicated otherwise: no change is visible to the naked eye or under the microscope. To test their hypothesis, the authors of this latest study exposed these tiny organisms to high temperatures in artificial sea water that contained only oxygen-18. Using a NanoSIMS (nanoscale secondary ion mass spectrometer), an instrument used to run very small-scale chemical analyses, they then observed the incorporation of oxygen-18 in the calcareous shells. The results show that the level of oxygen-18 present in the foraminifera tests can in fact change without leaving a visible trace, thereby challenging the reliability of their use as a thermometer: “What appeared to be perfectly preserved fossils are in fact not. This means that the paleotemperature estimates made up to now are incorrect,” says Sylvain Bernard, a CNRS researcher at the Paris-based Institute of Mineralogy, Materials Physics and Cosmochemistry and the study’s lead author.

For the French and Swiss team of researchers, rather than showing a gradual decline in ocean temperatures over the past 100 million years, these measurements simply reflect the change in oxygen-18 content in the fossil foraminifera tests. And this change appears to be the result of a process called re-equilibration: during sedimentation, temperatures rise by 20 to 30°C, causing the foraminifera tests to re-equilibrate with the surrounding water. Over the course of some ten million years, this process has a significant impact on paleotemperature estimates, especially those based on foraminifera that lived in cold water. Computer simulations run by the researchers suggest that paleotemperatures in the ocean depths and at the surface of the polar ocean have been overestimated.

For Meibom, the next steps are clear: “To revisit the ocean’s paleotemperatures now, we need to carefully quantify this re-equilibration, which has been overlooked for too long. For that, we have to work on other types of marine organisms so that we clearly understand what took place in the sediment over geological time.” The article’s authors are already hard at work.

原始論文：S. Bernard, D. Daval, P. Ackerer, S. Pont, A. Meibom. Burial-induced oxygen-isotope re-equilibration of fossil foraminifera explains ocean paleotemperature paradoxes. Nature Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-01225-9

引用自：Ecole Polytechnique Fédérale de Lausanne. "Current climate change unparalleled over the last 100 million years?."