冰河期的起因是什麼？微小的海洋生物化石提供了關鍵證據

 原文網址：https://www.princeton.edu/news/2020/12/10/what-caused-ice-ages-tiny-ocean-fossils-offer-key-evidence

Liz Fuller-Wright

地球最近數百萬年的歷史當中，有項特徵是頻繁的「冰期―間冰期」循環。這些極為劇烈的氣候動盪，造成涵蓋大片陸地的巨大冰層擴張或是縮減。引發這些循環的原因是地球公轉軌道以及自轉方式的微小振盪，但是軌道振盪的幅度卻太微小而難以解釋氣候的巨大變化。

本研究的樣品中最主要的矽藻種類Fragilariopsis kerguelensis活著的時候(左)以及變成化石之後(右)的圖片。圖片來源：左邊為Philipp Assmy (挪威極地研究所) and Marina Montresor (安東督宏動物研究站)；右邊為Michael Kloster(阿爾弗雷德·魏格納研究所)

「冰河期的成因是地球科學當中還未解開的最大難題之一，」普林斯頓大學地質與地球物理學系的教授Daniel Sigman表示。「解釋這種主要的氣候現象可以增進我們預測未來氣候變遷的能力。」

科學家在1970年代發現大氣中的溫室氣體――二氧化碳――的濃度在冰河期的時候會降低30%左右，促使他們推論大氣二氧化碳濃度下降，是造成冰期循環的關鍵因素之一，但是二氧化碳的變化是由什麼導致，還是沒能解答。有些數據指出冰河期二氧化碳會被封鎖在深海當中，不過這種現象的成因仍有所爭議。

普林斯頓大學和馬克斯普朗克化學研究所(MPIC)率領的國際科學團隊，最近發現冰河期南極洋表層海水發生的變化，使得深海可以儲存更多二氧化碳。研究人員的證據來自於矽藻，這是一種生活在海洋表層，死後會沉到海底的浮游藻類。他們利用從南極洋取得的沉積物岩芯，建立了被包在矽藻化石裡的有機物成分的詳細記錄。測量結果提供的證據顯示冰河期的南極洋，由風驅動的湧升流強度有減弱的趨勢。這項研究發表在當期的《科學》(Science)期刊。

研究人員數十年前就已經知道海洋藻類生長，並在死亡之後沉入海底的過程，可以將二氧化碳帶到海洋深處，一般稱這種過程為「生物幫浦」。生物幫浦在熱帶、副熱帶和溫帶海洋作用得最強，越靠近極區則效率不彰。極區的深層海水會迅速地被帶到海洋表層，使得二氧化碳被排回大氣。其中表現最糟的是南極洋：環繞南極大陸的強勁東風會把富含二氧化碳的深層海水帶到表層，使得二氧化碳「洩漏」到大氣當中。

研究人員數十年來也明白風成湧升流減弱造成海洋保留更多的二氧化碳，或許可以用來解釋冰河期二氧化碳的濃度為何會下降。然而，科學家直到最近都還缺乏方法可以鐵口直斷地證明這種變化是否曾經發生。

普林斯頓大學與MPIC合作之下發展出一套證明方法：運用微小的矽藻。矽藻是大量生長於南極海水表層的浮游藻類，它們死後的矽質殼體會堆積在深海沉積物。矽藻殼體的氮同位素含量會隨著海水表層未被利用的氮而變化。因此，普林斯頓大學與MPIC的團隊從矽藻化石的礦物質外壁中，取出了夾雜在內部的少量有機物。透過測量它們的氮同位素比，他們揭曉了南極洋表層海水過去十五萬年的氮濃度是如何變化，這段期間涵蓋了兩次冰河期以及兩次溫暖的間冰期。

研究第一作者，普林斯頓大學的研究生Ellen Ai表示：「分析矽藻這類化石中的氮同位素可以顯示出過去海洋表層的氮濃度，」她和Sigman以及MPIC 的Alfredo Martínez-García 與Gerald Haug共同進行了這篇研究。「這些藻類生長所需的氮在深層海水的濃度相當高。而南極的湧升流越強，海水表層的氮濃度就越高。因此我們的結果可以讓我們重建南極湧升流的變化。」

運用新的方法來定年南極沉積物使得他們的數據更加有力。他們從沉積物岩芯中重建海水表層的溫度變化，然後再跟南極冰芯的氣溫紀錄互相比較。

「這讓我們可以把矽藻氮同位素紀錄中的許多特徵，跟全球各處的氣候和海洋變化的發生時間連結起來，」Martínez-García表示。「特別是我們現在可以確定湧升流減弱以及氣候開始變冷的時間，同時還能把南極湧升流的變化與冰河期快速的氣候動盪之間的關係建立起來。」

這些更為精確的時間點使得研究人員可以準確指出風是造成湧升流變化的關鍵原因。

新發現也讓研究人員解開南極湧升流和大氣二氧化碳的變化，跟驅動冰河循環的軌道作用力有什麼樣的關聯，這讓科學家離建立冰河期起源的完整理論更近一步。

「我們的發現顯示出湧升流造成的大氣二氧化碳濃度變化在冰期循環裡佔據了核心地位，但它作用的方式並非跟我們許多人之前認為的全然一致，」Sigman說。「舉例來說，南極湧升流並沒有讓冰河期更快結束，而是讓二氧化碳產生變化，使得氣候最溫暖的階段得以延長。」

在預測未來全球暖化會讓海洋產生什麼變化的時候，他們的發現也能帶來啟發。關於極地風對於氣候變遷的敏感度，電腦模型得到的結果並非一致。研究人員觀察到在過去的暖期南極洋的風成湧升流有大幅增強的現象，意謂了未來的全球暖化也會讓湧升流變強。更強的湧升流可能會讓海洋加速吸收目前全球暖化所產生的熱能，同時也會影響到南極洋以及南極冰層上方的生物所處的環境。

「這項新發現指出下個世紀南極周遭的大氣和海洋將會面臨巨大的改變，」Ai表示。「然而，因為目前燃燒化石燃料產生二氧化碳是前所未見的現象，所以還需要更多研究才能了解南極洋的變化會對海洋吸收二氧化碳的速度產生什麼樣的影響。」

 

What caused the ice ages? Tiny ocean fossils offer key evidence

The last million years of Earth history have been characterized by frequent “glacial-interglacial cycles,” large swings in climate that are linked to the growing and shrinking of massive, continent-spanning ice sheets. These cycles are triggered by subtle oscillations in Earth’s orbit and rotation, but the orbital oscillations are too subtle to explain the large changes in climate.

“The cause of the ice ages is one of the great unsolved problems in the geosciences,” said Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences. “Explaining this dominant climate phenomenon will improve our ability to predict future climate change.”

In the 1970s, scientists discovered that the concentration of the atmospheric greenhouse gas carbon dioxide (CO₂) was about 30% lower during the ice ages. That prompted theories that the decrease in atmospheric CO₂ levels is a key ingredient in the glacial cycles, but the causes of the CO₂ change remained unknown. Some data suggested that, during ice ages, CO₂ was trapped in the deep ocean, but the reason for this was debated.

Now, an international collaboration led by scientists from Princeton University and the Max Planck Institute for Chemistry (MPIC) have found evidence indicating that during ice ages, changes in the surface waters of the Antarctic Ocean worked to store more CO₂ in the deep ocean. Using sediment cores from the Antarctic Ocean, the researchers generated detailed records of the chemical composition of organic matter trapped in the fossils of diatoms — floating algae that grew in the surface waters, then died and sank to the sea floor. Their measurements provide evidence for systematic reductions in wind-driven upwelling in the Antarctic Ocean during the ice ages. The research appears in the current issue of the journal Science.

For decades, researchers have known that the growth and sinking of marine algae pumps CO2 deep into the ocean, a process often referred to as the “biological pump.” The biological pump is driven mostly by the tropical, subtropical and temperate oceans and is inefficient closer to the poles, where CO2 is vented back to the atmosphere by the rapid exposure of deep waters to the surface. The worst offender is the Antarctic Ocean: the strong eastward winds encircling the Antarctic continent pull CO2-rich deep water up to the surface, “leaking” CO2 to the atmosphere.

The potential for a reduction in wind-driven upwelling to keep more CO2 in the ocean, and thus to explain the ice age atmospheric CO2 drawdown, has also been recognized for decades. Until now, however, scientists have lacked a way to unambiguously test for such a change.

The Princeton-MPIC collaboration has developed such an approach, using tiny diatoms. Diatoms are floating algae that grow abundantly in Antarctic surface waters, and their silica shells accumulate in deep sea sediment. The nitrogen isotopes in diatoms’ shells vary with the amount of unused nitrogen in the surface water. The Princeton-MPIC team measured the nitrogen isotope ratios of the trace organic matter trapped in the mineral walls of these fossils, which revealed the evolution of nitrogen concentrations in Antarctic surface waters over the past 150,000 years, covering two ice ages and two warm interglacial periods.

“Analysis of the nitrogen isotopes trapped in fossils like diatoms reveals the surface nitrogen concentration in the past,” said Ellen Ai, first author of the study and a Princeton graduate student working with Sigman and with the groups of Alfredo Martínez-García and Gerald Haug at MPIC. “Deep water has high concentrations of the nitrogen that algae rely on. The more upwelling that occurs in the Antarctic, the higher the nitrogen concentration in the surface water. So our results also allowed us to reconstruct Antarctic upwelling changes.”

The data were made more powerful by a new approach for dating the Antarctic sediments. Surface water temperature change was reconstructed in the sediment cores and compared with Antarctic ice core records of air temperature.

“This allowed us to connect many features in the diatom nitrogen record to coincident climate and ocean changes from across the globe,” said Martínez-García. “In particular, we are now able to pin down the timing of upwelling decline, when climate starts to cool, as well as to connect upwelling changes in the Antarctic with the fast climate oscillations during ice ages.”

This more precise timing allowed the researchers to home in on the winds as the key driver of the upwelling changes.

The new findings also allowed the researchers to disentangle how the changes in Antarctic upwelling and atmospheric CO₂ are linked to the orbital triggers of the glacial cycles, bringing scientists a step closer to a complete theory for the origin of the ice ages.

“Our findings show that upwelling-driven atmospheric CO₂ change was central to the cycles, but not always in the way that many of us had assumed,” said Sigman. “For example, rather than accelerating the descent into the ice ages, Antarctic upwelling caused CO₂ changes that prolonged the warmest climates.”

Their findings also have implications for predicting how the ocean will respond to global warming. Computer models have yielded ambiguous results on the sensitivity of polar winds to climate change. The researchers’ observation of a major intensification in wind-driven upwelling in the Antarctic Ocean during warm periods of the past suggests that upwelling will also strengthen under global warming. Stronger Antarctic upwelling is likely to accelerate the ocean’s absorption of heat from ongoing global warming, while also impacting the biological conditions of the Antarctic Ocean and the ice on Antarctica.

“The new findings suggest that the atmosphere and ocean around Antarctica will change greatly in the coming century,” said Ai. “However, because the CO₂ from fossil fuel burning is unique to the current times, more work is needed to understand how Antarctic Ocean changes will affect the rate at which the ocean absorbs this CO₂.”

原始論文：Xuyuan E. Ai, Anja S. Studer, Daniel M. Sigman, Alfredo Martínez-García, François Fripiat, Lena M. Thöle, Elisabeth Michel, Julia Gottschalk, Laura Arnold, Simone Moretti, Mareike Schmitt, Sergey Oleynik, Samuel L. Jaccard, Gerald H. Haug. Southern Ocean upwelling, Earth’s obliquity, and glacial-interglacial atmospheric CO2 change. Science, 2020; 370 (6522): 1348 DOI: 10.1126/science.abd2115

引用自：Princeton University. "What caused the ice ages? Tiny ocean fossils offer key evidence: