新理論解釋地球鐵的特性

原文網址：http://www.geologypage.com/2017/03/new-theories-nature-earths-iron.html

新理論解釋地球鐵的特性

新研究挑戰了現行理論中，認為地球鐵的特殊性質是由數十億年前的地核形成過程所導致。

這項研究導出了另一項具有競爭力的理論來解釋為何地球上某些較重的鐵，也就是某些鐵同位素的含量會比其他太陽系天體中的還要多。現今盛行的理論將地球奇異的鐵組成歸因於地球地核的形成過程，但2月20日刊登於《自然通訊》(Nature Communications)中的這篇研究，卻認為地球特殊的鐵同位素訊號是在地球歷史中較晚才發展出來，可能是地球與其他行星碰撞使得較輕的鐵同位素蒸發，或是攪動地函而造成較重的鐵同位素以不同比例從地函進入至地殼之內。

由冰與火交織而成的完美風暴造就了雪球地球

原文網址：www.sciencedaily.com/releases/2017/03/170313160813.htm

由冰與火交織而成的完美風暴造就了雪球地球

對數十億年一次的事件提出解釋

是什麼造成了地球史上稱做「雪球地球」(snowball Earth)的最劇烈冰河期事件？地質學家和氣候學家數年來不斷尋求解答，但此事件的根源仍然尚未明瞭。

哈佛大學的研究人員現在提出了一則新假說，來解釋是什麼造成了使地球兩極之間皆被冰雪所覆蓋的超級冰河期。

此研究刊登於《地球物理研究通訊》。

研究人員早已精確定出Sturtian雪球地球事件的起始時間約為7.17億年前(誤差約為數百萬年)。大約是同一時間，一起巨型火山爆發事件摧毀了從現今的阿拉斯加至格陵蘭的廣大地區。這會是件巧合嗎？

哈佛大學的教授Francis Macdonald和Robin Wordsworth並不這麼認為。

「我們知道火山活動可以對環境造成重大影響，因此關鍵問題是：這兩個事件之間有何關聯？」哈佛自然科學的John L. Loeb副教授Macdonald表示。

起初，Macdonald的團隊認為是會分解成鎂和鈣的玄武岩，跟空氣中的二氧化碳反應而導致了冷化。然而，若此理論為真，則冷化會在數百萬年之間逐漸發生。但從加拿大北極地區的火山岩得到的同位素定年結果，卻指出火山爆發跟冷化事件發生的時間更為接近。

Macdonald轉而尋求模擬地球以外行星的氣候的專家Wordsworth協助，並提出有沒有可能是由這些火山噴出的氣膠(aerosol)快速冷卻了地球？

答案是：在適當的情況下，確實如此。

「形成大型火山岩區域(large volcanic province)的噴發事件一點都不特別。」哈佛大學約翰·保爾森工程與應用科學院的環境科學與工程學助理教授Wordsworth表示。「這類型的火山噴發在整部地質史上一直反覆發生，但它們並非總是跟冷化事件有關。因此問題是：什麼造成了這個事件有所不同？」

對稱作Franklin大火成岩省進行的地質以及化學研究顯示，此區域噴發火山岩時，岩漿穿透了富含硫的沉積物，這些硫會在噴發過程中以二氧化硫的形式進入大氣層。當二氧化硫進入大氣上層時，可以對太陽輻射起到相當好的遮蔽作用。1991年菲律賓的Pinatubo火山將大約1000萬公噸的硫噴發至大氣當中，使得接下來一年全球氣溫下降了華氏1度左右。

對流層頂(tropopause)是分隔對流層和平流層的邊界，如果二氧化硫通過此處便能對太陽輻射起到最佳的屏蔽作用。若二氧化硫到達此等高度，它們就不太會經由降水作用或是跟其他粒子混和而回到地面，這會使它們待在大氣的時間延長數個禮拜到將近一年。對流層頂這道屏障的高度為何，全取決於當時地球的氣候背景值(background climate)。地球越冷，對流層頂的所在高度就越低。

「在地球歷史相當溫暖的時期，受到溫暖且位於高處的對流層頂保護，火山冷化不會有多大影響。」Wordsworth說，「在氣候較冷的情況下，地球會變得特別容易受到這類火山作用影響，使得氣候發生擾動。」

「我們的模型顯示事件本身的內容及發生背景都至關重要。」Macdonald表示。

另一個重點是二氧化硫氣柱到達對流層頂的位置。由於大陸漂移的關係，在7.17億年前發生噴發事件的Franklin大火成岩省當時是位於赤道附近，讓地球保持溫暖的太陽輻射大部份是在此處進入地球。

結論便是可以有效反射陽光的氣體於正確的地點到達大氣適當的高度，使得冷化效應發生。但還需要另一個要素才能孕育出這道「完美風暴」。畢竟，Pinatubo火山噴發也具有類似的特質，但其冷卻效應只不過持續了一年之久。

7.17億年前將硫拋射至空氣中的噴發事件可不是像Pinatubo這類由單一火山導致的單次爆發事件。研究中的火山群從加拿大綿延至格陵蘭，長度將近2000英哩。這些火山的噴發方式並非為單獨一次的爆裂式噴發，而是持續不斷的噴發，就像今日發生於夏威夷和冰島的形式。研究人員的論述中認為此種類型的火山噴發持續了十年左右，而將足以迅速顛覆氣候的大量氣膠傾倒至大氣層當中。

「由氣膠導致的冷化作用不需要將整個地球冰封起來，它只需要把冰層推進到某個緯度，冰層就會接手完成剩下的工作。」Macdonald表示。

冰層越廣，就越會將越多的陽光反射回太空，造成地球變得更冷。一旦冰層到達現今加州所在的緯度，正回饋效應造成的迴圈就會接手整個過程，超級雪球效應也變得勢不可擋。

「人們很容易將氣候想像成難以撼動的巨大系統，就很多方面來看也是事實。但在過往確實曾經發生十分劇烈的改變，同樣地，這樣的巨變也很可能在未來發生。」Wordsworth表示。

理解這些劇烈的氣候變遷事件如何發生，有助於研究人員更加了解大滅絕事件的發生原因，以及之前提出的地球工程方法可能對氣候造成的衝擊，還有其他星球的氣候變遷會是何種情況。

「這項研究提醒我們需要摒棄把地外行星看得過於單純的慣例，只考慮他們處於穩定平衡狀態和是否處於適居帶。」Wordsworth說。「我們知道地球是顆生氣勃勃，卻擁有激烈變化的行星。有充分理由可以相信像這樣的快速氣候變遷對行星來說是種常態，而非難得一見的例外。」

A perfect storm of fire and ice may have led to snowball Earth

Explaining a 'once-in-a-billion-year event'

What caused the largest glaciation event in Earth's history, known as 'snowball Earth'? Geologists and climate scientists have been searching for the answer for years but the root cause of the phenomenon remains elusive.

Now, Harvard University researchers have a new hypothesis about what caused the runaway glaciation that covered Earth pole-to-pole in ice.

The research is published in Geophysical Research Letters.

Researchers have pinpointed the start of what's known as the Sturtian snowball Earth event to about 717 million years ago -- give or take a few 100,000 years. At around that time, a huge volcanic event devastated an area from present-day Alaska to Greenland. Coincidence?

Harvard professors Francis Macdonald and Robin Wordsworth thought not.

"We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related," said Macdonald, the John L. Loeb Associate Professor of the Natural Sciences.

At first, Macdonald's team thought basaltic rock -- which breaks down into magnesium and calcium -- interacted with CO₂ in the atmosphere and caused cooling. However, if that were the case, cooling would have happened over millions of years and radio-isotopic dating from volcanic rocks in Arctic Canada suggest a far more precise coincidence with cooling.

Macdonald turned to Wordsworth, who models climates of non-Earth planets, and asked: could aerosols emitted from these volcanos have rapidly cooled Earth?

The answer: yes, under the right conditions.

"It is not unique to have large volcanic provinces erupting," said Wordsworth, assistant professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Science. "These types of eruptions have happened over and over again throughout geological time but they're not always associated with cooling events. So, the question is, what made this event different?"

Geological and chemical studies of this region, known as the Franklin large igneous province, showed that volcanic rocks erupted through sulfur-rich sediments, which would have been pushed into the atmosphere during eruption as sulfur dioxide. When sulfur dioxide gets into the upper layers of the atmosphere, it's very good at blocking solar radiation. The 1991 eruption of Mount Pinatubo in the Philippines, which shot about 10 million metric tons of sulfur into the air, reduced global temperatures about 1 degree Fahrenheit for a year.

Sulfur dioxide is most effective at blocking solar radiation if it gets past the tropopause, the boundary separating the troposphere and stratosphere. If it reaches this height, it's less likely to be brought back down to earth in precipitation or mixed with other particles, extending its presence in the atmosphere from about a week to about a year. The height of the tropopause barrier all depends on the background climate of the planet -- the cooler the planet, the lower the tropopause.

"In periods of Earth's history when it was very warm, volcanic cooling would not have been very important because Earth would have been shielded by this warm, high tropopause," said Wordsworth. "In cooler conditions, Earth becomes uniquely vulnerable to having these kinds of volcanic perturbations to climate."

"What our models have shown is that context and background really matters," said Macdonald.

Another important aspect is where the sulfur dioxide plumes reach the stratosphere. Due to continental drift, 717 million years ago, the Franklin large igneous province where these eruptions took place was situated near the equator, the entry point for most of the solar radiation that keeps Earth warm.

So, an effective light-reflecting gas entered the atmosphere at just the right location and height to cause cooling. But another element was needed to form the perfect storm scenario. After all, the Pinatubo eruption had similar qualities but its cooling effect only lasted about a year.

The eruptions throwing sulfur into the air 717 million years ago weren't one-off explosions of single volcanoes like Pinatubo. The volcanoes in question spanned almost 2,000 miles across Canada and Greenland. Instead of singularly explosive eruptions, these volcanoes can erupt more continuously like those in Hawaii and Iceland today. The researchers demonstrated that a decade or so of continual eruptions from this type of volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilize the climate.

"Cooling from aerosols doesn't have to freeze the whole planet; it just has to drive the ice to a critical latitude. Then the ice does the rest," said Macdonald.

The more ice, the more sunlight is reflected and the cooler the planet becomes. Once the ice reaches latitudes around present-day California, the positive feedback loop takes over and the runaway snowball effect is pretty much unstoppable.

"It's easy to think of climate as this immense system that is very difficult to change and in many ways that's true. But there have been very dramatic changes in the past and there's every possibility that as sudden of a change could happen in the future as well," said Wordsworth.

Understanding how these dramatic changes occur could help researchers better understand how extinctions occurred, how proposed geoengineering approaches may impact climate and how climates change on other planets.

"This research shows that we need to get away from a simple paradigm of exoplanets, just thinking about stable equilibrium conditions and habitable zones," said Wordsworth. "We know that Earth is a dynamic and active place that has had sharp transitions. There is every reason to believe that rapid climate transitions of this type are the norm on planets, rather than the exception."

原始論文：F. A. Macdonald, R. Wordsworth. Initiation of Snowball Earth with volcanic sulfur aerosol emissions. Geophysical Research Letters, 2017; DOI: 10.1002/2016GL072335

引用自：Harvard John A. Paulson School of Engineering and Applied Sciences. "A perfect storm of fire and ice may have led to snowball Earth: Explaining a 'once-in-a-billion-year event'." ScienceDaily. ScienceDaily, 13 March 2017.

發現世上最古老的化石

原始論文：www.sciencedaily.com/releases/2017/03/170301132012.htm

發現世上最古老的化石

由倫敦大學學院的科學家(UCL)領導的國際團隊發現了年代至少有37.7億年的微生物遺骸。此發現是顯示地球最早生命形式之一為何的直接證據。

在加拿大魁北克的努夫亞吉圖克表岩帶(Nuvvuagittuq Supracrustal Belt，NSB)的石英地層中，發現嵌有以鐵維生的微生物形成的微小絲狀和管狀痕跡。

NSB有些岩石是世上已知最古老的沉積岩。這些岩石的年代為37.7億年前至43億年前，部份形成於富含鐵質的深海熱泉系統，此處提供了地球第一批生物生存的棲地。「我們的發現支持了生物在地球形成不久之後即出現在海底熱泉的說法。這項證據顯示地球生命出現地相當快，也跟最近發現年代為37億年，由微生物形成的丘狀沉積物互相吻合。」第一作者Matthew Dodd解釋(其為UCL地球科學系和英國倫敦奈米科技中心的博士生) 。

由UCL、NASA、加拿大卡內基研究院和英國工程暨物理科學研究委員會贊助，今日刊登於期刊《自然》(Nature)的這篇研究，詳細描述了團隊發現的遺骸及分析結果。此團隊由UCL、挪威地質調查所、美國地質調查所、西澳大學、渥太華大學和里茲大學的科學家組成。

在此發現之前，其他論文發表的最古老微體化石是發現於澳洲西部，定年結果為34.6億年。但有些科學家認為它們是非生物作用在岩石中形成的產物。因此由UCL領導的團隊的首要任務便是要確認從加拿大出土的遺骸來源是否為生物。

研究人員有系統的觀察樣本中由赤鐵礦(hematite)――鐵的一種氧化物，也就是鐵鏽――組成的絲線與管束，是否有可能由非生物作用形成，像是沉積物埋藏過程中發生的溫度壓力變化。他們發現由非生物作用產生的可能性為零。

這些由赤鐵礦形成的結構及其擁有分支的特徵，跟今日發現於熱泉系統附近的鐵氧化菌(iron-oxidising bacteria)如出一轍。另外這些結構周遭有石墨(graphite)、磷灰石(apatite)和碳酸鹽類礦物，它們可以在生物體內像是骨頭和牙齒中發現，因此常被視作跟化石有關的礦物。

他們還發現這些已被礦物取代的化石伴有年代較近的化石中常出現的球狀構造，代表很有可能是利用氧化鐵來得到能量的細菌在岩石裡變成化石之後，赤鐵礦才形成。

「我們認為在這些數公分寬稱作結核(concretion)或團塊(nodule)的結構內部，以及其他稱作薔薇組織(rosette)或米粒組織(granule)的更小型球狀結構中發現的絲線和微管是腐敗作用(putrefaction)的產物。它們的礦物組成跟挪威、北美五大湖區和澳洲西部較年輕的岩石一致。」領導這項研究的Dominic Papineau博士表示(其任職於UCL地球科學系和英國倫敦奈米科技中心)。

「從非常早期直到近代的地質紀錄中都有腐敗作用的詳細記載，而此結構和礦物組成都符合腐敗作用會導致的結果。我們從已知最古老岩層之一中挖掘到的事物，意味著發現了地球最古老生命形式之一為何的直接證據。這項發現有助於我們拼湊出地球的真實歷史以及其上的不凡生命是如何發展，甚至未來可以帶領我們找出宇宙其他地方的生命蹤跡。」

Matthew Dodd總結：「這些發現在在顯示當地球和火星表面皆有液態水時，地球的生命已經發展出來了，這帶出了許多關於地外生命的有趣問題。因此，我們預期終將找到火星40億年前曾有生命存在的證據，如果沒有找到的話，代表地球或許是個特別的例外。」

World's oldest fossils unearthed

Remains of microorganisms at least 3,770 million years old have been discovered by an international team led by UCL scientists, providing direct evidence of one of the oldest life forms on Earth.

Tiny filaments and tubes formed by bacteria that lived on iron were found encased in quartz layers in the Nuvvuagittuq Supracrustal Belt (NSB), Quebec, Canada.

The NSB contains some of the oldest sedimentary rocks known on Earth which likely formed part of an iron-rich deep-sea hydrothermal vent system that provided a habitat for Earth's first life forms between 3,770 and 4,300 million years ago. "Our discovery supports the idea that life emerged from hot, seafloor vents shortly after planet Earth formed. This speedy appearance of life on Earth fits with other evidence of recently discovered 3,700 million year old sedimentary mounds that were shaped by microorganisms," explained first author, PhD student Matthew Dodd (UCL Earth Sciences and the London Centre for Nanotechnology).

Published today in Nature and funded by UCL, NASA, Carnegie of Canada and the UK Engineering and Physical Sciences Research Council, the study describes the discovery and the detailed analysis of the remains undertaken by the team from UCL, the Geological Survey of Norway, US Geological Survey, The University of Western Australia, the University of Ottawa and the University of Leeds.

Prior to this discovery, the oldest microfossils reported were found in Western Australia and dated at 3,460 million years old but some scientists think they might be non-biological artefacts in the rocks. It was therefore a priority for the UCL-led team to determine whether the remains from Canada had biological origins.

The researchers systematically looked at the ways the tubes and filaments, made of haematite -- a form of iron oxide or 'rust' -- could have been made through non-biological methods such as temperature and pressure changes in the rock during burial of the sediments, but found all of the possibilities unlikely.

The haematite structures have the same characteristic branching of iron-oxidising bacteria found near other hydrothermal vents today and were found alongside graphite and minerals like apatite and carbonate which are found in biological matter including bones and teeth and are frequently associated with fossils.

They also found that the mineralised fossils are associated with spheroidal structures that usually contain fossils in younger rocks, suggesting that the haematite most likely formed when bacteria that oxidised iron for energy were fossilised in the rock.

"We found the filaments and tubes inside centimetre-sized structures called concretions or nodules, as well as other tiny spheroidal structures, called rosettes and granules, all of which we think are the products of putrefaction. They are mineralogically identical to those in younger rocks from Norway, the Great Lakes area of North America and Western Australia," explained study lead, Dr Dominic Papineau (UCL Earth Sciences and the London Centre for Nanotechnology).

"The structures are composed of the minerals expected to form from putrefaction, and have been well documented throughout the geological record, from the beginning until today. The fact we unearthed them from one of the oldest known rock formations, suggests we've found direct evidence of one of Earth's oldest life forms. This discovery helps us piece together the history of our planet and the remarkable life on it, and will help to identify traces of life elsewhere in the universe."

Matthew Dodd concluded, "These discoveries demonstrate life developed on Earth at a time when Mars and Earth had liquid water at their surfaces, posing exciting questions for extra-terrestrial life. Therefore, we expect to find evidence for past life on Mars 4,000 million years ago, or if not, Earth may have been a special exception."

原始論文：Matthew S. Dodd, Dominic Papineau, Tor Grenne, John F. Slack, Martin Rittner, Franco Pirajno, Jonathan O’Neil, Crispin T. S. Little. Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, 2017; 543 (7643): 60 DOI: 10.1038/nature21377

引用自：University College London. "World's oldest fossils unearthed." ScienceDaily. ScienceDaily, 1 March 2017. 

從科羅拉多州的岩石找到「混沌太陽系」理論的證據

原始網址：www.sciencedaily.com/releases/2017/02/170222131512.htm

從科羅拉多州的岩石找到「混沌太陽系」理論的證據

科羅拉多州有一組年代為9000萬年，貌似千層派的沉積岩。威斯康辛大學麥迪遜分校和西北大學的科學家團隊深入剖析之後，發現的證據可以支持一則關於太陽系行星的繞日軌道如何運行的理論。

刊登於2017年2月23日的《自然》(Nature)，此發現的重要之處在於對科學家所稱的「渾沌太陽系」(chaotic solar system)理論首次提出了鐵證。這項理論於1989年提出，說明太陽系當前狀態的些微改變會在數百萬年後顯現出來，而對地球氣候造成巨大影響――這些影響會反映在地球歷史的岩石紀錄之中。

此發現不只有望讓我們更加了解太陽系運作的機制，還可以讓我們更精準地測量地質年代的特定時刻。此外，這也可以使我們進一步認識地球軌道和氣候變遷在地質時間尺度下彼此之間有何關聯。

在數千萬年前恐龍仍統治地球之時，北美淺海形成了一組由石灰岩和頁岩交互堆疊的岩層。由威斯康辛大學麥迪遜分校的地質科學教授Stephen Meyers，和西北大學行星與地球科學教授Brad Sageman領導的團隊，利用在這組岩石中找到的證據，發現了8700萬年前火星和地球之間發生的「共振變換」(resonance transition)所產生的訊號。共振變換是渾沌理論中「蝴蝶效應」產生的後果，其概念係指非線性系統中初始狀態的微小改變，經過一段時間後會產生巨大的影響。

就太陽系來說，當一顆行星在其環繞太陽的軌道跟另一軌道上的行星以較近距離擦肩而過時，兩顆繞日天體之間便會產生拉力，重複作用之後就會導致共振變換。行星軌道中這些微小而規律的擾動最終可以讓行星相對於太陽的位置，以及自轉軸的指向產生很大的變動。結果會造成行星特定面積接收到的太陽輻射量跟著變化，又行星受到的太陽輻射量多寡以及照射位置是氣候運作的關鍵要素。

「天文現象的循環能對氣候產生莫大影響。」Meyers特別以地球冰河期擁有的特定步調當作範例來解釋。現在我們已經可以將冰河期循環跟地球軌道形狀以及地軸傾斜方式的變化之間做出良好的匹配。「天文理論讓我們可以精確估算過去的氣候事件，或許也能使我們模擬未來的氣候。」

Meyers、Sageman和威斯康辛大學麥迪遜分校的研究生Chao Ma(本研究為其論文內容的一部份)為了找出共振變換的訊號，探討了科羅拉多州Niobrara岩層中的地質紀錄。此岩層為沉積物於數千年萬間逐層累積在白堊紀西部內陸海道(Cretaceous Western Interior Seaway)的寬廣淺海中而形成。這座淺海從現今的北極海延伸至墨西哥灣，將北美洲分成了東西兩半部。

Meyers指出：「由於黏土和碳酸鈣相對含量的改變，使得Niobrara層中的層狀岩石呈現出明確的交替變化。」他是天文年代學的專家，這門學問利用天文事件的循環來測量地質年代。「黏土(以頁岩的形式沉積)的來源是地表的風化作用，並經由河流運輸至海道。碳酸鈣(石灰岩)的來源則是生活在水層當中，體型多半相當微小的生物的殼體。」

Meyers解釋雖然氣候變遷和沉積作用之間的關係可以相當複雜，但基本概念十分單純：「氣候變遷會影響黏土相對於碳酸鹽輸入至海道的量，從而記錄了在此作用中的天文訊息。舉例來說，氣候處於相當暖濕的狀態時，河流會將大量黏土輸入至海道當中，而形成黏土含量豐富的岩石，也就是頁岩；另一種情況則是氣候處於乾冷的狀態時，海道收到的黏土會較少，因此形成碳酸鈣含量豐富的岩石，也就是石灰岩。」

這項新研究由美國國家科基金會贊助，主要建立於先前對Niobrara層的地層紀錄進行的縝密調查，以及天文年代學的重大研究成果。後者是西北大學Sageman的前研究生Robert Locklair撰寫論文期間所進行的研究。

由Ma、Meyers和Sageman發現的火星―地球間共振變換的年代也經由放射性定年得到佐證。放射性定年為一種定年方法，利用岩石中元素放射性衰變的速率，來定出岩石的絕對年代。近年來，威斯康辛大學麥迪遜分校的地質科學教授Bradley Singer以及其他科學家設計出準度及精度更加提高的放射性同位素定年法，它們被用在共振變換的定年而得到許多收穫。

自日心說――地球和行星皆繞著太陽轉動的學說――於16世紀問世以來，行星環繞太陽的運動方式就一直是科學家深感興趣的議題。從18世紀開始，對於行星環繞太陽的主流看法是將其視為一座時鐘，具有準週期(quasiperiodic)性且高度可預測的軌道。然而1988年，電腦對外行星的計算結果指出冥王星的軌道具有混沌性質。1989年，現任職於巴黎天文台的天文學家Jacques Laskar提出了混沌太陽系的概念。

Meyers表示自Laskar提出混沌太陽系理論之後，科學家便一直在熱切尋找可以支持這個想法的鐵證。

「其他研究也曾根據地質紀錄提出混沌性質的存在。」Meyers說。「但這是首度有人提出無庸置疑的證據。依靠高品質的放射性定年數據以及保存在岩石當中的強烈天文學記號才得以達成。」

From rocks in Colorado, evidence of a 'chaotic solar system'

Plumbing a 90 million-year-old layer cake of sedimentary rock in Colorado, a team of scientists from the University of Wisconsin-Madison and Northwestern University has found evidence confirming a critical of how the planets in our solar system behave in their orbits around the sun.

The finding, published Feb. 23, 2017 in the journal Nature, is important because it provides the first hard proof for what scientists call the "chaotic solar system," a theory proposed in 1989 to account for small variations in the present conditions of the solar system. The variations, playing out over many millions of years, produce big changes in our planet's climate -- changes that can be reflected in the rocks that record Earth's history.

The discovery promises not only a better understanding of the mechanics of the solar system, but also a more precise measuring stick for geologic time. Moreover, it offers a better understanding of the link between orbital variations and climate change over geologic time scales.

Using evidence from alternating layers of limestone and shale laid down over millions of years in a shallow North American seaway at the time dinosaurs held sway on Earth, the team led by UW-Madison Professor of Geoscience Stephen Meyers and Northwestern University Professor of Earth and Planetary Sciences Brad Sageman discovered the 87 million-year-old signature of a "resonance transition" between Mars and Earth. A resonance transition is the consequence of the "butterfly effect" in chaos theory. It plays on the idea that small changes in the initial conditions of a nonlinear system can have large effects over time.

In the context of the solar system, the phenomenon occurs when two orbiting bodies periodically tug at one another, as occurs when a planet in its track around the sun passes in relative proximity to another planet in its own orbit. These small but regular ticks in a planet's orbit can exert big changes on the location and orientation of a planet on its axis relative to the sun and, accordingly, change the amount of solar radiation a planet receives over a given area. Where and how much solar radiation a planet gets is a key driver of climate.

"The impact of astronomical cycles on climate can be quite large," explains Meyers, noting as an example the pacing of Earth's ice ages, which have been reliably matched to periodic changes in the shape of Earth's orbit, and the tilt of our planet on its axis. "Astronomical theory permits a very detailed evaluation of past climate events that may provide an analog for future climate."

To find the signature of a resonance transition, Meyers, Sageman and UW-Madison graduate student Chao Ma, whose dissertation work this comprises, looked to the geologic record in what is known as the Niobrara Formation in Colorado. The formation was laid down layer by layer over tens of millions of years as sediment was deposited on the bottom of a vast seaway known as the Cretaceous Western Interior Seaway. The shallow ocean stretched from what is now the Arctic Ocean to the Gulf of Mexico, separating the eastern and western portions of North America.

"The Niobrara Formation exhibits pronounced rhythmic rock layering due to changes in the relative abundance of clay and calcium carbonate," notes Meyers, an authority on astrochronology, which utilizes astronomical cycles to measure geologic time. "The source of the clay (laid down as shale) is from weathering of the land surface and the influx of clay to the seaway via rivers. The source of the calcium carbonate (limestone) is the shells of organisms, mostly microscopic, that lived in the water column."

Meyers explains that while the link between climate change and sedimentation can be complex, the basic idea is simple: "Climate change influences the relative delivery of clay versus calcium carbonate, recording the astronomical signal in the process. For example, imagine a very warm and wet climate state that pumps clay into the seaway via rivers, producing a clay-rich rock or shale, alternating with a drier and cooler climate state which pumps less clay into the seaway and produces a calcium carbonate-rich rock or limestone."

The new study was supported by grants from the National Science Foundation. It builds on a meticulous stratigraphic record and important astrochronologic studies of the Niobrara Formation, the latter conducted in the dissertation work of Robert Locklair, a former student of Sageman's at Northwestern.

Dating of the Mars-Earth resonance transition found by Ma, Meyers and Sageman was confirmed by radioisotopic dating, a method for dating the absolute ages of rocks using known rates of radioactive decay of elements in the rocks. In recent years, major advances in the accuracy and precision of radioisotopic dating, devised by UW-Madison geoscience Professor Bradley Singer and others, have been introduced and contribute to the dating of the resonance transition.

The motions of the planets around the sun has been a subject of deep scientific interest since the advent of the heliocentric theory -- the idea that Earth and planets revolve around the sun -- in the 16th century. From the 18th century, the dominant view of the solar system was that the planets orbited the sun like clockwork, having quasiperiodic and highly predictable orbits. In 1988, however, numerical calculations of the outer planets showed Pluto's orbit to be "chaotic" and the idea of a chaotic solar system was proposed in 1989 by astronomer Jacques Laskar, now at the Paris Observatory.

Following Laskar's proposal of a chaotic solar system, scientists have been looking in earnest for definitive evidence that would support the idea, says Meyers.

"Other studies have suggested the presence of chaos based on geologic data," says Meyers. "But this is the first unambiguous evidence, made possible by the availability of high-quality, radioisotopic dates and the strong astronomical signal preserved in the rocks."

原始論文：Chao Ma, Stephen R. Meyers, Bradley B. Sageman. Theory of chaotic orbital variations confirmed by Cretaceous geological evidence. Nature, 2017; 542 (7642): 468 DOI: 10.1038/nature21402

引用自：University of Wisconsin-Madison. "From rocks in Colorado, evidence of a 'chaotic solar system'." ScienceDaily. ScienceDaily, 22 February 2017.