原文網址：https://www.fau.edu/newsdesk/articles/dinosaur-extinction-chicxulub-asteroid
By Gisele
Galoustian
一項由佛羅里達大西洋大學的研究人員以及國際科學團隊主持的研究得到了突破性的發現，他們的結論證實了造成浩劫的希克蘇魯伯小行星撞擊事件，也就是6600萬年前導致恐龍和地球上75%生命滅亡的原因，是在一年當中的什麼時候發生。雖然春天是萬物之始的季節，但是恐龍1億6500萬年的王朝卻在此時落幕，並且改變了地球生命的演化方向。
象徵萬物之始的春天卻結束了恐龍1億6500萬年的王朝，並且改變了地球生命的演化方向。
這篇發表在期刊《科學報告》的研究成果讓科學家可以更加仔細地追查在這場災難的最初階段，地球上的生命承受了什麼樣的傷害。資深作者，佛羅里達大西洋大學理學院地球科學系的兼任教授Robert
DePalma、曼徹斯特大學的一名博士生以及第二作者，佛羅里達大西洋大學地球科學系的副教授Anton
Oleinik博士在科學方面得到的大幅進展，使他們可以更加瞭解這場造成恐龍滅亡的巨大撞擊事件。
「一年當中的時節對於生物的許多機能來說具有很重要的影響，像是繁殖、進食策略、寄生蟲與宿主的關係、季節性的休眠以及生育模式，」DePalma表示。「因此，擴及全球的災害會讓生物受到多麼嚴重的衝擊，在一定程度上取決於當下所處的時節並不令人意外。也因為這樣，白堊紀末滅絕的故事之中，希克蘇魯伯撞擊事件的發生季節便成為了相當重要的問題。目前為止，這項問題的答案還沒有定論。」
科學家從幾十年前就已經知道造成浩劫的希克蘇魯伯小行星是在6600萬年撞上猶加敦半島。撞擊造成了地球歷史上第三大的滅絕事件，全球生物群系因此產生的劇烈改變讓人直接聯想到今日的全球生態危機。但是，撞擊發生之後更加細部的情節以及這些事件如何導致地球歷史上第三大的滅絕事件，仍然十分模糊。
這項新研究是從2014年開始的長期工作，研究人員結合了傳統方法與最新科技來拼湊各條線索，使他們可以辨認希克蘇魯伯撞擊事件發生在哪個季節。DePalma為了瞭解滅絕事件的內部運作情形，他在北達科他州西南部的塔尼斯展開調查，此處是白堊紀―古近紀邊界在世上保存得最為詳細的地點之一。研究結果不只提供了重要的新數據，也建立了新的學術合作關係。
「這處位於北達科他州獨一無二的場址提供的大量新資訊讓我們十分興奮。經過努力分析該場址蒐集到的野外數據之後，我們得到令人難以置信、極為詳細的全新觀點，不僅得知白堊紀―古近紀邊界發生了什麼事，還有確切的發生時間，」Oleinik表示。「6600萬年前小行星撞擊地球發生在一年當中的哪個時節，有好幾條獨立線索都給出了非常明確的答案，這真的是再驚人不過了。科學最棒的其中一個地方就是讓我們看待似乎已經很熟悉的事實或者事件的時候，可以提供不同的角度並且看得更加仔細，使得我們的知識增長同時更瞭解自然界。此外，結果也證明即便到了21世紀，地質學和古生物學仍然可以帶來新的科學發現。」
DePalma 領導的研究團隊之前在2019年的研究中費盡心力，透過放射性定年、地層學、孢粉化石、指準化石以及富含銥的黏土蓋層，定出塔尼斯剛好就是在白堊紀末希克蘇魯伯撞擊事件發生時形成。
該篇研究表示撞擊引發的大洪水造成大量沉積物迅速掩埋了塔尼斯，將這場事件凝結在時間當中，保存了迄今所知唯一的白堊紀―古近紀邊界大規模死亡的脊椎動物化石組合。這些緊緊堆疊並交纏在一塊的植物、動物、樹木以及小行星噴濺出來的碎片提供了絕無僅有的機會，讓科學家釐清白堊紀―古近紀事件、因而死亡的生物相以及它們所處的環境等諸多細節。
「化石紀錄是瞭解全球規模的災難發生之後生物如何回應的關鍵。如果沒有這些知識，當現代發生類似的事件，我們便無法做足準備來做出適當的反應或回應，」DePalma表示。「有鑑於另外一次全球生物遭受威脅的情況似乎迫在眉睫，化石紀錄對現代人的上述用途便顯得更加重要。」
從塔尼斯找到的魚骨化石可以看到生長線，其獨特的構造以及模式就像條碼一樣，顯示他們調查的魚類全都是在春夏季的生長期死亡。利用高科技的同位素分析來檢驗生長線也獨立證實了這點，結果顯示同位素的年度震盪也終止於春夏季的生長期。
團隊更進一步地提出其他線索來支持他們的發現。其中一項是運用史丹佛同步輻射光源進行同步快速掃描―X射線螢光法(Synchrotron-Rapid-Scanning
X-Ray Fluorescence，SRS-XRF)，這種尖端技術提供了新的途徑來定年沉積物的季節，結果也支持了未成年魚類骨頭化石的檢驗結果。
團隊比較年紀最小的魚類身長和現生種類的生長速度，藉此預估這些魚類是在孵化多久之後被埋起來。他們接著將此對應到現代這些魚類已知的產卵季節，便可以推測坦尼斯的沉積物代表的季節範圍，一如骨頭指出的是春季至夏季。
「像這樣的偉大發現美好之處在於這不只回饋了科學社群，也是回饋社會大眾的機會，」DePalma表示。「研究不只回答了重要的問題，也讓新的想法激發出來，並且加以延伸與實現。」
此處能進行的研究遠遠不只如此，即將展開的其他計畫希望可以挖掘更多細節，以了解這塊有趣且重要的時間片段。除了最初的研究團隊之外，還有來自其他學術單位、未參與主要研究的人員加入調查此場址，如此熱絡的合作過程沒有任何跡象顯示會在短時間內結束。
Dinosaurs’ last spring: study
pinpoints timing of asteroid impact
A groundbreaking study led by researchers
at Florida Atlantic University and an international team of scientists
conclusively confirms the time year of the catastrophic Chicxulub asteroid,
responsible for the extinction of dinosaurs and 75 percent of life on Earth 66
million years ago. Springtime, the season of new beginnings, ended the
165-million-year reign of dinosaurs and changed the course of evolution on
Earth.
Results of the study, published in the journal Scientific Reports, greatly enhances the
ability to trace the first stages of damage to life on Earth. FAU’s Robert
DePalma, senior author and an adjunct professor in the Department of
Geosciences, Charles E. Schmidt College of Science, and a doctoral student at
the University of Manchester; and Anton Oleinik, Ph.D., second author and an
associate professor, FAU’s Department of Geosciences, contribute to a major
scientific advancement in the ability to understand the massive impact that
brought an end to the dinosaurs.
“Time of year plays an important role in many
biological functions such as reproduction, feeding strategies, host-parasite
interactions, seasonal dormancy, and breeding patterns,” said DePalma. “Hence,
it is no surprise that the time of year for a global-scale hazard can play a
big role in how harshly it impacts life. The seasonal timing of the Chicxulub
impact has therefore been a critical question for the story of the
end-Cretaceous extinction. Until now, the answer to that question has remained
unclear.”
For decades, it has been known that the cataclysmic
Chicxulub asteroid impact hit the Yucatan peninsula 66 million years ago. The
impact triggered the third-greatest extinction in Earth’s history, dramatically
changing global biomes in ways that directly relate to current global
ecological crisis. Yet, the finer details of what happened after impact and how
those events led to the third-worst mass-extinction in Earth’s history remain
very hazy.
The new study was a long-term effort that started in
2014 and applied a combination of traditional and cutting-edge techniques to
piece together a trail of clues enabling identification of the season for the
Chicxulub impact event. DePalma examined the Tanis research locality in
southwestern North Dakota, one of the most highly detailed Cretaceous-Paleogene
(KPg) boundary sites in the world, to understand the inner workings of the
extinction event. The research provides important new data while building new
academic bridges.
“This unique site in North Dakota had yielded a
wealth of new and exciting information. Field data collected at the site, after
hard work that went into analyzing it, provided us with new incredibly detailed
insight of not only what happened at the Cretaceous-Paleogene boundary, but
also exactly when it happened,” said Oleinik. “It is nothing short of amazing
how multiple lines of independent evidence suggested so clearly what time of
the year it was 66 million years ago when the asteroid hit the planet. One of
the great things about science is that it allows us to look at seemingly
well-known facts and events at different angles and with different precision,
therefore advancing our knowledge and understanding of the natural world. It
also proves that geology and paleontology is still a science of discovery, even
in the 21st Century.”
Using radiometric dating, stratigraphy, fossil
pollen, index fossils, and a capping layer of iridium-rich clay, the research
team laboriously determined in a previous study led by DePalma in 2019 that the
Tanis site dated from precisely the time of the end-Cretaceous Chicxulub
impact.
The same study documented that a massive surge of
water, triggered by the impact, was the cause for the rapidly deposited drape
of sediment that locked the event in time and preserved the only known
impact-caused vertebrate mass-death assemblage at the KPg boundary. The densely
packed tangle of plants, animals, trees, and asteroid ejecta enabled a unique
opportunity to work out the fine details of the KPg event, the biota that
succumbed to it, and the environment in which they lived.
“The fossil record is a key to understanding biotic
response to global-scale hazards, without which we would be ill-equipped to
properly respond or react to modern events,” said DePalma. “This modern utility
of the fossil record is highlighted by the fact that we currently appear to be
perched at the threshold of another episode of global biotic stress.”
The unique structure and pattern of the growth lines
in fossil fish bones from the site, similar to a barcode, showed that all of
the examined fish died during the spring-summer growth phase. High-tech
isotopic analysis of the growth lines provided independent confirmation of
this, showing a yearly oscillation that also terminated during the
spring-summer growth.
The team further supported their findings by
overlaying multiple additional lines of evidence. Examination of juvenile
fossil fish was supported in part by cutting-edge Synchrotron-Rapid-Scanning
X-Ray Fluorescence (SRS-XRF) carried out at the Stanford Synchrotron Radiation
Lightsource (SSRL), providing a novel way of seasonally dating the deposit.
Comparing the sizes of the youngest fish to modern
growth rates enabled the team to predict how long after hatching the fish were
buried. Comparing this to known modern spawning seasons enabled them to deduce
what seasonal range was represented by the deposit at Tanis – spring to summer,
just as indicated by the bones.
“The beauty of any great discovery such as this is
that it is a chance to give back to the scientific community, and to the world,”
said DePalma. “It not only answers important questions, but also sparks new
minds to reach forward and achieve.”
Research at the site is far from complete, and
additional upcoming projects hope to uncover more details about this
interesting and significant slice of time. Multiple researchers from other
institutions, who are not part of the main study, have examined the site in
addition to the primary research team, and there is no indication that this
rich collaborative process will end anytime soon.
原始論文：Robert A.
DePalma, Anton A. Oleinik, Loren P. Gurche, David A. Burnham, Jeremy J.
Klingler, Curtis J. McKinney, Frederick P. Cichocki, Peter L. Larson, Victoria
M. Egerton, Roy A. Wogelius, Nicholas P. Edwards, Uwe Bergmann, Phillip L.
Manning. Seasonal calibration of the end-cretaceous Chicxulub impact
event. Scientific Reports, 2021; 11 (1) DOI: 10.1038/s41598-021-03232-9
引用自：Florida Atlantic University. "
Dinosaurs’ last spring: study
pinpoints timing of asteroid impact.”
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