在小行星撞擊殺死恐龍之後,撞擊地點的生命卻迅速復原
在6600萬年前左右有顆小行星狠狠撞上地球,引發的大滅絕終結了恐龍的統治,也殲滅了75%的生物。
6600萬年前的小行星撞擊事件幾乎殺死了地球各處的生物。不過,一項新研究發現的證據顯示在導致生物滅絕的撞擊事件幾年之後,就有各式各樣的浮游動物和微生物棲息在該次事件造成的隕石坑。圖片左邊三種表面有毛的生物代表了棲息在隕石坑的浮游動物和微生物。左下角的幾何圖案則是一種藻類。引用自:德州大學奧斯汀分校傑克遜地球科學院/ John Maisano
雖然這顆小行星消滅了許多物種,但由德州大學奧斯汀分校領導的新研究卻發現在撞擊事件之後不到十年的時間,小行星撞擊留下的隕石坑已經成為許多海洋生物的家園,並且在三萬年之內就發展出十分繁盛的生態系――比地球上其他地方的復原速度還快上許多。
科學家對這項發現十分訝異,因為這違反了最靠近隕石坑的地方會因為撞擊釋放的物質汙染環境,像是有毒金屬等,而回復得最為緩慢的理論。反之,證據指出世界各地的復原狀況主要是受到當地條件影響,此發現對於今日受到氣候變遷而動盪的環境來說別有意義。
「我們發現在撞擊後的數年之內隕石坑內部就有生命出現,這真得快到讓人不敢置信。」領導這項研究,德州大學地球物理研究所(UTIG)的博士後研究員Chris Lowery表示,「這顯示環境的復原能力大體上是無法被準確預測的。」
這樣研究於5月30發表於期刊《自然》(Nature)。共同作者包含了UTIG的研究員Gail Christeson 和Sean Gulick,以及博士後研究員Cornelia Rasmussen,其他還有由國際科學家組成的團隊。UTIG是德州大學奧斯汀分校傑克遜地球科學院的研究單位。
從隕石坑取出的岩石中可以看到生命存在的證據,主要是內部的微體化石,也就是藻類和浮游生物等單細胞生物留下的殘骸,另一種則是較大型的生物挖出來的洞穴。這些岩石是國際海洋發現計畫(International Ocean Discovery Program)與國際陸地鑽探計畫(International
Continental Drilling Program)近期聯合進行的科學鑽井成果。
這些微小的化石不但是生物居住在隕石坑的鐵證,也可以用來作為撞擊事件後的許多年之間,對於環境適居性的通用指標。迅速的回復過程指出在撞擊發生不久之後,隕石坑內部除了微生物以外還有其他的生命形式居住於此。
「微體化石可以給出完整的圖像讓你知道當時的生物群集組成」。Lowery表示,「拿起一塊石頭,裡面就有千百具微體化石,所以我們十分有自信可以看出他們的族群數量是如何變化……同時我們也可以利用它們作為某種代用指標來觀察更大型的生物。」
科學家發現生命的證據首次出現在撞擊兩到三年之後,包括了由小型蝦子或蟲子挖出的洞穴。在撞擊發生30,000年之後,隕石坑內部已經擁有相當繁盛的生態系,大量的浮游植物供養了海洋表面以及海床上各式各樣的生物群集。相較而言,世上其他地區,像是北美洲和墨西哥灣的其他地點則花了將近300,000年才回復到相近的程度。
在2016年由傑克遜地球科學院共同主導的航次中,科學家從隕石坑取得了這根含有化石證據的岩芯。在此研究中,科學家聚焦於這根岩芯中的一個特殊段落,其以前所未見的詳細程度捕捉了撞擊過後的海床景象。從海洋其他地方取得的岩芯樣品中,撞擊事件後的那段時間沉積下來的物質只有幾毫米厚,但研究中從隕石坑取得的這段岩芯段落卻含有超過130公尺厚的沉積物,而最上方的30英吋(大約是76公分)是從混濁的水體中緩緩沉降而成。這些物質提供的紀錄保存了事件過後數天至數年之內海床的狀態。
「你可以看見此岩芯中有一層一層的構造,但其他岩芯的組成物質通常是混在一起,代表它們的化石和沉積物紀錄都攪和在一塊,使得我們無法分辨出相當短時間之內發生的事物。」共同作者,賓州州立大學的微體古生物學教授Timothy Bralower表示。「但我們在這裡得到的化石紀錄,能讓我們分辨它們在數年、數個月、數周甚至是數天之內的變化。」
耶魯大學地質和地球物理學的資深研究人員Ellen Thomas並未參與這項研究,她表示雖然這篇論文已經呈現出快速復原的強烈證據,她預期許多科研人員會有興趣親自來探討這些數據的意義。
Thomas表示:「依照我的看法,關於(復原過程的)特性、年代、沉積速率和微體化石組成,之後將會有許多爭辯……特別是在撞擊事件後數年之內就有會挖洞的生物返回撞擊地點的猜測。」
生命在隕石坑從谷底反彈的速度相對較快,顯示小行星雖然能造成大滅絕,卻無法阻止生命復原。科學家指出局部因素,從海水的循環、生物之間的交互作用,以至於可以佔據的生態棲位多寡,對於個別生態系統的回復速率來說似乎是最具影響力的因子。
這項發現代表在全球性的毀滅事件之後,不同地方的復原過程卻可能是各自為政。
Life Recovered Rapidly at Impact Site of Dino-Killing Asteroid
About 66
million years ago, an asteroid smashed into Earth, triggering a mass extinction
that ended the reign of the dinosaurs and snuffed out 75 percent of life.
An asteroid impact 66
million years ago wiped out most life across the planet. A new study has found
evidence for a diverse array of plankton and microorganisms inhabiting the
crater only a few years after the extinction-causing impact. The three
hair-covered forms (left) represent species of plankton found inside the
crater. The geometric form (bottom left) is a species of algae. Credit:The University of Texas at Austin Jackson School of Geosciences/John
Maisano
Although the asteroid
killed off species, new research led by The University of Texas at Austin has
found that the crater it left behind was home to sea life less than a decade
after impact, and it contained a thriving ecosystem within 30,000 years — a
much quicker recovery than other sites around the globe.
Scientists were surprised
by the findings, which undermine a theory that recovery at sites closest to the
crater is the slowest due to environmental contaminants — such as toxic metals
— released by the impact. Instead, the evidence suggests that recovery around
the world was influenced primarily by local factors, a finding that could have
implications for environments rocked by climate change today.
“We found life in the
crater within a few years of impact, which is really fast, surprisingly fast,”
said Chris Lowery, a postdoctoral researcher at the University of Texas
Institute for Geophysics (UTIG) who led the research. “It shows that there’s
not a lot of predictability of recovery in general.”
The research was published
May 30 in the journal Nature.
UTIG research scientists Gail Christeson and Sean Gulick and postdoctoral
researcher Cornelia Rasmussen are co-authors on the paper, along with a
team of international scientists. UTIG is a research unit of the Jackson School
of Geosciences.
The evidence for life comes
primarily in the form of microfossils — the remains of unicellular organisms
such as algae and plankton — as well as the burrows of larger organisms
discovered in a rock extracted from the crater during recent scientific
drilling conducted jointly by the International Ocean Discovery Program and
International Continental Drilling Program.
The tiny fossils are hard
evidence that organisms inhabited the crater, but also a general indicator
about habitability in the environment years after impact. The swift recovery
suggests that other life forms aside from the microscopic were living in the
crater shortly after impact.
“Microfossils let you get
at this complete community picture of what’s going on,” Lowery said. “You get a
chunk of rock and there’s thousands of microfossils there, so we can look at
changes in the population with a really high degree of confidence … and we can
use that as kind of a proxy for the larger scale organisms.”
The scientists found the
first evidence for the appearance of life two to three years after impact. The
evidence included burrows made by small shrimp or worms. By 30,000 years after
impact, a thriving ecosystem was present in the crater, with blooming
phytoplankton (microscopic plants) supporting a diverse community of organisms
in the surface waters and on the seafloor. In contrast, other areas around the
world, including the North Atlantic and other areas of the Gulf of Mexico, took
up to 300,000 years to recover in a similar manner.
The core containing the
fossil evidence was extracted from the crater during a 2016 expedition co-led
by the Jackson School. In this study, scientists zeroed in on a unique core
section that captures the post-impact seafloor in unprecedented detail. Whereas
core samples from other parts of the ocean hold only millimeters of material
deposited in the moments after impact, the section from the crater used in this
study contains more than 130 meters of such material, the upper 30 inches of
which settled out slowly from the turbid water. This material provides a record
that captures the seafloor environment days to years after the impact.
“You can see layering in
this core, while in others, they’re generally mixed, meaning that the record of
fossils and materials is all churned up, and you can’t resolve tiny time
intervals,” said co-author Timothy Bralower, a micropaleontology professor at
Pennsylvania State University. “We have a fossil record here where we’re able
to resolve daily, weekly, monthly, yearly changes.”
Ellen Thomas, a senior
research scientist in geology and geophysics at Yale University who was not
part of the study, said that although she thinks the paper makes a strong case
for a speedy recovery, she expects that the larger scientific community will be
interested in digging into the data for themselves.
“In my opinion, we will see
considerable debate on the character, age, sedimentation rate and microfossil
content … especially of the speculation that burrowing animals may have
returned within years of the impact,” Thomas said.
The relatively rapid
rebound of life in the crater suggests that although the asteroid caused the
extinction, it didn’t hamper recovery. The scientists point to local factors,
from water circulation to interactions between organisms and the availability of
ecological niches, as having the most influence on a particular ecosystem’s
recovery rate.
The findings indicate that
recovery after a global catastrophe could be a local affair.
原始論文:Christopher M. Lowery, Timothy J. Bralower, Jeremy D. Owens,
Francisco J. Rodríguez-Tovar, Heather Jones, Jan Smit, Michael T. Whalen,
Phillipe Claeys, Kenneth Farley, Sean P. S. Gulick, Joanna V. Morgan, Sophie
Green, Elise Chenot, Gail L. Christeson, Charles S. Cockell, Marco J. L.
Coolen, Ludovic Ferrière, Catalina Gebhardt, Kazuhisa Goto, David A. Kring,
Johanna Lofi, Rubén Ocampo-Torres, Ligia Perez-Cruz, Annemarie E. Pickersgill,
Michael H. Poelchau, Auriol S. P. Rae, Cornelia Rasmussen, Mario
Rebolledo-Vieyra, Ulrich Riller, Honami Sato, Sonia M. Tikoo, Naotaka Tomioka,
Jaime Urrutia-Fucugauchi, Johan Vellekoop, Axel Wittmann, Long Xiao, Kosei E.
Yamaguchi, William Zylberman. Rapid
recovery of life at ground zero of the end-Cretaceous mass extinction. Nature, 2018;
DOI: 10.1038/s41586-018-0163-6
引用自:University of Texas at Austin. "Life
recovered rapidly at impact site of dino-killing asteroid."
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