恐龍或許是被接連兩下重擊打倒在地的
By Jim Barlow
恐龍到底是被什麼事物消滅的？這項爭議仍然存在。
奧勒岡大學進行的新研究發現6600萬年前沿著深海洋脊發生了跟重力相關的擾動現象，其指出撞擊墨西哥猶加敦半島的巨大隕石可能引發全世界的火山噴出岩漿。這兩次接踵而來的重創或許決定了發生在恐龍身上的噩運。
「我們找到的證據顯示大滅絕事件期間，有段之前未辨識出來的全球火山活動高峰期。」之前在奧勒岡大學就讀博士學位的Joseph Byrnes表示。
這項由Byrnes和奧勒岡大學地球科學系的教授Leif Karlstrom進行的研究於2月7日刊登於《科學進展》(Science Advances)。這份研究詳細描繪了保存在中洋脊(板塊構造在海底的邊界)的火山活動紀錄，證據則來自於從海床上方測到的重力強度變化。
這項奧勒岡大學的研究由美國國家科學基金會資助。Karlstrom表示研究指出在希克蘇魯伯撞擊事件之後全球火山活動有加劇的現象，包括印度德干高原的噴發量也有所提升。位在印度中西部的德干高原是由一段歷時長久的大型火山噴發事件形成，其噴濺出來的熔岩層層堆積達到數千公尺厚，使德干高原成為地球最大型的火山地貌之一。
在恐龍滅絕的爭論中，德干高原地區曾屢次居於要角又退居幕後。科學家知道如此大規模的罕見火山噴發事件對地球氣候造成的擾動足以釀成重大災難，當它們發生時通常會跟大滅絕事件有所關聯。大型火山噴發事件會噴出大量灰燼和氣體至大氣當中，僅有少數植物能生存下來，進而破壞食物鏈造成動物滅絕。
自從在1980年代發現今日墨西哥的希克蘇魯伯附近曾遭到隕石撞擊的證據之後，科學家就一直在爭論殲滅所有非鳥類恐龍的滅絕事件的罪魁禍首到底是誰――隕石還是德干高原噴發事件。
日趨進步的定年方法顯示德干高原的火山在隕石撞擊之前就已經相當活躍。Karlstrom表示隕石撞擊產生傳遍全球的地震波，或許有助於加速這些噴發事件進行。
「我們的成果顯示這些分布在整個地球，極度稀有且破壞性十足的事件彼此之間互有關連。」Karlstrom表示，「隕石撞擊可能影響了正在進行的火山噴發活動，因而對恐龍造成了接連兩下重創。」
2015年加州大學柏克萊分校的研究人員提出這兩個事件可能有關而加強了此種說法。包括Karlstrom的該團隊認為隕石產生的強力地震波撼動了整個地球，或許能影響遠方的火山活動。
Karlstrom表示就像平常由構造活動產生的地震有時候會讓水井和溪流的流量增加，他們的研究認為震波產生的晃動可以使德干高原下方地函儲存的岩漿釋放出來，而在當地造成最具規模的火山爆發。
奧勒岡大學進行的這項新發現將震波引起噴發的發生範圍從印度拓展至全世界的海盆。
現為明尼蘇達大學博士後研究員的Byrnes從公開取用的資料庫分析全球自由空間重力、海底地形和板塊擴張速率的資料。
在他的分析當中，他以一百萬年為區間來劃分海床年代，建構出的紀錄可以回推至1億年前。他發現大約6600萬年前，自由空間重力的數據出現異常的頻率有所增加，顯示沿著古代洋脊發生的海底岩漿活動此時出現了短暫的高峰期。
自由空間重力異常是指重力加速度的變化量，計算單位是相當微小的毫伽。地球重力比較強的地方會聚集更多海水，運用衛星測量便能偵測到此種變化。Byrnes發現在隕石撞擊過後的首個100萬年之內形成的海床，其自由空間重力發生了5到20毫伽的異常變化。
A one-two punch may have helped deck the dinosaurs
The debate goes on: What killed off the dinosaurs?
New University of Oregon
research has identified gravity-related fluctuations dating to 66 million years
ago along deep ocean ridges that point to a “one-two punch” from the big meteor
that struck off Mexico’s Yucatan peninsula, possibly triggering a worldwide
release of volcanic magma that could have helped seal the dinosaurs' fate.
"We found evidence for
a previously unknown period of globally heighted volcanic activity during the
mass-extinction event," said former UO doctoral student Joseph Byrnes.
The study by Byrnes
and Leif Karlstrom,
a professor in the UO’s Department of Earth Sciences, was published Feb. 7 in Science Advances. It details a record of volcanism preserved along
the mid-ocean ridges, which mark the oceanic boundaries of tectonic plates. The
evidence comes from changes in the strength of gravity above the seafloor.
The findings of the UO's
National Science Foundation-supported study, Karlstrom said, point to a pulse
of accelerated worldwide volcanic activity that includes enhanced eruptions at
India's Deccan Traps after the Chicxulub impact. The Deccan Traps, in west-central
India, formed during a period of massive eruptions that poured out layers of
molten rock thousands of feet deep, creating one of the largest volcanic
features on Earth.
The Deccan Traps region has
been in and out of the dinosaur debate. Rare volcanic events at such a scale
are known to cause catastrophic disturbances to Earth's climate, and, when they
occur, they are often linked to mass extinctions. Huge volcanic events can
eject so much ash and gas into the atmosphere that few plants survive, disrupting
the food chain and causing animals to go extinct.
Since evidence of the
meteor strike near present-day Chicxulub, Mexico,
surfaced in the 1980s, scientists have debated whether the meteor or the Deccan
Traps eruptions drove the extinction event that killed off all nonavian
dinosaurs.
Progressively improving
dating methods indicate that the Deccan Traps volcanoes already were active
when the meteor struck. Resulting seismic waves moving through the planet from
the meteor strike, Karlstrom said, probably fueled an acceleration of those
eruptions.
"Our work suggests a
connection between these exceedingly rare and catastrophic events, distributed
over the entire planet," Karlstrom said. "The meteorite's impact may
have influenced volcanic eruptions that were already going on, making for a
one-two punch."
That idea gained strength
in 2015 when researchers at the University of California, Berkeley, proposed
that the two events might be connected. That team, which included Karlstrom,
suggested that the meteorite may have modulated distant volcanism by generating
powerful seismic waves that produced shaking worldwide.
Similar to the impacts that
normal tectonic earthquakes sometimes have on wells and streams, Karlstrom
said, the study proposed that seismic shaking liberated magma stored in the
mantle beneath the Deccan Traps and caused the largest eruptions there.
The new findings at the UO
extend this eruption-triggering in India to ocean basins worldwide.
Byrnes, now a postdoctoral
researcher at the University of Minnesota, analyzed publicly available global
data sets on free-air gravity, ocean floor topography and tectonic spreading
rates.
In his analyses, he divided
the seafloor into 1-million-year-old groupings, constructing a record back to
100 million years ago. At about 66 million years, he found evidence for a
"short-lived pulse of marine magmatism" along ancient ocean ridges.
This pulse is suggested by a spike in the rate of the occurrence of free-air
gravity anomalies seen in the data set.
Free-air gravity anomalies,
measured in tiny increments call milligals, account for variations in
gravitational acceleration, found from satellite measurements of additional
seawater collecting where the Earth's gravity is stronger. Byrnes found changes
in free-air gravity anomalies of between five and 20 milligals associated with
seafloor created in the first million years after the meteor.
原始論文：Joseph S.
Byrnes, Leif Karlstrom. Anomalous K-Pg–aged seafloor attributed to
impact-induced mid-ocean ridge magmatism. Science Advances,
2018; 4 (2): eaao2994 DOI: 10.1126/sciadv.aao2994
引用自：University of Oregon. "A one-two punch may
have helped deck the dinosaurs.”
