原文網址：https://www.ub.edu/web/ub/en/menu_eines/noticies/2021/09/030.html
發表在期刊《地質》(Geology)的研究否決了在白堊紀末的物種大量滅絕事件中，極端劇烈的火山活動期具有任何一絲影響。這項結果也證實了6600萬年前一次巨大的隕石撞擊對生物造成了重大危機，最終消滅了非鳥類恐龍的血脈以及其他陸上與海中生物的假說。
蘇馬亞海岸的岩壁保存了相當完整的地層段落，從中可以得出地球1億1500萬年到5000萬年前的地質歷史
這項研究的作者為巴塞隆納大學地球科學院的研究人員Sietske
Batenburg，以及阿拉貢環境科學大學研究所(IUCA，隸屬於薩拉戈薩大學)的專家Vicente
Gilabert、Ignacio
Arenillas和José
Antonio Arz。
白堊紀―古近紀邊界：記錄在蘇馬亞海岸的白堊紀大滅絕
這項研究的對象為巴斯克自治區的蘇馬亞海岸，此處保存良好的地層段落顯示了地球1億1500萬到5000萬年前的地質歷史。研究團隊在此分析了沉積於6640萬到6540萬年前、富含微體化石的沉積物與岩石。這段期間包括了6600萬年前的白堊紀―古近紀邊界，它不只分隔了中生代與新生代，同時也是地球五次大滅絕事件其中一次的發生時間。
研究分析了白堊紀―古近紀邊界代表的大滅絕事件前後不久發生的氣候變遷，以及它們跟該場大型生物危機的潛在關聯。這是研究人員首次從時間的角度來檢驗氣候變遷是否能對應到它的潛在原因：地質史上最劇烈的火山活動之一――印度德干的大型火山活動，以及地球公轉軌道的變動。
「蘇馬亞露頭的特別之處在於該地堆積的沉積物有兩種類型：有些富含黏土，其他的則富含碳酸鹽。它們現在變成了我們看到的這些地層，或者是由泥岩和石灰岩交互堆疊形成的韻律層，」巴塞隆納大學地球與海洋動力學系的研究人員Sietske
Batenburg如此解釋。「在沉積作用中，這種強烈的規律性和地球自轉軸的方向與傾角，以及繞日軌道橫向移動的周期性變化有關。」
這些天文學上的型態變化稱作米蘭科維奇循環，每過405,000、100,000、41,000和21,000年便會重複一次。它們控制了地球接收到的太陽輻射量，進而調節全球溫度以及進入海洋的沉積物種類。「透過從蘇馬亞沉積物中辨識出來的周期性，我們對於最後一群恐龍存活的時間點附近發生的氣候事件，得出了迄今最為精準的定年成果，」巴塞隆納大學地球科學系的博士生Vicente
Gilabert表示。他即將在今年底進行論文口試。
透露過往氣候的浮游有孔蟲
浮游有孔蟲的微體化石是一種十分精準的生物地層指標，藉由研究它們並分析岩石的碳13同位素，研究人員重建了蘇馬亞沉積物的年代表以及古氣候。蘇馬亞沉積物中的白堊紀浮游有孔蟲90%以上都在6600萬年前滅亡；於此同時，碳循環也發生了大幅變動，並且堆積了許多衝擊產生的玻璃小球，其源自於撞擊墨西哥猶加敦半島希克蘇魯伯的小行星。
此外，研究結果也顯示出三次跟希克蘇魯伯無關的強烈氣候暖化事件，稱為「超高溫事件」。第一次事件LMWE發生在白堊紀―古近紀邊界之前，年代為6625萬到6610萬年前之間。另外兩次則發生在大滅絕事件之後，分別稱作Dan-C2(6580萬至6570萬年前)與LC29n(6548萬至6541萬年前)。
德干火山活動的增強會排放大量氣體到大氣當中，是否導致了上述的超高溫事件？過去十年對此有很激烈的討論。這些專家接著表示：「我們的結果指出這些事件都跟極端的軌道型態――『偏心率極大期』同時發生。唯有估計讓全球氣溫升高2到5°C的LMWE從時間上來看與德干噴發活動有關，代表它的成因為火山活動帶來的影響加上白堊紀最後一次的偏心率極大期。」
地球繞日軌道的變化
發生在白堊紀末期與古近紀早期，也就是白堊紀―古近紀邊界之前25萬年到之後20萬年間的全球氣候變遷，成因為地球繞日軌道的偏心率到達極大值。
然而白堊紀―古近紀邊界前後，對氣候變遷造成影響的軌道偏心率與白堊紀末的物種大滅絕並沒有關係。這些起因自偏心率極大值並受到德干火山活動進一步增強的氣候變遷，是以數十萬年的時間尺度逐漸發生。
「這些數據證實了白堊紀大滅絕的起因完全來自地球系統的外部：小行星的撞擊比白堊紀末的氣候變遷(LMWE)還要萬了10萬年，」研究團隊表示。「此外，白堊紀―古近紀邊界之前的最後10萬年，環境處於相當穩定的狀態，沒有明顯的波動；而從地質時間上來看，物種大滅絕事件就像是瞬間發生的事情，」他們對此總結。
Extreme volcanism did not cause the massive
extinction of species in the late Cretaceous
A study published in the journal Geology rules out that extreme volcanic
episodes had any influence on the massive extinction of species in the late
Cretaceous. The results confirm the hypothesis that it was a giant meteorite
impact what caused the great biological crisis that ended up with the non-avian
dinosaur lineages and other marine and terrestrial organisms 66 million years
ago.
The study was carried out by the researcher Sietske
Batenburg, from the Faculty of Earth Sciences of the University of Barcelona,
and the experts Vicente Gilabert, Ignacio Arenillas and José Antonio Arz, from
the University Research Institute on Environmental Sciences of Aragon
(IUCA-University of Zaragoza).
K/Pg boundary:
the great extinction of the Cretaceous in Zumaia coasts
The scenario of this study were the Zumaia cliffs
(Basque Country), which have an exceptional section of strata that reveals the
geological history of the Earth in the period of 115-50 million years ago (Ma).
In this environment, the team analyzed sediments and rocks that are rich in
microfossils that were deposited between 66.4 and 65.4 Ma, a time interval that
includes the known Cretaceous/Paleogene boundary (K/Pg). Dated in 66 Ma, the
K/Pg boundary divides the Mesozoic and Cenozoic eras and it coincides with one
of the five large extinctions of the planet.
This study analysed the climate changes that occurred
just before and after the massive extinction marked by the K/Pg boundary, as
well as its potential relation to this large biological crisis. For the first
time, researchers examined whether this climate change coincides on the time scale
with its potential causes: the Deccan massive volcanism (India) ─one of the
most violent volcanic episodes in the geological history of the planet─ and the
orbital variations of the Earth.
“The particularity of the Zumaia outcrops lies in
that two types of sediments accumulated there ─some richer in clay and others
richer in carbonate─ that we can now identify as strata or marl and limestone
that alternate with each other to form rhythms”, notes the researcher Sietske
Batenburg, from the Department of Earth and Ocean Dynamics of the UB. “This
strong rhythmicity in sedimentation is related to cyclical variations in the
orientation and inclination of the Earth axis in the rotation movement, as well
as in the translational movement around the Sun”.
These astronomic configurations ─the known
Milankovitch cycles, which repeat every 405,000, 100,000, 41,000 and 21,000
years─, regulate the amount of solar radiation they receive, modulate the
global temperature of our planet and condition the type of sediment that
reaches the oceans. "Thanks to these periodicities identified in the
Zumaia sediments, we have been able to determine the most precise dating of the
climatic eepisodes that took place around the time when the last dinosaurs
lived", says PhD student Vicente Gilabert, from the Department of Earth
Sciences at UZ, who will present his thesis defence by the end of this year.
Planktonic
foraminifera: revealing the climate of the past
Carbon-13 isotopic analysis on the rocks in
combination with the study of planktonic foraminifera ─microfossils used as
high-precision biostratigraphic indicators─ has made it possible to reconstruct
the paleoclimate and chronology of that time in the Zumaia sediments. More than
90% of the Cretaceous planktonic foraminiferal species from Zumaia became
extinct 66 Ma ago, coinciding with a big disruption in the carbon cycle and an
accumulation of impact glass spherules originating from the asteroid that hit
Chicxulub, in the Yucatan Peninsula (Mexico).
In addition, the conclusions of the study reveal the
existence of three intense climatic warming events ─known as hyperthermal
events─ that are not related to the Chicxulub impact. The first, known as LMWE
and prior to the K/Pg boundary, has been dated to between 66.25 and 66.10 Ma.
The other two events, after the mass extinction, are called Dan-C2 (between
65.8 and 65.7 Ma) and LC29n (between 65.48 and 65.41 Ma).
In the last decade, there has been intense debate
over whether the hyperthermal events mentioned above were caused by an
increased Deccan volcanic activity, which emitted large amounts of gases into
the atmosphere. "Our results indicate that all these events are in sync
with extreme orbital configurations of the Earth known as eccentricity maxima.
Only the LMWE, which produced an estimated global warming of 2-5°C, appears to
be temporally related to a Deccan eruptive episode, suggesting that it was
caused by a combination of the effects of volcanism and the latest Cretaceous
eccentricity maximum", the experts add.
Earth's orbital
variations around the Sun
The global climate changes that occurred in the late
Cretaceous and early Palaeogene ─between 250,000 years before and 200,000 years
after the K/Pg boundary─ were due to eccentricity maxima of the Earth's orbit
around the Sun.
However, the orbital eccentricity that influenced
climate changes before and after the K/Pg boundary is not related to the late
Cretaceous mass extinction of species. The climatic changes caused by the eccentricity
maxima and augmented by the Deccan volcanism occurred gradually at a scale of
hundreds of thousands of years.
"These data would confirm that the extinction
was caused by something completely external to the Earth system: the impact of
an asteroid that occurred 100,000 years after this late Cretaceous climate
change (the LMWE)", the research team says. "Furthermore, the last
100,000 years before the K/Pg boundary are characterized by high environmental
stability with no obvious perturbations, and the large mass extinction of
species occurred instantaneously on the geological timescale", they
conclude.
原始論文：Vicente
Gilabert, Sietske J. Batenburg, Ignacio Arenillas, José A. Arz. Contribution
of orbital forcing and Deccan volcanism to global climatic and biotic changes
across the Cretaceous-Paleogene boundary at Zumaia, Spain. Geology,
2021; DOI: 10.1130/G49214.1
引用自：University of Barcelona. "Extreme
volcanism did not cause the massive extinction of species in the late
Cretaceous.”
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