全球暖化讓海洋動物難以呼吸，造成地球歷史上最慘重的滅絕事件
Hannah Hickey
遠在恐龍出現之前，當時大部分居住在地球的動植物都在西伯利亞發生的一連串大規模火山爆發中消失了。這起地球歷史上最大的滅絕事件發生在2億5200萬年前左右，其標記了二疊紀的結束。
這塊來自於中國南部，大約1.5呎(約45 cm)厚的石板顯示出二疊紀―三疊紀的交界。下段是在滅絕之前形成的石灰岩，上段則是滅絕之後由細菌沉積成的石灰岩。圖片來源：Jonathan Payne/Stanford University
古代海床形成的岩石中，化石展現出生機盎然且物種繁多的海洋生態系，但接著便是屍橫遍野。大約96%的海洋物種在這起「大死亡」事件裡消失殆盡，過了數百萬年之後生物的種類和數量才再次回復。
目前科學家仍在爭論海洋變得不適合生物居住的確切原因為何：是海水變得相當酸、金屬或硫化物造成毒害、氧氣完全消失、或者只是因為溫度升高？
美國華盛頓大學和史丹佛大學的新研究結合了模擬海洋環境因子的模型、已發表的動物代謝實驗數據、古海洋紀錄，顯示海裡的二疊紀大滅絕起因是全球暖化造成動物無法呼吸。當溫度升高造成海洋動物的代謝作用加快，較為溫暖的海水所含有的氧氣並不足以讓牠們活下去。
這項研究於12月7日發表在《科學》。
第一作者，華盛頓大學海洋學的博士生 Justin Penn表示：「我們首次能以物理原理來預測生物滅絕的原因並用化石紀錄直接加以驗證。之後我們能以此預測未來生物滅絕的可能原因。」
二疊紀時地球的陸塊結合成盤古超大陸。研究人員以此為條件來模擬當時的氣候。在西伯利亞接連不斷的火山爆發讓地球變成一個充滿溫室氣體的星球之前，海洋的溫度和氧含量跟現在頗為相似。研究人員接著將模型的溫室氣體濃度升高，使得熱帶海洋的表層溫度比現在還要高出10℃左右，以符合當時的狀況。
模型重現出海洋因此而發生的劇烈變化。海洋失去了將近百分之80的氧氣。而半數的海床(特別是深度較深的地方)更是變成完全無氧的狀態。
為了分析對海洋生物造成的影響，研究人員參照了61種現代的海洋生物，包括甲殼類、魚類、貝類、珊瑚和鯊魚對不同氧含量和溫度的敏感度。資料來自已發表的實驗數據。他們認為能夠忍受高溫低氧環境的現代動物跟二疊紀的動物比較相近，因為當時的動物是在類似的環境條件中演化而成。研究人員接著把這些生物的特徵跟古氣候模型結合起來，藉此預測滅絕情形的地理分布。
第二作者，華盛頓大學的海洋學副教授Curtis Deutsch說：「海洋生物很少待在牠們原本住的棲地――不是逃走就是死了。」
模型顯示高緯度地區對氧氣最為敏感的生物受到了最沉重的打擊。雖然模型中許多生活在熱帶的物種也滅亡了，但預測結果顯示高緯度地區的物種幾乎完全消失無蹤，尤其是需氧量高的物種。
為了證實模型的預測結果，史丹佛大學的共同作者Jonathan Payne和Erik Sperling 運用彙整了已發表化石紀錄的線上資料庫――Paleoceanography Database，來分析二疊紀晚期的化石分布。化石紀錄顯示了滅絕之前的生物分布情形，滅絕事件使哪些生物完全消失或者退縮至過往棲地的一部分。
化石紀錄證實了高緯度生物在事件中的處境最為艱難。
「模型預測與化石分析得到的地理模型透露出滅絕機制、氣候暖化與氧氣流失的訊息。」Penn表示，「這兩個方法得到的一致之處指出氣候暖化與氧氣流失是導致生物滅絕的主要機制。」
這項研究的基礎是之前由Deutsch進行的研究，它認為海洋暖化造成海洋動物的代謝加快，意謂著動物需要更多氧氣，然而暖化卻會使海水攜帶的氧氣卻減少。這篇前人研究顯示出海洋暖化如何迫使動物離開熱帶地區。
新的研究進一步結合了海洋環境因子的變化跟許多種動物在不同溫度下的代謝需求。結果顯示住在極區的動物因為氧氣削減而受到的影響是最嚴重的。
「由於熱帶生物的代謝作用已經了適應相當溫暖且低氧的環境，所以牠們可以搬離熱帶而在他處找到跟原本環境一樣的地方。」Deutsch表示，「但對已經適應寒冷且富含氧氣環境的生物來說，牠們再也無法在淺海地區找到這種條件。」
「死區」(dead
zone)是指完全缺乏氧氣的區域。因為所在深度大多位在動物生存的深度以下，故對存活率的影響也不大。
「最後，結果證明死區的大小在滅絕事件中的重要性其實不高。」Deutsch表示，「我們通常認為完全缺乏氧氣(缺氧，anoxia)是讓大部分地區變成不適合居住的條件。但觀察生物對低氧條件的忍受程度，大部分的生物在離缺氧還有一段距離的情況下，就無法在海水中繼續生存了。」
海洋生物多樣性的喪失有一半以上可以用暖化造成氧氣不足來解釋。作者表示其他原因可能還包括了別的變化，像是海洋酸化或光合作用生物的產量改變。
二疊紀末期的狀況跟現在十分相似，都是大氣中的溫室氣體增加使得地球升溫。
「如果我們以目前的速度繼續排放二氧化碳，到了2100年海洋上層的暖化程度會趨近於二疊紀晚期的百分之20，2300年則會到達百分之35至50。」Penn表示，「這項研究顯現出人為導致的氣候變遷有可能透過類似的機制而引起大滅絕。」
研究經費來自於高登和貝蒂•摩爾基金會，以及美國國家科學基金會。
Biggest extinction in Earth’s history caused by
global warming leaving ocean animals gasping for breath
The largest extinction in Earth’s history marked the end of the
Permian period, some 252 million years ago. Long before dinosaurs, our planet
was populated with plants and animals that were mostly obliterated after a
series of massive volcanic eruptions in Siberia.
Fossils in ancient seafloor
rocks display a thriving and diverse marine ecosystem, then a swath of corpses.
Some 96 percent of marine species were wiped out during the “Great Dying,”
followed by millions of years when life had to multiply and diversify once
more.
What has been debated until
now is exactly what made the oceans inhospitable to life – the high acidity of
the water, metal and sulfide poisoning, a complete lack of oxygen, or simply
higher temperatures.
New research from the
University of Washington and Stanford University combines models of ocean
conditions and animal metabolism with published lab data and paleoceanographic
records to show that the Permian mass extinction in the oceans was caused by
global warming that left animals unable to breathe. As temperatures rose and
the metabolism of marine animals sped up, the warmer waters could not hold
enough oxygen for them to survive.
The study was
published Dec. 7 in Science.
“This is the first time
that we have made a mechanistic prediction about what caused the extinction
that can be directly tested with the fossil record, which then allows us to
make predictions about the causes of extinction in the future,” said first
author Justin Penn, a UW doctoral student in oceanography.
Researchers ran a climate
model with Earth’s configuration during the Permian, when the land masses were
combined in the supercontinent of Pangaea. Before ongoing volcanic eruptions in
Siberia created a greenhouse-gas planet, oceans had temperatures and oxygen
levels similar to today’s. The researchers then raised greenhouse gases in the
model to the level required to make tropical ocean temperatures at the surface
some 10 degrees Celsius (20 degrees Fahrenheit) higher, matching conditions at
that time.
The model reproduces the
resulting dramatic changes in the oceans. Oceans lost about 80 percent of their
oxygen. About half the oceans’ seafloor, mostly at deeper depths, became
completely oxygen-free.
To analyze the effects on
marine species, the researchers considered the varying oxygen and temperature
sensitivities of 61 modern marine species — including crustaceans, fish,
shellfish, corals and sharks — using published lab measurements. The tolerance
of modern animals to high temperature and low oxygen is expected to be similar
to Permian animals because they had evolved under similar environmental conditions.
The researchers then combined the species’ traits with the paleoclimate
simulations to predict the geography of the extinction.
“Very few marine organisms
stayed in the same habitats they were living in — it was either flee or
perish,” said second author Curtis Deutsch, a UW associate professor of
oceanography.
The model shows the hardest
hit were organisms most sensitive to oxygen found far from the tropics. Many
species that lived in the tropics also went extinct in the model, but it
predicts that high-latitude species, especially those with high oxygen demands,
were nearly completely wiped out.
To test this prediction,
co-authors Jonathan Payne and Erik Sperling at Stanford
analyzed late-Permian fossil distributions from the Paleoceanography Database,
a virtual archive of published fossil collections. The fossil record shows
where species were before the extinction, and which were wiped out completely
or restricted to a fraction of their former habitat.
The fossil record confirms
that species far from the equator suffered most during the event.
“The signature of that kill
mechanism, climate warming and oxygen loss, is this geographic pattern that’s
predicted by the model and then discovered in the fossils,” Penn said. “The
agreement between the two indicates this mechanism of climate warming and
oxygen loss was a primary cause of the extinction.”
The study builds
on previous work led by Deutsch showing that as oceans warm, marine
animals’ metabolism speeds up, meaning they require more oxygen, while warmer
water holds less. That earlier study shows how warmer oceans push animals away
from the tropics.
The new study combines the
changing ocean conditions with various animals’ metabolic needs at different
temperatures. Results show that the most severe effects of oxygen deprivation
are for species living near the poles.
“Since tropical organisms’
metabolisms were already adapted to fairly warm, lower-oxygen conditions, they
could move away from the tropics and find the same conditions somewhere else,”
Deutsch said. “But if an organism was adapted for a cold, oxygen-rich
environment, then those conditions ceased to exist in the shallow oceans.”
The so-called “dead zones”
that are completely devoid of oxygen were mostly below depths where species
were living, and played a smaller role in the survival rates.
“At the end of the day, it
turned out that the size of the dead zones really doesn’t seem to be the key
thing for the extinction,” Deutsch said. “We often think about anoxia, the
complete lack of oxygen, as the condition you need to get widespread
uninhabitability. But when you look at the tolerance for low oxygen, most
organisms can be excluded from seawater at oxygen levels that aren’t anywhere
close to anoxic.”
Warming leading to
insufficient oxygen explains more than half of the marine diversity losses. The
authors say that other changes, such as acidification or shifts in the
productivity of photosynthetic organisms, likely acted as additional causes.
The situation in the late
Permian — increasing greenhouse gases in the atmosphere that create warmer
temperatures on Earth — is similar to today.
“Under a business-as-usual
emissions scenarios, by 2100 warming in the upper ocean will have approached 20
percent of warming in the late Permian, and by the year 2300 it will reach
between 35 and 50 percent,” Penn said. “This study highlights the potential for
a mass extinction arising from a similar mechanism under anthropogenic climate
change.”
The research was funded by
the Gordon and Betty Moore Foundation and the National Science Foundation.
原始論文：Justin L.
Penn, Curtis Deutsch, Jonathan L. Payne, Erik A. Sperling. Temperature-dependent
hypoxia explains biogeography and severity of end-Permian marine mass
extinction. Science, 2018 DOI: 10.1126/science.aat1327
引用自：University of Washington. "Biggest mass
extinction caused by global warming leaving ocean animals gasping for
breath."
沒有留言:
張貼留言