倫敦大學學院、瑞典自然史博物館、佛羅倫斯大學和倫敦自然史博物館組成的國際團隊，發現了一種從古至今幾乎被完全忽略的特殊化石種類。
鬼魂化石(左)以及它們的虛擬翻模(右)。這些化石的長度大約只有5微米，是人類頭髮寬度的15分之1。圖片來源：S.M Slater et al.
這種化石是數百萬年前生活在海裡、稱為鈣板藻的單細胞浮游生物留下的「鬼魂」——一種微小的印痕。發現它們使我們了解氣候變遷對海洋浮游生物會造成什麼樣的影響時，有了不一樣的理解。
鈣板藻在現今的海洋具有很重要的功能：它們提供了我們呼吸的大部分氧氣，支撐海洋的食物網，並且把碳封鎖在海床的沉積物裡面。鈣板藻是一種微小的浮游生物，細胞周圍環繞著堅硬的鈣質薄板，稱為「鈣板」。在岩石裡變成化石的通常就是這些鈣板。
在過去幾段全球暖化事件，紀錄顯示鈣板藻的化石數量有所下降，意味它們會因為氣候變遷與海洋酸化而受到很嚴重的影響。
然而今日發表於期刊《科學》(Science)的研究，表示侏儸紀與白堊紀的三段暖化事件當中(分別發生在9400萬年、1億2000萬年與1億8300萬年前)這種新發現的鬼魂化石紀錄在全球各地都相當多，意味過去發生氣候變遷的時候，鈣板藻的恢復能力比之前認為的還強。
共同作者Paul
Bown 是倫敦大學學院地球科學系的教授，他說：「這些鬼魂化石的保存方式真的讓人非常驚訝。雖然鈣板本身已經溶解掉了，但它們壓在其他古代有機物表面的痕跡卻保存了下來。這些鬼魂化石的尺寸非常微小，長度大約只有五千分之一公厘，也就是人類頭髮寬度的十五分之一！但仍然可以清楚看到鈣板原本的細節。」
「這些美麗的鬼魂化石完全是意料之外的發現，」瑞典自然史博物館的Sam
Slater博士表示。「最初我們發現它們保存在花粉化石的表面，接著很快就看到一般的鈣板藻化石很少出現或是消失的期間，其實有很多鬼魂化石——這令我們非常訝異！」
雖然鈣板藻非常微小，但在現今的海洋當中它們可能大量出現，形成從太空中看起來像是雲朵一般的藻華。鈣板藻死後它們的鈣質外骨骼會沉到海床，大量堆積之下便形成了白堊這類的岩石。
這些鬼魂化石形成於海床上的沉積物被埋藏並轉變成岩石的過程：隨著上方的泥巴逐漸堆積，壓力也跟著增強，使得鈣板和其他有機物質被擠壓在一塊。堅硬的鈣板會被壓入花粉、孢子和其他較軟的有機物質的表面。之後岩石孔隙中的酸性水體可能會把鈣板溶解，只留下它們的印痕——就像鬼魂一樣。
顯微鏡下，這種浮游生物細胞壁的表層壓進了古代有機物的表面，形成了「鬼魂」般的印痕化石而保存下來(年代為1億8300萬年前)。圖片顯示崩散的細胞壁表層(鈣板球)留在一塊古代有機物碎片表面的印痕(左)。將個別的薄板(鈣板)放大之後，可以看到次微米尺度的構造精巧地保存了下來(右)。藍色圖片是將顏色反轉，以虛擬的方式來翻模化石，也就是原先的立體型態。原本的鈣板受到溶解作用而從沉積物消失，只留下這些鬼魂般的印痕。圖片來源：S.M.
Slater, P. Bown et al / Science journal
「古生物學家一般只會尋找鈣板本體的化石，如果都沒有找到的話，他們通常會推測過去這些浮游生物群集瓦解了，」瑞典自然史博物館的教授Vivi
Vajda解釋。「這些鬼魂化石顯示化石紀錄有時候會捉弄我們，其實這種鈣質超微浮游生物還能透過其他方法保存下來。當我們試著瞭解它們如何應對過去的氣候變遷，應該要把這種化石考慮進去。」
佛羅倫斯大學的教授Silvia
Danise說：「這些鬼魂般的超微化石在化石紀錄可能十分常見，但因為十分微小而且保存的方式相當隱密，使得我們一直以來都對它們視而不見。我們認為這種奇特的化石種類未來十分有用，特別是在研究化石紀錄沒有原本的鈣板的地質時代。」
這篇研究的重點放在侏儸紀早期(1億8300萬年前)一段全球迅速暖化的時期——托阿爾階海洋滅絕事件(Toarcian
Oceanic Anoxic Event , T-OAE)，成因為南半球的大型火山活動造成大氣裡的二氧化碳增加。研究人員從英國、德國、日本和紐西蘭發現與T-OAE有關的鬼魂超微化石；此外在其他兩個發生在白堊紀、類似的全球暖化事件中也發現了它們，分別是海洋缺氧事件1a(1億2000萬年前)在瑞典的紀錄以及海洋缺氧事件2(9400萬年前)在義大利的紀錄。
「之前的化石紀錄顯示過往在侏儸紀和白堊紀的暖化事件中，超微浮游生物因為海洋酸化的關係而瓦解，但是鬼魂化石卻指出它們的數量和種類其實很多而且非常旺盛，」倫敦自然史博物館的教授Richard
Twitchett解釋。「這些化石改寫了我們對於鈣質超微浮游生物如何因應暖化事件的認知。」
Sam Slater博士最後解釋：「我們的研究顯示在過去的暖化事件當中有非常多的浮游藻類，並且促成了海洋死區擴張，也就是海床氧含量太低，大部分物種都無法生存的區域。在我們逐漸暖化的全球海洋中，這些浮游藻華和死區的出現情形未來可能也會擴張。」
Discovery of 'ghost' fossils reveals
plankton resilience to past global warming events
An international team of scientists from
UCL, the Swedish Museum of Natural History, the University of Florence and
Natural History Museum have found a remarkable type of fossilization that has
remained almost entirely overlooked until now.
The fossils are microscopic imprints, or “ghosts”, of
single-celled plankton, called coccolithophores, that lived in the seas
millions of years ago, and their discovery is changing our understanding of how
plankton in the oceans are affected by climate change.
Coccolithophores are important in today’s oceans,
providing much of the oxygen we breathe, supporting marine food webs, and
locking carbon away in seafloor sediments. They are a type of microscopic
plankton that surround their cells with hard calcareous plates, called
coccoliths, and these are what normally fossilize in rocks.
Declines in the abundance of these fossils have been
documented from multiple past global warming events, suggesting that these
plankton were severely affected by climate change and ocean acidification.
However, a study published today in the journal Science presents new global records of
abundant ghost fossils from three Jurassic and Cretaceous warming events (94,
120 and 183 million years ago), suggesting that coccolithophores were more
resilient to past climate change than was previously thought.
Co-author Professor Paul Bown (UCL Earth Sciences)
said: “The preservation of these ghost nannofossils is truly remarkable. The
ghost fossils are extremely small ‒ their length is approximately five
thousandths of a millimetre, 15 times narrower than the width of a human hair!
‒ but the detail of the original plates is still perfectly visible, pressed
into the surfaces of ancient organic matter, even though the plates themselves
have dissolved away”.
“The discovery of these beautiful ghost fossils was
completely unexpected”, said Dr Sam Slater from the Swedish Museum of Natural
History. “We initially found them preserved on the surfaces of fossilized
pollen, and it quickly became apparent that they were abundant during intervals
where normal coccolithophore fossils were rare or absent – this was a total
surprise!”
Despite their microscopic size, coccolithophores can
be hugely abundant in the present ocean, being visible from space as cloud-like
blooms. After death, their calcareous exoskeletons sink to the seafloor, accumulating
in vast numbers, forming rocks such as chalk.
The ghost fossils formed while the sediments at the
seafloor were being buried and turned into rock. As more mud was gradually
deposited on top, the resulting pressure squashed the coccolith plates and
other organic remains together, and the hard coccoliths were pressed into the
surfaces of pollen, spores and other soft organic matter. Later, acidic waters
within spaces in the rock dissolved away the coccoliths, leaving behind just
their impressions – the ghosts.
“Normally, palaeontologists only search for the
fossil coccoliths themselves, and if they don’t find any then they often assume
that these ancient plankton communities collapsed,” explained Professor Vivi
Vajda (Swedish Museum of Natural History). “These ghost fossils show us that
sometimes the fossil record plays tricks on us and there are other ways that
these calcareous nannoplankton may be preserved, which need to be taken into
account when trying to understand responses to past climate change”.
Profesor Silvia Danise (University of Florence) said:
“Ghost nannofossils are likely common in the fossil record, but they have been
overlooked due to their tiny size and cryptic mode of preservation. We think
that this peculiar type of fossilization will be useful in the future,
particularly when studying geological intervals where the original coccoliths
are missing from the fossil record”.
The study focused on the Toarcian Oceanic Anoxic
Event (T-OAE), an interval of rapid global warming in the Early Jurassic (183
million years ago), caused by an increase in CO2-levels in the
atmosphere from massive volcanism in the Southern Hemisphere. The researchers
found ghost nannofossils associated with the T-OAE from the UK, Germany, Japan
and New Zealand, but also from two similar global warming events in the
Cretaceous: Oceanic Anoxic Event 1a (120 million years ago) from Sweden, and
Oceanic Anoxic Event 2 (94 million years ago) from Italy.
“The ghost fossils show that nannoplankton were
abundant, diverse and thriving during past warming events in the Jurassic and
Cretaceous, where previous records have assumed that plankton collapsed due to
ocean acidification,” explained Professor Richard Twitchett (Natural History
Museum, London). “These fossils are rewriting our understanding of how the
calcareous nannoplankton respond to warming events.”
Finally, Dr Sam Slater explained: “Our study shows
that algal plankton were abundant during these past warming events and
contributed to the expansion of marine dead zones, where seafloor oxygen-levels
were too low for most species to survive. These conditions, with plankton
blooms and dead zones, may become more widespread across our globally warming
oceans.”
原始論文：Sam M. Slater,
Paul Bown, Richard J. Twitchett, Silvia Danise, Vivi Vajda. Global
record of “ghost” nannofossils reveals plankton resilience to high CO 2 and
warming. Science, 2022; 376 (6595): 853 DOI: 10.1126/science.abm7330
引用自：University College London. "Discovery of
'ghost' fossils reveals plankton resilience to past global warming events:
Scientists have discovered a new type of fossilization."
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