藻類化石揭露了長達五億年的氣候變遷史
地球科學家可以穿梭至遙遠的過去，重建地質史與古氣候，藉此更加精準地預測未來的氣候狀況。荷蘭海洋研究所(NIOZ)和烏特勒支大學的科學家成功發展出可以指示古代二氧化碳含量的新型代用指標(proxy)，方法是利用葉綠素產生的微小碎屑，稱作植烷(phytane)的微小分子。這種新的有機物代用指標不只提供了迄今最為連續的二氧化碳濃度紀錄，它所涵蓋的時間也破紀錄地到達五億年。新的數據證實了目前的想法：過去花費數百萬年才能到達的二氧化碳上升幅度，如今卻在一個世紀之內就發生了。成果發表11月28日的《科學進展》(Science Advances)。
由於現在大氣裡的二氧化碳不斷增加，因此瞭解這些變化會造成什麼後果是相當重要的。為了對未來做出更精準的預測，我們必須先瞭解在漫長的地質歷史中二氧化碳的長期變化過程。雖然有方法可以直接量測過去的二氧化碳多寡，比方量測冰芯裡的氣泡含有的古代大氣。然而，冰芯涵蓋的時間長度大概侷限在一百萬年。為了追溯至更久遠的歷史，地球科學家從不同代用指標，像是儲存在古代沉積物裡的藻類、葉子、古土壤與化學物質，發展出許多間接測量二氧化碳的方法來重建出古代的環境因子。
時光旅行的新工具：植烷
地球化學家利用新的代用指標――一種葉綠素分解之後的產物――而得到許久之前持續記載二氧化碳含量的歷史紀錄。NIOZ的科學家最近把植烷發展成一種相當有潛力的新型有機代用指標，它揭露了海洋的二氧化碳含量從寒武紀到近代這五億年間的變化。
利用這個新的代用指標，他們可以建構出迄今最為連續的古代二氧化碳紀錄。「我們證實了我們發展出的新工具可以讓我們追溯至更古老的歷史，並帶領我們前往更多地方。」NIOZ的科學家Caitlyn Witkowski表示。「植烷給了我們單一一種海洋代用指標所得出的最長二氧化碳紀錄。這項新數據對於模擬人員來說極具價值，藉此他們可以對未來做出更精準的預測。」
Witkowski和同事從深海岩芯的海洋沉積物以及世界各地的石油中挑出了超過300份樣品，它們代表了最近五億年來的多數地質時代。
分子化石
分子化石可以「儲存」過往的化學反應，因此它們也許能反映出許多種過去的環境因子。地球化學家便可以從分子化石「讀取」這些環境因子，比方海水溫度、pH、鹽度和二氧化碳含量。像植烷這類的有機物反映的是海水或大氣裡的二氧化碳分壓(pCO2)。
微小的綠色奇蹟
雖然任何有機物都有潛力能反映出二氧化碳含量，植烷卻非泛泛之輩。植烷這種色素是我們的世界之所以為綠色的原因。任何吸收陽光來進行光合作用的生物，包括植物、藻類以及某些細菌都含有植烷的來源――葉綠素。植物和藻類會吸收二氧化碳並產生氧氣。如果沒有這些微小的綠色奇蹟，我們的世界會截然不同。
由於世上每個角落都可以找到葉綠素，因此植烷也是無所不在，同時也是生質分解與形成化石之後的主要成分。Witkowski說：「植烷的化學性質不會隨著時間改變，即使經歷了數百萬年的時光。」
碳同位素分化
分析植烷這類的有機物中光合作用時發生的碳同位素分化現象，就能預估過往的二氧化碳含量。植物和藻類吸收二氧化碳時偏好輕的碳同位素(12C)多於重的碳同位素(13C)。只有在周圍的水體或是大氣中的二氧化碳含量較低的時候，它們才會使用重的碳同位素。因此，這兩種同位素之間的比例便能反映出它們生長時環境中的二氧化碳含量。
同位素分化也可以解釋為什麼Witkowski的研究幾乎只利用從海洋環境(化石)中取出的植烷，而不用陸生植物產生的。植物王國可以區分成C3和C4植物，它們有各自的輕重同位素比例。相較而言，浮游植物的同位素比例就清一色地十分類似。Witkowski說：「透過只選擇來自海洋的植烷，我們可以減少資料中因植烷來源而產生的不確定性。」
「我們的數據中可以看到二氧化碳含量很高的時期，相較於今日的410ppm，當時可以到達1000ppm。從這個角度來看，二氧化碳當今的濃度並不突出，但變化速度之快卻是前所未見。過往一般要用上數百萬年才能發生的變化，如今卻在一個世紀內就發生了。這些新的二氧化碳數據也許可以幫助我們瞭解地球的未來。」將來的研究也許可以透過在20億年前的樣品中找到的最古老植烷，而回溯到更久以前，比顯生元還老的時代。
Fossil algae reveal 500 million years of climate change
Earth scientists are able to travel far back in time to
reconstruct the geological past and paleoclimate to make better predictions
about future climate conditions. Scientists at the Netherlands Institute for
Sea Research (NIOZ) and Utrecht University succeeded in developing a new
indicator (proxy) of ancient CO2-levels, using the organic molecule
phytane, a debris product of chlorophyll. This new organic proxy not only
provides the most continuous record of CO2-concentrations ever, it
also breaks a record in its time span, covering half a billion years. The data
show the present idea that rises in CO2-levels that used to take
millions of years, are now happening in a century. These findings are published
in Science Advances on
November 28th.
As CO2 increases
today, it's vital to understand what impact these changes will have. To better
predict the future, we must understand long-term changes in CO2 over
geologic history. Direct measurements of past CO2 are
available, e.g. bubbles in ice cores containing ancient gases. However, ice
cores have a limited time span of one million years. To go farther back in
time, earth scientists have developed various indirect measurements of CO2from
proxies e.g. from algae, leaves, ancient soils and chemicals stored in ancient
sediments to reconstruct past environmental conditions.
Phytane, a new way to
travel in time
A new proxy, using a
degradation product of chlorophyll, allows geochemists to infer a continuous
record of historic CO2-levels in deep time. Scientists at NIOZ have
recently developed phytane as a promising new organic proxy that uncovers half
a billion years of CO2-levels in the oceans, from the Cambrian until
recent times.
Using the new proxy, they
were able to make the most continuous record of ancient carbon dioxide levels
ever. "We developed and validated a new way to time travel -- going
farther back in time and to more places," says NIOZ-scientist Caitlyn
Witkowski. "With phytane, we now have the longest CO2-record
with one single marine proxy. This new data is invaluable to modelers who can
now more accurately make predictions of the future."
Witkowski and colleagues
selected more than 300 samples of marine sediments from deep sea cores and oils
from all over the globe, reflecting the majority of geological periods in the
last 500 million years.
Fossil molecules
Past chemical reactions can
be 'stored' in fossil molecules, and so they may reflect various ancient
environmental conditions. Geochemists are able to 'read' these conditions, such
as seawater temperature, pH, salinity and CO2-levels. Organic
matter, such as phytane, reflects the pressure of CO2 in ocean
water or the atmosphere (pCO2).
Little green miracles
Although all organic matter
has the potential to reflect CO2, phytane is special. Phytane is the
pigment responsible for our green world. Anything that uses photosynthesis to
absorb sunlight, including plants, algae, and some species of bacteria, has
chlorophyll from which phytane comes. Plants and algae take in CO2 and
produce oxygen. Without these little green miracles, our world just wouldn't be
the same.
Because chlorophyll is
found all around the world, phytane is also everywhere and is a major
constituent of decayed and fossilized biomass. "Phytane doesn't chemically
change over the course of time, even if it is millions of years old,"
Witkowski says.
Carbon isotope
fractionation
CO2 of the
past is estimated from organic matter, such as phytane, through the phenomenon
of carbon isotope fractionation during photosynthesis. When taking up CO2,
plants and algae prefer the light carbon isotope (12C) over the heavy carbon
isotope (13C). They only use the heavy carbon isotope when CO2-levels
in the surrounding water or atmosphere are low. The proportion between these
two isotopes therefore reflects the level of carbon dioxide in the environment
at the moment of growth.
This also explains why
Witkowski didn't use terrestrial plants as a source for her research,
exclusively using phytane from (fossilized) marine sources. The plant world is
divided into so-called C3- and C4-plants, each with their own unique ratio of
light-to-heavy carbon. Phytoplankton all have very similar ratios compared to
their plant counterparts. Witkowski: "By choosing only marine sources, we
could limit uncertainty of the phytane source in the dataset."
"In our data, we see
high levels of carbon dioxide, reaching 1000 ppm as opposed to today's 410 ppm.
In this respect, present day levels are not unique, but the speed of these
changes have never been seen before. Changes that typically take millions of
years are now happening in a century. This additional CO2-data may
help us understand the future of our planet." In future research, phytane
can be used to go even further back in time than the Phanerozoic, the earliest
found in two billion-year-old samples.
原始論文：Caitlyn R.
Witkowski, Johan W. H. Weijers, Brian Blais, Stefan Schouten, and Jaap S.
Sinninghe Damst�. Molecular fossils from phytoplankton
reveal secular Pco2 trend over the Phanerozoic. Science Advances,
2018 DOI: 10.1126/sciadv.aat4556
引用自：Royal Netherlands Institute for Sea Research.
"Fossil algae reveal 500 million years of climate change."
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