原文網址：http://www.geologypage.com/2017/05/sea-level-metronome-earths-history.html
數著地球歷史節奏的海平面
沉積岩層記錄著地球的歷史。它含有的地層其循環及組成模式精確地顯露出數千年來，氣候和構造活動的環境如何不停變化，讓我們可以藉此更加瞭解並預測地球的演化歷程。瑞士日內瓦大學的研究人員，跟洛桑大學和美國、西班牙的科學家合作，結合現地觀察以及測量碳的輕重同位素之間的比例，來分析形成於深海的沉積岩層。他們發現這些沉積岩層序列中反覆出現的間斷，並非像他人認為的僅因盆地周遭的山脈侵蝕所造成，而更要歸因於海平面變化。在這篇可在期刊《地質》(Geology)上讀到的文章，為同位素方法在探勘地質學的新應用中打下了基礎。
庇里牛斯山的南部地區特別適合研究沉積岩層。此處的岩石大範圍地裸露出來，讓研究人員得以直接進行觀察。在這裡可以看到濁流岩(turbidite)：一種由過往的海底山崩造成，包含大量礫石和砂石的厚層沉積物。日內瓦大學理學院地球科學系的教授Sébastien Castelltort解釋：「我們注意到濁流岩層以大約100萬年的周期反覆出現。於是我們開始思考造成這種循環的原因。」
海平面的上升下降決定了沉積循環
地質學家將他們的注意力放在於氣候特別炎熱的始新世(Eocene, 距今大約5000萬年前)形成的岩石，並且著手繪製沿著沉積岩層剖面的同位素變化。日內瓦大學的研究人員Louis Honegger表示：「我們每10公尺便採集一次樣品，測量它們的13C(較重的穩定碳同位素)和12C(較輕的穩定碳同位素)的含量。有機碳是12C的主要來源，因此兩者之間的比例可以告訴我們岩石中有多少有機碳，而此比例在海平面高時會比較大。因此，同位素比例有助於我們探討岩層跟海平面之間的潛在關聯。」研究團隊發現濁流岩較多的區間跟高含量12C有關，且幾乎可以全部對應至海平面低的時期。看來沉積岩的循環主要是由海平面的上升下降導致，而非山脈的分段抬升。
當海平面較高時，陸地的邊緣地帶會被淺水淹沒。由於河流到此便無法流動，它們會開始將它們攜帶的沉積物堆積於此。這就是此時幾乎沒有物質可以往下到達深海盆地的原因。反之，當海平面降低時，河流會侵蝕河床來讓河口的位置下降到較低的海平面，使它們直接將攜帶的沉積物運送到深海盆地邊緣的大陸斜坡，而形成富含砂礫的海底山崩。因此，如果知道海平面的變化歷程，就有機會可以預測由濁流岩形成的厚層沉積岩出現位置。由於濁流岩層通常具有大量碳氫化合物，使得它們成為地質探勘夢寐以求的目標之一。
測量穩定碳同位素：儲油岩的新指標
這項研究讓碳同位素有了新的用途。Castelltort繼續說道；「從現在開始，我們知道從類似靠近陸地的大陸斜坡沉積物中，測量樣品中的碳同位素可以用來指示海平面變化，意謂我們有機會可以對我們腳下的沉積岩分布做出更好的預測。」此外，執行這種測量方法相對來說較為簡單，但卻可以提供高精確度的數據，這對科學界和礦業公司而言都是相當有用的資產。Honegger總結說：「當然，構造變形作用和侵蝕作用在沉積岩層的形成中都是重要因子。但是對於主要跟海平面變化有關的濁流岩來說，它們在濁流岩的形成過程中僅扮演了次要腳色。」
Sea level
as a metronome of Earth’s history
Sedimentary
layers record the history of Earth. They contain stratigraphic cycles and
patterns that precisely reveal the succession of climatic and tectonic
conditions that have occurred over millennia, thereby enhancing our ability to
understand and predict the evolution of our planet. Researchers at the
University of Geneva (UNIGE), Switzerland, — together with colleagues at the
University of Lausanne (UNIL) and American and Spanish scientists — have been
working on an analytical method that combines observing deep-water sedimentary
strata and measuring in them the isotopic ratio between heavy and light carbon.
They have discovered that the cycles that punctuate these sedimentary
successions are not, as one might think, due solely to the erosion of mountains
that surround the basin, but are more ascribable to sea level changes. This
research, which you can read in the journal Geology,
paves the way for new uses of isotopic methods in exploration geology.
The area south
of the Pyrenees is particularly suitable for studying sedimentary layers. Rocks
are exposed over large distances, allowing researchers to undertake direct
observation. Turbidites can be seen here: large sediment deposits formed in the
past by underwater avalanches consisting of sand and gravel. “We noticed that
these turbidites returned periodically, about every million years. We then
wondered what the reasons for this cyclicity were,” explains Sébastien
Castelltort, professor in the department of earth sciences in UNIGE’s faculty
of sciences.
The ups and downs of oceans regulate
sedimentation cycles
The geologists
focused their attention on Eocene sedimentary rocks (about 50 million years
ago), which was particularly hot, and undertook the isotopic profiling of the
sedimentary layers. “We took a sample every 10 metres,” says Louis Honegger, a
researcher at UNIGE, “measuring the ratio between 13C (heavy carbon
stable isotope) and 12C (light carbon stable isotope). The ratio
between the two tells us about the amount of organic matter, the main consumer
of 12C, which is greater when the sea level is high. The variations
in the ratio helped us explore the possible link with the sea level.” The
research team found that the turbidite-rich intervals were associated with high
12C levels, and almost always corresponded to periods when the sea
level was low. It seems that sedimentary cycles are mainly caused by the rise
and fall of the sea level and not by the episodic growth of mountains.
When the sea
level is high, continental margins are flooded under a layer of shallow water.
Since the rivers are no longer able to flow, they begin to deposit the
sediments they carry there. This is why so little material reaches the deep
basins downstream. When the sea level is low, however, rivers erode their beds
to lower the elevation of their mouth; they transfer their sediment directly to
the continental slopes of the deep basins, creating an avalanche of sand and
gravel. Consequently, if the variations of the sea level are known, it is
possible to predict the presence of large sedimentary accumulations created by
turbidites, which often contain large volumes of hydrocarbons, one of the holy
grails of exploration geology.
Measuring stable carbon isotopes:
a new indicator of reservoir rocks
The research
provides a new role for the use of carbon isotopes. “From now on, continues
Castelltort, we know that by calculating the ratio between 13C and 12C
sampled in similar slope deposits close to continents, we can have an
indication of the sea level, which means it’s possible to better predict the
distribution of sedimentary rocks in our subsurface.” In addition, this
measurement is relatively simple to perform and it provides accurate data — a
real asset for science and mining companies. The study also highlights the
importance of sea levels, which are a real metronome for Earth’s sedimentary
history. “Of course,” concludes Honegger, “tectonic deformation and erosion are
important factors in the formation of sedimentary layers; but they play a
secondary role in the formation of turbidite accumulations, which are mainly
linked to changes in the sea level.”
原始論文：Sébastien
Castelltort, Louis Honegger, Thierry Adatte, Julian D. Clark, Cai
Puigdefàbregas, Jorge E. Spangenberg, Mason L. Dykstra, and Andrea Fildani.
Detecting eustatic and tectonic signals with carbon isotopes in deep-marine
strata, Eocene Ainsa Basin, Spanish Pyrenees. Geology, May 2017 DOI: 10.1130/G39068.1
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