原文網址：http://www.geologypage.com/2017/05/magnesium-within-plankton-provides-tool-taking-temperatures-past-oceans.html
浮游生物中的鎂提供了得知過去海洋溫度的利器
科學家無法旅行至過去直接測量地球的溫度，所以他們使用代用指標來辨識過往的氣候狀況。其中一種最常用來取得這類資料的方法，來自於從海洋沉積物岩芯中發現的微小海洋生物，稱作有孔蟲的(foraminifera)遺骸。
有孔蟲是砂粒大小，會形成方解石殼體的海洋原生生物。當它們成長時，會從海水中吸收鎂元素至它們的殼體中。當海洋溫度比較高時，有孔蟲會吸收更多的鎂；而在較低溫時，吸收的鎂就比較少。因此科學家可以利用它們的鎂含量，來訴說在幾千年前，甚至數百萬年以前海水的溫度有多高。對於了解過去的氣候來說，這些指標是至關重要的工具。
然而，對現生有孔蟲的研究顯示，就算在海洋溫度相當恆定的情況下，殼體中的鎂含量還是會有所變化。本週刊登在期刊《自然通訊》(Nature Communications)的新研究，確認了在簡單的單一殼室(chamber)有孔蟲中出現的鎂含量變化，跟日夜(光暗)循環有關，且這項發現可以延伸至更加複雜的多殼室有孔蟲身上。
由美國奧勒岡州立大學、加州大學戴維斯分校、華盛頓大學和西北太平洋國家實驗室組成的科學家團隊，為了瞭解有孔蟲的發育過程，以及鎂含量變化的成因，他們在高度控制的條件下，於實驗室養殖稱作Neogloboquadrina
dutertrei的多殼室有孔蟲。他們利用高解析度的影像技術，來繪製這些在實驗室成長的樣品的殼體成分「地圖」。
「我們發現鎂含量高的區塊是在夜間沉澱出來，而鎂含量低的部分則是在白天加入至殼體當中，這跟單一殼體種類的生長模式十分相似。」此研究的主要作者，奧勒岡州立大學的海洋生化學家與古海洋學家表示。「這證實佔有兩種不同生態區位的兩種生物身上，鎂含量的變化是由相同機制造成。我們現在可以有一定程度的信心聲稱，鎂的帶狀變化是殼體形成過程的固有結果，完全沒有牽涉其他環境因子。」
「有孔蟲殼體的鎂含量變化並不會改變它們作為溫度指標的使用價值。相反地，我們的研究對有孔蟲如何建構它們的殼體提供了新的觀點，也帶給科學家在使用有孔蟲來重建過去的溫度時有更多信心。」
Magnesium
within plankton provides tool for taking the temperatures of past oceans
Scientists
cannot travel into the past to take the Earth’s temperature so they use proxies
to discern past climates, and one of the most common methods for obtaining such
data is derived from the remains of tiny marine organisms called foraminifera
found in oceanic sediment cores.
These
“forams,” as they are called, are sand-grained-sized marine protists that make
shells composed of calcite. When they grow, they incorporate magnesium from
seawater into their shells. When ocean temperatures are warmer, forams
incorporate more magnesium; less when the temperatures are cooler. As a result,
scientists can tell from the amount of magnesium what the temperature of the
seawater was thousands, even millions of years ago. These proxies are important
tools for understanding past climate.
However,
studies of live forams reveal that shell magnesium can vary, even when seawater
temperature is constant. A new study published this week in the journal Nature Communications affirms that
magnesium variability is linked to the day/night (light/dark) cycle in simple,
single-celled forams and extends the findings to more complex multi-chambered
foraminifera.
To understand
how forams develop and what causes magnesium variability, the team of
scientists from Oregon State, University of California, Davis, University of
Washington and Pacific Northwest National Laboratory grew the multi-chambered
species, Neogloboquadrina dutertrei,
in a laboratory under highly controlled conditions. They used high-resolution
imaging techniques to “map” the composition of these lab-grown specimens.
“We found that
high-magnesium is precipitated at night, and low-magnesium is added to the
shells during the day, similar to the growth patterns of the single-chambered
species,” said Jennifer S. Fehrenbacher, an ocean biogeochemist and
paleoceanographer at Oregon State University and lead author on the study.
“This confirms that magnesium variability is driven by the same mechanism in
two species with two different ecological niches. We can now say with some
level of confidence that magnesium-banding is intrinsically linked to shell
formation processes as opposed to other environmental factors.
“The
variability in magnesium content of the shells doesn’t change the utility of
forams as a proxy for temperature. Rather, our results give us new insights
into how these organisms build their shells and lends confidence to their
utility as tools for reconstructing temperatures.”
原始論文：Jennifer S.
Fehrenbacher et al. Link between light-triggered Mg-banding and chamber
formation in the planktic foraminifera Neogloboquadrina
dutertrei, Nature Communications
(2017). DOI:
10.1038/ncomms15441
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