原文網址：https://news.ucsc.edu/2021/03/ocean-chemistry.html
鍶同位素的最新分析結果揭露了在地質時間當中，氣候和海平面的變化如何影響全球碳循環
By Tim Stephens
分析海洋沉積物中鍶同位素的最新結果讓科學家得以重建過去3500萬年以來，海洋化學成分的波動跟氣候因子的變化之間有什麼樣的關係。
這幅掃描式電子顯微鏡的影像是從深海沉積物岩芯中萃取出來的海洋重晶石，其提供了海洋化學性質隨著地質時間經過而變動的紀錄。(圖片來源：Adina Paytan)
這項3月26日發表在《科學》(Science)上的成果讓我們對於全球碳循環的內部運作方式有了新的見解，特別是碳酸鹽沉積下來而把環境中的碳移除的過程。
「鍶的性質和鈣相當類似，因此海洋生物製造碳酸鈣外殼時也會把鍶摻雜進去，」主要作者，加州大學聖塔克魯茲分校海洋科學研究所的教授Adina
Paytan如此解釋。
Paytan和共同作者探討了不同鍶同位素的比例，包括放射性同位素(產生自放射性衰變)與穩定同位素，這讓他們得到地球化學作用的資訊可以互相補足。他們發現海洋裡穩定鍶同位素的比例在過去3500萬年當中發生了可觀的變化，而且直到今日依然持續著，代表海水鍶濃度的變化幅度也很劇烈。
「鍶並非處在穩定的狀態，也就是說進入海洋的鍶和離開海洋的並不一致，」Paytan表示。「海水裡鍶的組成會依據碳酸鹽沉積的方式與地點而變化，這些作用又會受到海平面和氣候變化的影響。」
這項分析裡鍶同位素比例的波動反映了從全球來看，不同地質作用之間的平衡改變之後帶來的綜合影響。這些作用包括了陸上岩石的風化、海底熱泉活動，以深海與近岸的海洋環境中形成碳酸鹽的作用。
碳酸鹽的沉積物
在開放海洋中碳酸鹽的沉積物源自於海洋浮游生物，像是鈣板藻和有孔蟲，它們會製造成分為碳酸鈣的礦物方解石來建造殼體。而在大陸棚的淺海海域當中則有豐富的硬珊瑚，它們建造骨骼時則是利用另外一種碳酸鈣礦物――霰石，其夾帶的鍶多於方解石。
「珊瑚形成時會帶走鍶。如果它們之後露出海面，這些鍶就會被沖刷出來然後回到海裡，」Paytan表示。「海平面改變會讓長有珊瑚的大陸棚露出來或者遭到淹沒，因而影響海水的組成中有多少鍶。」
碳酸鹽的沉積作用也會回饋到氣候系統當中，原因是海洋可以吸收大氣裡的二氧化碳，因此從地質時間尺度來看，碳酸鹽的沉積作用可以把碳從氣候系統當中移除。不管是從長期來說，或者在近代冰期循環裡氣溫反覆的上下震盪之中，全球碳循環與大氣二氧化碳濃度都跟氣候變遷的關係十分密切。
「我們從穩定鍶同位素讀到的新資訊，讓我們可以更瞭解當碳進入海底的碳酸鹽而從環境中移除的時候，會對全球碳循環產生什麼樣的實質效應，」研究共同作者，加州大學聖塔克魯茲分校地球與行星可學系的助理教授Mathis
Hain表示。
「這項發現讓我們可以透過一條新的管道來觀察在地質時間當中，全球碳循環如何隨著海平面變化與氣候變遷而調適，」他接著說。「我們需要這些觀點來指引我們應該如何應對目前的氣候緊急狀態，並且降低海洋酸化帶來的最糟後果。」
海洋裡的重晶石
研究人員藉著分析從深海沉積物岩芯裡萃取出來的海洋重晶石，成功重建出海水鍶同位素變化的詳實記錄。
「要瞭解地球在地質時間當中是以怎樣的方式運作時，這樣的記錄是非常重要的，」共同作者，俄亥俄州立大學的Elizabeth
Griffith表示。「我們的國際團隊合作建立起這份獨特的紀錄，並且透過數學模擬來解釋它重要的地方，因此才能重建出過往氣候條件不同時的變化歷程。希望成果可以讓我們更加理解我們的藍色行星未來可能的運作方式。」
除了Paytan、Hain、Griffith，論文共同作者還包括了德國亥姆霍茲海洋研究中心的Anton
Eisenhauer和Klaus
Wallmann；加州大學河濱分校的Andrew
Ridgwell。研究經費來自美國國家科學基金會。
Changes in ocean chemistry show how
sea level affects global carbon cycle
New analysis of strontium isotopes
reveals how the global carbon cycle has responded to changes in climate and sea
level through geologic time
A new analysis of strontium isotopes in
marine sediments has enabled scientists to reconstruct fluctuations in ocean
chemistry related to changing climate conditions over the past 35 million
years.
The results, published March 26 in Science, provide new insights into the
inner workings of the global carbon cycle and, in particular, the processes by
which carbon is removed from the environment through the deposition of
carbonates.
“Strontium is very similar to calcium, so it gets
incorporated into the calcium carbonate shells of marine organisms,” explained
lead author Adina Paytan, research professor in the Institute of Marine
Sciences at UC Santa Cruz.
Paytan and her coauthors looked at the ratios of
different isotopes of strontium, including radiogenic isotopes (produced by
radioactive decay) and stable isotopes, which provide complementary information
about geochemical processes. They found that the stable isotope ratio of
strontium in the ocean has changed considerably over the past 35 million years,
and it is still changing today, implying large changes in seawater strontium
concentration.
“It’s not in a steady state, so what’s coming into
the ocean and what’s leaving don’t match,” Paytan said. “The strontium
composition of seawater changes depending on how and where carbonates are
deposited, and that is influenced by changes in sea level and climate.”
The fluctuations in strontium isotope ratios analyzed
in this study reflect the combined effect of shifts in the global balance of
geologic processes including weathering of rocks on land, hydrothermal
activity, and the formation of carbonate sediments in both deep-sea and
shallow, nearshore marine environments.
Carbonate
deposition
Carbonate deposition in the open ocean comes from
marine plankton like coccolithophores and foraminifera, which build their
shells of the calcium carbonate mineral calcite. In shallow water on the
continental shelves, hard corals are more abundant, and they build their
skeletons of a different mineral of calcium carbonate, aragonite, which
incorporates more strontium than calcite does.
“When corals form, they remove strontium, and when
they are exposed, this strontium washes out and goes back into the ocean,”
Paytan said. “With changes in sea level, more or less of the continental shelf
where corals grow is exposed, so that impacts the strontium composition of
seawater.”
Carbonate deposition also feeds back into the climate
system, because the ocean absorbs carbon dioxide from the atmosphere, and
carbonate deposition on geological timescales removes carbon from the system.
The global carbon cycle and atmospheric carbon dioxide are tightly coupled to
climate change, both in the long-term and during the recurring ups and downs of
recent ice age cycles.
“The new type of information we can read from the
stable strontium isotopes now allows us to take a close look at the business
end of the global carbon cycle, when carbon is removed from the environment and
laid down into marine carbonate beds,” said coauthor Mathis Hain, assistant
professor of Earth and planetary sciences at UCSC.
“These findings throw open a new window to let us see
how the global carbon cycle adjusted to sea level and climate change through
geologic time,” he added. “We will need these insights in guiding our response
to our current climate emergency and to mitigate the worst effects of ocean
acidification."
Marine barite
The researchers were able to reconstruct a robust and
detailed record of strontium isotope variations in seawater based on an
analysis of marine barite extracted from deep-sea sediment cores.
"Records like this are critical to understanding
how our earth operates over geologic times," said coauthor Elizabeth
Griffith at Ohio State University. "Our international team worked together
to both create this unique record and explain its significance through
mathematical modeling, so we can reconstruct changes in the past when the climate
conditions were different. The hope is to gain insight into how our blue planet
might operate in the future."
In addition to Paytan, Hain, and Griffith, the
coauthors of the paper include Anton Eisenhauer and Klaus Wallmann at the
GEOMAR Helmholtz Center for Ocean Research in Germany, and Andrew Ridgwell at
UC Riverside. This work was supported by the National Science Foundation.
原始論文：Adina Paytan,
Elizabeth M. Griffith, Anton Eisenhauer, Mathis P. Hain, Klaus Wallmann, Andrew
Ridgwell. A 35-million-year record of seawater stable Sr isotopes
reveals a fluctuating global carbon cycle. Science, 2021; 371
(6536): 1346 DOI: 10.1126/science.aaz9266
引用自：University of California - Santa Cruz.
"Changes in ocean chemistry show how sea level affects global carbon
cycle."
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