https://news.unm.edu/news/unm-usf-scientists-find-stable-sea-levels-during-last-interglacial#
科學家發現上次間冰期的海平面變化相當穩定
這項研究對於了解氣候變遷來說相當重要
By Steve Carr
試著在腦海中想像下列情境：地球的氣候來回擺盪於寒冷的冰河期和溫暖的間冰期之間。末次冰河期大約是在20,000年前，當時的海平面比現在低了大約126公尺；而最近12,000年來的氣候變遷則是發生於身為間冰期的全新世。
末次間冰期大約是在距今127,000年前至116,000年前，這段時期的海平面是地球歷史上最近一次跟現今的海平面差不多甚至更高的時期。末次間冰期的氣候比現在還溫暖一些，因此若要預測海平面未來因全球暖化而升高的情形，了解末次間冰期的海平面變化是非常重要的研究領域。
末次間冰期精確來說又稱為海洋氧同位素第5e階(Marine
Isotope Stage 5e, MIS-5e)，科學家仍不確定這段時間海平面的變化幅度和過程。目前為止科學家的共識認為當時海平面可能比現在高了6到9公尺，這需要把格陵南和西南極冰蓋融化才能達到；同一時間海平面可能也發生了一或多次幅度達到數公尺的上下變動。
然而，美國新墨西哥大學和美國南佛羅里達大學的科學家及其他合作人員組成的國際研究團隊，卻無法肯定這精準地表達出海平面變動的情形。在他們今日發表於期刊《自然―地質科學》(Nature Geoscience)的新研究「地中海西部在末次間冰期期間對於海平面相對變化的高解析度紀錄」(A highly resolved record of
relative sea-level in the western Mediterranean Sea during the Last
Interglacial period)中，這群科學家呈現了相對海平面在MIS-5e期間的紀錄。這些有良好定年的紀錄位於地中海西部的馬略卡島(Mallorca)，刻劃在洞穴灰華(speleothem)靠近海平面時形成的潛流增生(phreatic
overgrowth)之中。
「稱為海洋氧同位素第5e階的末次間冰期發生於127,000年前至116,000年前，其中有段時間的全球氣溫大概比現在還高了1
到2 °C。」這項研究的第一作者和計畫共同主持人，新墨西哥大學地球和行星科學系的研究員Victor
Polyak表示。「關於MIS-5e的研究雖然很多，但我們仍不曉得這段時間之內海平面的確切行為。我們可以肯定的是12萬年前氣溫比現在高了1
到2 °C，同時海平面也比較高。有鑑於此，若我們想要知道現在的海平面隨著氣候暖化未來會出現什麼變化，則MIS-5e的海平面變化歷史會是相當重要的類比。」
目前為止，用來得出過往海平面的最佳標記是珊瑚，因為某些種類的珊瑚會生長在非常靠近海平面的地方，而且珊瑚可以用鈾釷定年法來測出年代。迄今許多關於MIS-5e海平面的重要重建結果都是靠著研究珊瑚來得到。然而，珊瑚的研究有兩個科學上的問題：第一個是沒有人能確定這些珊瑚可以長到海面下多深的地方；第二個問題是珊瑚的組成為生物來源的碳酸鈣，它們受到換質作用後會輕微影響到鈾釷定年的測量結果，使得此方法的準確度受到質疑。
「幾篇最為詳細的研究指出MIS-5e期間的海平面比現在高出6到9公尺，而且在高水位時期海平面可能有發生幾次大幅下跌的現象。」Polyak表示。「這讓人相當憂心，因為這意謂如果我們讓氣溫升高1到2℃，海平面可能會升高6到9公尺。而且發生速度可能相當快，使得海平面變的不穩定。」
「127,000年前至116,000年前是地球的上一個暖期，我們的研究目的是要精確重建出這段期間的海平面位置，利用的材料是馬略卡島沿岸洞穴中沉澱出來的特殊碳酸鹽結殼(encrustation)。」共同作者，南佛羅里達大學的喀斯特地質學家Bogdan
P. Onac表示。「其他探討這段期間海平面的研究多數指出海平面的上升幅度為6到9公尺。我們想要更加了解海平面上升的規模、時間以及穩定度，因為預估海平面未來在全球溫度提升1.5至2℃的狀況下會如何變化時，這類資訊相當重要。」
Polyak、Onac,和新墨西哥大學的教授Yemane
Asmerom研究了地中海西部，屬於西班牙的馬略卡島東側海岸的洞穴。他們利用一種特殊的洞穴沉積物，稱為「洞穴灰華的潛流增生」(phreatic
overgrowths on speleothems, POS)。這種沉積物在自然情況下形成於半鹹水洞穴裡的水面附近，如果這些洞穴位在海岸附近，則水面就剛好等同於海平面。馬略卡島的半鹹水洞穴水文跟地中海相通，當洞穴水中的二氧化碳逸散時，就會形成洞穴灰華的潛流增生。
洞穴灰華的潛流增生可以讓研究人員得到定年良好的紀錄顯示末次冰河期期間的相對海平面變化，該段時期精確來說稱作「海洋氧同位素第5e階」。圖片來源：UNM
Newsroom
「這些POS的保存情況十分良好，」Polyak表示。「此外我們還可以定出它們的年代，所以能十分準確地得到現今以及過去的海平面高度。它們含有的無機碳酸鈣(方解石和霰石)可以用鈾釷定年法來定年。跟珊瑚不同，這類碳酸鈣不易受到換質作用，因此得出來的鈾釷年代更為精確。POS的另一個優點是只要海平面一直維持在同一高度，它們就可以持續生長。」
Polyak和Asmerom在8個不同的洞穴中，距現今海平面上方約2公尺處找到POS，他們對其中的11個POS進行定年。45個鈾釷年代顯示當時的相對海平面十分穩定。「這是末次間冰期MIS-5e最為精準、解析度也最高的海平面紀錄。」Polyak表示。「結果十分精確地呈現了MIS-5e期間海平面變化的歷史進程：大約在127,000年前海水升高至比現今海平面高6公尺的地方，然後逐漸下降，到了122,000年前海平面比現今的高了2公尺。之後一直到116,000年前，這段高水位時期的海平面都維持在此高度。」Onac表示。「這代表未來的氣溫如果比工業革命前高出1.5
至2℃，海平面會隨之升高2到6公尺。」
波士頓學院的助理教授Carling Hay運用九種不同的冰河均衡(glacial
isostatic)模型，來對這份海平面紀錄進行冰河均衡調整。綜合這些模型顯示馬略卡島的ice-equivalent海平面在MIS-5e開始的時候達到高峰，之後逐漸下降直到12萬2千年前，然後維持穩定到11萬6千年前結束這段高水位時期。
「未來有許多事物是我們明顯無法確定的，最重要的其中之一是全世界有多少土地會因為全球暖化而消失，發生速度又有多快。」Asmerom表示。「這是一個十分困難的科學問題，其中涉及許多事物，規模廣及全球。我們除了要知道海平面會升到多高，也必須詳加了解升高速度有多快。」
「如果你採用的是之前的數據，有些指出溫度僅升高2度就會讓海平面上升9公尺，這對我們現今的城市分布以及某些島國來說會是重大災難。這項研究最為重要的發現是它指出海平面不會忽上忽下。如果溫度變化不大的話，海平面也會處在相當穩定的狀態。」
這項研究是由南佛羅里達大學主導跟各方合作之下的結果。南佛羅里達大學的Onac進行了野外工作，包含前往洞穴採集樣品並量測其高度；接著對採集回來的礦物樣品進行礦物學和結晶學分析。
「雖然我們已經有這些十分重大的發現，但是關於海平面變化仍有許多層面需要進一步探討。」Polyak表示。「因此，國家科學基金會資助新墨西哥大學和南佛羅里大學繼續在地中海地區進行相關研究。」
Scientists find stable sea levels during last
interglacial
Research vital to
understanding climate change
Visualize
the following: The Earth’s climate swings between cold glacial and warm
interglacial periods; the last glacial interval was about 20,000 years ago; sea
level was about 126 meters (413 feet) below modern sea level at that time; and
the Holocene, which represents the last 12,000 years of climatic change, is an
interglacial period.
The last interglacial period about
127,000 to 116,000 years ago was the last time sea level was as high as or
even higher than present-day sea-level. Understanding sea level change
during the last interglacial period, a time when the earth was slightly warmer
the present, is an important research area for understanding future sea level
rise due to global warming.
The magnitude and trajectory of sea-level
change during the Last Interglacial, more specifically Marine Isotope Stage
(MIS) 5e, is uncertain. To date the consensus view has been that sea-level may
have been six to nine meters above present sea level, values that require
additional melting of Greenland and the West Antarctic ice sheet and that there
were one or more oscillations of up to several meters superimposed.
However, scientists at The University of
New Mexico (UNM) and the University of South Florida (USF) and their
international team of collaborators aren’t so sure that those sea level
fluctuations are accurate. According to new research published today in the
journal Nature Geoscience
titled, A highly
resolved record of relative sea-level in the western Mediterranean Sea during
the Last Interglacial period, these scientists present a
well-dated relative sea level record from the island of Mallorca in the western
Mediterranean Sea for MIS-5e based on the occurrence of phreatic overgrowths on
speleothems forming near sea level.
“Globally, the climate was warmer by 1 to
2 °C during the part of the Last Interglacial Period referred to as Marine
Isotope Stage 5e (MIS-5e) between 127,000 and 116,000 years ago,” said Victor
Polyak the first author and co-principal investigator and senior research scientist
in the UNM Department of Earth and Planetary Sciences. “While this is a
well-studied period, we still do not know the exact behavior of sea level
during MIS-5e. What we know for certain is that sea level was higher when
climate was 1 to 2 °C warmer 120,000 years ago. For this reason, the
history of MIS-5e sea level is important as an analog for what will happen to
current sea level with warming climate into the future.”
Until now, the best sea level markers
were corals, because some species grow very close to sea level, and corals can
be dated with the uranium-thorium method. Important reconstructions of MIS-5e
sea level have been made from such coral studies. However, the scientific
problem with coral is no one can ever be certain of how deep corals grow below
sea level, and because they are made up of biogenic calcium carbonate, they are
subject to alteration that slightly affects the uranium-thorium age
measurements casting doubt regarding the accuracy of this method.
“The best studies of MIS-5e sea level
have suggested that sea level during this period was 6 to 9 meters above
present sea level, and that there were likely significant drops during the
MIS-5e sea level highstand,” said Polyak. “This is alarming because it suggests
that if we warm our climate by 1 to 2 °C, we might cause sea level to rise
6 to 9 meters (20 to 30 feet). This could happen quickly causing sea level to
be unstable.”
“The purpose of this research was to
precisely reconstruct the sea level position during the last warm period,
between 127,000 and 116,000 years ago, using some peculiar carbonate
encrustations precipitated in littoral caves along the coast of Mallorca,” says
co-author and USF School of Geosciences karst geologist Bogdan P. Onac.
“Most other known sea level studies for this period report elevations between 6
and 9 m – so we wanted to better understand the magnitude, timing, and
stability of sea level since such information is critical for future
projections of sea level change under a 1.5-2ºC global warming scenario.”
Polyak, Onac, along with UNM Professor
Yemane Asmerom, studied caves along the eastern coast of the Spanish island of
Mallorca in the western Mediterranean. They utilized unique cave formations
called phreatic overgrowths on speleothems (POS) that form naturally at the
brackish cave water surface, which happens to be exactly equivalent with sea
level in those caves near the coast. Phreatic overgrowths on speleothems form
as carbon dioxide degasses from brackish cave water hydrologically connected to
the Mediterranean Sea.
“POS are well-preserved,” said Polyak,
“and given that they can be dated, they provide very accurate sea level
elevations for current and pre-existing sea levels. They consist of inorganic
calcium carbonate (calcite and aragonite) and are dateable by the
uranium-thorium method. Unlike corals, they are not prone to alteration, and
therefore the uranium-thorium dates are accurate. Another benefit of POS is
that they will continue to grow as long as sea level is stable at a
given elevation.”
Polyak and Asmerom dated 11 POS from
eight different caves that exhibited POS at ~2 meters above present sea level.
Forty-five uranium-thorium dates show that relative sea level remained
stable. “This is the most accurate, best-resolved sea level record for MIS-5e
of the last interglacial period,” said Polyak. “It provides
exceptionally accurate timing of the sea level history during the above
mentioned period and shows that it rose to 6 meters above present sea level
~127,000 years ago, it would have gradually fell to 2 meters by 122,000 years
ago, and would have stayed at that elevation for the remainder of the sea level
highstand to 116,000 years ago,” says Onac. “The results suggest that if
the pre-industrial temperature will be surpassed by 1.5 to 2°C, sea level
will respond and rise 2 to 6 meters (7 to 20 feet) above present sea level.”
Dr. Carling Hay, assistant professor at
Boston College University, corrected the relative sea level record for glacio
isostatic adjustment using nine different glacial isostatic models. Together,
these models suggest that ice-equivalent sea level in Mallorca peaked at the
start of MIS-5e then gradually decreased and stabilized by 122,000 years ago,
until the highstand termination 116,000 years ago.
“One of the biggest obvious unknowns
about the future is how much global real-estate we are going to lose to global
warming and how fast it could happen,” said Asmerom. “This is pretty awesome
science; it’s big stuff, global in scale. Beyond how high sea level is going to
rise, we are also acutely in need of knowing how fast it will rise.
“If you take the older data, in some
cases the suggestion that's how sea level rise by nine meters by mere warming
of two degrees would be catastrophic for our present configuration of cities
and in some cases island countries. This work clearly shows the most important
findings that sea level didn't just go up and down. You had small changes in
temperature and sea level stayed pretty stable.”
The research reported in this study is
the result of a collaborative NSF project in which USF is the lead
organization. Field activities (cave visits for collecting samples and
measuring their elevation), mineralogical, and crystallographical
investigations on the mineral samples were performed by Onac at USF.
“Although these are significant findings,
there are many aspects of sea level change that need further exploration,” said
Polyak. “As a result, the UNM and the University of South Florida team was
awarded a new National Science Foundation Grant to continue the work in the
Mediterranean.”
Victor J. Polyak, Bogdan P.
Onac, Joan J. Fornós, Carling Hay, Yemane Asmerom, Jeffrey A. Dorale, Joaquín
Ginés, Paola Tuccimei, Angel Ginés. A highly resolved record of
relative sea level in the western Mediterranean Sea during the last
interglacial period. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0222-5
University of New Mexico.
"Scientists find stable sea levels during last interglacial: Research
vital to understanding climate change."
https://news.unm.edu/news/unm-usf-scientists-find-stable-sea-levels-during-last-interglacial#
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