覆蓋以色列境內一半陸地的內蓋夫沙漠極度乾燥，部分地區每年的降水量不到3英寸，稱為超乾燥（hyper-arid）。不過其實直到現今，內蓋夫沙漠下方仍有含水層。
了解這個含水層的來源、儲量以及正在發生的變化，對於保護和分配關鍵資源至關重要。
以色列內蓋夫本古里安大學（Ben-Gurion University of the Negev）研究人員與美國能源部與芝加哥大學阿貢國家實驗室（Argonne National Laboratory）合作，深入研究藏在內蓋夫沙漠和以色列大部分國土下的努比亞砂岩含水層系統。
內蓋夫沙漠的拉蒙坑景觀。攝（CC BY 2.0）
結合阿貢實驗室無線輻射測年技術與水成分同位素指紋分析，研究人員不僅可分辨出水沉積的時間，還知道它來自何處以及近40萬年前水層產生時的氣候條件，也是科學家首次利用地下水描繪出古老的水文氣候。
7月29日刊登了一篇介紹該研究的文章〈〉。
「內蓋夫沙漠下面的含水層今日並沒有在補充水份，因此很明顯地，該地區過去曾有過多雨的時期，」阿貢實驗室放射性同位素分析中心（TRACER）首席物理學家穆勒（Peter Mueller）說。
為了確定這個含水層的補水時期和機制，研究團隊從當地的20多口井中收集水，分離出氪氣，並使用阿貢實驗室為核子物理測量發展出的「Atom Trap Trace Analysis」（ATTA）的分析方法進行分析。
ATTA測量水中微量的稀有氪（Kr）同位素81Kr，可以測定4萬年前到150萬年前的水，這遠超出了放射性碳年代測定的範圍，放射性碳測年超過4萬年便不準確。
ATTA分析顯示，井中的水來自兩次主要的補水事件，分別發生在4萬年前和近36萬年前。兩個時期剛好都是較涼爽的氣候。這些「區域潮濕時期」為當時暴風雨的發展提供足夠的水量，補充了內蓋夫含水層。
雖然81Kr很適合確認時間，但分佈資料出乎意料地複雜費解。它顯示出氪與氘之間有有趣的共變。氘是一種氫同位素，比一般水中的氫更重。
「我們正在尋找δD（氘，音同刀），即氘與一般氫的比值，」TRACER中心博士後研究扎帕拉（Jake Zappala）說。「這個數值在不同的水體之間，會因水的來源和天氣狀況不同會有所差異。」
因為氘的質量比氫大，所以有不同的特性，蒸發和凝固點都不同。
例如，當蒸發快速發生時，如地中海上空，氘呈現出獨特的特徵，與全球降水趨勢有所不同。儘管和氫相比，氘非常罕見，10,000個水分子中只有一個含有一個氘原子而不是氫，卻可以非常精確地測量。
因此，科學家們可以根據其穩定同位素的特徵分別不同水體，就像水體的「指紋」。研究人員解釋，每種氣候模式都有自己的印記。
芝加哥大學地球物理科學系研究副教授、研究第一作者Reika Yokochi說：「這個研究顯示這些工具深具潛力，比過去更能追踪水的運動。」
根據資料的共變和空間分佈，研究團隊確定來自兩次補水事件的水來源不同。
大約40萬年前，該地區比現在更涼爽，水分是以熱帶羽流的形式從大西洋傳來。
前一次的回補在不到4萬年前，可能是最近一次重大冰川事件，又稱末次冰期期間，地中海颶風所導致。
Yokochi指出，另一個值得注意的地方是，含水層的水來自地震斷層帶附近，這顯示斷層可以作為一堵「牆」，保存數十萬年來相對較新的水份。
她說，「世界各地其他斷層帶也可能存在類似的水層。」
Krypton Reveals Ancient Aquifer Under the Negev Desert BEERSHEBA, Israel, August 5, 2019 (ENS)
The Negev Desert, which covers half of Israel’s landmass, is so dry that parts of it get less than three inches of water a year; so dry it is called hyperarid. Yet today, there is still water in an aquifer beneath the Negev Desert.
Understanding where it came from, how much is there, and what’s happening to it is critical to the security and allocation of that crucial resource.
Researchers at Israel’s Ben-Gurion University of the Negev are collaborating with colleagues at the U.S. Department of Energy’s Argonne National Laboratory in Chicago and the University of Chicago to better understand the Nubian Sandstone Aquifer System, which lies beneath a large portion of the Negev desert and other parts of Israel.
By combining Argonne’s pioneering radiokrypton dating technique with other isotopic fingerprints of the water’s composition, the researchers are not only able to tell when that water was deposited, but where it came from and the climate conditions that produced it nearly 400,000 years ago.
The result marks the first time that scientists have been able to use groundwater to build a picture of ancient hydro-climates dating back that far.
An article describing the research, “Radiokrypton unveils dual moisture sources of a deep desert aquifer,” was published July 29, in “Proceedings of the National Academy of Sciences” online.
“The aquifers beneath the Negev don’t get replenished today, so apparently there were times when there was much more rain in the region that collected underground,” says Peter Mueller, principal physicist at Argonne’s Trace Radioisotope Analysis Center (TRACER).
To determine when and how that might have occurred, the team collected water from more than 20 wells in the area, then separated out the krypton gas and analyzed it using a technology called Atom Trap Trace Analysis, ATTA, a technique first developed at Argonne to support nuclear physics measurements.
ATTA measures water for traces of the rare krypton (Kr) isotope 81Kr, which can date water within a range of 40,000 to 1.5 million years old. This extends it well beyond the range of radiocarbon dating, which cannot reach accurately beyond about 40,000 years.
The ATTA analysis suggested that the water in the wells accumulated by means of two major “recharging” events that occurred less than 40,000 and near 360,000 years ago. Both periods coincided with generally cooler climates. These “regional humid periods” were ripe for the development of storms that could provide rainfall adequate to replenish the Negev aquifers.
While the 81Kr usually provides an excellent window into the time frame, the distribution data was unexpectedly complex and puzzling. But it showed interesting covariation with deuterium, an isotope of hydrogen heavier than that found in “regular” water.
“We were looking for the delta deuterium, which is a measure of the difference in the ratio of heavy hydrogen to regular hydrogen,” says Jake Zappala, postdoctoral appointee at the TRACER Center. “That number is going to vary for different bodies of water depending on where the water came from and what the weather conditions were, which is important.”
Because deuterium has a heavier mass than hydrogen, it behaves differently, evaporating and condensing at different temperatures.
For example, when evaporation happens quickly, as over the Mediterranean Sea, it exhibits a peculiar signature compared to global precipitation trends. Even though it is very rare relative to hydrogen — only one in 10,000 water molecules contains one deuterium atom instead of hydrogen — it can be measured very precisely.
Thus scientists can “fingerprint” such bodies of water based on the particular signature of its stable isotopes. Every climate pattern places its own imprint in that signature, the researchers explained.
“This project shows us these tools could be really transformative, tracing water movement much further than we’ve previously been able to,” said Reika Yokochi, research associate professor in the Department of Geophysical Sciences at the University of Chicago, and first author of the new study.
From the covariation and the spatial distribution of the data, the team determined that water from the two recharge events came from two distinct sources.
About 400,000 years ago, the region was cooler than the present, and moisture is believed to have been delivered from the Atlantic Ocean in the form of tropical plumes.
The more recent recharge, less than 40,000 years ago, may have been the result of Mediterranean cyclones during the most recent major glacial event, or Last Glacial Maximum.
Another interesting point is that the water came from near an earthquake fault zone, notes Yokochi, suggesting that faults can serve as a “wall” that preserves relatively fresh water over hundreds of thousands of years.
She says, “It’s possible that similar water repositories may exist along other fault zones all over the world.”
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