沙塵可能決定了古代人類的文明發展

 原文網址：https://www.geosociety.org/GSA/News/pr/2020/20-25.aspx

沙塵可能決定了古代人類的文明發展

數萬年前早期人類開始走出非洲並散布至歐亞大陸時，地中海東部稱為黎凡特(Levant)的地區因為土地肥沃而成為北非至歐亞大陸之間的重要通道。最近發表在《地質》(Geology)的新研究表示決定這處綠洲存在與否的是我們幾乎從沒想到的東西――沙塵。

以色列地質調查局的Rivka Amit博士和其團隊最初想解決的問題十分單純：為什麼地中海附近的土壤有些較薄有些較厚？他們進行研究之後不僅發現了黎凡特形成厚層土壤的過程中，沙塵堆積扮演了相當重要的腳色，也顯示出如果二十萬年前沙塵的來源沒有發生變化的話，早期人類在離開非洲時可能會過得艱辛許多，一部份的肥沃月灣也不會變得如此適合人類居住而成為文明發源的地點。

一般來說濕潤地區形成的土壤偏厚，風化速率低的乾燥地區形成的土壤則偏薄。但地中海附近因為底岩大部分都是容易溶解的碳酸岩，因此這裡的情況相反過來――北邊濕潤地區的土壤較薄且貧瘠，東南方乾燥地區的土壤則較厚且肥沃。有些科學家認為人類活動導致侵蝕速率有所差異是這種模式的成因。但是對於研究此區多年的Amit來說，只用侵蝕速率高低並無法完全解釋。她對現有理論感到懷疑，認為在風化速率低到底岩難以形成土壤時，具有關鍵地位的是另外一種因素――沙塵的輸入量。

Amit和團隊為了評估地中海地區土壤受到沙塵的影響有多高，他們需要先追查這些沙塵最初的來源。他們從此區的土壤中蒐集沙塵的樣品，同時也採集了附近與遠方來源區域的沙塵樣品，接著再比較它們的粒徑分佈。團隊發現土壤較薄和較厚的地區之間有個關鍵差異：薄層土壤只有從遠方(像是撒哈拉沙漠)來的最細顆粒；而厚層的肥沃土壤則有稱為黃土的較粗顆粒，它們來自附近內蓋夫沙漠裡的廣袤沙丘。地中海東部的厚層土壤形成於二十萬年前，此時冰河覆蓋住大片陸地，磨碎了岩石，形成了大量細顆粒的沉積物。Amit表示：「當時整個地球的塵埃比現在還要多出許多。」這讓內蓋夫之類的地區得以堆積出廣大的沙丘而成為新的沙塵供應來源，最後使黎凡特等地區形成了厚厚的土壤。

因此Amit得出了自己的答案：土壤較薄的地區不過是因為無法得到足夠的黃土來形成適合農業的厚層肥沃土壤，反之地中海東南部則具有夠多的黃土。「侵蝕作用在此的重要性並不高，」她說。「重要的是有沒有粗顆粒的沙塵來到這裡，如果沒有的話，你就只能擁有又薄又貧瘠的土壤。」

Amit並沒有就此打住。此處土壤最厚的地區因為農業生產力相當肥沃而有「流奶與蜜之地」的稱號。既然她知道成因是有大量的粗粒沙塵來到這裡，她接下來的問題是：從古至今都是如此嗎？

他們觀察土壤剖面之後發現黃土之下的層位竟然缺乏細顆粒的沉積物，這讓她感到相當訝異。「在黃土堆積之前這裡的土壤相當薄，」她說。「這真的讓人十分驚訝……過去這裡的地貌和現在截然不同。更早之前這裡的環境相當艱困，幾乎是寸草不生。我無法肯定如果沒有大量土壤的話，人們是否會選擇在此定居。」假使風未曾改變而且內蓋夫也沒有形成沙丘的話，這條讓早期人類散佈出去的通道可能會變得十分險惡，讓人無法活著通過。

現今的地中海已經無法繼續堆積土壤。「沙塵的來源已經沒了，」Amit解釋。自從進入全新世，冰河消融之後，「現在我們只能重複利用過去的黃土。」即便有沙塵的來源，也需要數萬年的時間才能重新堆積土壤，因此這些山區的土壤處境相當脆弱，居住在此的人們必須在保育和農業利用之間取得平衡。如果人們想要繼續在此務農就需要保護土壤，實施對環境負責的農耕方法，像是已有數千年歷史的梯田，便是相當重要的措施。

 

Dust may have controlled ancient human civilization

When early humans began to travel out of Africa and spread into Eurasia over a hundred thousand years ago, a fertile region around the eastern Mediterranean Sea called the Levant served as a critical gateway between northern Africa and Eurasia. A new study, published in Geology, shows that the existence of that oasis depended almost entirely on something we almost never think about: dust.

Dr. Rivka Amit, at the Geological Survey of Israel, and her team initially set out with a simple question: why are some soils around the Mediterranean thin and why are some thick? Their investigation led them to discover not only that dust deposition played a critical role in forming thick soils in the Levant, but also that had the source of dust not changed 200,000 years ago, early humans might have had a much tougher time leaving Africa, and parts of the Fertile Crescent wouldn’t have been so hospitable for civilization to take root.

Thick soils tend to form in areas with wet, humid climates, and thin soils form in arid environments with lower weathering rates. But in the Mediterranean, where much of the bedrock is dissolvable carbonate, the opposite is true: wetter northern regions have thin, unproductive soils, and more arid southeastern regions have thick, productive soils. Some scientists have attributed these patterns to differences in the rates of erosion, driven by human activity. But for Amit, who has been studying the area for years, a high erosion rate alone didn’t make sense. She challenged the existing hypotheses, reasoning that another factor—dust input—likely plays a critical role when weathering rates are too slow to form soils from bedrock.

To assess the influence of dust on Mediterranean soils, Amit and her team needed to trace the dust back to its original source. They collected dust samples from soils in the region, as well as nearby and far-flung dust sources, and compared the samples’ grain size distribution. The team identified a key difference between areas with thin and thick soils: thin soils comprised only the finest grain sizes sourced from distant deserts like the Sahara, whereas the thicker, more productive soils had coarser dust called loess, sourced from the nearby Negev desert and its massive dune fields. The thick soils in the eastern Mediterranean formed 200,000 years ago when glaciers covered large swaths of land, grinding up bedrock and creating an abundance of fine-grained sediments. “The whole planet was a lot dustier,” Amit said, which allowed extensive dune fields like those in the Negev to build up, creating new sources of dust and ultimately, thicker soils in places like the Levant.

Amit, then, had her answer: regions with thin soils simply hadn’t received enough loess to form thick, agriculturally productive soils, whereas the southeastern Mediterranean had. “Erosion here is less important,” she said. “What’s important is whether you get an influx of coarse [dust] fractions. [Without that], you get thin, unproductive soils.”

Amit didn’t stop there. She now knew that the thickest soils had received a large flux of coarse dust, leading to the area’s designation as the “land of milk and honey” for its agricultural productivity. Her next question was, had it always been like this?

She was surprised at what they found. Looking below the loess in the soil profile, they found a dearth of fine-grained sediments. “What was [deposited] before the loess were very thin soils,” she said. “It was a big surprise… The landscape was totally different, so I’m not sure that people would [have chosen] this area to live in because it was a harsh environment and [an] almost bare landscape, without much soil.” Without the changing winds and formation of the Negev dune field, then, the fertile area that served as a passage for early humans may have been too difficult to pass through and survive.

In the modern Mediterranean, the soils aren’t accumulating any more. “The dust source is cut off,” Amit explained, since the glaciers retreated in the Holocene, “now we’re only reworking the old loess.” Even if there were a dust source, it would take tens of thousands of years to rebuild a soil there. That leaves these mountainous soils in a fragile state, and people living there must balance conservation and agricultural use. Employing responsible agricultural practices in the region, as terracing has been used for thousands of years, is critical for soil preservation if agriculture is to continue.

原始論文：Rivka Amit, Yehouda Enzel, Onn Crouvi. Quaternary influx of proximal coarse-grained dust altered circum-Mediterranean soil productivity and impacted early human culture. Geology, 2020; DOI: 10.1130/G47708.1

引用自：Geological Society of America. "Dust may have controlled ancient human civilization."