冰川融化使得阿爾卑斯山增高

原始網址：www.sciencedaily.com/releases/2016/11/161110085844.htm

冰川融化使得阿爾卑斯山增高

阿爾卑斯山以大概每年1~2釐米的速率穩定「長高」。同樣地，在過往被冰河覆蓋的北美以及斯堪地那維亞次大陸也正在經歷持續地向上成長過程。這是因為18,000年前的末次冰盛期(Last Glacial Maximum, LGM)結束時，冰河消融使得之前冰河施加於地表的龐大重量跟著減輕的緣故。雖然冰河對氣候變遷的反應相當迅速，然而直到今日，地殼還在回應相對而言突如其來的冰川融化。於末次冰盛期，阿爾卑斯山也被冰帽所覆蓋，範圍甚至一度遠達阿爾卑斯前緣地區。但此處的冰河覆蓋程度遠遠不及北美和斯堪地那維亞次大陸上發生的。這是科學家長久以來假設冰帽後退對今日阿爾卑斯山的穩定成長來說，並沒有多少重要性的原因。然而，德國地質研究中心(GFZ)的科學家Dirk Scherler和Taylor Schildgen參與在內的國際研究團隊，表示現在阿爾卑斯山的持續成長，有90%仍要歸因於末次冰盛期時形成的冰帽消失。

地球地殼發生垂直運動的主要因素為板塊移動造成的構造變形作用，另外還有火山作用以及水、冰、沉積物的減壓作用(unloading)。我們可以利用衛星及地面測站進行大地測量來得知地殼的運動。對於北美和斯堪地那維亞次大陸這種古老且構造穩定的陸塊，科學家很早便瞭解它們的垂直運動幾乎都是由所謂的「冰後回彈效應」(postglacial rebound effect)造成，也就是上方冰河融化使得下方地殼往上隆起。然而，在阿爾卑斯山這種年輕的造山帶中，會有許多複雜機制共同作用而互相影響。此處發生的作用有非洲板塊隱沒至歐亞板塊下方，同時亞德里亞板塊(Adriatic Plate，非洲板塊的次板塊)則在歐亞板塊下方以逆時針方向移動。另外，如同在斯堪地那維亞以及北美發生的，此處也具備由侵蝕和沉積物搬運，以及「冰河消退」造成的減壓作用。現今阿爾卑斯山持續抬升的因素究竟為何，已經爭論超過四分之一個世紀之久。

長久以來科學家推測抬升主要是由侵蝕作用，和以河川為主的搬運作用將沉積物運送至山脈前緣地區造成。這項研究比對了侵蝕、冰河減壓以及區域構造運動對阿爾卑斯山垂直運動的貢獻程度。科學家利用電腦模型輔以鑽井資料，顯示冰川消融之後，也就是冰盛期結束以後大多數侵蝕下來的物質仍沉積於造山帶內部。故阿爾卑斯山隆起的主因中，可以將此作用屏除在外。另一方面，模型則顯示如同斯堪地那維亞和北美地區，抬升訊號的最佳解釋是上次冰盛期結束時，阿爾卑斯山的冰河於短短3000年間消退了將近80%而造成的減壓補償運動。今日的抬升中僅約10%得肇因於沉積物的減壓作用。在某些區域，特別是奧地利某些地區，構造運動也是抬升的重要因素，可能是由亞德里亞次板塊的旋轉運動導致。利用他們的模型科學家顯示冰河的荷重高達將近62兆噸，但冰河期之後的沉積物減壓作用僅有4兆噸。

Thawing ice makes the Alps grow

The Alps are steadily "growing" by about one to two millimeters per year. Likewise, the formerly glaciated subcontinents of North America and Scandinavia are also undergoing constant upward movement. This is due to the fact that at the end of the Last Glacial Maximum (LGM) about 18,000 years ago the glaciers melted and with this the former heavy pressure on Earth's surface diminished. The ice reacted rapidly to climate change at that time whereas Earth's crust is still responding today to this relatively sudden melting of ice. During the LGM the Alps were also coated with an ice cap that temporarily reached far into the alpine foreland. The extent of glaciation was much smaller here than on the subcontinents of North America and Scandinavia. This is why it was assumed for a long time that the retreat of the ice cap back then did not play a significant role in the steady uplifting of the Alps today. However, an international team with the participation of the GFZ scientists Dirk Scherler and Taylor Schildgen have now been able to show that the loss of the LGM ice cap still accounts for 90 percent of today's uplifting of the Alps.

Vertical motions of the Earth's crust are mainly caused by tectonic deformation due to movements of tectonic plates, and by volcanism, and unloading of water, ice, and sediments. The movement of the crust can be measured by geodetic methods via satellites and ground stations. For old, tectonically stable continents like the subcontinents of North America and Scandinavia it has been known for a long time that vertical motion is almost exclusively caused by the so called postglacial "rebound effect" -- i.e. the upward motion of the crust due to the thawing of the glaciers. In young mountain belts such as the Alps, however, complex mechanisms come into play that mutually effect each other: The African Plate subducts below the Eurasian Plate, and the Adriatic Plate -- a sub-plate of the African Plate – moves counterclockwise below the Eurasian Plate. Furthermore, as in Scandinavia and North America, there is unloading due to erosion and sediment transport, and "deglaciation." The causes for today's uplift of the Alps has been a matter of debate for over a quarter of a century.

For a long time it was assumed that the uplift is primarily caused by erosion and sediment transport, mainly by rivers, towards the foreland. The new study compares by how much erosion, ice unloading, and local tectonics contribute to the vertical motion of the Alps. The scientists use models supported with drill core data to show that the better part of postglacially, and therewith after the end of the main glacial phase, eroded material was deposited within the orogen. Hence, this process can be excluded as a main cause for the alpine uplift. The models, however, show that, just like in Scandinavia and America, the uplift-signal is best explained with a relieving compensatory movement after the decline of the LGM-glaciers: Within only 3,000 years the glaciation of the Alps decreased by about 80 percent. Only about 10 percent of today's uplift can be attributed to sediment unloading. Locally, especially in parts of Austria, tectonic effects add to the uplift, likely caused by the circular motion of the Adriatic sub-plate. With their models the scientists are able to show that the glacial load weighed about 62,000 gigatonnes, while the postglacial sedimentary unloading only accounts for about 4,000 gigatonnes

原始論文：Jürgen Mey, Dirk Scherler, Andrew D. Wickert, David L. Egholm, Magdala Tesauro, Taylor F. Schildgen, Manfred R. Strecker. Glacial isostatic uplift of the European Alps. Nature Communications, 2016; 7: 13382 DOI: 10.1038/ncomms13382

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "Thawing ice makes the Alps grow." ScienceDaily. ScienceDaily, 10 November 2016.