由化石述說的冰層歷史

原文網址：http://www.nature.com/nature/journal/v547/n7661/full/547035a.html

由化石述說的冰層歷史

微體化石顯示10400年前到7500年前，稱作繞極深層水的水團上湧使得南極冰棚崩解。這項發現有助於我們更加瞭解冰層後退的現象。

瞭解過往驅使南極冰層後退的原因，是找出影響現在和未來冰層穩定性的因子為何的關鍵問題。南極西部冰層含有的冰足以讓全球海面升高1.2公尺，而南極西部的阿蒙森海灣(ASE)是現今南極西部冰層流失的主要原因。冰棚可以阻止冰層的冰塊往外流出，但在阿蒙森海灣，由於較溫暖的水團――繞極深層水(Circumpolar Deep Water, CDW)往上流進冰棚下方的大陸棚水域，使得冰棚逐漸從底部融化。科學家認為繞極深層水的流入也在過去造成了冰層往後退縮。從最早10400年前開始到7500年前，冰棚崩解可能造成了冰層迅速變薄。在本期《自然》(Nature)的第43頁，Hillenbrand 等人首度提出確切證據指出當時有更多的繞極深層水上湧，使得阿蒙森海灣的冰棚消融。作者也認為此過程是1940年代以來，當地冰量減少的主要因素。

圖一：冰層後退的一種機制

Hillenbrand等人發表的證據顯示稱作繞極深層水的相對溫暖水團造成南極西部阿蒙森海灣地區的冰層消融。在此過程中，繞極深層水往上流至阿蒙森海灣冰棚之下的大陸棚區水域，造成冰棚下方開始融化，進一步使得冰棚崩解，可能導致冰層往後退縮。圖中的接地線(grounding line)標記了陸地上的冰層和浮在海上的冰棚之間的交界。箭頭代表繞極深層水的流動方向。

現今西南極冰層能否持續存在取決於它的冰棚帶來的穩定作用。在末次冰河期期間(111000至11700年前)，繞極深層水冷化、流到此區域的量減少或兩者兼具，可能造成南極冰棚底部的融化量減少，促成冰層擴張也讓它變得較為穩定。過往一項研究提出冰河期結束之後，阿蒙森海灣的冰層於大約8000年前後退至目前的大小。該論文的作者猜測溫暖的海水流入造成冰棚變得不穩定，連帶使得冰層後退。

雖然科學家可以運用衛星和其他儀器來觀測現今南極西部的冰層、冰棚和周遭海域，但他們還是必須仰賴地質紀錄中代用指標的測量結果來重建過去的變化。Hillenbrand和同事研究從阿蒙森海取回的沉積物岩芯，藉此重建繞極深層水湧升到阿蒙森海灣大陸棚的變化情形，並測定過去11000年來繞極深層水湧升對冰層後退具有何種影響。作者分析的對象為生活於海水上層的浮游有孔蟲和生活在海床的底棲有孔蟲，兩種微體化石殼體的化學成分和物種組成。

Hillenbrand等人以底棲有孔蟲組成元素的比例當作代用指標，顯示在早於7500年之前，阿蒙森海灣大陸棚底層的海水較為溫暖，代表這段時間溫暖的繞極深層水一直都有湧入此區域。之後海水溫度下降顯示繞極深層水的流入量有所減少，直到近期才又升高。作者藉由測量浮游和底棲有孔蟲中代表水團的示蹤劑來證實這項發現。繞極深層水具有獨特的化學訊號，作者發現阿蒙森海灣的大陸棚直到8000年前左右，一直都有此訊號出現；此後由於其他水團跟繞極深層水混和，使得繞極深層水的專有訊號降低。

作者接著運用底棲有孔蟲的群集(物種組成)變化，推測冰棚是否覆蓋到阿蒙森海灣的表面。他們發現直到距今7500年以前，化石群集中的主要種類都是生活在冰棚下方的種類，但之後卻發生了顯著改變，群集中的主要種類象徵此處的環境變成位在冰棚邊緣。

將上述線索整合起來，強烈支持了在大約8000至7500年前，阿蒙森海灣的冰棚瓦解是由海洋造成的說法。尤其是有更多溫暖的繞極深層水最早從10400年前開始流往阿蒙森海灣的大陸棚，可能使冰棚底部融解，最後造成整座冰棚瓦解。Hillenbrand 等人將7500年之前繞極深層水的流入量增加歸因於南半球的西風帶往南方移動。此研究的主要優點之一在於研究方法同時採用了多種代用指標，讓他們可以個別驗證各項數據代表的意義。

前人研究發現從西南極冰層流入阿蒙森海灣的兩條主要冰河之一――松島冰河在8000年前曾經迅速地減薄，而Hillenbrand和其同事的發現顯示此現象跟海洋之間有重要關聯。他們發現冰層後退的同時，也有更多繞極深層水流入阿蒙森海灣大陸棚。海水的流動造成冰棚崩解，冰棚對冰層的支撐作用減少使得冰層後退的速率加劇。

作者也重建了過去一世紀繞極深層水流入阿蒙森海灣的變化情形，雖然這部分研究根據的資料有限，他們還是發現了自1940年代開始，繞極深層水往上流入阿蒙森海灣大陸棚的量有再次增加的現象。如果事實真是如此，便可以確認過去數個世紀以來在阿蒙森海灣區域發生的冰棚崩解以及冰層後退，主要原因就是海洋作用力和繞極深層水流入。

Hillenbrand和其同事的發現受限於沉積物紀錄的長度以及底層水溫度數據的低解析度。由於沉積物紀錄只能追溯至大約11000年前，但此時阿蒙森海灣的大陸棚已經有繞極底層水流入，因此難以確定是什麼時候開始有較多的繞極深層水流入阿蒙森海灣。若可以界定開始的時間點，則有助於確認繞極深層水流入量的增加是否由南半球西風帶的南移導致。此外，底層水溫度的數據跟研究用的其他資料相較而言解析度比較低，且時間無法延伸至超過8000年前。利用新的沉積物岩芯進行更多分析可以幫助作者更加確定他們的成果。即便如此，他們的研究代表科學家對冰棚崩解的背後因素有了更為長足的瞭解。

Ice-sheet history revealed by fossils

Microscopic fossils show that, from 10,400 to 7,500 years ago, upwelling of a water mass called Circumpolar Deep Water destabilized Antarctic ice shelves — a finding that advances our understanding of ice-sheet retreat. 

Understanding the past drivers of Antarctic ice-sheet retreat is key to recognizing the constraints on present and future ice-sheet stability. The Amundsen Sea Embayment (ASE) region of West Antarctica is a dominant contributor to the present mass loss from the West Antarctic Ice Sheet¹, containing enough ice to raise the global sea level² by 1.2 metres. In the ASE, ocean-driven melting of the undersides of ice shelves, which restrain the flow of ice from the ice sheet, is mainly caused by the inflow of a relatively warm water mass, Circumpolar Deep Water (CDW), onto the continental shelf beneath the ice shelf³ (Fig. 1). This inflow is thought to have led to ice-sheet retreat in the past. On page 43, Hillenbrand et al.⁴ provide the first definitive evidence that enhanced upwelling of CDW forced deglaciation of the ASE from at least 10,400 years ago until 7,500 years ago, when ice-shelf collapse could have caused rapid ice-sheet thinning. The authors also suggest that this process has been responsible for ice loss in the region since the 1940s.

Figure 1: A mechanism for ice-sheet retreat.

Hillenbrand et al.⁴ report evidence that a relatively warm water mass called Circumpolar Deep Water (CDW) led to past deglaciation of the Amundsen Sea Embayment (ASE) region of West Antarctica. In this process, CDW flows onto the continental shelf beneath the ASE ice shelf. This causes melting of the underside of the shelf, leading to ice-shelf collapse and possible ice-sheet retreat. The grounding line marks the transition from the grounded ice sheet to the floating ice shelf. Arrows indicate the direction of travel of CDW.

The persistence of the modern West Antarctic Ice Sheet relies on the stabilizing influence of its ice shelves⁵. During the last glacial period (between 110,000 and 11,700 years ago), cooling or decreased presence of CDW, or both, would have reduced melting beneath the Antarctic ice shelves, contributing to ice-sheet growth and stability⁶. One study has suggested that the postglacial retreat of the ice sheet to modern levels in the ASE occurred about 8,000 years ago, and its authors speculated that the inflow of warmer ocean waters led to ice-shelf instability, which in turn drove ice-sheet retreat⁷.

Although scientists can use satellites and other instrumentation to investigate the modern West Antarctic Ice Sheet, ice shelves and surrounding seas, they must rely on proxy measurements from archives to reconstruct past changes. Hillenbrand and colleagues studied sediment cores recovered from the Amundsen Sea to reconstruct CDW upwelling onto the ASE continental shelf and to determine the role of CDW upwelling in driving ice-sheet retreat over the past 11,000 years. The authors analysed the chemical composition and assemblage of the microscopic fossil shells of organisms known as planktic and benthic foraminifera, which live in the upper ocean and on the sea floor, respectively.

Hillenbrand et al. used the ratios of chemical elements in benthic foraminifera as a proxy to show that relatively warm bottom water persisted on the ASE continental shelf before 7,500 years ago, suggesting that warm CDW flooded the region until that time. Subsequently cooler temperatures indicate that CDW inflow was reduced until modern times. The authors confirmed these findings using measurements of a water-mass tracer on planktic and benthic foraminifera — CDW has a distinct chemical signature, and the authors found that its presence on the ASE continental shelf is recorded until about 8,000 years ago, after which the pure CDW signature is reduced as a result of the mixing of CDW with other water masses.

The authors then used variations in the assemblage (species composition) of benthic foraminifera to infer the presence or absence of an ice shelf covering the ASE. They found that species indicative of a sub-ice-shelf environment dominate the assemblage until 7,500 years ago, when a distinct change — to an assemblage dominated by species attributed to an ice-shelf edge environment — occurred.

Taken together, these lines of evidence provide strong support for an oceanic driver of ice-shelf collapse in the ASE between about 8,000 and 7,500 years ago. Specifically, the enhanced inflow of warm CDW onto the ASE continental shelf, beginning at least 10,400 years ago, probably contributed to the melting of the undersides of ice shelves, leading to their collapse. Hillenbrand et al. attribute the intensification of CDW inflow before 7,500 years ago to a southerly position of the Southern Hemisphere westerly wind belt⁸. A major strength of the authors' study is the multi-proxy approach taken, because it allows independent validation of data.

Hillenbrand and colleagues' findings provide a crucial oceanic link to a previous study that found that Pine Island Glacier, one of two main glaciers that drain the West Antarctic Ice Sheet into the ASE, experienced rapid thinning about 8,000 years ago⁷. This ice-sheet retreat coincided with the strengthened CDW inflow on the ASE continental shelf found by Hillenbrand and collaborators. Such inflow probably caused the ice shelf to collapse, reducing its stabilizing effect on the ice sheet and leading to increased rates of ice-sheet retreat.

The authors also reconstructed CDW inflow to the ASE over the past century, and although their work is based on limited data, they found a renewed strengthening of CDW inflow onto the ASE continental shelf since the 1940s. If that is the case, it would confirm oceanic forcing and CDW inflow as the main drivers of ice-shelf collapse⁹ and ice-sheet retreat in the ASE region⁷ in the past few decades.

Hillenbrand and colleagues' findings are limited by the length of the sediment records and the low resolution of the bottom-water temperature data. Because the sediment records extend back about 11,000 years, when CDW was already present on the ASE continental shelf, it is difficult to determine when the enhanced CDW inflow began. Constraining this timing could help to confirm whether the southerly shift of the Southern Hemisphere westerly wind belt led to the intensification of CDW inflow. Furthermore, the bottom-water temperature data are of lower resolution than the other data used, and do not extend back beyond about 8,000 years. Additional analyses from new sediment cores would help to confirm the authors' findings. Nevertheless, their study represents a major advance in our understanding of the drivers of ice-shelf collapse.

原始論文：Claus-Dieter Hillenbrand, James A. Smith, David A. Hodell, Mervyn Greaves, Christopher R. Poole, Sev Kender, Mark Williams, Thorbjørn Joest Andersen, Patrycja E. Jernas, Henry Elderfield, Johann P. Klages, Stephen J. Roberts, Karsten Gohl, Robert D. Larter, Gerhard Kuhn. West Antarctic Ice Sheet retreat driven by Holocene warm water incursions. Nature, 2017; 547 (7661): 43 DOI: 10.1038/nature22995

引用自：Science. “Ice-sheet history revealed by fossils”.

原文網址：http://www.nature.com/nature/journal/v547/n7661/full/547035a.html