重大天災可以把大片森林帶到數千公里外的海底埋藏起來
By Darrin S. Joy
地球科學家表示颶風與季風降下的滂沱大雨，以及其他的重大天災可以把大量的新鮮木頭帶到海洋深處並埋藏在海底。
這項研究10月21日刊登在《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)，其中呈現的證據首度指出樹木可以從它們山上的家園，旅經數千公里之後停歇在河口至深海的厚層沉積物中。
從深海找到的古老木塊
由南加州大學文理學院的地球科學副教授Sarah
Feakins領導的研究團隊，檢視了從孟加拉灣海底鑽取出來的岩芯樣本，此地距離孟加拉海岸有數千公里之遠。
他們搭乘的船隻為國際海洋發現計畫的研究船之一――隸屬於美國的聯合果敢號。當船隻到達目標位置便會伸出鑽井儀器，先從海洋表面穿過比2英里(約3.2公里)還深的海水到達海床，然後再往下鑽取超過半英里(約0.8公里)的沉積物。接著研究團隊把取回來的沉積物帶回實驗室進行徹底研究，他們發現在砂層中找到的木片年代可以追溯到距今1900萬年以前。
分析結果顯示大多數層位裡的木頭是來自於低海拔地區，但有一個層位的卻是來自於高山上的樹木。
「我們找到了幾塊完好無缺的針葉樹，」Feakins表示，「這些樹生長在海拔3200公尺以上的喜馬拉雅山區。」
這些生活於末次冰河期的樹木會被連根拔起，可能是冰河、山崩形成的天然壩或是類似地形潰堤而導致。不論如何這勢必造成了巨大的洪水，使得木頭可以順著水勢旅行數千公里，從尼泊爾一路穿過孟加拉，最後沉沒在孟加拉扇――世上最巨厚的海底沉積物內部。
科學家研究岩芯樣品的其他層位也發現了來自低海拔樹木的木頭。在他們研究的這1900萬年之間可能經常有季風或氣旋造成大雨與洪水，這些木片可能就是由這些事件帶來的。
河川運送的木頭
研究除了顯示自然事件讓河川把樹木運送到海洋的距離之遠有多麼令人驚訝，也發現這類木頭是碳循環中相當重要的一部分。
地球大部分的碳都存在於岩石當中，但是其他部分則會以各種形式流動在陸地、空氣與海洋之間；而在動物、植物和微生物之間也是如此。
空氣中的碳一部分是二氧化碳、甲烷或者其他溫室氣體，它們能困住大氣裡的熱能，造成溫室效應。
另一方面，跟植物結合的碳則會與大氣隔絕，但植物死後不久通常會遭到動物代謝、受到分解或是燃燒，使得碳又返回大氣。然而河川造成的快速搬運事件卻可以跳過正常來說樹木倒塌之後會發生的分解作用，而把新鮮的木頭埋藏於海床上堆積的沉積物。
這項新發現指出了一種先前不為所知的作用，可以把碳持續封鎖起來，有效地把碳從碳循環中隔離數百萬年之久。之前曾有人估計布拉馬普特拉河及恆河目前運送至海洋的碳有多少。這篇研究中大量的木頭顯示先前認定的數值有所低估，由這兩條河川運送並埋藏起來的碳可能還要再多出50%。
「一直以來我們都在試著計算碳循環中每一個部分的碳有多少，但我們之前並不曉得海床裡埋藏著這些殘破的樹木碎片。」Feakins表示，「現在我們應該要把它們給納入方程式之中。」
消耗大氣中的碳
Feakins也特別指出這項發現強調了自由流動的河川，在碳封存這方面具有相當重要的地位。
Feakins說：「過去5000萬年來地球的平均氣溫下降了很多，」而大氣的二氧化碳濃度也是如此。這是因為岩石風化(包括喜馬拉雅山)和植物碳埋藏在近海等作用，使得碳被抽離大氣。研究指出這些鮮為人知、含有木頭的沉積層數百萬年來封存的碳應該也要列入上述作用，否則溫室氣體的含量還是會讓氣候相當溫暖。
另一方面，5000萬年來的冷化趨勢在最近數十年因為人類活動而迅速逆轉，使得二氧化碳回升到將近300萬年前的濃度。Feakins強調在人類減少排放二氧化碳的同時，也要了解自然界的生態系和河川如何把碳封存起來。這可以讓我們從河川和生態系具有潛能來幫助我們管理碳循環的觀點上，在管理森林、拆除水壩以及其他議題上做出適當的決定。
Catastrophic events carry forests of
trees thousands of miles to a burial at sea
Flooding from torrential rains caused by
cyclones and monsoonal storms, as well as other catastrophic events, are
responsible for moving huge amounts of fresh wood to a watery grave deep under
the ocean, according to Earth scientists.
Their research, published in the Proceedings of the National Academy of Sciences on Oct. 21, shows
the first-ever evidence that trees may travel thousands of miles from their
mountain homes to settle in the vast sediments extending under the sea from
river mouths.
Wood from ancient
trees found in the deep sea
Researchers led by Sarah Feakins, associate professor
of Earth sciences at USC Dornsife College of Letters, Arts and Sciences,
examined core samples taken from the ocean floor over a thousand miles offshore
from Bangladesh, in the Bay of Bengal.
Once at the target point at sea, the U.S.-operated
research ship R/V Joides Resolution, which is part of the International Ocean Discovery
Program, extended a drill mechanism more than two miles down from the ocean’s
surface to its floor and drilled more than a half a mile down into the
sediments. They then carried the samples back to the lab, where the research
team combed through the resulting core samples. They discovered wood chips in
the sandy layers dating back as far as 19 million years.
Their analysis showed that wood in most layers came
from lowland sources, but one layer contained wood from trees high in the
mountains.
“We found pristine pieces of conifers,” Feakins said.
“These trees grow two miles above sea level, up in the Himalayas.”
The trees likely were uprooted during the last ice
age by a massive release of water from the breach of a natural dam created by a
glacier, landslide or similar land feature. In what must have been a surge of
water, the trees rode rivers thousands of miles from Nepal through Bangladesh
and into the Bengal Fan, the largest underwater sediment accumulation in the
world.
The scientists, searching through other layers in the
core sample, found wood from the lowlands, as well. These wood chips likely
were carried to the sea by torrential rains and flooding during monsoons or
cyclones that occurred many time across the 19-million-year time span.
Rivers export
trees
Aside from revealing the astounding distance trees
may be carried by rivers to the sea due to natural events, the study finds this
tree wood is an important part of the carbon cycle.
Most of the planet’s carbon resides in rock, but the
remainder flows in various forms between land, air and ocean, making its way
through plants, animals and microbes as it does so.
In the air, carbon may be part of carbon dioxide, methane
or other greenhouse gases, which can trap heat in the atmosphere, causing the
greenhouse effect.
The carbon bound by plants, on the other hand,
remains sequestered from the atmosphere until metabolism, decay or burning
releases it, usually shortly after death. Yet, rapid transport events by rivers
bypass decomposition that normally follows tree-fall and instead deliver fresh
wood to be entombed in sediment on the seafloor.
The new findings point to a previously unrecognized
way carbon can remain locked away, effectively removed from the carbon cycle,
for millions of years. The abundance of wood suggests that the prior estimates
of carbon exported by modern Ganges-Brahmaputra rivers were low, and now
accounting for wood, the amount of carbon exported and buried may be 50%
greater than previously thought.
“As we’ve tried to calculate the amount of carbon in
all parts of the carbon cycle, we didn’t know about this forest of fragmented
trees buried in the ocean floor,” Feakins said. “Now we need to add it to the
equation.”
Carbon drawdown
Feakins also noted that the findings emphasize the
importance of free-flowing rivers in carbon sequestration.
“Over the past 50 million years, average global
temperatures fell significantly,” Feakins said, as did atmospheric CO2
levels. This was due to natural processes — such as weathering of rocks
(including the Himalayas) and burial of plant carbon offshore — that draw
carbon out of the atmosphere. This study points to the need to inventory
millions of years of carbon sequestration in these elusive woody sediment
layers without which greenhouse gases would have kept temperatures high.
Further, the 50 million-year-long cooling trend has
rapidly reversed in recent decades due to human activity, which has raised CO2
levels close to those of 3 million years ago. Feakins notes that, in parallel
to emissions reductions, understanding the carbon sequestration services that
natural ecosystem and rivers perform can inform decisions about forest
management, dam removal and other issues with regard to their potential to help
manage the carbon cycle.
原始論文：Hyejung Lee,
Valier Galy, Xiaojuan Feng, Camilo Ponton, Albert Galy, Christian
France-Lanord, and Sarah J. Feakins. Sustained wood burial in the
Bengal Fan over the last 19 My. PNAS, 2019 DOI: 10.1073/pnas.1913714116
引用自：University of Southern California.
"Catastrophic events carry forests of trees thousands of miles to a burial
at sea."
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