24億年前陸地抬升出海面，也改變了地球
一篇由奧勒岡大學領導的研究探討頁岩中經由風化所形成的訊號，而指出改變地球氣候、地質和生物的事件發生於何時。
頁岩是地球上最常見的沉積岩，其中的化學訊號指出24億年前陸地以相當快的速度抬升出海面，可能對氣候和生物造成了巨大的影響。
此篇研究發表在5月24日發行的《自然》(Nature)期刊，研究人員表示他們從世界各地採集的頁岩樣品含有品質絕佳的證據，顯示地球最早從35億年前就開始有極為微量的雨水對陸地造成了風化作用。
主要作者，奧勒岡大學的地質學家Ilya Bindeman表示，氧17與氧18和較為常見的氧16之間的比例出現的顯著變化，讓研究人員得以解讀這些岩石化學成分的變化歷史。
藉此他們可以確定從什麼時候開始，新形成的地殼表面會受到物理和化學作用造成的風化，或者更廣義來說，在今日進行的水文作用中，水流經大片陸地途中蒸發成水氣的過程是從什麼時候開始發生。
證據來自於三種氧同位素的分析結果，尤其是稀有的穩定同位素氧17。他們分析的278具頁岩樣品來自於全球各地的露頭和鑽井，年代則涵蓋了地球歷史當中的37億年；分析進行的地點為Bindeman的穩定同位素實驗室。
Bindeman表示根據他自己先前進行的模擬以及其他研究，24億年前地球陸塊的總體積或許已經達到今日可觀測體積的三分之二了。而且這些新生陸地出現的速度相當快，同時地函動力學也發生了大規模變化。
此時的頁岩樣品中的同位素紀錄也吻合理論中陸塊碰撞而形成地球第一座超大陸――凱諾蘭大陸(Kenorland)――的時間點，並形成了高聳的山脈和高原。
「地殼要有足夠的厚度才能凸出水面，」Bindeman表示。「厚度除了跟地殼本身的體積有關，也和地函的熱力學機制和黏滯性有關。當地球溫度較高而地函較軟弱時，地函無法支持高山形成。我們的數據指出情況在24億年前出現了劇變，地函溫度降低使得地函可以支撐大片陸地挺出海面。」
他說當時從海面冒出的新生陸地，表面溫度可能比今天的還要高上數十度。
研究發現氧的三種同位素在這段時間附近出現了階段式變化。科學家表示這解決了過往對於11億年前至35億年前，陸地究竟是逐漸形成還是分段出現的爭議。Bindeman表示在24億年前，這些新形成的陸地開始藉由化學風化作用消耗大氣中的二氧化碳。
這個時間點也和太古代過渡至元古代時的其他轉變同時發生，此時地球上的生命從存活於海裡的簡單原核生物，包括古菌和細菌，開始出現真核生物，像是藻類、植物和真菌。
「在此研究中，我們著眼於35億年來風化作用的歷程。」Bindeman表示，「抬升出海面的陸地會改變地球的反照率(albedo
)。地球最初從宇宙看起來是一顆有些許白雲繚繞的深藍色星球。新形成的陸地會增加地球反射陽光的能力。現在我們的陸地顏色較深是因為有大量的植被覆蓋。」
他說新生陸地遭受風化作用之後或許可以儲集二氧化碳之類的溫室氣體，因而打亂地球的輻射平衡，造成地球在24億至22億年前出現一連串冰河期。他說這段冰河期可能又釀成了之後的大氧化事件，大氣變化使得大量自由氧進入到空氣當中。岩石因為氧化作用而變成赤紅色，反觀太古代的岩石則是灰色的。
Bindeman表示在沒有太多陸地的情況下，從太陽來的光子主要是跟水互相作用並使其溫度提高。新生陸地呈現的明亮表面會把陽光反射回太空，使得輻射作用和溫室氣體之間的平衡出現新的變數，造成氣候變化。
「我們猜測大型陸塊一旦出現，就會有許多陽光被反射回太空，而啟動一發不可收拾的冰河期。」Bindeman表示，「地球因此第一次看見雪。」
頁岩是由地殼風化的產物組成。
「它們(頁岩)可以告訴你許多關於地殼受到空氣、光和降水作用的情形。」Bindeman表示，「形成頁岩的過程中會捕捉到有機物，最終會成為土壤形成過程的一部份。頁岩提供了我們關於風化作用的連續紀錄。」
Land rising above the sea 2.4 billion years ago
changed planet Earth
A
University of Oregon-led study of chemical signatures in shales formed through
weathering points to changes that transformed climate, geology and life on the
planet
Chemical signatures in shale, the
Earth's most common sedimentary rock, point to a rapid rise of land above the
ocean 2.4 billion years ago that possibly triggered dramatic changes in climate
and life.
In a study published in the May 24 issue
of the journal Nature,
researchers report that shale sampled from around the world contains archival
quality evidence of almost imperceptible traces of rainwater that caused
weathering of land from as old as 3.5 billion years ago.
Notable changes in the ratios of oxygen
17 and 18 with more common oxygen 16, said lead author Ilya Bindeman, a
geologist at the University of Oregon, allowed researchers to read the chemical
history in the rocks.
In doing so, they established when newly
surfaced crust was exposed to weathering by chemical and physical processes,
and, more broadly, when the modern hydrologic process of moisture distillation
during transport over large continents started.
The evidence is from analyses of three
oxygen isotopes, particularly the rare but stable oxygen 17, in 278 shale
samples drawn from outcrops and drill holes from every continent and spanning
3.7 billion years of Earth's history. The analyses were done in Bindeman's
Stable Isotope Laboratory.
Based on his own previous modeling and
other studies, Bindeman said, total landmass on the planet 2.4 billion years
ago may have reached about two-thirds of what is observed today. However, the
emergence of the new land happened abruptly, in parallel with large-scale
changes in mantle dynamics.
Isotopic changes recorded in the shale
samples at that time also coincides with the hypothesized timing of land
collisions that formed Earth's first supercontinent, Kenorland, and
high-mountain ranges and plateaus.
"Crust needs to be thick to stick
out of water," Bindeman said. "The thickness depends on its amount
and also on thermal regulation and the viscosity of the mantle. When the Earth
was hot and the mantle was soft, large, tall mountains could not be supported.
Our data indicate that this changed exponentially 2.4 billion years ago. The
cooler mantle was able to support large swaths of land above sea level."
Temperatures on the surface when the new
land emerged from the sea would have likely been hotter than today by several
tens of degrees, he said.
The study found a stepwise change in
triple-isotopes of oxygen around that time frame. That, the scientists said,
resolves previous arguments for a gradual or stepwise emergence of land between
1.1 and 3.5 billion years ago. At 2.4 billion years ago, Bindeman said, the
newly emerged land began to consume carbon dioxide from the atmosphere amid
chemical weathering.
The timing also coincides with the
transition from the Archean Eon, when simple prokaryotic life forms, archaea
and bacteria, thrived in water, to the Proterozoic Eon, when eukaryotes, such
as algae, plants and fungi, emerged.
"In this study, we looked at how
weathering proceeded over 3.5 billion years," Bindeman said. "Land
rising from water changes the albedo of the planet. Initially, Earth would have
been dark blue with some white clouds when viewed from space. Early continents
added to reflection. Today we have dark continents because of lots of
vegetation."
Exposure of the new land to weathering,
he said, may have set off a sink of greenhouse gases such carbon dioxide,
disrupting the radiative balance of the Earth that generated a series of
glacial episodes between 2.4 billion and 2.2 billion years ago. That, he said,
may have spawned the Great Oxygenation Event in which atmospheric changes
brought significant amounts of free oxygen into the air. Rocks were oxidized
and became red. Archean rocks are gray.
In the absence of much land, he said,
photons from the sun interacted with water and heated it. A bright surface,
provided by emerging land, would reflect sunlight back into space, creating
additional torque on radiative-greenhouse balance and a change in climate.
"What we speculate is that once
large continents emerged, light would be reflected back into space and initiate
runaway glaciation," Bindeman said. "Earth would have seen its first
snowfall."
Shales are formed by the weathering of
crust.
"They tell you a lot about the
exposure to air and light and precipitation," Bindeman said. "The
process of forming shale captures organic products and eventually helps to
generate oil. Shales provide us with a continuous record of weathering."
原始論文：I. N. Bindeman, D. O.
Zakharov, J. Palandri, N. D. Greber, N. Dauphas, G. J. Retallack, A. Hofmann,
J. S. Lackey, A. Bekker. Rapid
emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5
billion years ago. Nature,
2018; 557 (7706): 545 DOI: 10.1038/s41586-018-0131-1
引用自：University of Oregon.
"Land rising above the sea 2.4 billion years ago changed planet Earth: A
study of chemical signatures in shales formed through weathering points to
changes that transformed climate, geology and life on the planet." ScienceDaily.
ScienceDaily, 23 May 2018.
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