過去10年,，幾十項研究工作報告了新元古代碳酸鹽巖石中碳同位素的變化，并將它們與全球碳循環(huán)的波動聯(lián)系起來,。
Paul Knauth 和Martin Kennedy從一個側(cè)面對這些數(shù)據(jù)進行了分析,，主要關(guān)注氧同位素的測量結(jié)果（是從超過2萬份樣品取得的）。氧同位素的測量結(jié)果是作為碳同位素分析的一部分必然要獲得的,，但卻常常被忽略,。他們得出一個引人注目的結(jié)論：這些氧和碳同位素的組合體系與我們非常了解的顯生宙樣品中的相同；顯生宙樣品巖化在了沿海的孔隙液中,，接受著含有來自陸地植物質(zhì)的光合作用碳的地下水流入,。被廣泛報道的新元古代碳酸鹽中13C/12C比例的下降并非是碳周期的波動，它們更容易通過與顯生宙樣品類比來理解,。這個結(jié)果可能表明,，在前寒武紀(jì)晚期，受由能夠進行光合作用的藻類,、苔蘚和真菌構(gòu)成的一層貼地覆蓋物的影響,，當(dāng)時的地球正在發(fā)生“綠化”。這樣一個產(chǎn)生氧和植物質(zhì)的事件,，甚至可能要對（地球）從前寒武紀(jì)的一個實質(zhì)上為微生物的世界向寒武紀(jì)的一個后生動物世界的關(guān)鍵過渡負(fù)間接責(zé)任,。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 460, 728-732 (6 August 2009) | doi:10.1038/nature08213
The late Precambrian greening of the Earth
L. Paul Knauth1 & Martin J. Kennedy2
1 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
2 Department of Earth Science, University of California, Riverside, Riverside, California 92557, USA
Many aspects of the carbon cycle can be assessed from temporal changes in the 13C/12C ratio of oceanic bicarbonate. 13C/12C can temporarily rise when large amounts of 13C-depleted photosynthetic organic matter are buried at enhanced rates1, and can decrease if phytomass is rapidly oxidized2 or if low 13C is rapidly released from methane clathrates3. Assuming that variations of the marine 13C/12C ratio are directly recorded in carbonate rocks, thousands of carbon isotope analyses of late Precambrian examples have been published to correlate these otherwise undatable strata and to document perturbations to the carbon cycle just before the great expansion of metazoan life. Low 13C/12C in some Neoproterozoic carbonates is considered evidence of carbon cycle perturbations unique to the Precambrian. These include complete oxidation of all organic matter in the ocean2 and complete productivity collapse such that low-13C/12C hydrothermal CO2 becomes the main input of carbon4. Here we compile all published oxygen and carbon isotope data for Neoproterozoic marine carbonates, and consider them in terms of processes known to alter the isotopic composition during transformation of the initial precipitate into limestone/dolostone. We show that the combined oxygen and carbon isotope systematics are identical to those of well-understood Phanerozoic examples that lithified in coastal pore fluids, receiving a large groundwater influx of photosynthetic carbon from terrestrial phytomass. Rather than being perturbations to the carbon cycle, widely reported decreases in 13C/12C in Neoproterozoic carbonates are more easily interpreted in the same way as is done for Phanerozoic examples. This influx of terrestrial carbon is not apparent in carbonates older than 850 Myr, so we infer an explosion of photosynthesizing communities on late Precambrian land surfaces. As a result, biotically enhanced weathering generated carbon-bearing soils on a large scale and their detrital sedimentation sequestered carbon5. This facilitated a rise in O2 necessary for the expansion of multicellular life.
