过去10年，几十项研究工作报告了新元古代碳酸盐岩石中碳同位素的变化，并将它们与全球碳循环的波动联系起来。
Paul Knauth 和Martin Kennedy从一个侧面对这些数据进行了分析，主要关注氧同位素的测量结果（是从超过2万份样品取得的）。氧同位素的测量结果是作为碳同位素分析的一部分必然要获得的，但却常常被忽略。他们得出一个引人注目的结论：这些氧和碳同位素的组合体系与我们非常了解的显生宙样品中的相同；显生宙样品岩化在了沿海的孔隙液中，接受着含有来自陆地植物质的光合作用碳的地下水流入。被广泛报道的新元古代碳酸盐中13C/12C比例的下降并非是碳周期的波动，它们更容易通过与显生宙样品类比来理解。这个结果可能表明，在前寒武纪晚期，受由能够进行光合作用的藻类、苔藓和真菌构成的一层贴地覆盖物的影响，当时的地球正在发生“绿化”。这样一个产生氧和植物质的事件，甚至可能要对（地球）从前寒武纪的一个实质上为微生物的世界向寒武纪的一个后生动物世界的关键过渡负间接责任。（生物谷Bioon.com）
生物谷推荐原始出处：
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.
