The late Precambrian greening of the Earth

Dozens of studies in the past decade have reported carbon isotope variations in Neoproterozoic carbonate rocks and linked them to perturbations of the global carbon cycle. Paul Knauth and Martin Kennedy have taken a sideways look at the data by concentrating on the oxygen isotope measurements (for over 20,000 samples) that are necessarily obtained as part of the carbon isotope analysis but are often overlooked. They arrive at the striking conclusion 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 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 by analogy to the Phanerozoic examples. And that could suggest a 'greening' of the Earth under a ground-hugging mat of photosynthetic algae, mosses and fungi in the late Precambrian. Such an event, producing oxygen and phytomass, could even be indirectly responsible for the critical transition from the essentially microbial world of the Precambrian to the metazoan world of the Cambrian. The low 13C/12C ratio in some Neoproterozoic carbonates is considered to be evidence of carbon cycle perturbations unique to the Precambrian. Here, all published oxygen and carbon isotope data for Neoproterozoic marine carbonates are compiled. The combined isotope systematics are found to be identical to those of well-understood Phanerozoic examples, suggesting an influx of photosynthetic carbon rather than perturbations to the carbon cycle — and implying an explosion of photosynthesizing communities on late Precambrian land surfaces. 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.