The late Precambrian greening of the Earth

Abstract

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.