"Plants" 850 Million Years Ago?

The late Precambrian greening of the Earth

L. Paul Knauth (1)
Martin J. Kennedy (2)

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

Correspondence to: L. Paul Knauth1 Correspondence and requests for materials should be addressed to L.P.K. (Email: Knauth@asu.edu).

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 approx 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.

To say this is controversial would be putting it mildly. I mean, plants, in the Cryogenian/Tonian Boundary?! Or even substantial terrestrial anything would be a shocker. If plants or some terrestrial oxygenerating ...something...existed, then you might be able to argue that the Cryogenian glaciations were like the Devonian and possibly tied to something kerchunking its way through the atmospheric carbon. MIGHT.

Here's a pop sci treatment.