Greening of the Late Precambrian Landscape

Although isotopic and microfossil evidence document the presence of photosynthesizers on land as far back as 1.2 Ga, the extent of the occupation prior to the advent of Phanerozoic vascular land plants has not been previously assessed.  Recently, thousands of C isotope analyses have been published for late Precambrian carbonates in attempts to correlate strata and assess possible perturbations to the global C cycle.  The O isotopes are usually dismissed as altered, but the combined O and C isotope systematics of Neoproterozoic carbonates are identical to those of Phanerozoic marine precipitates that converted to limestone/dolostone in coastal pore fluids and then experienced various degrees of later metamorphic alteration. Carbon isotope variations resulted from variable input of low ¹³C in the coastal terrestrial phytomass rather than from perturbations to the global C cycle. Carbonate cements, concretions, and beds in clastic sequences also display isotope systematics identical to their Phanerozoic counterparts.  For coastal examples, low ¹⁸O results from meteoric waters in phreatic lenses, and low ¹³C is contributed from the terrestrial phytomass.  Carbonate that formed during deep burial has low ¹⁸O from equilibration with pore fluids at elevated temperatures and low ¹³C results from thermal decarboxylation of buried organic matter. These Phanerozoic-style systematics are absent in carbonates older than ~850 Ma but are evident in carbonates immediately preceding the Cryogenian ice ages. We therefore infer a remarkable explosion of photosynthesizing communities on late Precambrian coastal land surfaces.  These provided the transition to the important role terrestrial vegetation plays today in biogeochemical cycling, including critical feedbacks necessary to sustain animal life through biotically enhanced chemical weathering, transpirational cooling, changes in albedo, and landscape stabilization.  If correct, the greening event was a key trigger for the transition from the Precambrian to the Phanerozoic world.