Hypothesis: Lichen-Fungal Colonization of the Land Drove NeoProterozoic Oxidation Event?

Hypothesized link between Neoproterozoic greening of the land surface and the establishment of an oxygen-rich atmosphere

Author:

Kump

Abstract:

Considerable geological, geochemical, paleontological, and isotopic evidence exists to support the hypothesis that the atmospheric oxygen level rose from an Archean baseline of essentially zero to modern values in two steps roughly 2.3 billion and 0.8–0.6 billion years ago (Ga). The first step in oxygen content, the Great Oxidation Event, was likely a threshold response to diminishing reductant input from Earth’s interior. Here I provide an alternative to previous suggestions that the second step was the result of the establishment of the first terrestrial fungal–lichen ecosystems. The consumption of oxygen by aerobes respiring this new source of organic matter in soils would have necessitated an increase in the atmospheric oxygen content to compensate for the reduced delivery of oxygen to the weathering environment below the organic-rich upper soil layer. Support for this hypothesis comes from the observed spread toward more negative carbon isotope compositions in Neoproterozoic (1.0–0.542 Ga) and younger limestones altered under the influence of ground waters, and the positive correlation between the carbon isotope composition and oxygen content of modern ground waters in contact with limestones. Thus, the greening of the planet’s land surfaces forced the atmospheric oxygen level to a new, higher equilibrium state.

3D Video Reconstruction of Retallack's Problematic "Terrestrial" Fossils

 

The original post about Retallack's latest controversial fossil is here.  This video is from the supplements.

Diskagma buttonii: Evidence of Paleoproterozoic Terrestrial Life?

Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa

Authors:

1. Gregory J. Retallack (a)
2. Evelyn S. Krull (b)
3. Glenn D. Thackray (c)
4. Dula Parkinson (d)

Affiliations:

a. Department of Geological Sciences, University of Oregon, Eugene, Oregon

b. C.S.I.R.O. Land and Water, Waite Campus, Glen Osmond, South Australia 5064

c. Department of Geosciences, Idaho State University, Pocatello, Idaho 83209

d. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720

Abstract:

Small (0.3-1.8 mm long), locally abundant, urn-shaped fossils within surface horizons of a paleosol in the 2.2 Ga Hekpoort Formation near Waterval Onder, South Africa, are here described and named Diskagma buttonii Retallack gen. et sp. nov. The fossils are from fresh rock of a deep highway cutting, and have been metamorphosed to upper greenschist facies like their matrix. Despite metamorphic alteration, total organic carbon of the samples was 0.04% and its isotopic composition (δ13C) was–25.6 ± 0.08 ‰ (two standard deviations) versus Vienna Pee Dee belemnite standard. Organic outlines of the fossils are also accentuated by recystallized berthierine and opaque oxides. The fossils are locally clumped within surface swales of a Vertisol paleosol, identified from characteristic penecontemporaneous deformation (clastic dikes between swales of mukkara structure) and from pronounced geochemical differentiation (phosphorus and copper strain-corrected mass-depletion characteristic of an oxidized biologically active soil). This paleosol's chemical composition is evidence of temperate humid climate (mean annual temperature 11.3 ± 4.4 °C, and mean annual precipitation 1489 ± 182 mm). Associated paleosols indicate atmospheric CO2 of 6640 +12880/-4293 ppm (0.6%) and 0.9-5% atmospheric O2. The best preserved examples of Diskagma are shaped like an urn with a flared rim, and closed below the flare. Observation of hundreds of specimens in thin section reveals substantial variation in growth (elongation) and decay (shredding and deflation). They had a hollow ellipsoidal interior that is unusually devoid of opaque debris, unlike the matrix. Diskagma is superficially comparable with lichens such as Cladonia (Ascomycota) and Geosiphon (Glomeromycota). Definitive reproductive structures remain unknown. They predate the oldest other likely fossil eukaryotes (1.9 Ga) and fungi (1.5 Ga), and current molecular clock estimates for eukaryotes (1.6 Ga) and fungi (1.1 Ga). Lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks. Although biological affinities of Diskagma are uncertain, these fossils reveal the general appearance of Paleoproterozoic life on land.

I gotta add that I am skeptical.  Not that there is life, but...lichens?  That early?  The paper even notes there are problems with the timing...