The massive die-off at the end of the Palaeozoic Era (251.5 Ma) has focussed attention from a variety of geoscientists for over a decade. Theories for the cause abound, including the climatic influence of the huge Siberian continental flood basalt province, which formed around the same time, explosive release of sea-floor methane, oceanic anoxia, continental aridity and a massive belch of sulphur from the deep mantle. There is now another candidate, asphyxiation (Weidlich, O. et al. 2003. Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop. Geology, v. 31, p. 961-964). The explosion of land plants in the Carboniferous and early Permian that led to the world’s great coal deposits drove up atmospheric oxygen levels to their all-time peak. The occurrence at that time of giant insects, whose metabolism depends on direct diffusion of oxygen, suggests levels of as high as 35%. By the end of the Permian oxygen levels may have been as low as 15%. One line of support for such low concentrations is the growing abundance of fungal spores in the late Permian, which the authors suggest may have been related to a decline in insect populations which consume vast amounts of plant debris. Another is the widespread evidence of anoxic conditions in the Permian oceans, including isotopic features that support a “Strangelove” ocean at the P-T boundary. How oxygen was removed from the atmosphere in the Carboniferous to end-Permian is hard to assess. At levels above around 25% green vegetation catches fire easily, so large firestorms may have been characteristic of the coal-forming era. However, that would not drop levels much below those that prevail at present. Yet the Permian is famous for its continental red beds, the red coloration being due to iron oxide (hematite). Perhaps the missing oxygen became locked in Fe2O3 as the Earth took on a distinct reddishness as the Permian progressed.
“Archaean” ironstone pods prove to be very young
For a number of reasons, including evidence that the cell-chemistry of the most primitive bacteria includes heavy metals and sulphur, the most popular current theory for the place of life’s origin suggests ocean-floor hydrothermal vents. This has led to a search for remains of such “black smokers” in Archaean greenstone belts. One of the most celebrated sites is in the 3.5 Ga Barberton greenstone belt on the South Africa-Mozambique border. Within it are bodies rich in iron oxides, known as “ironstone pods” (not banded iron formations) that show many of the characteristic features of hydrothermal processes. As well as spurring many authors into concluding that the complex organic compounds in them indicate highly developed microbial ecosystems around early-Archaean seafloor vents, scientists have used fluids included in them to speculate on Archaean oceans, and the prevailing temperatures so long ago. They will be dismayed by a re-appraisal of the pods by Donald Lowe of Stanford University and Gary Byerly of Louisiana State University, which casts doubt on their antiquity (Lowe, D.R. & Byerly, G.R. 2003. Ironstone pods in the Archean Barberton greenstone belt, South Africa: Earth’s oldest hydrothermal vents reinterpreted as Quaternary hot springs. Geology, v. 31, p. 909-912). These pods are composed mainly of ferric hydroxide (goethite), which survives only at low temperatures, and are full of open pore spaces that include banded goethite indicating that it formed with the pores’ present orientation, The Barberton Archaean rocks are highly deformed and were metamorphosed at greenschist facies. The pods cut the foliation, and goethite is seen to partly replace Archaean cherts and serpentinised ultramafic lavas. As if these features were not sufficient to rule out the pods’ formation during Archaean times, Lowe and Byerly found one that is clearly related to a now inactive modern spring that formed terraces of botryoidal goethite. These show clear evidence of having formed as a result of modern bacterial action; they are biofilms. In places, modern landslide debris is cemented by goethite. Watch out for interesting correspondence in future issues of Geology from groups who stuck out their necks too far.
Artificial Archaean “fossils”
Debate on the existence of the world’s oldest microfossils from the 3.5 Ga Warrawoona cherts in Western Australia (see Doubt cast on earliest bacterial fossils, April 2002 EPN) has been stoked up by the creation of similar filamentous objects in vitro by geochemists from Spain and Australia (Garcia-Ruiz, J.M. et al. 2003. Self-assembled silica-carbonate structures and detection of ancient microfossils. Science, v. 302, p. 1194-1197). They did this by mixing soluble barium salts in an alkaline sodium silicate solution (pH 8.5-11) exposed to CO2 in the atmosphere. At high alkalinity CO2 dissolves to enrich solutions in carbonate and bicarbonate ions. Filaments made up of precipitated barium carbonate (witherite) and silica soon form. They take on shapes very similar to the tiny segmented worm-like structures that in 1996 were trumpeted as fossils in a now notorious Martian meteorite, as well as those from Warrawoona that are disputed by Schopf and Brazier. The experimenters went a step further, by immersing the filaments in a formaldehyde-phenol mixture and heating them to 125ºC. They then became coated in brownish, kerogen-like carbonaceous material, much as the Warrawoona structures are. Such organic coatings can also be produced by heating iron carbonate (siderite) to 300ºC in water vapour. These “test-tube” analogues of microfossils formed in plausible chemical compositions under not particularly special physical conditions. Interestingly, the Warrawoona chert contains both baryte and iron carbonate. Reaction to the paper was mixed!
Eucarya missing from Mesoproterozoic
Naively, I am always surprised to learn of Precambrian oilfields, even though petroleum in the vast fields of Saudi Arabia partly had its source in Neoproterozoic sediments and migrated into the overlying cover. Provided oil has not been degraded by later biological activity, it contains chemical traces of the organisms whose original decay produced the hydrocarbons, even a breakdown product of cholesterol (cholestane) that is characteristic of the former presence of Eucarya. In the Northern Territories of Australia, Mesoproterozoic sediments (~1430 Ma) that formed in a shallow marine basin are a target for oil exploration. Potential reservoir rocks contain bitumen in pore spaces, but there are fluid inclusion in fractures, which host liquid oil and brines. Organic geochemists at CSIRO, the University of Sydney and Macquarie University have analysed the oil’s molecular structure (Dutkiewicz, A. et al. 2003. Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia. Geology, v. 31, p. 981-984). Mass chromatography reveals a wealth of complex organic compounds, that are biomarkers for the kinds of organisms that were buried and then thermally matured to form the oil. These are exclusively those which point to prokaryotes, especially the cyanobacteria. Evidence for eukaryotic organism is completely absent. This is useful evidence in assigning a maximum age for the rise of the Eucarya that evolved into all modern complex organisms. The earliest likely eukaryote fossil is Grypania, a glossy carbonaceous spiral, found occasionally in sediments around 1400 Ma old, although dubious finds may indicate an origin as far back as 2100 Ma. The dominance of evidence for photosynthesising blue-green bacteria indicates that the oil-forming organisms thrived in an oxygenated, shallow environment. So there seems every reason to believe that Eukarya would have been capable of thriving as part of the trophic pyramid, had they arisen before 1430 Ma.