Nitrogen isotopes and a change in the Archaean biosphere

All life forms require nitrogen fixation; pretty obvious since they are largely made of C, H, O, N and P. It happens through two main processes in the nitrogen cycle: anaerobic reduction of dinitrogen (N2) to ammonium ions (NH4+) and the degradation of that by oxidation to nitrite (NO2) or nitrate (NO3) ions (nitrification). Both kinds of process allow nitrogen to enter cells today, but before the Earth’s biota evolved oxygen production through photosynthesis only the first, anaerobic process was possible. As with many elements that have several stable isotopes – nitrogen has two: 14N and 15N – such chemical processes favour one isotope over the others leading to fractionation in the overall environment. A measure of the relative proportions of nitrogen isotopes is δ15N, and its mean value in modern seawater is +5‰ due mainly to the reduction of nitrite and nitrate ions by denitrification. In an oxygen-free ocean δ15N would be significantly lower. Nitrogen-isotope studies of the organic matter in ancient sediments should therefore be a test for the presence of free oxygen in the environment.

In Archaean shales that have not been much metamorphosed δ15N is generally low, as expected. However, there have been hints of higher values from the youngest Archaean strata that do indicate oxygen. The Hamersley Group of Western Australia, famous for its vast reserves of banded ironstone formations (BIFs), includes a 50 m thick carbonaceous shale deposited at the very end of the Archaean around 2.5 Ga (Garvin, J. et al. 2009. Isotopic evidence for an aerobic nitrogen cycle in the latest Archaean. Science, v. 323, p. 1045-1048). Detailed geochemical analyses through the shales and enveloping BIFs, including nitrogen isotopes, show considerable variations ascribed to environmental changes. Aerobic denitrification is marked by a shift from 1 to 7.5‰ in δ15N within the shales, which correlates with shifts in molybdenum and the proportions of sulfur isotope. The real significance of the paper is not that the study detected evidence of free oxygen in the Archaean – the BIFs formed by combination of iron-2 ions with oxygen. It shows that before 2.5 Ga prokaryote organisms had already to perform aerobic nitrification as well as denitrification, of which there are only three groups nowadays, two of Bacteria the other of Archaea.

 The Palaeocene Snake of Death and torrid times

As a reader of anything connected with exploration of the Amazon as a kid, I developed a perfectly rational fear of snakes, especially anacondas that ate pigs. To my horror I awoke one snowy February morning to an item on the BBC Radio 4 Today programme about the biggest snake that ever lived (Head, J.J. and 7 others 2009. Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures. Nature, v.  457, p. 715-717). At 13 m long and weighing in at over a ton, Titanoboa could have eaten an entire family at one sitting, and gone next door for seconds: and it would probably get in the house with the booid’s celebrated stealth. Becoming calmer, I saw how interesting this gigantic people crusher must have seemed to its discovers. Seemingly the maximum size of snakes is governed by ambient temperature. The anaconda that gave me bad dreams gets to a maximum length of around seven metres in present equatorial South America (mean annual temperature in the upper 20s). Modelling based on a range of snakes now living at different latitudes suggests that Titanoboa grew Topsy-like at hotter Palaeocene tropical latitudes (a mean around 33ºC at least). We can all be thankful that such tropical temperatures would require atmospheric CO2 levels around 2000 parts per million, but this century’s possible global warming will probably mean bigger anacondas and boas for the Amazonian explorer to grapple with.

Snowball Earth and the major division among animals

There are two basic kind of animals: those whose embryos show bilateral symmetry – bilaterians like ourselves, sea urchins and lobsters, for instance – and those that don’t, such as corals and sponges. Evidence from genetic differences among living animals suggests that the evolutionary separation of the two fundamental groups was probably during the Proterozoic Eon. Calibrating molecular clocks based on DNA sequences of living organisms is possible to some extent for animal groups and the ancestral kinds preserved as fossils, for instance humans and domesticated chickens share a common ancestor that lived during the Carboniferous Period. (A propos of very little, mammals have uvulas dangling in their throats that have no other function than to make one throw up if they are tickled, and we share the uvula with birds who still use them to sing: food for the imagination there.) However, the separation of bilaterians from the others, and a great many living phyla, must have taken place in Precambrian times among ancestors with no hard parts and therefore no palpable trace of their existence. Thus, any evidence of when one or another was around is highly useful in phylogenic studies. Most such evidence is likely to come from resistant kerogen and bitumen hydrocarbons found in reduced facies sediments that occur as far back as the Archaean.

Biomarkers include organic molecules that can sometimes be linked to specific phyla, and distinctive ones are associated with either side of the bilaterian-‘others’ split (Love, G.D. and 12 others 2009. Fossil steroids record the appearance of Demospongiae during the Cryogenian period. Nature, v.  457, p. 718-721). The US-UK-Australia team sampled kerogen and bitumen from reduced carbonate sediments in the now famous Omani sequence that almost continuously spans times from the Cryogenian Period of Snowball Earth episodes, through the trace-fossil rich Ediacaran and across the Cambrian boundary. Incidentally, strata like these are source rocks for petroleum reserves in many parts of the Arabian Peninsula. Among the various kinds of molecule identified by chromatography are 24-isopropylcholestanes, degraded remnants of steroids based on 30 carbon atoms per molecule. These are characteristic of one group of sponges, i.e. non-bilaterians, and occur in the oldest samples (around 700 Ma). This shows clearly that the big evolutionary divergence predated that time and may have happened during the climatically dramatic Cryogenian.

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