Author: zooks777

Most geologists would answer, ‘The continents after the start of the Silurian Period’, and from now on they could be wrong. Evidence for an earlier ‘greening’ of the land comes from a detailed analysis of thousands of oxygen- and carbon-isotope measurements in Neoproterozoic carbonate rocks (Knauth, L.P. & Kennedy, M.J. 2009. The Neoproterozoic greening of the Earth. Nature, v. 460, p. 728-732). An important consideration in understanding the geochemistry of limestones is that however they originally formed as wet sediments at some later stage their constituents were largely transformed into crystalline aggregates by lithification through the intermediary of pore fluids. During lithification chemistry is equilibrated between crystals and the pore fluids, so if pore fluids are chemically (in this case isotopically) different from the sediment the resulting rock will have been changed isotopically. Studies of Cenozoic carbonates strongly suggest that the place where carbonate sediments are lithified most quickly is in coastal areas where terrestrial groundwater mixes with marine formation water in sediments. Since colonisation of the land by photosynthesising organisms groundwater C- and O-isotopes evolves in equilibrium with those organisms. The terrestrial biomass fixes ¹²C preferentially thereby depleting their proportion of ¹³C by up to 20‰. Groundwater, having originated as water vapour evaporated from the oceans that acts preferentially on ¹²O is also depleted in ¹⁸O. Consequently, low δ¹³C and δ¹⁸O signatures are passed on to groundwater and thence to carbonate rocks when groundwater participates in lithification.

Neoproterozoic carbonates plot in the same δ¹³C vs δ¹⁸O fields as those from the Phanerozoic. Earlier Precambrian carbonate data plotted in the same way show depletion in δ¹⁸O but not in δ¹³C, which signifies no terrestrial life, but normal preferential evaporation of ¹⁶O from the ocean surface to form rain and then groundwater. Knauth and Kennedy’s results suggest a strong likelihood that carbonates of the late Precambrian were lithified by groundwater from a land surface where photosynthetic organisms were well-established and abundant. There is likely to be a sceptical backlash to this remarkable conclusion, largely because it seems that the terrestrial biomass in the Neoproterozoic would have needed to be of the same order as that in later times. Yet molecular evidence from modern fungi, lichens, liverworts and mosses suggests that they evolved in the Neoproterozoic and Chinese scientists have found traces of what look remarkably like lichens in the 600 Ma Doushantuo lagerstätte – fungus-like hyphae and cells that resemble those of cyanobacteria (see The earliest lichens in May 2005 issue of EPN). In an earlier paper, Martin Kennedy had noted that around 700 Ma, the record of marine limestones show increasing ⁸⁷Sr/⁸⁶Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. That may have coincided with biological agencies helping break down bare rock chemically to swelling clays that show a surge in Neoproterozoic sedimentary sequences (see Clays and the rise of an oxygenated atmosphere in March 2006 issue of EPN). The same paper pointed out that such clays increase the chances of preservation of buried organic matter, thereby boosting build-up of atmospheric and dissolved oxygen, as would terrestrial photosynthesisers. The feedback of increased oxygen to other eukaryotes that had evolved as heterotrophic animals would have enabled them to increase in size. Interestingly the earliest fossil animals occur in the same Chinese lagerstätte as the putative terrestrial photosynthesisers.

See also: Arthur, M.A. 2009. Carbonate rocks deconstructed. Nature, v. 460, p.698-699; Hand, E. 2009. When Earth greened over. Nature, v. 460, p.161.

It is pretty clear that events in the deep Earth, which give rise to surface changes, such as topographic uplift and increases or decreases in the pace of continental drift, feed into changes in the biosphere. A convincing example of that is the manner in which uplift of the flanks of the East African Rift System led to climate change that favoured bipedal apes. But is there a more direct link involving chemical influences?

It is likely that the earliest autotrophic organisms performed a variety of chemical tricks in order to create energy and chemical conditions that moved matter back and forth through their cell walls. As well as photoautotrophs of different kinds, including those that release oxygen as waste there would have been chemautotrophs, such as sulfate-sulfide reducers, methanogens and considerably more. Oxygenic photosynthesis apparently was functioning almost 3500 Ma ago, long before the Great Oxidation Event (see Early signs of oxygen…but in the wrong place in this issue) yet it was slow to make any impact on the atmosphere. In the Archaean oceans free oxygen would have been consumed by oxidation of soluble iron-II, probably creating banded iron formations. But photosynthesis has to take place in shallow sunlit water, so it would have been easy for oxygen to enter the atmosphere. Since carbon dioxide in the atmosphere is unable to react with oxygen, oxygen build up in the air might be expected to have built far faster than it did. That is, unless there was a reducing gas present in sufficient amounts to consume oxidation. The most likely buffering agent holding back an oxygen-bearing atmosphere is methane produced by methanogen autotrophs, and it has been suggested that falling methane levels towards the end of the Archaean and start of the Proterozoic Aeons eventually permitted atmospheric oxygen to remain unreacted. Since very little methane is produced by inorganic processes, that hypothesis has a corollary; that there was a decline in methanogen Bacteria and Archaea. So, how might that be tested?

A cunning piece of lateral thinking presents a test, and suggests a mechanism linked to processes in the Late Archaean – Palaeoproterozoic mantle (Konhhauser, K.O. and eight others 2009. Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature, v. 458, p. 750-753). The first cunning bit comes from the biochemistry of modern methanogens: Methyl-coenzyme M reductase (MCR) catalyses the formation of methane from methyl-coenzyme M and coenzyme B in methanogenic Archaea. This enzyme contains the nickel-centred porphinoid F430 tightly bound in its structure. Needless to say, the olivine-rich mantle contains abundant nickel, so the greater the percentage of mantle partial melting, the more nickel enters the surface environment. Archaean stratigraphy, especially its earlier parts, contains abundant ultramafic lavas known as komatiites, associated with some of the world’s big nickel mines. From the Late Archaean onwards, komatiites are rare rocks. The second master stroke by the authors is to find a means of charting the varying abundance in Archaean and Proterozoic seawater: they analysed the Ni content relative to that of Fe in banded iron formations. To as late as 2700 Ma the Ni/Fe ratio remains high in BIFs, but thereafter it falls sharply. That seems to support the hypothesis that a decline in the mass of methanogens did allow oxygen to build up in the atmosphere, and that decline reflected a fall in the supply of mantle nickel to the oceans. The next step would be to exploit the recently demonstrated ability of methanogen Archaea to fractionate nickel isotopes during their metabolism of dead organic matter. That would ideally be done using Ni-rich BIFs, as in this study.

Hadean not so hellish for life

Although the Earth’s history before 4 Ga is not the mystery that it was, following the discovery of 4.3 Ga-old metasedimentary rocks in NE Canada (see At last, 4.0 Ga barrier broken in November 2008 issue of EPN), the early history of the Moon suggests that it was hectic and plagued by very large asteroid and comet impacts. The mightiest events occurred around 3.9 Ga, forming the huge mare basins on the Moon. Scaling up for the Earth’s greater gravitational pull even larger catastrophes would have pounded our planet, although its turbulent tectonics has removed all tangible traces of them. From detailed studies of rocks and impact melts from the Moon – much of the lunar regolith comprises glass spherules produced by cratering over its entire history – the late heavy bombardment (LHB) was not prolonged in geological terms, lasting 20 to 200 Ma. Yet it involved the most extreme delivery of kinetic energy since the giant Moon-forming event around 2.45 Ga, which generated stupendous power – the rate of energy delivery by impactors moving at a minimum of 15 km s^-1 is about a second. This has encouraged speculation that the Earth was effectively sterilised for a second time in its history. The 500-600 Ma of Hadean history may have witnessed emerging life forms of the most basic kind, only to see them wiped out, perhaps more than once. It has been assumed, therefore, that the earliest living things which left descendants, including us, had a universal ancestor that appeared only after 3.9 Ga. Now it seems a serious rethink is needed (Abramov, O. & Mojzis, S.J. 2009. Microbial habitability of the Hadean Earth during the late heavy bombardment. Nature, v. 459, p. 419-422).

Feeding the impact data from the Moon and terrestrial planets into new modelling software run on a super-fast computer, Oleg Abramov and Stephen Mojzis of the University of Colorado have been able to model the degree of thermal metamorphism that the Earth’s crust may have undergone during the LHB. Interestingly, they reveal that less than 10% of the surface would have been heated above 500ºC, and only 37% would have been sterilised, even if all the huge impacts predicted for Earth landed at the same time. Assuming that any basic life forms that had arisen in the Hadean were randomly distributed at the surface and in the subsurface – a variety of extremophile bacteria still live at depths down to 4 km – populations would survive to leave descendants. If they could survive temperatures up to 110ºC, which modern hyperthermophiles do, then so much the better for life as a whole. Although based on modelling, the work by Abramov and Mozjis, gives palaeobiologists another half billion years in which inorganic processes could have assembled the immensely complex molecules the living processes demand. The earliest possible signs of life, based on carbon isotopes locked in stable minerals of a Greenland metasediment, date to 3.8 Ga. Previous assumptions about life’s slate being wiped clean by the LHB therefore left only a few tens of million years for that assembly by some kind of thermodynamic miracle. The new vista will please Mike Russell of the University of Strathclyde in Glasgow. Russell is an economic geochemist turned palaeo-biochemist set on testing the Oparin-Haldane hypothesis of the origin of life using apparatus and approaches that are much more sophisticated than those used by Miller and Urey who created amino acids in vitro during the early 50s. The 21 May 2009 issue of Nature includes an account of Russell’s plans and the views of those with a more cautious outlook (Whitfield, J. 2009. Nascence man. Nature, v. 459, p. 316-319).

See also: Rothschild, L.J. 2009. Life battered but unbowed. Nature, v. 459, p. 335-336.

Irresistible brevia

Surprisingly, the most abundant crustacean fossils are those of ostracodes, which have two carapace shells. They reach back as far as the Ordovician. Although modern ostracodes are an ecologically very diverse group, much used in assessing changing environmental conditions, they are not the most prepossessing creatures being small and externally smooth. Ostracode bodies and appendages are rarely found as fossils, but a German, Japanese, Czech, British and French team has set out to find soft parts using X-ray synchrotron tomography on a Brazilian ostracode of Cretaceous age (Matzke-Karasz, R et al. 2009. Sexual intercourse involving giant sperm in Cretaceous ostracode. Science, v. 324, p. 1535). A third of the ostracode’s body is devoted to reproduction, males having large Zenker organs or sperm pumps. This is unsurprising, when one is informed that the ostracode sperm are sometimes longer than an individual creature. Indeed, Matzke-Karasz et al. assign some significance to them; ‘persistence of reproduction with giant sperm through geological time may add a criterion to test for the pressure of sexual selection’…

Gas source for flood basalts

Although there are several coincidences between flood basalt eruptions from large igneous provinces and mass extinction, not all basalt flood events made an impact on the biosphere and not all mass extinctions link to a LIP. Where there is a connection, two mechanisms dominate discussion: dust and noxious gas such as SO₂, stratospheric aerosols from which can also induce global cooling, or global warming stemming from CO₂ emissions. The odd thing is that most flood eruptions in LIPs are of tholeiitic basalt magma, which is generally low in gas content. Of sizeable flood basalt provinces, the Ethiopian (30 Ma), Karoo (~180 Ma), Parana (130 Ma) and North Atlantic (55-60 Ma) had no truly significant impact on life. Those that certainly did were the Siberian Traps implicated in the end-Permian devastation, those of Emeishan in China at the time of35 % of all genera went extinct around 260 Ma, the Central Atlantic Province the main suspect for the end-Triassic extinctions and the Deccan Traps that coincided with the Chicxulub impact at the K-T boundary. Two of these massive tholeiitic magma events have been assessed in terms of how they might have emitted gases.

The Emeishan LIP emerged through crust that contains large volumes of carbonates of Proterozoic to Silurian age. Conceivably the magma might have released carbon dioxide by inducing thermal metamorphism (Ganino, C. & Arndt, N.T. 2009. Climate change caused by degassing of sediments during the emplacement of large igneous provinces. Geology, v. 37, p. 323-326). Clément Ganino and Nick Arndt of the University of Grenoble, France investigated a monstrous sill almost 2 km thick in the deeply eroded Emeishan province. It proved to have a 300 m contact aureole dominated by brucite (Mg(OH)₂) marble, evidence of melting of carbonates and calc-silicate marbles, production of which by metamorphism would have yielded huge amounts of CO₂. They go on to discuss other possibilities for gas generation by magmatism, involving thermal metamorphism of coals, oil shales and evaporites. The last is a distinct possibility in the case of the Siberian Traps (Li, C. et al. 2009. Magmatic anhydrite-sulfide assemblages in the plumbing system of the Siberian Traps. Geology, v. 37, p. 259-262). A large stratiform intrusion associated with the end-Permian flood basalts contains around 7% sulfides; truly huge for mafic magma and making it a major exploration target for platinum-group metals, yet unusual for a tholeiite. It also contains abundant anhydrite, calcium sulfate that is more usually found in sedimentary evaporites. The isotopic composition of sulfur in the intrusion is enriched in ³⁴S, suggesting that at least 50 % was derived from a sedimentary rather than a mantle source. The sedimentary sequence through which the Siberian flood basalt magmas passed contains evaporites around 5 km thick. That would be a suitable source for the sulfur in the intrusion, but would also yield stupendous amounts of SO₂ if carried to the surface by erupting magma. An example of a LIP that had little if any effect on the biosphere is that which mantled both side of the North Atlantic with flood basalts in the Palaeocene. The magma that was involved moved through almost entirely crystalline ancient continental crust. The same set-up characterised the Ethiopian, Parana and Karoo provinces.

Social behaviour among giant trilobites

There’s something about a trilobite that causes outbreaks of hyperbole: as far as I know they are the only class of animals to warrant an expletive in serious literature (Fortey, R. 2001. Trilobite! Flamingo). The title conjures a vision of a three-lobed, segmented alien hurtling for one’s nether regions, hideous malice in its compound eye. Well, most trilobites were little, albeit with anorak-rending diversity in form and habit: they ranged from burrowing bottom feeders to inhabitants of the ocean meniscus, rather like early water boatmen. If you want to use an exclamation mark for an invertebrate, then it might be better to reserve it for the fearsome Eurypterids or sea scorpions. At up to 2 m, with mighty pincers and capable of galloping across a beach, they certainly would have best been avoided in the Ordovician to Permian. Yet, from time to time big trilobites do turn up, such as Paradoxides, Ogyginus and Hunioides that break the metre barrier. Rather a lot of them have been found in a Portuguese lagerstätte of Middle Ordovician age (Gutiérrez-Marco, J.C. et al. 2009. Giant trilobites and trilobite clusters from the Ordovician of Portugal. Geology, v. 37, p. 443-446). They were up to something, as the locality described by Gutiérrez-Marco et al. contains huge numbers that were apparently having been overwhelmed by a sudden turbidity flow once they had gathered together. Some of them are in single file… It could be some sexual frenzy; fearfulness when moulting synchronously or something at which we cannot even guess. Whatever, it seems likely that the gigantism in the deposit is something to do with these being high-latitude animals.

The Cambrian Explosion of shell-bearing animals and the preceding, diverse and very odd Ediacaran fauna that left imprints and moulds in the Late Neoproterozoic both posed two puzzles for early palaeontologists. What organisms evolved so that unmistakable traces of animal life were able to leave fossils after about 600 Ma, and what pace did evolution take to present us with virtually all the animal phyla, including some not around nowadays, ‘fully separated’? Molecular genetic studies of living animals are beginning to throw up some answers (Holmes, R. 2009. The mother of us all. New Scientist, v. 202 (2 May Issue), p. 38-41). It is a complex and growing field, so Bob Holmes’ review of current ideas on the last common ancestor of the animals is welcome for non-specialists. It does look as though the radiation was long before the Ediacaran, but may well have been very rapid. The genetically closest single-celled organism to metazoan animals are the rare choanoflagellates; filter feeders with a collar-like structure and a tail. They bear some resemblance to the feeding cells of sponges, but sponges in their current form seem highly unlikely as the Ur-creature, totally lacking any organs and really just a coexistence of clone-like cells. Gene sequencing from 42 animal groups puts sponges at the bottom of a relatedness tree, yet at the bottom of two of the main branches. So the sponges do indeed seem to have it as our ultimate ancestors. Yet the flurry of ever-more detailed sequencing, for more and more groups using increasingly sophisticated statistical analysis has fired up controversy. Jellyfish-like ctenophores now have a look-in too, as do mysterious placozoans, according to one or other researcher. This field is throwing up an object lesson for hubristic scientists used to counting their chickens… No, the votes are never all in, and surprises always lie ahead for both the unwary and the patient. 

Luckily, Holmes closes by looking at a careful proposal for the ‘How’. Claus Nielson of the University of Copenhagen, a major ‘player’ in this field, has suggested how starting with a slab-like choanoflagellate, with all its function cells on the outside, might have evolved be curling to enclose a tube of inward facing cells; a precursor of a gut. One next step from there could be specialisation of some cells as nerves, then the development of a ‘mouth’ and ‘anus’ – the basis for the bilateral symmetry of all higher animals including ourselves. As for the ‘When’, there are sufficient leads from a molecular clock approach to settle on the oddest climatic events of the last 1.5 Ga of the Proterozoic, the near global glaciations or ‘Snowball Earth’ events that began around 750 Ma ago.

Photosynthesis from way back when: the hunt for RuBisCO

Charles Darwin had an abiding fascination with plants, though one that was essentially practical through observation and breeding. That is sufficient excuse in his bicentenary for reviews, but a good way to honour his legacy is again to push essays to the leading edge of present understanding (Leslie, M. 2009. On the origin of photosynthesis. Science, v. 323, p. 1286-1287). Being able to convert sunlight, water and carbon dioxide to the basis of their own life and that of the rest of the planet, plants and other photosynthesising organisms are the fundamental essence of the living world. Land plants are recent developments, emerging in the Silurian around 425 Ma ago with presumed terrestrial spores some 50 Ma earlier. Their forbears were almost certainly marine algae. Yet they are highly evolved, and it is not to separate precursors that palaeobotanists can look  for origins, but to the internal chloroplasts that look remarkable like cells in their own right with separate DNA and RNA. They perform the astonishing trick of breaking the extremely strong OH-H bonds that form the water molecule otherwise achieved either by extremely high temperatures or by electrolysis. The trick is for an organism to grab an electron thereby releasing the bond and both hydrogen and oxygen. The hydrogen links to carbon and oxygen from CO₂, and the other oxygen is freed. Similar to a magician’s trick with smoke and mirrors, photosynthesis uses pigments. Colour in any object or material results from photons of one wavelength range in sunlight being absorbed so that those reflected make up the colour. The most familiar is chlorophyll which absorbs two wavelength ranges: the red and the blue regions to leave green to be reflected for us to see. It is actually a bit of quantum mechanics, as the absorbed photons carry the energy needed to stoke up that of electrons so that they can break free of the OH-H bond in water and split the molecule. The chain of organic chemistry which follows this trick is hugely complex, and it seems to have taken several forms reflected in specific genes in a growing array of photosynthesising bacteria of various genetic antiquities. There are green ones, blue ones, the reds, yellows and oranges.

Luckily the chemical remnants of photosynthesising bacteria are pretty robust, and also distinctive. The central one for most photosynthesising organisms is an enzyme that is complicated, called Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO for short. Euan Nisbet of Royal Holloway, University of London has been hunting RuBisCO for most of the latter part of his career as a Precambrian geologist. he and colleagues found relics of it in 2.7 Ga Archaean sediments from Zimbabwe and Canada (Nisbet, E.G. et al. 2007. The age of Rubisco: the evolution of oxygenic photosynthesis. Geobiology, v. 5, p. 311-335) and claim there are signs far older. Needless to say.

A fluffy grazing dinosaur

The Cretaceous of NE China is becoming a favoured destination for palaeobiologists interested in well-preserved vertebrates, little dinosaurs, especially. An increasing number turned up by fossil hunters have skin relics covered in feathers, although they are rarely if at all equipped for flight, are. Recently, something even more bizarre was unearthed (Zheng, X.-T. et al. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature, v. 458, p. 333-336). In plain-speak, Tianyulong confuciusi was fluffy. And as readers really ought to know, the heterodontosaurs were largely Jurassic herbivorous creatures, 70 Ma older than T. confuciusi; a good example of a ‘living fossil’ in its own time. They evolved to large Cretaceous herbivores, such as the famous duck-billed hadrosaurs, Triceratops and Stegosaurus, members of the Ornithischia as opposed to the more commonly carnivorous Saurischia. It was the latter that were widely believed to have been evolutionary branch from which birds sprang. There is a complex argument surrounding T. Confucius, based on which is a proposal that the ancestral dinosaurs were themselves fluffy. First, thoughts of brightly coloured ‘monsters’ and now the possibility that some may even have looked cuddly.

See also: Witmer, L.M. 2009. Fuzzy origins for feathers. Nature, v. 458, p. 293-295.