Lots of oxygen on the Archean Earth?

ResearchBlogging.orgMost geological evidence indicates that significant amounts of oxygen only began to accumulate in the Earth’s atmosphere and oceans during a ‘Great Oxygenation Event’ at the beginning of the Proterozoic, between 2.3 and 2.4 billion years ago. However, distinctive organic biomarkers found in 2.7 billion year-old sediments in northwest Australia [1] indicate that the ultimate source of all that oxygen, photosynthetic cyanobacteria, first emerged at least 300 million years before the Great Oxygenation Event; and if oxygen producers apparently laid low for that long without apparently making much of a mark on the Earth’s atmosphere, it’s possible that they could have emerged even earlier in the history of our planet – which is exactly what evidence published a few months ago Nature Geoscience might suggest. The authors claim that minerals in Archean chemical sediments from the same part of northwest Australia precipitated from oxygenated seawater; if they’re right, this would potentially push the emergence of photosynthesis back another 700 million years, to almost 3.5 billion years ago.
Masamichi Hoashi and his colleagues have closely examined iron minerals in the Marble Bar Chert, from the Warrawoona Group of the Pilbara Craton. The stratigraphic column below (adapted from [2]) shows that this sequence has yielded a number of other controversial claims about early life, including the oldest known “stromatolites“, which may or may not have been inorganically precipitated (see [3] and subfigure b below), and, even more contentiously, putative bacterial microfossils (subfigure a – see [4] and [5] for pro- and anti-bacterial views, respectively).

Pilbarastrat.png
Warrawoona Group, Pilbara Craton, NW Australia

The silica that makes up the bulk of the Marble Bar Chert is peppered with iron minerals.

Marble Bar Chert in outcrop
The upper part of the Marble Bar Chert; the red stripes are rich in the iron oxide haematite


Iron mineral precipitation is a common feature of Archean sediments, with banded iron formations being the most impressive examples. Because Archean oceans generally appear to have contained little or no dissolved oxygen, Archean seawater contained large amounts of soluble reduced iron (Fe2+) produced by hydrothermal vents. Under the right geochemical conditions, possibly with the help of bacteria, the iron was eventually converted to insoluble forms that settled out over extensive tracts of the ocean floor. The particular iron mineral that precipitated is potentially a valuable source of information about the amount of oxygen in Archean oceans. In the lower part of the Marble Bar Chert, the dominant iron mineral is siderite, iron carbonate (FeCO3), which forms in the absence of oxygen and is therefore probably the result of iron-rich hydrothermal fluids reacting with anoxic and acidic Archean seawater.

anoxicBIF.png

In the upper part of the chert, however, the most common iron mineral is haematite (Fe2O3), which is the source of the deep red colouration in the picture above, and can only form in oxygen-rich environments. At first glance, then, this change in the type of mineralisation might indicate increased amounts of free oxygen during deposition of the Marble Bar Chert. However, there is an important caveat when studying iron minerals: the very sensitivity to changes in redox conditions that make them such a valuable tool in interpreting past environments also makes them prone to chemical alteration following deposition. Most of the haematite found in iron formations (and there is often a lot of it) appears to have formed some time after deposition, by recrystallisation of the originally precipitated iron minerals (usually iron oxyhydroxides, that can precipitate under either oxic or anoxic conditions). It is therefore a testament to the prevailing geochemical conditions during these later diagenetic changes, and tells us nothing about conditions when these formations were originally deposited.
In the case of the Marble Bar Chert, however, Hoashi et al. argue that there is strong evidence that the haematite within it did directly precipitate from Archean seawater. Firstly, the samples used in this study were as pristine as could possibly be hoped for, having been retrieved from a borehole 200 m below the current surface, far below where modern weathering has penetrated. Secondly, the haematite – concentrated in microscopic bedding parallel bands – is in the form of small single crystal grains with distinct boundaries, rather than the larger aggregations of fused crystals that usually result from the alteration of other iron minerals during diagenesis. Thirdly, the presence of unaltered grains of other reactive iron minerals, like pyrite and siderite, in the thin sections examined demonstrate that no later oxidation has occurred; if it had, these minerals would have been altered as well. In the case of the siderite, it often seems to be growing around the haematite, which is strong evidence that the haematite is indeed one of the earlier forming mineral phases.

Marble Bar Chert under the microscope
The Marble Bar Chert under the microscope

If the haematite did precipitate directly from Archean seawater, it could only have done so in the presence of oxygen, presumably produced by some early bacterial photosynthetic pioneers. Whilst this does potentially tie neatly – and rather excitingly – into the other rather controversial signs of early life found within this section, the potential implications of these results actually stretch further – much further. This is because there are no features consistent with reworking by waves or ocean currents in these rocks, implying they were deposited in a deep water environment – a long way from any site of putative photosynthetic oxygen production. Somehow oxygenated water must have penetrated into the deep oceans and reacted with hydrothermal fluids to produce the observed haematite – which implies that rather a lot of it was being produced, and was at large in the Archean oceans.

oxicBIF.png

This paper is actually the latest salvo in a broader, and ongoing, scientific debate between geologists who think that they evidence points to an oxygen-free early Earth prior to an early Proterozoic Great Oxygenation Event and those who argue that, to the contrary, there were geochemically significant amounts of oxygen present in the atmosphere and oceans even at the very beginning of the Earth’s history. It might seem odd that both conclusions can be legitimately argued from the available evidence, given the rather profound geochemical effects that the presence of free oxygen has on the chemical processes occurring in the atmosphere and oceans. However, in the unfortunate absence of direct samples of Archean air or seawater, scientists are forced to rely on geochemical fingerprints like sulphur isotopes, and the disagreement is actually over which of these different proxies is providing the most reliable picture – the one least obscured by unrelated and unaccounted for chemical effects, or later alteration.
The other thing to bear in mind is that there is no one oxidation state for a system as complex as the entire Earth. There are many stable low-oxygen environments to be found on, or close to, the surface of todays ‘oxygenated’ planet, so it follows that the converse could be the case back in the Archean: a mostly ‘anoxic’ earth, as suggested by things like the presence of detrital pyrite in many ancient sediments, with some isolated more oxygen-rich environments. For example, if the haematite in the Marble Bar Chert was indeed one of the initially precipitated minerals, one possibility is that the oxygen-rich environment that it formed in was a restricted rift basin, cut off from, and therefore not representative of, the rest of the Archean oceans. Nonetheless, even if that is the case, it is strong evidence that photosynthetic cyanobacteria may already have established themselves back in the earliest Archean 3.5 billion years ago – an important fact to consider when you’re trying to understand how, and under what conditions, photosynthesis might have emerged in the first place. But it does raise another important – and as yet unanswered question: why did it take 1.2 billion years before the atmosphere responded to their presence?
MBCtimescale.png
Hoashi, M., Bevacqua, D., Otake, T., Watanabe, Y., Hickman, A., Utsunomiya, S., & Ohmoto, H. (2009). Primary haematite formation in an oxygenated sea 3.46 billion years ago Nature Geoscience DOI: 10.1038/NGEO465
Other references
[1] Brocks, J. (1999). Archean Molecular Fossils and the Early Rise of Eukaryotes Science, 285 (5430), 1033-1036 DOI: 10.1126/science.285.5430.1033
[2] Marshall, C., Love, G., Snape, C., Hill, A., Allwood, A., Walter, M., Van Kranendonk, M., Bowden, S., Sylva, S., & Summons, R. (2007). Structural characterization of kerogen in 3.4Ga Archaean cherts from the Pilbara Craton, Western Australia Precambrian Research, 155 (1-2), 1-23 DOI: 10.1016/j.precamres.2006.12.014
[3] Allwood, A., Walter, M., Kamber, B., Marshall, C., & Burch, I. (2006). Stromatolite reef from the Early Archaean era of Australia Nature, 441 (7094), 714-718 DOI: 10.1038/nature04764
[4] Schopf, J., Kudryavtsev, A., Agresti, D., Wdowiak, T., & Czaja, A. (2002). Laser‚Äö?Ñ?¨Raman imagery of Earth’s earliest fossils Nature, 416 (6876), 73-76 DOI: 10.1038/416073a
[5] BRASIER, M., GREEN, O., LINDSAY, J., MCLOUGHLIN, N., STEELE, A., & STOAKES, C. (2005). Critical testing of Earth’s oldest putative fossil assemblage from the ‚Äö?†¬?3.5Ga Apex chert, Chinaman Creek, Western Australia Precambrian Research, 140 (1-2), 55-102 DOI: 10.1016/j.precamres.2005.06.008

Categories: Archean, geochemistry, geology, paper reviews, past worlds, rocks & minerals
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Comments (20)

  1. D. C. Sessions says:

    Thanks, Chris — oxygenation is a fascinating topic (says /me, who managed to not study geology when it would have been easy.)
    What I still don’t get is where the oxygen came from in the first place. Barring hydrogen loss to space, whatever it was bound to is still here and I (in my admitted ignorance) am not aware of any masses of reducing materials (barring nitrogen, which doesn’t add up as far as my rusty chemistry can see) on the surface of the Earth that would have been left by all that freed-up oxygen.

  2. The idea that archaic earth had isolated pockets of oxygenation, akin to anoxic pockets in today’s earth, makes a lot of sense to me.
    @DC
    I’m not a geologist, but since most of the free oxygen on earth is created by photosynthesis, I would guess that the hydrogen and carbon that were once bound to that oxygen, are now bound to each other in the form of organic compounds.

  3. DrA says:

    I’m not a geochemist either, but the long lag in atmospheric oxygen accumulation has been attributed to the precipitation of dissolved iron in the oceans resulting in the banded iron formations. So until the very reactive iron is removed from the oceans, free oxygen wouldn’t accumulate in the atmosphere, and then of course, levels would be way below today’s concentration.

  4. D. C. Sessions says:

    What bothers me is that (carbon dioxide being in the news lately) there doesn’t seem to be remotely enough carbon in the biosphere, coal, and oil deposits to account for all of the Earth’s free oxygen — much less the oxygen tied up in banded iron deposits.
    That’s why I posited hydrogen loss — but that’s a slow, gradual process.
    Baffled.

  5. Robert says:

    The Earth was largely molten and it was being bombarded constantly by asteroids and comets during. On the other hand, the Archean lasted for over a billion years and who can say life didn’t happen during all that time.
    In my opinion, life during the Archean would suggest it was seeded from somewhere else since the conditions were perhaps too harsh for spontaneous development.

  6. Chris Rowan says:

    DC: The ultimate source of the oxygen is carbon dioxide from volcanoes [water, actually - thanks to everyone who corrected me]. This is then split into oxygen and reduced organic carbon by photosynthesis, and if the carbon is then kept separated by burial, and the oxygen left behind is produced faster than it can be consumed by other oxidation processes (especially reacting with iron and sulphur) it will build up.
    Regarding the amount of carbon, this piece nicely sums it up – organic carbon is incorporated into all marine sediments as dead algae etc. settle out of the water column, and whilst the concentrations are low compared to something like coal, when you integrate over the volume of all that sedimentary rock you get a very large number indeed. To quote:
    “Organic matter in shales is the dominant reduced carbon reservoir. The earth’s crust contains 1.1 x 10^21 moles of reduced carbon…The total amount of organic carbon needed to account for all the oxygen in the atmosphere is only 0.038 x 10^21 moles.
    Of course, these marine sediments are then mainly subducted back into the mantle and thus really buried.
    As for the iron formations, as I understand it, what they’re actually telling us is not quite as simple as them being a huge oxygen sink – not least because, as I mentioned in the post, their present mineralogy is largely the result of later alteration. It’s one of those subjects where you’re perfectly happy until you start reading the literature, and then you develop a headache.
    Robert: the Late Heavy Bombardment ended about 3.8 billion years ago; these rocks formed 300 million years later. That’s a lot of time to play with – enough in recent geological history for life to go through 2 major mass extinctions and subsequent evolutionary radiations. ‘Spontaneous’ should used lightly, methinks.

  7. amphiox says:

    #6: I thought the atmospheric oxygen came from water, and oxygen in CO2 stays bound with the carbon after reduction with the hydrogen atoms stripped from water.
    DC: If all the earth’s reserves of fossil fuels (ie sequestered organic carbon) were burned, including all the inaccessible ones existing as individual molecules scattered through stones like shale, etc, and the ones buried deep in the crust and mantle, you would, in fact, consume all the oxygen in the atmosphere. The accessible organic carbon in the biosphere and the usable fossil fuel reserves make up just a tiny fraction of the total organic carbon on earth.

  8. Robert says:

    Do you disagree that early life implies a higher probability for some kind of seeding?
    I can imagine life in the Archean if there were a constant supply of life forms arriving from outside the Earth and waiting for a rare opportunity to appear in an otherwise very hostile environment. Or, if not life itself, then at least major building blocks already assembled.
    A few hundred billion years is a long time, but were the conditions ever stable enough for life? Don’t you need stable pH and stable salt levels and things like that? How do you propose stability with molten crust and continuous changes on scale that we can’t even imagine?

  9. Greg Laden says:

    That’s some nice chert, by the way. (Sorry, palaeoanthropologists tend to like to look at chert.)

  10. Robert says:

    Oh, rats. I meant to say a few hundred million years is a long time.

  11. Lab Lemming says:

    It is very sad that there are no Australian academics on this paper. Funding for basic research has been declining for a while, but the fact that we are getting geocolonized drives that home.

  12. AD says:

    isn’t the oxygen source water, as well as CO2??? as in water is oxidized to molecular oxygen in photosynthesis?
    Great post- keep it up- more about Archaean biogeochemistry please!

  13. amphiox says:

    #8: Since we don’t know yet what processes can give rise to life, what conditions they require, or how robust they might be regarding extreme fluctuations we cannot cogently say anything about the probability of life arising early in earth’s history versus arriving from elsewhere. Evidence for early life on earth cannot be used to say anything at all regarding panspermia versus terretrial abiogenesis until we actual learn something about this.
    For example, at the most extreme, there is nothing that says that a self-replicating system cannot arise in an environment of molten rock. It couldn’t be carbon/water based of course, but there’s nothing that says the first replicators had to be carbon/water based. A carbon/water system could have been evolved later as an adaptation to a cooling earth, and then displaced the original replicating system.
    Secondly, the early earth was not monolithic. Scattered puddles of water could easily have formed very early on in sheltered microenvironments even if the majority of the planet remained molten. Whatever the microenvironments necessary for abiogenesis (and nothing says that it had to be just one type, either), even if they were very rare early on, it only takes one.
    Thirdly, it hasn’t been conclusively demonstrated that the late heavy bombardment would have been a guaranteed showstopper for life. Even in the scenario of a complete evaporation of the oceans after an impact, some subterraneon microenvironments are likely to remain habitable. And all it takes is for one small microenvironment to escape obliteration to act as a reservoir for repopulating the planet.
    Finally, we can turn the question around and ask what is the likelihood for abiogenesis and/or the synthesis of complex organic molecules in the environment of space, which was in its own way just as hostile.

  14. Robert says:

    I don’t know if they were synthesized in space. Perhaps they were sent out by an intelligent species to a solar system that was in its early stages of forming. I’m not saying that happened, only that there are so many possibilities that are beyond our ability to imagine. We know that some Mars rocks ended up on Earth. Given enough time, and enough collisions between bodies, pieces of stuff can get sent everywhere. A whole planet could get flung out of one solar system and crash into another.

  15. Chris Rowan says:

    Yes, you’re all right: the oxygen molecules have been liberated from water in photosynthesis, not carbon dioxide. My A-level biology teacher would be ashamed of me. The important point is that the products of photosynthesis get separated so that they can’t react back together again, by the organic carbon produced getting buried.
    As for the ‘seeding’ discussion: this research has no bearing on it whatsoever; go and bother the people discussing possible geochemical tracers of life in 3.8 billion year-old rocks in Greenland. Indeed, it’s one of those subjects where you can speculate all you like, but hard evidence is rather hard to come by. Come back to me when you find DNA-based organisms on another planet.

  16. Andrew Dodds says:

    Robert -
    As far as Abiogenesis goes, the process must be relatively rapid. None of the the postulated environments for abiogenesis are going to remain stable for over >100k years (usually much less), so either or prebiotic chemistry is going to spiral in complexity into something living in that time, or it will be dispersed and fizzle out. The complex molecules you have to get ‘half way there’ are not stable enough to hang around for millions of years waiting for a next step.
    As any creationist will tell you, a random process (i.e an ocean of ammino acids and nucleotides) as effectively zero chance of ever having life emerge. They probably won’t continue to go on to say that you need chemical energy gradients, Fe-S reaction centers, abiotic nucelotide synthesis, etc, etc.. which exist in marine hydrothermal systems.. and possibly other situations.

  17. Brad says:

    How much oxygen do you need to account for the hematite and other minerals? Mars’ atmosphere is 0.13% oxygen now; I don’t think anyone is claiming that is due to photosynthesis. And lots of oxygen on Mars has been tied up as… um… iron oxides, that’s why it is the ‘red planet’. I am guessing that this oxygen is due to photolysis of water vapor in the atmosphere, something that would be much more prevalent on Earth than Mars – warmer, more liquid water to evaporate, closer to the Sun.

  18. Noumenon says:

    That timeline at the bottom of your posts is really helpful and I’m going to encourage other science bloggers to take it up. I know so little about the earth’s history because of my religious education that the era names alone mean little to me.

  19. DD says:

    Green plants can’t uptake carbon from soil, because they use water to move nutrients, water + carbon = carbonic acid (eats limestone -> caves). Soil fungi do decompose carbs, as do animals, and exude CO2. Plant leaves “breathe” O2 just like animals and fungi. Plant leaves “eat” CO2, using solar energy to decompose it into O2 & C, the C is then combined with H et al to make carbs for plant tissue. Plants (grasses) require wind to reduce stratification of gases (gas stratification starves them of C or chokes them of O2), tropical rainforests need mobile animals not only for fruit dissemination but also gas mixing (flying insects/fruit bats/fruit birds), this is the major advantage of angiosperm flora over non-fruiting gymnosperms (seeded but non-fruiting conifers) and the reason that flowering plants and symbiotic fauna “won” the competitive war against cycads, conifers, ferns, etc. Biological textbooks say little about gas stratification prevention, but it is critical in closed-canopy ecosystems (same reason that sealife comes in both mobile jetsam (tail/fin) & immobile photosynthetic flotsam).
    Regarding Archean earth: Where was the oxygen earlier? Locked up in superthick glacial ice, as earth’s hydrospheric crust, during a cool cosmic period?

  20. AD says:

    Gas stratification is not really a problem in forests, even molecular diffusion is sufficient to mix nearsurface air with canopy air, and plus you have small air currents due to daily heating and due to rainfall, mixing even the most constant environment. Anyway, the air around you is in constant flux and the air you breathe today may not be the airmass you breathe tomorrow- that’s how it can be 70 degrees one day and 50 the next.
    In answer to your question, the oxygen was locked up in water H2O and CO2. Photosynthesis is the reduction of CO2 by H2O (+solar energy), producing carbohydrate CH2O and oxygen O2.