Evidence for Hadean Eon Life?
(Originally posted March 02, 2017 on Blogger)
In my last blog we briefly discussed the possible biosignatures of life on exoplanets. In this blog we'll discuss the possible biosignatures of the earliest existing life here on Earth.
Yesterday a paper was published in the journal, Nature, that provides evidence for what may be the earliest life on Earth. The paper by Dodd et al., details the analysis of reputed fossilized microorganisms found in the Nuvvuagittuq Greenstone Belt (NGB) in Quebec.
The NGB is an extensive sequence of metamorphosed silicate-rich rock, that contains some of the oldest-known rocks on Earth. It's a remnant of primeval silica-rich ocean crust that has managed to survive subduction to this day.
In very simplistic and general terms, we can think of Earth's surface as being covered by two types of igneous rocks; basalt, and granite. Basalt is denser than granite (has a higher specific gravity), and we can imagine it covering the entire planet's surface. Granite 'floats' on top of that layer of basalt. Those massive 'pieces' of granite floating around on all that basalt are the continents. The parts not covered are the exposed basalt ocean floors.
Because of movement within Earth's mantle*, the basalt surface moves around in all sorts of directions relative to itself; it could be split apart causing new basalt material to come to the surface (divergent boundary), it can slide alongside itself in different directions (transform boundary), or it can converge into itself with the older, heavier basalt being overrun and pushed back into the mantle for re-assimilation (convergent boundary). All this moving around means the 'floating' chunks of granite (continents) are along for the ride.
*I should clarify this state (about movement within Earth's mantle). The mantle is a rheostatic solid, which is to say in simplest terms, it deforms (moves) to slow stress, and acts as a solid (rigid) to fast stress. This is how the mantle creeps (diffusion creep) under slow, steady stress over geologic timescales, yet can act as a solid when a fast moving stress like the S-waves from earthquakes pass through. Silly putty acts much this way. If you pull it apart slowly, it will stretch. But if you try to pull it apart quickly it will snap apart.
Whereas basalt can be reassimilated at convergent boundaries back into the upper mantle from whence it originally came due to its relatively-higher density, granite (continents) with its lower density cannot. They just keep on floating around, colliding with other chunks of granite on opposing basalt trajectories creating such features as the Andes, or Himalayas. In other words, they survive subduction whereas the basaltic ocean floors do not.
As such, it isn't easy to find geologically-archaic mafic ocean crusts; yet the NGB is just that! Some of its rocks have been dated back to at least 3.774 billion years, even as far back as ~4.3 billion years by some dating techniques analyzing gabbro; basalt's course-grained subsurface cousin. In fact, some of the oldest, if not the oldest, iron formations on Earth are found here.
Dodd et al., have conservatively dated their finds to 3.77 billion to as old as 4.28 billion years. If this latter age is confirmed, that would translate to Hadean eon rocks, which therefore could mean their rock samples crystallized out of Hadean crustal melt. Caveat: these dates have yet to be confirmed. As of the writing of this blog, the oldest confirmed microfossils date back 3.5 billion years. Mind-boggling time scales to put it lightly.
With regard to age, Dodd's paper cites another paper (O'Neil et al. 2008) that provides evidence for Hadean crust melt being a possible source of NGB parent rock. The dating technique described in that paper analyzes the stable isotope, Neodymium-142 which itself is formed by the alpha decay of Samarium-146.
Samarium-Neodymium (Sm-Nd) dating is unique for dating extremely old rock in that these two elements are rare earth elements. This isn't to say they're actually rare, they aren't (only rare earth, promethium is). But as rare earth elements, they tend to occur together in nature and resist partitioning during metamorphism and melting of silicate-rich rocks (mafic and ultramafic rocks).
This resistant characteristic of Sm and Nd makes it possible for these rare-earths to maintain their ratios despite melting, or intensive heating and pressure situations such as what metamorphic rocks have undergone. The concentration of Sm-Nd in silicate minerals increase in proportion to temperature as they crystallize out from a melt. This is best understood under the framework of Bowen's Reaction Series, which is a temperature-related, two-branch crystallization sequence illustrating the formation of rocks of different compositions. As a melt cools, minerals form; some at higher temperatures, others at relatively lower temperatures as the melt becomes cooler and cooler.
Whereas analyzing minerals in igneous rocks is good for dating materials, it isn't so good for finding evidence of early life. The whole melting thing sort of undermines the preservation of fossils, as does the whole heat and pressure thing experienced by metamorphic rocks. Though chances are obviously much higher of finding evidence for early life in sedimentary rocks, this isn't to say it isn't fraught with its own set of difficulties.
Natural contamination and mineral alterations governed by the laws of thermodynamics, can alter their chemical composition (and crystallography) such that they become similar to biologically-produced minerals, but are in fact not. In the paper, Dodd gives two examples where discoveries initially thought to be biologically derived, may not be at all.
The first example he gives is that of Schists that contained a percentage of graphitic carbon that has decayed to isotopic carbon-13. This could be a biosignature process, or it could be non-biological in origin. Another example is that of graphite-coated apatite in iron formations; derived from slightly metamorphosed biological matter, or abiotic fluid deposits? The science continues...
These conundrums make Dodd's and others' work extremely tedious and difficult, yet fascinating,
So with all that said, let's proceed cautiously, yet hopefully that the work of Dodd et al. is ultimately confirmed, because if it is, that would have incredible implications for how life got started, as well as paving the way back to how it evolved. These realizations, of course, would extend to any planet here or in extra-solar systems throughout our galaxy.
I want to add one important point glossed over by many of the sources reporting on his and his colleagues' findings that I think is really quite important. Even if their work is confirmed not to be biologically-derived microfossils, it will nonetheless be of utmost value, and here is why...
Their work would still likely demonstrate new and/or improved ways to distinguish biologically-derived evidence from that which only appears to be biologically derived. An important science in and of itself, and one that will certainly need to be employed on future life-seeking missions to Mars.
We should all be aware of this, because some sources with a wide audience reach might take early, yet-to-be-fully-analyzed discoveries from a future life-searching mission, and immediately post headlines that might read something like, "Evidence of Life Discovered on Mars", or "Proof that Mars once Harbored Life", only for that evidence to be confirmed months or years later as having been derived from non-biological sources. By that point, I foresee convincing the masses otherwise might be as difficult as changing the mind of an ideologue... or a doorknob for that matter. :/
Furthermore, like with some of the other disciplines in science, this sort of public misunderstanding can and has lead to unjust and unfounded accusations of conspiracy, hoax, well-orchestrated political agenda, or even claims that scientists cannot be trusted. None of which have anything to do with the nature of science, or how it's employed in fields from astrophysics to zoology. There is power in peer review!
Dodd picked his site carefully, finding iron formation units in a geologic setting conducive to seafloor hydrothermal activity, itself evidenced with having "seawater-like chemical signatures". Though most of the NSB underwent metamorphism 2.7 billion years ago, Dodd's site was less affected and therefore better preserved, as evidenced by the presence of chalcopyrite crystals within jasper and carbonate iron formations.
The presence of chalopyrite demonstrates that post-deformation oxidation has not occurred, because chalcopyrite oxidizes when exposed to air. Had it done so, instead of chalcopyrite, Dodd would have found a variety of oxides, hydroxides and sulfates... but not chalcopyrite.
Finding a well-preserved site that once played host to seafloor hydrothermal vents is exciting, because as we can see today, hydrothermal vents and their environs provide a thriving environment for microorganisms.
We know that the vents' cylindrical iron oxyhydroxide morphology are products of iron-oxidizing bacteria (life!), and it might be inferred that similar structures in NSB rocks were formed similarly as well. Similarly as in biologically.
Dodd found such structures. Well, not full-sized vents like what we see above, but microscopic ones (no bigger than 500 micrometers... the length of the width of like 5 thick human hairs side-by-side). Being so small, Dodd et al. used optical microscopy, scanning electron microscopy, and confocal Raman spectroscopy to look into this miniature world in great detail.
But just like with Pluto, size is not proportional to importance! So don't fret my friends, these tiny filaments are not insignificant. Quite the contrary. They're in fact similar to microbial filaments in modern-day hydrothermal vents. Evidence of which is also found in preserved Phanerozoic filaments.
Dodd believes these filaments may have been able to survive the rigors of geologic-scale heat and pressure because microcrystalline quartz reinforcement, further strengthened by tiny grains of hematite which form the inner walls of the filaments (and tubes). Like reinforcing something with iron.
Dodd points out that this hematite structure within the filaments is similar to iron-oxidizing bacterial attributes found in other hydrothermal jaspers. Even their microstructures, such as terminating in tiny knobs, uniform tube diameters, and even tube alignment relative to other tubes, are all reminiscent of other, more geologically recent, seafloor-hydrothermal jaspers that have been proven to be biogenic in origin. Not to mention these filaments are found among other carbon-based material suggestive of biologic origins.
To determine the authenticity of his finds, Dodd employed the null hypothesis, where he and his colleagues looked for all possible abiogenic (not produced by life) processes that could have created such filaments and tubes. After extensive experimentation and research, they were unable to find any.
Not a single abiogenic process could explain these formations, not even a combination of abiogenic processes could do this. This obviously bodes well for the likelihood these formations were formed by Earth's earliest forms of life!
Time will tell... and on a human time scale.
As always, thanks for reading.