Monthly Archives: March 2017

Ediacara; Welcome to the revolutionary world of animals

Mistaken Point on the Atlantic coast of Newfoundland (Canada) acquired its unfortunate reputation by fooling mariners.  In a celebration of a different kind, UNESCO, in July 2016 designated the Mistaken Point coast a World Heritage Site; it is the graveyard of exquisitely preserved animals known as the Ediacaran Fauna, and at 575 million years they are the oldest known, structurally complex, multicellular creatures.

From an evolutionary context, life forms during the previous 3 billion years were dominated by much simpler algal-bacteria like organisms that constructed mats, mounds and columns (stromatolites) and even reef-like structures, all made by single-cell prokaryotes.  The Ediacaran fauna thus represents a kind of evolutionary paradigm shift – to real animals.  As Guy Narbonne (Queens University, Ontario) has suggested, this unique fauna probably formed the “root stock” of the more recent and familiar animal kingdom, but also includes some fossils that represent failed evolutionary experiments – creatures having unique form, phylum, and genetic codes that simply didn’t go anywhere.

The complete 2016 Mistaken Point UNESCO Heritage Site dossier by Richard Thomas and Guy Narbonne can be found here, but NB, it is a large file!

What kind of animals were they?

Although discovered in Namibia, the age and evolutionary significance of the fauna were first recognised in Flinders Range strata, Australia. The name Ediacara is probably Aboriginal.  Ediacaran fossils range from 575-542 million years; the period immediately prior to what is commonly called the Cambrian Explosion. Ediacaran fossils are now found on all continents except Antarctica.

The iconic Ediacaran fossils are those that appear petal-, feather-, or sea-pen-like, creatures that in some beautifully preserved examples exhibit complex growth patterns. Guy Narbonne has described these growth patterns as “quilted fractals”, an analogy that is quite apt. They were soft-bodied animals; fossils with hard parts, shells or hard skeletal frames did not appear until the very end of the Precambrian, becoming abundant in the Cambrian.  The petal-like structures had a central stem that was attached to or grew into the sea floor; in some cases only these holdfasts are preserved. Other forms that appear frond-like grew to almost 2m in length. Some were fan-, bush-, and comb-shaped; others simple domes or discs. Imagine the ancient seafloor covered in a forest of these soft, delicate forms, swaying in the wash of gentle sea currents.  It must have been quite stunning.

Trace fossils are also present, becoming abundant in rocks younger than about 555 million years.  These are not static impressions of animals, but tracks and burrows of worm-like creatures that moved on or through soft sediment.  Many traces resemble those made by animals in much younger strata, and if the same interpretation is applied to the Ediacaran types, then they too represent animal behaviours such as feeding, or burrowing a new home.

 

Preservation – an interesting conundrum

Paleontologists frequently consider the preservation potential of the fauna and flora they study.  Animals having hard parts are more likely to be preserved than those without.  However, even skeletal remains may not survive the vagaries of scavenging or dislocation.  Complete dinosaur skeletons, although celebrated, are rare; after death the animal is prone to being eaten, crunched by powerful jaws, or dismembered by flooding rivers. Preservation of soft-bodied animals is even more fraught – they tend to decay rapidly, are eaten by scavengers, or are dismembered by ocean currents and waves.

Most Ediacaran fossils were preserved as impressions in sediment. The uniqueness of the Ediacaran fossil record is a testimony to the absence of scavengers during this geological period.  Many, like the Mistaken Point communities (and also in Mackenzie Mountains) lived in relatively deep water where currents were subdued but strong enough to ensure a continuous supply of nutrients.  That the fossils are intact means that they were buried by sediment before decay set in.

Those animal communities that lived in shallower seas (there are examples in Australia, Namibia and Russia) were periodically subjected to stronger currents and waves and had correspondingly lower preservation potential.  The buried parts of stems and fronds, and some animal burrows could be preserved (after all they were already buried), but the more delicate structures above the sea floor were easily broken up.   In some environments, such as those now found in the Flinders Range, the dead fronds or bushes were covered by a thin microbial mat that enhanced preservation.  Elsewhere (Newfoundland and England), volcanic ash falling into the sea filtered quickly through the water column, gently smothering the live animals – a bit like Pompeii.

In the grand scheme of things It is generally understood that complex, multicellular animals like the Ediacara fauna require oxygen.  For much of the preceding 3 billion years, free oxygen was in short supply. By about 1800 million years the oxygen levels are thought to have been about 10% of the concentration in our modern atmosphere (based mainly on stable isotope chemistry).  The biomass back then was dominated by single cell, prokaryotic microbes (such as cyanobacteria).  There is good evidence that simple, multicell eukaryotes were present at least 1300 million years ago, for example in forms like red algae, but they were in the minority. Sudden appearance of the Ediacaran fauna indicates that oxygen levels may have increased abruptly 600-580 million years ago, creating the right conditions for evolutionary expansion; some estimates put oxygen concentrations at about 50% present atmospheric levels.

Continued research will probably refine these numbers. Regardless, the Ediacaran fauna provides fantastic evidence of significant evolutionary trajectories and ancient environmental conditions for one of the most crucial periods in the history of our earth.

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A Watery Mars; Canals, a duped radio audience, and geological excursions

Deceptive news is the art of pulling wool over the eyes of the populace, a tool (recently resurrected by certain politicians) for persuasion or dissuasion.  Orson Welles got more than he bargained for when, on October 30, 1938, he orchestrated a radio adaptation of H.G. Wells The War of The Worlds, a 1898 sci-fi that pits intelligent Martians against Victorian Britain.  Welles broadcast created a mix of amusement in some commentators, and in others panic and anger; panic in the unwitting, anger in the duped (especially other broadcasters), and amusement in all the above.

Well’s novel, apart from being the product of an agile mind, was influenced by some of the popular astronomical ideas of his time.  Italian astronomer Giovanni Schiaparelli produced, in 1888 a wonderfully detailed map of Mars showing (above image), among features such as seas, islands, and other landmasses, a network of ‘canali’, or channels.  Canali was misinterpreted in English as canals, and along with all its connotations of intelligent life, the idea of Martian canals entered popular belief.

Were the anthropomorphic connotations of the word canali deliberate?  Percival Lowell, a self-made American astronomer certainly thought so.  In 1894 Lowell announced his own findings, that there were indeed canals, 100s of them, many of them straight, intricately networked, and all artificial, which of course meant intelligent beings.  H.G. Wells simply expanded Lowell’s ideas to the point of delightful absurdity.  That O. Welles would later foist his version of events on an unsuspecting public seems quite reasonable.

Since the 1970s we have been projecting our own intelligence and sense of puzzlement on Mars, using satellites and landed vehicles.  There are no artificial canals, but there are canyons, channels and gullies, landforms that bear an uncanny resemblance to terrestrial analogues.  There is now a significant body of evidence to indicate that Mars was once watery.

On earth, sediment is distributed far and wide by flowing water. Very fine sediment from rivers or wind-blown dust is commonly suspended in water; the sediment gradually settles on the sea or lake floor.  Coarser sediment, like sand and gravel tends to be ‘entrained’ close to the sea floor or river bed by fast flowing water.  Sediment that is moved in this way forms a variety of structures such as ripples and larger dune-like structures.

Rivers in particular, generally move sediment to larger repositories, or basins such as seas or lakes; The kinds of landforms that represent these processes are very distinctive.  On Mars, there are several landform-indicators of flowing liquid (most likely water), most of which have direct terrestrial counterparts; deltas, straight and meandering river, point bars, alluvial fans, and gullied crater margins.  One such Martian landform, imaged by NASA’s Mars Global Surveyor, is the Eberswalde Delta which contains many of the ingredients that also make up terrestrial deltas.  In this case, sediment making up the delta was probably derived from outside the Eberswalde Crater and subsequently transported by rivers into the crater:

  • The delta consists of one or two main river channels (left side) that split into many smaller channels,
  • Bifurcating channels form distinct lobes – there are at least 6 of these, where each lobe represents a specific period of delta formation.
  • Switching of delta lobes is common in terrestrial delta. Each lobe represents a period of sediment movement and deposition, in this case into the deep crater basin.  At a certain point in time, the channel will switch direction and begin to build a new lobe.
  • Each new lobe partly overlaps older lobes, such that the younger deposits appear to lie on top of older deposits.
  • The Lena Delta in Russia provides a nice analogue for the overall shape of channels, with some active parts of the delta (especially the centre-right) juxtaposed with less active segments.

The Eberswalde Delta has another remarkable set of structures.  Meander loops (opposite image), seemingly identical to those seen in meandering rivers on Earth, contain patterns of progressive channel movement.  The Martian meander channel loop was eventually cut off, perhaps forming an oxbow lake like its terrestrial counterpart.

Martian landforms like these are mostly found in regions assigned to the Noachian Period, a geological interval that extended from 4.1 to 3.7 billion years.  All the evidence (so far) points to a time when surface water was common as rivers, lakes, and possibly seas; groundwater can be added to this mix.  If this was the case, there must also have been water vapour in the atmosphere.  The surface must have been significantly warmer than the present frigid temperatures; water vapour probably provided some degree of greenhouse protection.  Overland flow of water also produced sediment, much of which ended up in impact craters and broad lowlands.

 

However, some extremely large outflow channels, such as the Kasei Valles formed sporadically during a later, generally drier and colder time known as the Hesperian Period (3.7 to 2.9 billion years).  This massive system of channels and canyons extends about 3000km from its source in the Tharsis volcanic region, and through about 4km of topographic relief. The overall form of the channels, plus more detailed images of flow-like structures within the channels, indicates possible catastrophic outbursts of humongous volumes of water.  One popular hypothesis to account for this involves massive volumes of frozen groundwater being released either during meteorite impact or volcanic activity and heating.

The comparison between Eberswalde Delta and Kasei Valles mega-floods is quite stark; the delta represents relatively continuous river flow over a long period of time, into a crater.  The Kasei Valles outflow formed almost instantly, driven by the forces of impact and directed away from the crater.

Scientific understanding of Martian geology will continue to evolve; some hypotheses will stand the test of experimental and observational rigour; others will become history.  Modern science has developed the technology to actually do the field work, albeit remotely.  Perhaps we shouldn’t be too hasty to consign Schiaparelli’s and Lowell’s ideas to the theatrically amusing; their observations and explanations were not without context. Keep in mind the possibility that another H.G. Wells may point a satirical finger at 21st century science.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The Introductory chapter by John Grotzinger and Ralph Milliken provides an excellent technical summary of the Martian sedimentary realm.

 

 

 

 

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Sand dunes but no beach; a Martian breeze

When James Hutton, in 1785 presented to the Royal Society of Edinburgh his ideas on the uniformity of earth processes (over vast tracts of time), he did so with both feet planted firmly on good Scottish ground.  Hutton’s Principle, for which Archibald Geikie later (1905) coined the phrase “The present is the key to the past” gave to geologists a kind of warrant to interpret the geological past using observations and experiments of processes we see in action today (see an earlier post for a bit more discussion on this philosophy).  One wonders whether either of these gentlemen gave thought to the Principle being used to interpret processes elsewhere in our solar system.

There is of course, no logical reason why we cannot use terrestrial environments and physical-chemical-biological processes to unravel the geology on our solar neighbours.  We may need to extend our thinking beyond purely earth-bound processes, but the Principle remains a starting point for scientific thinking, interpretation, and discovery.  Mars provides the perfect opportunity for this scientific adventure.

I am constantly amazed with the images that various satellites and extraterrestrial vehicles provide of our celestial neighbours.  The surface of Mars is no exception.  There is clear evidence for ancient processes involving water; landforms and landscapes, strata, and minerals, all pointing to a once-watery planet – ancient rivers, lakes, and seas, beset by Noachian floods.  But all this aqueous excitement was in the distant past; more than 3.7 billion years past.  The Martian surface is now cold and bone dry (although the solid carbon dioxide ice does melt from time to time).  Active surface processes on Mars, i.e. those operating now, mainly involve loose sediment (sand, dust), and wind.  On earth, our experience tells us that if there is enough wind and enough sand, then sand dunes will form. Martian sand dunes look pretty much like our earth varieties.

The formation of terrestrial ripples and dunes requires certain (well understood) sediment attributes and physical processes.  We define sand grains as particles of rock or mineral that range in size from about 0.1mm to 2mm diameter.  Most dunes and ripples on earth form in sands that are in the 0.1mm to 0.3mm range.

  • The sand needs to be dry.
  • Sand grains must be loose, and not stick to one another (in technical jargon they are cohesionless).
  • Wind speed must be greater than the threshold for moving the grains. Stronger winds can move larger grains.

Satellite imagery of the Martian surface has revealed that sand dunes and ripples are common. They have formed, and continue to form in crater floors, broad flat plains, and ancient river valleys; basically anywhere there is sufficient loose sand available.

The variety of dune sizes and shapes is similar to that seen on earth.  Dune shape, like the Barchan dunes shown in the above images, provides clues to prevailing wind directions.  Other dune forms suggest greater variability in wind direction and strength, like the smaller, ripple-like structures (below) imaged by Curiosity ; here, changes in wind direction and speed are indicated by the very small ripples that move in different directions across the larger structures.  These patterns of sand movement are identical to those we commonly see on earth.

The larger dune imaged by Curiosity (below) is moving directly towards the camera.  Sand blown over the top of the dune (the crest) tumbles or slides down the steep front, or lee face, a process that continues as the dune moves forwards.  The inset, taken with a special camera that acts like a small magnifying glass (or hand lens), shows that the sand grains are indeed loose and cohesionless.

All the above images show dunes and ripples that are currently active.  It is also possible to identify fossil dunes in much older strata, using structures within the rocks.  In the examples below, the Martian fossil dune structures are very similar to those we see in some familiar rocks at Zion National Park (Utah).  The kind of layering visible in both Martian and earth examples is called crossbedding (see here for an explanation of how crossbedding forms).

Despite the obvious similarities between Martian and Terrestrial dunes, one in-depth analysis has led a couple of researchers (Gary Kocurek and Ryan Ewing, see below) to suggest that there are also important differences.  The Martian atmosphere is much less dense than that on Earth; there is no oxygen or water vapour.  Mars is consistently much colder; permafrost is probably widespread but in this case the ice is solid carbon dioxide.  Even if the CO2 melts, it will sublime (to gas) rather than form a liquid.  In some cases, permafrost may even stabilise, or solidify dunes.

One important difference between Earth and Mars is in the availability of loose sand.  On earth, much sand is produced by weathering of rock, by all the elements of weather plus chemical reactions that commonly involve oxygen.  Sand is available along most ocean, lake and river shores.  On Mars, physical weathering is very slow; sand is no longer moved by water, as it is on Earth.  Chemical weathering on Mars is also extremely slow.  Thus, the only sand available will likely be from local permafrost melt, the very slow breakdown of rocks, and the occasional meteorite impact.  Some sand may be left over from the period on Mars when water was abundant, the Noachian Period 4.1 to 3.7 billion years ago, but that seems like an awfully long time for sand grains to hang around.

Fossil dunes on Mars, like those on earth, provide clues to the kinds of environments that once existed. Some of these very ancient dunes may even have been associated with Martian lakes and seas.  Modern, active dunes on Mars have little or no association with water.  But they do provide information on sediment availability, wind direction, and wind strength.  The latter may be important when deciding where to locate future sites for human habitation.

NASA, ESA and other organizations have multiple sites to access imagery and general information on all space missions.  SEPM (Society for Sedimentary Research) also has a Special Publication (number 102; 2012) with 12 papers that describe aspects of sediments and sedimentary rocks on Mars.  The paper by Kocurek and Irwin, cited above, is one of these papers.

Kocurek and Ewing, 2012.  Source-to-sink; an Earth/Mars comparison of boundary conditions for eolian dune systems, p. 151-168.

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Comets; portents of doom or icy bits of space jetsam?

Omens, God’s wrath, or just plain misfortune; comets were seen by our Medieval forebears as a disturbance in the natural state of the heavens, portending disaster, pestilence, or famine, and if you were really unlucky, all three.  Harold, Earl of Wessex and later King, before he did battle against William of Normandy in 1066, must have had some misgivings with Halley’s comet nicely lighting up the northern sky (we now know it was comet Halley); he probably should have kept both eyes on the battle. Portent indeed; the Norman conquest changed irrevocably the history of Britain.

It seems that the ancient Chinese were a little more rational in their deliberations on comets – they referred to them as brush stars, and as early as 613 BC were computing approximate orbits.  In fact it is ancient Chinese astronomy records that have enabled modern astronomers to confirm calculated orbit periodicities for comets like Halley. Continue reading

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Striped oceans and drifting continents

German meteorologist Alfred Wegener, in a fit of creativity, presented his ideas about drifting continents to a 1912 geological meeting in Frankfurt.  He published an expanded version of his theory in 1915 – The Origin of the Continents and Oceans, to muted applause and resounding derision.  I found this quote by Russian contemporary, Vladimir Beloussov, in Arthur Holmes Principles of Geology wherein Beloussov quips in less than glowing terms of the …total vacuousness and sterility of the hypothesis.  The tenor of this remark was probably typical of Wegener’s detractors.  In fact, even today, certain scientific hypotheses and theories suffer the slings and arrows of equally vacuous remarks from the scientifically challenged.  As it turned out, Wegener’s revolutionary ideas were the vital spark that gave birth to modern Plate Tectonics. Continue reading

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