Tag Archives: Stromatolites

Determining stratigraphic tops

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a.k.a. Stratigraphic way up, or younging direction

This post is part of the How To… series

Stratigraphy is all about succession in the rock record – which events preceded other events; which is older, which younger. Nicolas Steno (1638-1686) surmised, and four centuries of geologists since have confirmed that in an uninterrupted succession of strata, the youngest layer is at the top.  However, tectonic hiccups and upheavals have frequently turned successions of strata sideways or on their head. In this case knowing which way is ‘up’ will confirm which strata were overturned. Continue reading

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In the field: Windows into two billion year-old rocks

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My early geological education was very much New Zealand centered; the gamut of sedimentary, igneous and metamorphic rocks (there are no Precambrian rocks in New Zealand), in the context of a landmass (and attached submerged bits) still rent by active faults and erupting volcanoes. The timing was fortuitous. We learned at the cusp of the ‘new tectonics’, sea-floor spreading, and the morphing of continental drift into plate tectonics.  The fixists were a disappearing breed; now everything was on the move, attached in some way to one tectonic plate or another, rifted, drifted, and eventually subducted. Now, the rock formations, faults (particularly the Alpine Fault), and the volcanoes, were all connected in one, all-encompassing global, plate tectonic system.  Geologically active New Zealand had a place in this grand scheme.

Admittedly, not all our professors found it easy to teach these revolutionary ideas. We would be exhorted to go and read the latest journal papers, and come back with questions – I guess this gave the teachers time to read the articles themselves. But it was an exciting time, reading the claims and counterclaims. It really was a (Thomas Kuhn) paradigm shift.

Landing on the shores of Belcher Islands (Hudson Bay) was also something of a mind warp; from a country that straddles a plate boundary, has a volcanic rift zone in central North Island, and faces a subduction zone within a stone’s throw of the east coast, to a part of the Canadian Shield where not much has happened over the last two billion years.  Perhaps that’s a bit of an exaggeration, but this prolonged period of stasis had its advantages.  The rocks, despite being about 2000 million years old, are loaded with beautifully preserved structures and fossils.  They were not cooked by metamorphism during the time they spent being buried, nor fractured beyond recognition by tectonic forces. Basically, everything was intact. Stunning.

For someone interested in deciphering ancient sedimentary environments, being parachuted into the Belchers and being told to take the rocks apart, layer by layer, sequence by sequence, was initially a tad scary; an emotional response that quickly dissipated once the measuring, observation, and interpretations had begun. On finishing the work on one set of exposures, we couldn’t wait to get to the next, and the next.

If you were to stand all the Belcher strata in a single pile, it would be almost 9 km thick. But this pile was subsequently tipped on its side. Over the eons, the rocks were eroded by rivers and scraped by ice, fortuitous levellers that provided windows into each layer. Geologists are enticed to enter these portals, at least in their mind’s eye; the rewards are huge.  We can envisage times when there were broad platforms of limestone (now all converted to the mineral dolomite), that harboured a massive biomass of primitive algae, stromatolites of all shapes and sizes; layers as thin as a fingernail, and reefs 10s of metres high. The platforms were covered by warm, seas that shoaled into tidal flats and (deserted) beaches. Some areas infrequently inundated by high tides, became desiccated; there are remnants of minerals like gypsum and halite (common salt) that attest to salty seas. Walking over rocks like these kindles the imagination; a beach stroll, waves rolling in like they have done for billions of years, or parched landscapes exposed to the full effects of sunlight uninhibited by oxygen and the UV dampening effects of ozone (the incidence of UV light must have been intense). The experience is humbling.

However, idylls have a tendency to dissipate in the fog of time or, as was the case here, a smothering by erupting ash columns and lava flows. Now we get to walk across the tops of really ancient lava flows, around piles of pillow lavas, or along catastrophic pyroclastic flows of ash and pumice.  The earlier tropical paradise had been obliterated, but even in this volcanic brutality there is wonder.

Other strata tell of deep seas fed by turbulent mud flows cascading down an ancient submarine slope, and of sandy rivers turned red by iron oxidized by the gradually increasing levels of oxygen in the ancient atmosphere (deposits like this are commonly referred to as red beds). In every layer, every rock we looked at, there were mysteries waiting to be unravelled. A geologist cannot hope to solve all such questions, but finding a solution to even one of them is incredibly satisfying.

I spent a total of 5 months in the field during the 1976-77 summers. This was not the kind of location where, if I’d forgotten to do something, I could whip back for a couple of days to sort things out. Several of my student colleagues were doing similar kinds of research in remote parts of the country – field seasons were long. Once you had arrived, you were there for the duration. And despite the sense of excitement and discovery, it was always good to get back home.

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Ediacara; Welcome to the revolutionary world of animals

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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.

Addendum: Although many who study these fossils do consider them to be the most ancient’animals’, others disagree; suggestions such as fungi, large protists, or some kind of lichen (the latter does seem unlikely). The form Dickinsonia (shown above) has come into its own once again where, in a recent study, fossil biomarkers were discovered in samples taken near the White Sea, Russia. Identification of fossil steroid markers confirms a direct link to the animal kingdom (Science, Sept 21, 2018). It seems likely that many other Ediacaran fossils were also the very first true animals, precursors to the myriad forms that appeared during the Cambrian explosion.

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Extreme living conditions; the origin of life and other adventures

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Extreme events are fascinating.  Extreme sports may give us a vicarious thrill, at least until something goes awry at which point we might comment about the foolishness of the act.  Extremes in the natural world are the stuff of movies; asteroids, tsunamis, tornadoes, plagues.  Perhaps our morbid fascination with such events derives from the realization that they can be real.

Over the last 2-3 decades, science too has developed a fascination for extreme living, for creatures that happily thrive in conditions that most other life forms, including us, would find inclement.  They are extremophiles, life forms like bacteria, algae and small critters that can endure extremes of temperature, pressure (e.g. deep sea black smokers), radioactivity, darkness, low levels of oxygen, high acidity or alkalinity, and even lack of water. The variety of extreme environments in which these life forms have evolved is, from a scientific perspective, quite stunning in that it provides us with many different analogues for our quest to understand the origin of life on earth, and whether life can exist on other planets.  A few examples are noted below. Continue reading

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Biomarkers; forensic tools for hydrocarbon fingerprinting

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I like a good detective thriller. Danish, Norwegian, Swedish and Britain’s BBC networks have produced some quality shows over the past few years.  Forensics is usually equated with ‘who dunnit?’ but science also makes use of forensic-like tools to help unravel mysteries and solve problems.  This post looks at certain chemical compounds found in hydrocarbon deposits.  The compounds are specific, complex organic molecules called biomarkers.  Biomarkers provide scientific fingerprints of oil deposits, that help scientists and oil explorationists decipher the where, when and how such deposits formed, and environmental scientists monitoring the migration and degradation of spilled oil. Continue reading

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The Shunga Event; did a Precambrian mass extinction give rise to an ancient supergiant oil field?

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shunga-time-line

Earth continues to evolve.  So far it has taken, notwithstanding Bishop Ussher’s different view of things, about 4.6 billion years for the atmosphere, hydrosphere, lithosphere (the solid earth), and biosphere to get to where they are today.  Over that time there have been (rewording a well-known expression) long geological periods of inexorably slow change punctuated by catastrophes.  Mass extinctions, caused by blink-of-an-eye bolides and episodes of rampant volcanism (e.g. the Deccan Traps), completely changed the course of biological evolution.  Contrast events like these with the life and death of oceans counted over 100-200 million years.  Meteorites, volcanoes, and glaciations have all played their part in moulding our planet. Continue reading

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The Ancient Earth 5. Life and all that… Where, How, When? Part 1

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The Origin of Life?

This is a vexing question; perhaps the ultimate puzzle! It invokes wonder and intrigue for many, but for others it’s a question that invites derision and disbelief. Of course, we may never know the complete answer, or answers to the question “How Did Life Come About?“, but there is also no reason why science shouldn’t persist in asking it, in looking for the geological evidence, either here or in some other solar system, or devising experiments and models to help explain it.  For all we know, it could be as simple an answer as it was to Richard Adams’ ultimate question in Restaurant At The End Of The Universe. The next two posts look at some scientific experiments that help us imagine how it might all have begun.

The oldest fossils

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Ancient earth. 3 The air we breath; how our atmosphere evolved

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land sea air

The really ancient earth: How our atmosphere evolved

Take a deep breath. Savour it.  One of the few absolutes of our physical world (that we probably haven’t looked after as well as we might have).  This post continues the theme “The Really Ancient Earth” by looking at what we know about the origin of our atmosphere; some of the evidence and some of the hypotheses.  What was it like on day 1 (about 4600 million years ago) and how did it evolved into our breath-taking world today? Continue reading

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Ancient earth. 1 A time-line for the first 4 billion years

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A time-line for the first 4 billion years of Earth history

The Cambrian, that relatively brief period in geological history (40 odd million years) was witness to one of the most amazing series of biological events in the entire history of the Earth; the rapid, almost explosive appearance of marine critters with preservable shells and skeletons – a real first.  Trilobites are probably the best known fossils from that period, but there are also some pretty weird and wonderful looking soft-bodied creatures (one famous fossil locality is the Burgess Shale near the town of Field, British Columbia).  Most animal life today can track its origin to those early life forms.  These events began about 540 million years ago (how easy these numbers roll off the tongue, or pen).  But we also know that our Earth is pretty close to 4600 million years old (4.6 billion – How old is Earth); in other words there is almost 4 billion years, a humongous period of time in which, seemingly, not much happened.  4000 million years worth of boredom!  This period is know as the “Pre” Cambrian, or Precambrian.  Most Precambrian events did take place pretty slowly, but these events also determined the kind of world we now live in: the air we breath, the oceans and rivers, the biosphere and indeed life itself, all originated and evolved over this, the deepest of geological time. Continue reading

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