Canadian Earth Science for @PMHarper 10 – a question of Iron

The preamble to this review series is here

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Halverson, G.P., F. Poitrasson, P.E. Hoffman, A. Nédélec, J.-M. Montel and J. Kirby, 2011, Fe-isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation. Earth and Planetary Science Letters, v. 309, p. 100-112.

Front of Mackenzie Mountains SW of Norman Wells

The MacKenzie Mountain front in Canada’s NW Territories as seen from the river plain near Norman Wells. Image from Google Earth (Panoramio). The fieldwork for this paper was carried out in these mountains. 

Iron is Earth’s most common element by weight. There is so much of it that a compass needle (yes, also the one in your smartphone) lines up with the earth’s magnetic field. We also know that Iron changes form, because we are familiar with rust. Rust is oxidized iron and oxidation happens when Ferrous Iron (Fe2+) is exposed to the atmosphere (rich in oxygen) without being protected by some sort of coating. Under the burning influence of atmospheric oxygen,Ferrous Iron it changes to Ferric Iron (Fe3+) and turns red. A lot of rocks are rust-coloured because of this reason.

Iron also has one unstable isotope: 57Fe. The amount of this isotope varies every so slightly depending on oceanic and atmospheric chemistry. This change can tell us something about conditions in the past, if we know the age of the rocks. This article uses that knowledge to unravel conditions about 700 million years ago.

The authors of this article are Snowball Earth (official site here) experts: their research focuses on testing the hypothesis that earth around this time experienced a global ice age during which continental ice caps covered most of the planet as far as the equator.

oosnowballearth  Artist impression of Snowball Earth

The most obvious evidence for Snowball Earth is the ubiquitous presence fossilized glacial till in rocks of this age. Till (another word is diamictite) is the chaotic sedimentary mass that forms beneath a continental ice cap: it consists of pulverized rocks and pebbles in hard clay. A dead give-away of a former ice cap over the ocean (such as Antarctica’s Ross Ice shelf today), is the presence of dropstones in marine sediments. Dropstones are rock fragments, eroded from underlying bedrock by the icecap, transported by that same ice cap and subsequently melted out, fallen to the sea floor below and incorporated in the sediment.

The authors of this paper studied a sequence of rocks known as the Rapitan Group in the Mackenzie mountains in the Northwest Territories (latitude 640N). The Rapitan Group is ca. 700 million years old and shows evidence of ice age conditions, such as till and dropstones. When these sediments were deposited, this part of earth was situated at about 180S! Not a latitude where glaciers occur today or in our more recent Ice Ages when they reached as far as about 400 (North or South).

Not only show the rocks of the Rapitan Group evidence of glacial conditions, they are also unusually enriched in Iron.

Rapitan group till dropstone in Orphan basin core Rapittan hemipelagics

Left: A dropstone in iron-rich sediments of the 700 million year old Rapitan Group, Mackenzie mountains, NWT (pen for scale below the dropstone; photo from article).  Centre: a core with 20,000 year old sediments and dropstones from the Orphan Basin offshore Newfoundland for comparison (vertical scale in centimeters) (Image Source Tripsanas et al, 2007). Right: finely laminated Iron-rich marine mudstones and siltstones from a different interval in the Rapitan Group.

Sedimentary rocks with this much Iron are called ‘Banded Iron Formations’ (BIF). The most widespread BIFs are much older, between 2.4 and 1.8 billion years and they represent the bulk of the world’s iron ore deposits. Those earlier BIFs are thought to have originated biologically: massive Iron fertilization of the world’s oceans through ocean floor vents caused equally massive blooming of cyanobacteria, which were almost the only living organism at the time, fixing the iron in these bacterial sedimentary rocks. In the course of that process, earth’s atmosphere got its first major oxygen injection because cyanobacteria are photosynthesizers (if you think this is one of these completely irrelevant ivory tower types of research, I’m happy to remind you of this ocean iron fertilization controversy).

But the BIFs of the Rapitan Group are different than those much older BIFs. The BIFs of the Rapitan Group were deposited in greater water depths and, unlike the older BIFs, their origin is linked to ice caps (dropstones and the like). Why suddenly all this iron-fixing again after it had ended 1 billion years earlier? Were Snowball Earth conditions the cause of this return of massive iron fixing?

After an immensely complex series of chemical analyses, the interpreted scenario is as follows:

Ice caps covered most of Earth during this time and the remaining ocean waters were cold. Iron was supplied to the oceans by interaction between ocean water and the ocean floor. Because ice covered most of the water surface, remaining ocean waters didn’t circulate or overturn much, so that any deeper ocean waters (just below the thin, oxygenated photic zone) were oxygen-deprived. The underside of the ice caps would nevertheless melt in contact with ocean water and send dropstones and occasional gravity flows (high density currents that hug the bottom) to greater depths, interfingering with the deeper finely laminated silts and muds.

The Iron that was released from the ocean floor precipitated either as a result of fixing by bacteria that thrived in low-oxygen conditions or as a result of abiologic oxidation: the research is inconclusive about which process was responsible.

The lower part of the Rapitan Group sequence is dominated by intervals that show finely laminated iron-rich mudstones and dropstone-dominated intervals. These were deposited and precipitated on the bottom of the sea under oxygen-poor conditions. The 57Fe isotope has low values. Upwards these abruptly make place for tills deposited below a continental ice cap and at this boundary the isotope 57Fe shows a marked increase, which must be explained by an increase in the amount available oxygen.

It’s difficult to interpret element cycling conditions on our planet this long ago because it was such a different place from today: bacteria were the only life forms, the land was barren. And yet the authors have found a modern analogue to the strange conditions that made deposition of the Rapitan Group rocks possible. The modern analogue is Lake Nyos, in Cameroon.

I was at work in the summer of 1986 when news spread that a volcanic eruption in Cameroon had killed nearly 2000 thousand people and a lot of cattle. A few days later (it was the pre-digital age), it turned out that this catastrophe had not been caused by a volcanic eruption. Lake Nyos is a deep crater lake of an inactive volcano. Because it is located in the seasonless tropics, the waters of this lake don’t overturn and hence the lake is anoxic and rich in carbon dioxide below the thin oxygen-rich surface layer. When a small landslide slid into the lake, oxygen-rich surface and anoxic deeper waters suddenly became mixed, allowing previously locked up toxic gases to escape over the crater edge and down the slope of the volcano (carbon dioxide is heavier than air and hugs the ground), suffocating people and cattle in its way. A unique and disturbing disaster (read more about it here).

In order to prevent such a catastrophe from happening again, the lake is now ventilated by a metal pipe through which deep water is pumped to the lake surface. The pipe delivers Ferrous Iron to the surface where it immediately oxidizes. At the anoxic-oxic boundary, the same change in 57Fe isotope is detected as in the Rapitan Group rocks. This observation strengthens the interpretation that conditions changed from anoxic to more oxygenated 700 million years ago.

Limnic_degassing1 Lake Nyos being degassed. (Wikipedia)

This is challenging research. Not only does it require travel to the remote Mackenzie Mountains, where you must sample inaccessible sections of rocks and then store and transport the samples without polluting them, you must also carry out very complex chemical analyses. Samples were shipped all over the world to specialized laboratories. The authors go to great lengths to explain all the ifs and buts and potential pitfalls of this kind of work and how they avoided them. Hats off.

This paper is an elegant contribution to understanding the incredibly intricate and complex cycling of elements and nutrients on our planet over time. A small trigger suddenly caused the deposition of massive iron deposits during an utterly exotic global glaciation. Cause for reflection on our role as humans and the potential pitfalls of human intervention in system Earth.

 

Reference

Tripsanas, E.K., D.J.W. Piper and K.A. Jarrett, 2007, Logs of piston cores and interpreted high-resolution profiles, Orphan Basin. Geological Survey of Canada Open File 5299, 339 pages.

Posted in Canadian Earth Science for @PMHarper, climate change, General geoscience | Tagged , , , , , , , , , , , , | Leave a comment

Nova Scotia’s own Great Unconformity: my new banner photo for 2015

Rainy Cove unconformity annotated

The Angular Unconformity (U) at Nova Scotia’s Rainy Cove, separating intensely folded and faulted early Carboniferous shales and sandstones of the Horton Group (labeled 1 below the unconformity) and gently inclined, undeformed sandstones and conglomerates of the Wolfville Formation (2) at Rainy Cove, Nova Scotia.

A New Year, a new blog banner! My banner pictures are from around Minas Basin, a stunning estuary on which shores I live (this to my daily surprise and gratitude). My previous banner picture and associated blog post is here.

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The unconformity in this banner photo is exposed along the eastern shores of Minas Basin (location image at the end of this post). It is one of my favourite places to explore along these shores: there is so much to see here! Not only in the exposed cliff face, but also on the tidal flats that extend for several kilometers west of the cliff for about 2 hours (during low tide).

Here are two close-ups of the dashing exuberance of rock units 1 and 2:

???????????????????????????????  Gibling 7

The contrast between the two units below and above the Unconformity at Rainy Cove. Left: intensely folded organic-rich shales and sandstones of the lower Carboniferous Horton Group (ca 340 million years old, unit ‘1’ in top photo). The image is about 2 x 2 meters. Right: massive, undeformed sandstones and conglomerates of the Wolfville Formation (unit ‘2’ in top photo – person for scale). 

What is an unconformity, how does it come about and why is that interesting or even relevant? This is not an exhaustive coverage of the subject, so here I will discuss only the type of unconformity we see here, the “Angular Unconformity”.

Here is a schematic image of how such an angular unconformity is formed:

AU

How an Angular Unconformity comes about (image source here). First (central panel, bottom of figure) sediments are deposited in a sea. The different colours and patterns indicate different kinds of sediments, such as sandstones and limestones. Next (lower left hand corner), the sediments, having become lithified, are uplifted and tilted during a mountain building episode. During and after uplift, the mountains are eroded down (central left hand panel) until the land surface is once again the bottom of a sea and new sediments are deposited on top (upper left hand panel). When those sediments are once again lithified and uplifted during a subsequent tectonic event, the end result is two juxtaposed rock formations, each with a different orientation (largest panel). There may be millions of years between the time of deposition of the first and the second set of rocks. The surface between the two formations (which shows as a line in a two-dimensional rock face) is the Angular Unconformity. The Angular Unconformity marks a gap in time from which rocks are missing at the outcrop. 

So – what happened here in Nova Scotia? This is what happened:

Early Carboniferous 340 ma from LBY

A. During the early Carboniferous, about 340 million years ago, Nova Scotia (red star) was located in a southern tropical latitude. Rich tropical forests bordered a shallow tropical sea, which was becoming narrower as the continents fused together to form the supercontinent Pangea (the green star is where Nova Scotia is located today). Thick piles of sediments of what we now call the Horton Group, consisting of organic-rich shales and sandstones, rife with unique fossil assemblages (especially at Blue Beach and Joggins), were deposited during this time. Image adapted from The Last Billion Years.

 Late Carboniferous 305 ma from LBY

B. By the late Carboniferous, ca. 305 million years ago, Pangea had formed and the sediments from that shallow, tropical sea had become sedimentary rocks. They were folded and faulted during the mountain building episode (called the Acadian orogeny) that was the result of the collision of these pieces of continent. Note that Nova Scotia is now solidly at the Equator. Image adapted from The Last Billion Years.

Permian 225ma from LBY

C. By the Permian, about 255 million years ago, things look fairly similar to the late Carboniferous, but Nova Scotia has yet again inched further north. It is still ‘highland’ and is eroding and shedding sediment to regions further away. Image adapted from The Last Billion Years.

late Triassic 215 ma from LBY

D. By the late Triassic, ca. 215 million years ago, a crack is appearing in Pangea because of early break-up of the supercontinent (this process will eventually lead to the formation of the Atlantic Ocean). Nova Scotia is now at 15N, the latitude where deserts occur (similar to today’s situation). The former highlands are eroding by sending alluvial fans off into the newly formed low area. These are the sedimentary rocks of the Wolfville Formation, which overly the unconformity (unit 2 in the top photo).  Image adapted from The Last Billion Years.

The Angular Unconformity at Rainy Cove represents a gap in the geologic record, a period of ca. 135 million years (between early Carboniferous and late Triassic). In other words, there are no rocks of 340-215 million years old in this area. Sediments of this period exist elsewhere: all of Prince Edward Island consists of Permian sedimentary rock, deposited while sedimentation bypassed what is now Nova Scotia, which was being eroded and whittled down.

Why is this interesting or even relevant?

Reconstructing such large-scale processes helps us understand the evolution of the earth and its tectonic activity. These processes have not only contributed to what our landscape looks like, they also help us understand the occurrence of our own natural resources and those in other regions. Opening and closing of oceans and associated mountain building processes explain the occurrence of much of our mineral and fossil fuel occurrences and comparing these outcrops with other parts of the world, contributes to that understanding elsewhere. For example, the article by Leleu et al, cited below, was the result of a study that looked at the Wolfville Formation as an analogue to hydrocarbon bearing rocks in the subsurface of the North Sea.

 

Rainy Cove location

Minas Basin, Nova Scotia (images Google Earth). Left: overview image – the green arrow points to the location of Rainy Cove. Right: same image, zoomed in. The Rainy Cove outcrop runs more or less North-South. It’s early morning in this image because the cliff is in the shade. The tide is fairly high, because the tidal flats aren’t all the way exposed, but a portion of the rocky tidal flat is exposed, showing intricate structures in the exposed rocky beach. 

 

References

Leleu, S., X.M.T. van Lanen, and A.J. Hartley, 2010, Controls on the architecture of a Triassic sandy fluvial system, Wolfville Formation, Fundy Basin, Nova Scotia, Canada: implications for the interpretation and correlation of ancient fluvial successions. Journal of Sedimentary Research, v. 80, p. 867-883

Waldron, J.W.F., C. Roselli and S.K. Johnston, 2007, Transpressional structures on a Late Paleozoic intracontinental transform fault, Canadian Appalachians. Geological Society of London, Special Publications, v. 290, p. 367-385

 

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2014 in review: my world according to WordPress

The WordPress.com stats helper monkeys prepared a 2014 annual report for this blog.

Here’s an excerpt:

A New York City subway train holds 1,200 people. This blog was viewed about 6,200 times in 2014. If it were a NYC subway train, it would take about 5 trips to carry that many people.

Click here to see the complete report.

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A #tsunami is really a tidal wave, except it isn’t

1024px-Great_Wave_off_Kanagawa2

Katsushika Hokusai, Great Wave off Kanagawa. Image from Wikimedia. Original in the Metropolitan Museum of Art, New York, USA

This week marks the 10-year anniversary of the Great Sumatra earthquake which triggered the devastating Indian Ocean Tsunami that killed a quarter million people. A rare and devastating event in itself, it was followed in March 2011 by an even larger earthquake and ditto tsunami in Japan, now known as the Tohoku event.

Two-thirds of the world population live at or below sea level. Almost all tsunamis are generated by oceanic earthquakes and are therefore a serious risk in many parts of the world.

People know that tsunamis are serious natural hazards (see my earlier post here on what makes a natural hazard). What makes them so dangerous is the relentless run-up of a massive wall of water that takes everything in its path. But how does that wall of water become so relentless and destructive?

Let’s look at what kind of waves there are:

types of waves

Different types of waves, the forces that cause them (‘disturbing force’) and the forces that restore them. In this diagram, wave height is shown along the vertical axis in centimeters and wave period along the horizontal axis. Wave period is defined as the time interval between the passing of two successive wave crests (figure compiled from P. Pinet “Invitation to Oceanography”  and from J.A. Knauss “Introduction to physical oceanography”)

On the left hand side of the diagram are wind-generated waves. To the right are Tsunamis and Tides (in between are “Seiches”, which I won’t discuss: here is a good link). Both wind-generated waves and Tsunamis & Tides can have wave heights of more than ten meters (1,000 cm). But wind-generated waves have fairly short wave periods, less than 100 seconds, whereas Tsunamis have wave periods anywhere from a few minutes to two hours. That means that the distance between two successive wave crests (the wavelength) of tsunamis is very long, much longer than for wind-driven waves. Tides, which are largely controlled by the gravitational effects of the Moon, have a wave period of twice 12.5 hours, because it takes the moon 25 hours to orbit around the earth.

Now let’s look at waves from a different perspective.

When a wave (energy) passes through a water body, the individual water particles make circular motions as shown in the image below, and this is what causes the wave, i.e. the energy, to travel. The image below shows how that looks in a so-called Deep-water Wave.

deep_wave

A deep-water wave. Water particles move in circular fashion. The force that drives the wave is usually the wind. The wavelength is the distance between two successive wave crests. For a deep-water wave, the water depth is greater than half the wavelength. 

Below the water surface, the circular motion of the water particles decreases due to friction (they rub against each other). At a depth of half of the wave’s wavelength, the water particles no longer move. We call this depth ‘wave base‘. So the depth of the wave base varies with the height of the waves. Storm waves are higher than calm weather waves, and their corresponding wave base will be deeper than for calm weather conditions.

Wind-driven waves in the open ocean can have wavelengths up to a few hundred meters. Therefore their wave base is always in the water column (it doesn’t hit bottom) because the open ocean is on average 4 km deep. From the wave’s perspective the water is always deep. That’s why these waves are called “Deep Water Waves”. Deep Water waves travel with a velocity that is proportional to their wavelength, and because their wavelength is at most a few hundred meters, they don’t travel very fast. They move unobstructed across major water bodies until they arrive in water depths that are shallower than half their wavelength. Then their character changes to that of a Shallow-water Wave.

What is a Shallow-water Wave?

A shallow water wave is one of which the wavelength is greater than 20 times the water depth. The average depth of the oceans is 4 km, so their wavelength can be 80 km or more! The speed of a shallow-water wave is proportional to the water depth and independent of its wavelength. Hence, in great water depths, they travel very fast. Tsunamis and tides are shallow-water waves. A tsunami typically has a speed of about 800 km/hr in the open ocean. But their wave height in the open ocean may not be more than 1 m (see for example this link).

When a wind-driven deep-water wave approaches the shore, it becomes a shallow-water wave at some point. It starts to slow down due to friction against the bottom. But it doesn’t lose energy, at least not right away. What happens when a wave slows down but retains its energy? The wave height increases (this makes the surfers happy). At some point, the wave becomes higher than the water is deep (remember, we’re running up to shore). At that moment the wave becomes unstable and breaks (this makes the swimmers happy), and that’s when it loses energy quickly. The beach is formed in equilibrium with the average breaking wave height in a specific area (when the occasional exceptional storm hits the coast, the beach is not in equilibrium and the result is erosion).

The shallow-water waves that are Tsunamis travel at velocities of hundreds of kilometers per hour. They have such speed that they won’t be slowed down by the regular beach, they are not in equilibrium with the existing coast. As they run up the shore and before they break, their height increases massively and they destroy everything in their path until their energy is finally lost.

People are used to tides and tides can be predicted with great precision. For example, here in my back yard, in the Bay of Fundy, where the tides are higher than anywhere else in the world, high tide at Saint John occurs exactly 1 hour before high tide at Burntcoat Head, which is 180 km away across the water (see figure below). In other words: the-wave-that-is-the-tide travels at a speed of 180 kilometers per hour (yes, this creates potential for tidal power generation, about which I wrote an earlier post here). It runs up the coastal plain and submerges everything but because it does so twice every 25 hours, the shape of the coastline is in equilibrium with this phenomenon: it consists of vast expanses of tidal flats.

Bay of Fundy

Bay of Fundy, Canada. The distance from Saint John to Burntcoat Head is 180 km over the water. High tide in Saint John occurs 1 hour before high tide at Burntcoat Head. 

So a tsunami behaves exactly like the incoming high tide, with an even higher velocity. But because tsunamis happens to rarely, there are no tidal flats to indicate where you shouldn’t put your dwelling. In essence, people in tsunami-risk zones live in an overgrown and built-up ‘tsunami flat’.

People will continue to live near the sea for many reasons. There is now a tsunami warning system for the Indian Ocean, but lives will still be lost the next time a Great Wave materializes.

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Four Billion Years and Counting: Canada is as old as the Earth and this book tells all

4by   book-cover-fr

Just published! 400-pages on Canada’s geologic heritage in both official languages for only $39.95! Order your English language copy here and your French copy here

The book’s website has tons of freely downloadable illustrations and other materials for educators

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One day last summer, a 40-ish well-educated woman visited our house. She makes a living in gastronomy, is a good visual artist and an avid ocean sailor. She asked me about my professional background and I told her that I am an earth scientist. She looked puzzled and said: “and what do you do with that, other than teach?”

Ai…….

I was dumbstruck for only a second, then noticed her nice shiny and stylish watch and said “well, let’s begin with your watch, where do you think its component materials came from?” Then it was her turn to look puzzled. And so evolved a conversation about steel, nickel, Sudbury, and on about the diesel that fueled the bus that she had taken that morning and about earthquakes that may cause tsunamis in her beloved Pacific Ocean.

This little incident is only one reason why we need earth science outreach products for the general public and why we as a society must cherish and promote recognition of and access to our shared geoheritage (here is an earlier post that I devoted to geoheritage in Canada).

“Four Billion Years and Counting” (4BY) is one of those products. What is unique about this book is that it’s only one of two products that covers all of Canada, the other one being a book called “Canada Rocks”, which came out in 2007 as the tangible result of the CBC television series with the same name. But unlike “Canada Rocks”, 4BY came out in both French and English at the same time, a fantastic accomplishment.

4BY is the result of eight years of work by more than 100 geoscientist authors from all over the country (full disclosure: I had a small role in the production of the book, but none in putting its content together). So yes, a book written by a committee! About 25% of the book’s authors are women. Given that Canadian Universities employ just over 20% women in academic positions in earth science departments, this is encouraging.

4BY is also, in a way, a sequel to “The last Billion Years”, a book about the geoheritage of Canada’s Maritime provinces, which came out in 2001, is now in its 10th print run and continues to sell steadily. That success was the reason for its editors to start researching a similar product that would cover the geoheritage of our entire enormous and geologically complex and fascinating country. 4BY is the result.

But starting from the history of the success of “The Last Billion Years” also carried a risk: that book came out 4 years before Google Earth was launched. Everyone who has a remote interest in the earth has Google Earth on their digital device of choice. Together with every earth science prof in the world, I started using it in my classes right away and life became different from that magical moment in 2005. Many earth science organizations now include linkages to Google Earth. For example, the Ontario Geological Survey allows you to download their geologic maps over your own Google Earth software. Just go here and click on ‘download bedrock geology’ and the bedrock geological map of Ontario will open over your own Google Earth.

I know that this is a book and not a software system. But I don’t really see how a book like this can go without even referring to Google Earth once. Or to Aeromagnetic surveying, a crucial technology for understanding tectonic history and – ultimately – for finding mineral resources of which we know that Canada has lots. The book does have introductions about multibeam bathymetric mapping and about seismic surveying, however.

Miocene 15ma

North America in the middle Miocene, ca. 15 million years ago. Image as in the book, from the image database by Ron Blakey

While 4BY is not intended as a textbook, it is organized as one: part I (15% of the book) covers the Foundations of geologic science, part II (50%) the Evolution of Canada, and part III (35%) is called Wealth and Health and pertains to the practical applications of geology to our economic and physical well-being. Wealth and Health has chapters on Canada’s Mineral Resources, Energy resources (coal, hydrocarbons, uranium and a tiny section on renewables), a wonderful section on building stones with a special inset about the building stones of Québec City, an extensive chapter on water resources, one that covers all the aspects of coasts (erosion, management, etc.), one on earthquakes (and landslides and tsunamis), one on impacts from outer space (I happen to have a post about those here), and one on environmental challenges. I think I’d recommend every neophyte geoscientist reader to start with this third part of the book, because – like our visitor last summer – that’s where the uninitiated reader gets the idea that this stuff just might be relevant for …….. well… their wealth and health!

The book is aimed at the general public so its language had to be carefully crafted to be both understandable and inviting. Most chapter and section headings certainly accomplish that. Wouldn’t you be curious to read on after headings entitled “Spheres of Influence”, “Continental bulldozer”, “Hell on Earth” or “Rolling up the Rim”? However, the text itself does require a reasonably initiated person, because it is in places rich in jargon. Fortunately there is an exhaustive index.

The photographs are a lust for the eyes: hundreds of pictures were submitted by armies of happy snap-shooting earth scientists, so the editors (one of whom is an accomplished photographer himself) were able to select the very best ones from a true horn of plenty. It is an appetite-wetting virtual geologic road trip through our country. There are also portraits of famous Canadian geoscientists, going back to 18th century Abraham Gesner, but I did miss a portrait of the great (20th century) J. Tuzo Wilson. In addition to photographs, there are dozens of explanatory diagrams and quite a few artwork reproductions. Especially the latter are worth mentioning: some of them are from museums, so we get to look at images of spectacular museum dioramas of and of the iconic Beringia paintings of George “Rinaldo” Teichmann. The paleogeographic maps are all by Ron Blakey, certainly the best.

The book will be accompanied by a website, which isn’t up yet. I understand that the website will be targeted especially to teachers and I can see that this book will be extremely valuable, together with the website, for high school science / earth science / evolutionary biology classes. I look forward to the site, because together with the site, the body of work may become a little easier to navigate. I missed that figures aren’t numbered and when the text refers to a certain section, it will not give the page number of that section. Many pages contain a string of places names, assuming that the reader knows exactly where those are. Most average citizens don’t, so a bit more geographic indexing would be helpful. As for me, I studied the book while sitting behind my computer and using the superb Atlas of Canada for finding my way around.

Overall, this is a exceptional contribution to the documentation of our amazing Canadian geoheritage legacy and I encourage everyone to buy this book pronto, for yourself and for whoever is on your gift-giving list.

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Canadian Earth Science for @PMHarper 9 – measuring the thickness of polar sea ice through time

The preamble to this reviews series, categorized as “Canadian Earth Science for @PMHarper”, is here.

de Vernal, A., R. Gersonde, H. Goosse, M.-S. Seidenkrantz, and E.W. Wolff, 2013, Sea ice in the paleoclimate system: the challenge of reconstructing sea ice from proxies – an introduction. Quaternary Science Reviews. v. 79, p. 1-8.

Climate is warming, ice caps are melting, thinning and retreating. The Arctic ocean shows dramatic declines in summer sea ice every year, leading to increased development pressure (shipping, mining, tourism). After the all-time low of 2007, Arctic summer ice in 2013 was at its 4th smallest extent (well explained here). In Antarctica, however, sea ice has been on the increase in the last few years. This phenomenon is not yet totally understood, but may be related to the faster movement of Antarctic ice caps caused by them melting away from the bedrock below (but this is difficult to determine and not the subject of this review, see for a recent overview of this issue this page).

The ice conditions in the Arctic and Antarctic are related to the land configurations and both poles are total opposites: the North Pole is an ocean surrounded by land whereas the South Pole is a continent surrounded by ocean. Couldn’t be more different.

north and south pole

Polar maps of the northern and southern hemisphere. The arrows illustrate the main drift patterns, which are also responsible for sea ice dispersal and melt towards low latitudes. In the Arctic map, BG and TPD stand for Beaufort Gyre and Trans Polar Drift. The pink line corresponds to the 1979-2000 average maximum sea ice cover extent in March (northern hemisphere) and September (southern hemisphere), the months when sea ice is at its maximum in either hemisphere (illustration and caption from the article under review).

If you want to predict the future, you must understand the past. Geoscientists understand that as no-one else: our entire science is based on the principle that “the present is the key to the past and vice-versa”.

Why is it important to understand past sea ice conditions? Because sea ice “acts as an amplifier: it influences the energy budget at the surface of the Earth because it reflects a significant part of the incoming solar radiation (it is white) and because it limits the heat exchange between the ocean and the atmosphere” (quote from the article). Therefore: if long term (Ant)Arctic sea ice cover changes drastically, that will have an effect on long-term weather patterns and thus on climate.

Also: if a lot of sea water freezes and the sea ice cover expands, then the ocean becomes more salty, because salt doesn’t freeze (sea ice is not salty) and salt water is heavier than fresh water, so it starts to sink, and that affects ocean circulation and thus…. climate.

And: saltier water supports a different population of (micro)organisms than less salty water and most of these organisms breathe by absorbing CO2, so when you change the population, the amount of CO2 absorbed in the ocean changes and this …..  affects climate.

And so on.

So – it would be really good to know how sea-ice changed over time so we can better understand our past climate changes and – eventually – better model what the future holds for us.

ice age temperature changes

This graph illustrates how global temperature and polar ice volume (both on the vertical scale) varied over the last 450,000 years (horizontal scale). The reconstruction is based on the analysis of two ice cores (Vostok and EPICA) from Antarctica (image source here). The present day average temperature is set at 0 (zero) because the graph shows the deviation from the present. The symbol Δ means ‘change’ or ‘deviation’ so the vertical axis of the top 2 curves (blue and green) indicates the deviation from the average present-day temperature in Antarctica during the last 450,000 years. The lowermost curve (pink) indicates the estimated change in global ice volume over this time period.

This paper by Anne de Vernal and others is the introductory article of a 230-page special issue of the journal Quaternary Science Reviews of which they were the guest editors. The title of this article is also the title of the entire issue; this paper is the State-of-the-Art summary of this important new research field. The entire list of articles of this volume is here.

Problem: we only have direct observations of sea ice conditions from satellites since about 35 years and through a variety of other direct measurements since maybe the end of WWII. “Direct observations” are measurements of actual ice conditions. Which are difficult enough even with the sophisticated equipment of today: see this video for an impression of those challenges. If we want to know sea ice conditions from before WWII, we must find trustworthy and measurable indicators for sea-ice conditions. Such indicators are called proxies. Another word for proxy is substitute. To help you wrap your mind around this: imagine you couldn’t measure summer temperature, but you did want to know what kind of summer it had been. Imagine local beaches tracked the number of visitors each summer. If you collected the number of visitors for each beach, you would get an indication of which days were really nice, because there would be more visitors on hot days (you would have to account for holidays and weekends). Beach visitors would be a proxy for summer weather.

The most useful proxies for oceanic conditions are generally microscopic organisms. When they die, they fall to the ocean floor and become part of the sediment (some of them disintegrate or get eaten, but there are so many of them that a lot of them end up on the ocean floor). When we sample that sediment by taking cores off a research vessel, we can measure (later, in the lab) all kinds of properties of those organisms and these properties give us clues about the conditions (light, temperature, salinity) under which the organisms lived.

piston_core    core1_en_24948

Left: A piston corer is launched over the side of a research vessel (image source here). Right: a core that’s cut open lengthwise to show finely layered sediment (image source)

But organisms on land also react to changing climate conditions. Trees grow faster or slower depending on the seasons and tree rings tell us. Dendrochronology (from Greek: ‘dendro’=tree, ‘chronos’= time and ‘logos’ = knowledge) is the scientific term for tree ring studies. Tree rings are excellent proxies. Trees grow one ring per year. The thickness of the rings tells you something about temperature and humidity conditions. Here is a link to the International Tree Ring Data Bank. It’s very important to be able to try to correlate ocean- and land-based proxy data.

treering

When tree rings are widely spaced, climate was moist and trees grew faster than during dry years, when the rings are more closely spaced.

A little more than three years ago, a number of climate scientists agreed that sea-ice is an important but insufficiently understood climate driver. They got together for a workshop at UQAM in Montreal, home base of Anne de Vernal and decided to form a working group to share their sea-ice methods and results to see if they could improve our understanding of this phenomenon.

This volume of articles is an outcome of that exercise. What an accomplishment! You get together in the summer of 2011 and you get 18 articles published in one volume two years later. Wow.

Nine sea-ice condition proxies are identified in this paper and each is evaluated for its advantages and disadvantages. Some proxies are brand-new discoveries, some have been known for a few decades, but in general you can safely state that this is a 21st century field of research.

The authors emphasize that individual proxies cannot be used in isolation, but should be considered complementary to each other. In other words, if you want to draw conclusions with respect to past sea-ice conditions, you must use a combination of proxy data. This is because conditions vary regionally and between ocean and land; also the sea-ice thickness varies regionally and seasonally and this has an effect on organisms. Also: some organisms only live in either polar region. In addition, we don’t really know enough about how organisms react to changing conditions, so scientists must experiment with different proxies and compare the results. If the results line up, you’ve got working methods, you’ve got a tool.

Considering the current State-of-the-Art, the authors conclude that there is every reason to be confident that sea ice conditions during the last half million years can be confidentially reconstructed in the coming years using these different proxy methods. They admit that there are still many challenges and they aim to address those in the coming years. Altogether a very admirable result.

Predicting future climate is hideously difficult because there are so many factors that play into climate. Sea-ice is just one of those factors and it will take a few more years before that parameter can be built into future climate models. It’s science, after all: nobody promised it would be quick and easy.

It’s somewhat humbling to realize that this research was financially supported by PAGES (Past Global Changes, a research program funded by the US and Swiss National Science organizations and by NOAA) and by a grant from the European Union’s 7th framework programme.

sea ice cartoon

Cartoon from the Australian Department of the Environment / Antarctic Division. Image source here

Posted in Canadian Earth Science for @PMHarper, climate change, General geoscience | Tagged , , , , , , , , , | Leave a comment

Did you know? #Louisiana is disappearing

SE_LA_landloss

Land loss map of South Louisiana. Image source here. Click on image to enlarge. 

Is it the weather? No fewer than three long, detailed and well-researched articles in important media discussed the continuing story of increasing land loss in South Louisiana. The Globe and Mail’s Omar el Akkad wrote an insightful piece about disappearing Louisiana in the October 18 paper. The October 5 New York Times Magazine’s main article was a heart-sinking rendering of the fight of a few individuals against the sheer unwillingness of anyone to do anything to save the State of Louisiana. The New Republic Magazine ran an article on September 30. The latter two articles particularly focused on corruption.

All three articles were excellent, so why should I want to add anything?

I am a sedimentary geologist. I worked for the Louisiana Geological Survey from 1981 to 1986. Most of that time I worked on marshes – disappearing marshes. I was one of a team of about 6 young ambitious geologists, brought together under the Louisiana Geological Survey’s Coastal Geology Program, an initiative to inventory and understand the causes of the State’s land loss.  We were not the only official program working on these issues in the State at the time. We cooperated with the Louisiana Universities Marine Consortium (LUMCON), with scientists at Louisiana State University’s Coastal Studies Institute and with its Center for Wetland Resources. And that’s not counting the – Federal – US Geological Survey and the US Army Corps of Engineers. We worked on barrier islands, hurricane impacts, sediment budgets, marsh dynamics, nearshore currents, river dynamics, you name it.

Did we come up with new information? I think we did, if only for the sheer volume of new data that we gathered, analyzed and published, both as technical reports and as articles in internationally peer-reviewed scientific literature. There were a couple of new ideas and new insights, but I think it’s realistic to state that we mostly added lots of data and lots of detail to a story that was basically known and accepted by the mid 1960’s: the Mississippi Delta consists of ephemeral land because the river is – as Omar El Akkad wrote – a ‘side-winder’ and has been prevented from behaving as such for several centuries because of levee-building by its inhabitants (European colonizers). Thus, vast amounts of river sediment end up in 4,000 m water depth off the present-day river mouth (which is completely artificial) rather than being available for nourishing the delta plain’s marshes and bays (important nursery grounds for the fisheries) and its skimpy but crucially important barrier islands and beaches (I wrote earlier about the fragility of Gulf of Mexico barrier islands here).

Miss D lobes with Atchafalaya

The Mississippi Delta is a side-winder. Depicted are the 5 main Holocene (less than 10,000 years old) delta complexes each with a number of individual delta lobes. The oldest delta complex, the Maringouin, has subsided and is below sea level. If the Mississippi would not be confined by artificial levees, it would have switched back to the Maringouin area by now: the present day Atchafalaya river and delta (blue arrow) occupies that position but river locks further north only permit it to carry about 1/3 of the total river load. The Atchafalaya route is 250 km shorter and significantly steeper to the sea than its current route and thus the preferred route from the river’s perspective. Image by Louisiana Geological Survey.

We knew this 30 years ago. We were never prevented from communicating these observations to the general public by the way. In fact, we were encouraged to do so and I clearly remember a TV crew in our core facility. Our director (who later became a USGS director) didn’t tell us what to say to the press – ever. The most vocal member of our team even got a New York Times Obituary when he passed away 8 years ago.

We did not think climate change back then. But you don’t even need rising sea level to declare an emergency: the Mississippi Delta is sinking because its sediment is waterlogged. Natural compaction squeezes the water out and makes the land sink. If you prevent the delta and the coastline from being nourished by its own sediment, the land loss due to compaction will be exponentially worse. If you allow the oil and gas industry to dig thousands of kilometers of canals, which disturb the delta’s hydrology, enabling salt water to penetrate landward, thus killing marshes and generating open water bodies, then you really have a problem.

Lies in Flotant 1

Author negotiating a disappearing marsh in the Mississippi Delta in 1984. This particular area was becoming infiltrated with salt water at the time, and this killed the fresh-water vegetation. It is now open water.

Hurricane Katrina slammed into Louisiana 9 years ago this summer (August 28). You’d think that that catastrophe would have led to some action. It didn’t. Some engineering companies got a few contracts. But soon after Katrina, people in power got really tired of scientists telling them why the land was disappearing. Louisiana State University even fired a tenured professor who told the truth, but had to pay him a hefty sum in damages a few years later. In the Globe and Mail article, Louisiana Governor Bobby Jindhal is quoted as saying that the oil and gas industry provides 60,000 jobs and therefore shouldn’t be the only actor pay the price for this catastrophe. He suggests maybe the fishers should pay a price as well. Really? Since when have fishers wealthy shareholders and corporate exit bonuses? That was just political distraction tactics by th governor. Because the truth is, you can’t allow the river to switch to its preferred location (the Atchafalaya course) because from Baton Rouge southward, the river would fill with sediment, New Orleans as a harbour would disappear and since it’s the lagest bulk port of the US, that’s unaffordable.

Is Louisiana an exception in the world? On the margin, I don’t really think it is. It’s true that Louisiana is an eccentric State. It is the only State in the US with a legal system based on the Code Napoleon rather than on the Anglo-Saxon Code and so all its legal experts are educated in Louisiana and this leads to professional inbreeding and corruption. That’s not a secret. So it’s a bit of a banana republic. It also has way too many very poor people, which is entirely unnecessary given its petroleum wealth, but there you go – that’s Louisiana.

After Hurricane Katrina, I put together a talk on the catastrophe. I gave that talk several times as a fundraiser. The money was for a public school in Baton Rouge that saw its population doubled in the first week of the school year (the hurricane hit on August 28) and had no funding for the additional required supplies. To this day, I am told what an eye-opener that talk was and that the story was in essence so simple that it was hard to believe nothing was ever done to mitigate the situation. Nice. Thank you. That was 9 years ago.

Is Louisiana exceptional in this respect? As a global society, we haven’t been willing or able to reduce green house gas emissions one bit, never mind overwhelming evidence that we must, if only for being at a serious risk of losing exponentially more land than is being lost in Louisiana. I can run off a list as long as my arm with examples of environmental hazards and disasters waiting to happen and elected decision-makers (aka politicians) sitting on their hands. Maybe it’s the weakness of democratic society or the general tendency of the public to stick its head in the sand and elect officials who are good at that too. Louisiana or the world, we will all still be discussing mitigation efforts when the water is at our lips.

ACordal

“Politicians discussing Global Warming”. Installation by Spanish artist Isaac Cordal (Berlin, Germany)

 

 

 

Posted in Natural hazards, Uncategorized | Tagged , , , , , , , , , , , , , , , , , , , | 1 Comment