February 23, 2011

Papers I'm reading: Declining sand dune activity in the southern Canadian prairies

"Visually, the dunes were incredible. They were at least four to five meters in height standing from the top, and there were about four-five rows of dunes. Most of them ran north-south in length. The wind picked up while we were there and I can understand how the dunes are moving so many mm's/cm's each year. The area surrounding the dunes was very desert-like (we spent one night nearby and I managed to get a cactus stuck in my foot, hurt very much). We could also hear the coyotes howl all night and in the morning witnessed several hawks flying by. The area was extremely dry, so much so that I even came across a sort of animal grave yard at the far north part of the main dunes. There was several skeletal remains of what I assumed to be cattle, though the bones were quite small and could have been deer. There were also plenty of little bugs wandering around the dunes, leaving their tracks in the sand, though I couldn't say what kind of bugs they were (although there were many "tiger bugs" I think that's what they are called)."
The above excerpt is courtesy of an aspiring journalist friend who has a keen instinct for the geographic/geologic, detailing an experience among some spectacular sand dunes: Writer's Fidelity

Her description of majestic dunes in southern Saskatchewan segues into the latest dozen-page intellectual nugget, coming courtesy of the Journal of Aeolian Research. The article is titled Declining sand dune activity in the southern Canadian prairies: Historical context, controls and ecosystem implications, authored by Chris Hugenholtz, Darren Bender, and Stephen Wolfe. The paper goes into detail about how the patchy sand dunes located in Southeast Alberta-Southwest Saskatchewan have been seeing a slow but steady decline for the past hundred years or so, and how this decline affects the ecosystem balance and the organisms that reside in the relict landforms:
Sandhills are islands of biodiversity in the southern Canadian prairies that sustain habitat for many rare and endangered species. These unique areas consist of large expanses of dune fields now mostly stabilized by grassland vegetation. Historically, the number of active dunes has decreased significantly due to vegetation stabilization, resulting in a dramatic decline of open-sand habitat for a variety of dune-dependent species. Without a certain level of wind erosion, opportunities for establishment of early-stage, species-rich vegetation types are diminished and open-sand habitat decreases by encroachment of the surrounding grassland vegetation. The current trend of dune stabilization, however, implies that wind erosion is decreasing, thereby threatening the continued existence of a variety of dune-dependent plants, arthropods and vertebrates, as well as other less-specialized species that benefit indirectly from these habitats. By reviewing factors contributing to the historical decline of active dunes, as well as the ecological implications of dune stabilization, the aim of this paper is to establish the biophysical context for new land management strategies that conserve valued landscape components, such as active dunes, and the processes therein. As dune stabilization continues management interventions will be required to sustain or re-establish open sand and the species that rely on these habitats.
Blowout dunes of Great Sand Hills, SW Saskatchewan
You can take a look in Google Earth @  50° 41.326'N 109° 17.069'W; most of the dunes are of the parabolic variety. The dunes are essentially relics from end of Pleistocene ice ages, notably the Wisconsonian, that have persisted due to a semi-arid precipitation regime and infrequent prolonged droughts typical of the prairies in SE Alberta/SW Saskatchewan. The southern prairies exhibit heavily glaciated terrain, wherein these glaciomarine-glaciodeltaic-glaciofluvial sands were derived from meltwaters at glacial fronts, and the region was subjected to katabatic winds flowing down off the front. The cold, sweeping winds were an excellent local atmospheric mechanism for sorting the sand into dunes.

The paper shows dune activity in 1900's declining due to dune stabilization via vegetation. Provincial action plans to reduce soil erosion has had a lateral effect upon the fragile ecosystem of the dunes and their plant & animal inhabitants. Thus in many ways, it is a "Damned if you do, Damned if you don't" issue, with anthropogenic activities being vastly responsible for currently affecting dune stabilization or proliferation. If left alone, the current climate change trend towards warming & disruption of precipitation regimes would likely lead to an increase in dune areal coverage, but only if decade-long droughts consistently occur. How does that work, though? If humans don't interfere with the dunes, our interference in the atmosphere will allow them to flourish?? In any case, the latter is already assured due to the lag of GHG's.

Ord's Kangaroo rat on sand hills
The real crux of the paper outlines what the implications are for the endemic flora & fauna. Since sand transport is effectively eliminated when vegetation cover exceeds 15%, the fragility of the dunes is acute due to the presence of rare, endangered species that habituate on blowout dunes. Within the paper there is listed over a dozen species of plants and animals in danger of extirpation. Examples include liverworts, arthropods, and the unique Ord's kangaroo rat. The latter is under threat due to dune stabilization allowing predators to corner the rats into an increasingly smaller territory with less space for effective burrowing. The dunes are stabilized by vegetation in a positive feedback mechanism that allows ruderal plants to proliferate on the dune perimeter by increasing surface roughness and changing soil properties to suite further veg expansion → if this goes too far, it will push out endangered dune-loving species like Ord's rat. The endemic animal plays a key role in the food web ecology of the southern prairies, extending an indirect influence outside the dune hills.

Co-operative efforts are underway in the prairies to allow some wind-driven erosion to facilitate natural dune migration & extent, though there is some back-forth wrangling with farm associations that prefer stabilization that protects soil resources. When looking at, for instance, the Great Sand Hills near Leader using Google Earth, you can see how farm lots completely surround the dunes. To conclude my observations, I generally get 3 things out of a peer-reviewed paper: 
  1. A new term learned; in this case it was ruderal
  2. A new question raised; in this case how does food security fit into the mix? If prairie farmers are wishing to avoid migrating dunes overtaking their lots, how do we address their concerns while allowing the dunes to flourish?
  3. A new realization; in this case connecting elements to learn how glaciers can form picture-perfect dunes. I knew of katabatic winds, I knew that wind is the greatest terrestrial sorter, and that glaciofluvial is second greatest & capable of transporting & sorting fine sand. Integrate the mechanisms together to make the dunes?....I did not realize that til now.
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    February 17, 2011

    Accretionary Wedge #31: "What the heck?!"

    This will be my first contribution to the Accretionary Wedge blog carnival, and as a senior undergrad the topic suites me perfectly: What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)?

    basic concept of gravity
    when applied to rock density
    Thanks to Jim Lehane of the Geology P.A.G.E. for hosting AW#31

    For my first two years at uni, previously unknown concepts were a given on a weekly basis. But as my studies get more in-depth, the concepts are becoming more esoteric and/or specific. One concept from those early years comes strongly to mind, as it took me some time to grasp before the metaphoric light-bulb turned on: Gravity Anomalies

    My physics teachers would drill it into us that acceleration due to gravity is a constant (@ 9.807 m/s2). GRAVITY, G,  IS A CONSTANT! ad nauseum. Then I was eventually presented with an alternate view of the consistency of the constant by my geophysics teacher. I, in my infinite lack of wisdom, and stubbornly sticking by what was told to me by my physics teachers, shirked off his silly idea of minute differences in gravity based on crustal thickness and rock types. I didn't really understand the mechanics of it the way he explained it, and it was never really tested on us students.

    The true revelation came during a summer volunteer expedition with a local CGS glaciologist. As one of three heading up to the Matier Glacier within Joffre Lakes park, I got a taste of what experts do, and what instruments they use to analyze receding glaciers and the mountains they rest on. I found out that one such device we lugged up to the top, a microgravimeter, measures the gravitational field at a point. So the glaciologist operated it, got the reading in milligals, and I stood there dumbfounded. He was gracious enough to explain to me the concept of gravity anomalies...how it relates to crustal properties (such as thick, light mountain roots & thin, dense old oceanic trenches), and how we get the Bouguer anomaly, derived from measurements/corrections.
    "Rocks have different densities, eg. felsic - ultramafic, and compression of the crust via plate tectonics can thicken the crust, developing structural mountains with or without igneous intrusions. These mountains have thick roots, sometimes 30+km from peak - basement. When such a thick mass is composed primarily of lighter granites, its lighter density is less of an attraction than when compared to a thinner, denser mass of heavier basalts. Denser basalts are found near subduction zones, especially above deep trenches. The phenomenon is comparable on continents as well: The Deccan Traps of Maharashtra measure a noticeable difference in gravity compared to the Himalaya ranges in Uttarakhand."
    Upper Joffre lake. Behind would be the tongues of the Matier Glacier.
    This is where I learned about gravity anomalies, luckily not by tumbling down to the tarn
    Thus I was enlightened to a concept that had previously been muddled in my brain. I've even had a couple opportunities to apply it to my personal and work projects (ie. Cornwall geology post). It's become an increasingly seen concept in higher level textbooks I've perused, and more and more diagrams & cross sections that have caught my eye combine topographical profiles and Bouguer anomaly milligal values (example below).
    Simple profile showing Bouguer anomaly values with general topography across the US
    In retrospect, I wish I had a time machine, so I could go back and tell my junior undergrad self about how not to take anything for granted in the scientific studies. Geology always seems to smash preconceptions built up by the other science disciplines, and that's something I love about it. For anyone interested, Britain & Ireland geological survey's have done some extensive gravity surveying, and documented some interesting positive & negative zones. Check out the additional links for it.

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    February 14, 2011

    Picking the best place to live, geologically speaking

    I rarely get to have discussion with human geography students, but the natural hazards class I recently completed had a good spread of the physical geeks with the human geeks. One human friend mentioned how she lives in a neighborhood on the edge of a broad alluvial fan, and that her parents have little clue as to the potential dangers of living on such a precarious foundation protected only by modest dikes, which I might add have had increasingly shrinking freeboard values for the past two decades. The deflecting reason they gave her is that "developers would not have built here if it wasn't safe". 

    Oh no, they would. Developers hedge their bets, and most in our regional district tend to downplay the importance of geophysical reports. Developers will push through proposals with hard cash to get the required zoning, especially if the 'view' and location is something that will bring them big bucks. This is not to say that they will cut corners wherever possible, as most of the time construction is top-notch, corresponding to strict standards for quake-proofing and packing down the sediment if the foundation happens to be laid upon Holocene alluvium.

    Where is this post leading? Well, in my humble opinion prospective home owners have to take on some proactive responsibility themselves. Common sense dictates that the buyer beware, and there is no excuse for an educated westerner who does have the rights and the access to geophysical information to not take some time and look at it. Where I live in beautiful British Columbia, our own provincial division of the Canadian geological survey has done some excellent work on investigating factors that determine how damaging certain natural hazards are in certain areas. Below you'll notice fragments of a poster that outlines the traffic-light approach to categorizing hazards:
    Slide hazard map of Greater Vancouver.
    Light green circles indicate known slide occurrences
    Liquefaction hazard map of Greater Vancouver.
    Red zones mostly correspond to deltaic and alluvial deposits
    Flood hazard map of Greater Vancouver.
    The red zones are deltaic and alluvial plains, and have experienced little uplift
    since the fill is mostly Holocene. The grey zones are Pleistocene glacial uplands,
    mostly till, and benefit from tens of meters of thickness + isostatic rebound
    Tsunami wake from Jan 26, 1700
    megathrust quake. Ghost forests
    attest to unbuckling of plates,
    causing meters of subsidence
    Looking at this one source of credible information already alleviates a lot of concerns a potential home owner would have, especially in our tectonically active region. Sure we haven't had a big one in living memory, but that doesn't mean we are out of any proverbial woods. On the contrary, the lack of moderate quake activity off the coast of Vancouver Island has led geophysicists to investigate and conclude that the Juan de Fuca plate & North America plate are locked at the shallow subduction contact, and tension is building to an inevitable unbuckling. Whether that results in one huge snap of a megathrust akin to the 1700 Cascadia quake, or a series of smaller unbuckling motions is yet to be seen.
    I was asked if I chose where I live because it is among the safest spots in the region. I reside on the glacial uplands, on the crest of a street with a gentle slope. Alas, as a student with a limited budget and only part-time jobs, I cannot claim that I was so smart to have chosen such a safe spot. It's pure coincidence, I chose it because I could afford the rent. But I have dodged a bullet, so to speak, by digging for this knowledge and taking it to mind before I do make my first home purchase years from now. I've already compiled a short list of personal rules for where a home should be, to which additions will slowly be added:
    • At least 300m from shore
    • At least 50m asl
    • Not adjacent to any slope greater than 25°
    • Not on any faults, not even ones considered “inactive”
    • Not on any META bedrock that has extreme foliation
    • Not on any SED bedrock where bedding planes are poorly indurated
    • Not within the boundaries of a 1 in 50 year floodplain
    • Not within the boundaries of a 1 in 100 year floodplain if recent evidence has shown urban development has forced flow to concentrate, and thus freeboard values for dikes are consistently less than 2 feet
    • Not on any foundation that has a layer that swells when saturated (ex. Bentonite)
    • Observe wind direction patterns if living on or near a sandy desert or high altitude snowy area, to avoid home being buried by windblown material
    • Not on any bedrock that has a lens of limestone beneath (GPR should discover it), lest it dissolves and a sinkhole forms
    • Not within a radius of 300m from the edge of a forest prone to fires. Consider making the perimeter of your property fuel-free
    I know a few of these are not economically feasible, but I've tried to arrange them in what I consider their order of importance, and the first several are free provided some time and effort and some light education. The GPR might not be an option, but if buying a $seven figure$ home, it's worth it for peace of mind. Above all else, look at the hazard history of the area you're moving into.

    I'm not touching on atmospheric hazards, but they should be looked into as well. They're kind of a big deal too.

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    February 8, 2011

    To study or not to study: a Metamorphic Genesis or a Dynamic Creek?

    Lately I've been thinking about what topic I should choose for doing my senior undergraduate major project. This project is essentially worth 3 credits, or one full course. The synopsis outlines it as almost a thesis but with a narrower scope of time, and only to be peer reviewed by my university professors. I'm planning on it for this semester next year, but want to utilize the coming summer months for field work.

    The first idea that comes to mind is to measure/analyze something local. I don't want to synthesize and regurgitate some phenomenon that is a world away where I can't put my hands on it (I do that enough on this blog). After all, one of the big reasons I chose geology is because on occasion the laboratory is the great outdoors. The second thing that comes to mind is that I want to break new ground. Not by finding new analytical methods or a stunning new hypothesis (I won't have the time, funds, knowledge nor experience to do that for several years), but by covering something in greater detail than has been.

    I might have access to basic hydrological equipment: current meter plus pH, conductivity, and DO probes. In that case I wish to study the fluvial geomorphology of a particular large creek that has not had a thoroughly holistic study. Plus my favorite geology book is Leopold's, and this would give me an excuse to read it again. Its geomorphology is of particular interest because the creek cut a channel down through glacial till and exposed a volcanic basalt layer underneath, thus a small portion of its mid-reaches has step-pool falls. More details found in the earthcache I created for it.
    Cliff falls in Maple Ridge, BC. Kanaka creek pinches
    here due to eroding down to a resistant lava flow
    If access to equipment or most of the drainage basin is not possible, I am considering doing a petrological study of a pre-Jurassic [mostly] metamorphic group on the north shore. There is surprisingly little information about it, beyond a few journal papers making slight references to it when describing other groups/formations that interact with it. This would involve a thorough investigation of approximately 8 sites of varying size (see map below) where outcrops have been identified, plus I'd get to use snazzy terms like aureoles and metamorphic facies. All I'd need in that case is a rockhammer, a handlens, maybe a petrographic microscope.
    Surficial geology map of North & West Vancouver
    #1 is the Twin Islands Group I'm considering for study
    I'll likely put this on the back burner until late spring, when I will have time to investigate the creek and gauge how I can approach it from various access points. In the meantime, professorial advice and anecdotes will likely sway me towards one or the other.

    Terminology note: It took me a while to wrap my head around the phenomenon of roof pendants. Sometimes a blatantly obvious mechanism or concept eludes me, until a light bulb goes on. This has happened with things like pedimentation, cyclothems, Benioff zone.  My best attempt to explain to myself the idea of roof pendants - When a batholith forms below overlying strata, it penetrates upwards, and thus makes contact with those layers. The layers will likely undergo contact metamorphism, and metamorphose under high temperature & low-moderate pressure. Erosion of subsequent material leaves the META members as isolated crops dangling/resting above the batholith. Essentially roof pendants have a parent rock, and are akin to xenoliths on a large scale but did not get included into the igneous mass, simply resting above it instead.

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    February 3, 2011

    Field Photo Tryout

    It's inevitable that a geo-geek will want to show off their collection of field photos. They generally show rocks, structures, facies, principles etc... tells a story and is a lot more interesting than showing a box filled with collected rocks & minerals, which in my opinion is tedious through the internet.

    Thinking about how to present them, I'm stuck between two methods I've seen other bloggers use: show a photo and give a complete explanation of what/when/where/how, or allow readers a shot at figuring out the what & how of the elements in the photo, and filling them in on the rest later on. So as a tryout, I will initially go with my preference, which is the latter. Below are two field photo's I've taken in southern British Columbia:

    Field Photo #1 (height of photo =10m)
    What do you see here? (hint: two distinct depositional environments are shown)

    Field Photo #2 (GPS = 15cm tall)
    What do you see here? (hint: mechanism at work on both rocks)
    Dunno if this format will work or not. Heck, I don't know if I have enough readers yet. But those who do read, feel free to take a stab (or rockhammer) at it in the comments. I'll follow-up later on depending on response, and change method if preferred.

    February 1, 2011

    Learning from the gentry: Banded Iron Formations

    I've only ever seen one of these, when vacationing around Lake Vermilion in Minnesota in my late teen years. At the time I had no idea the origins of the rock nor the processes that went into making the multibanded swirls. Luckily I found an old photo taken around a shore of the lake during a respite when my family was canoeing.
    BIF outcrop on Lake Vermilion, Minnesota

    The outcrop I happened to stop at shows a Banded Iron Formation that's quite deformed, with an identifiable fault, folding, and a series of what appears to be aplite cross-cuts that, if I recall correctly, were a few cm's thick. This was before the days I knew to put an object in the frame to define scale.

    I have a novice understanding of BIF's, but novice doesn't cut it anymore, especially in the latter half of my degree pursuit. So, armed with more experience and resources, I delved into a deeper understanding of these interesting Precambrian rocks. Hopefully it helps for my Structural Geology course, and takes me from BIF novice - BIF intermediate.

    Banded Iron formations are a unique rock strata that is formed of alternating bands of iron-rich layers interbedded with iron poor silica layers of mostly chert. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. The bands always alternate, and have minimal comingling of particles/minerals at the boundaries. Studies have found that the boundary between the iron-rich layers and the iron-poor silica layers is distinctly abrupt, even on a micrometer level. Each band can be millimeters thin to meters thick.

    The mineral content of the iron-rich layers have hematite and/or magnetite and/or siderite, whilst the iron-poor silica layers can have varying mineral content depending. This diverse mineral makeup of the siliceous bands, coupled with an additionally diverse array of trace & guide fossils, has spurred plenty of research into the origins of individual BIF's....whether the silica content is derived from continental erosion/deposition or oceanic currents or a dynamic mix of the two. Most Banded Iron Formations are dated from 3.8Ga (early Archaean) - 1.8Ga (late Paleoproterozoic). That's plenty of time on Earth, and thus most are of a highly deformed variety (discussed below). There is a widespread connection of deformed BIF's to the major greenstone belts of the world, thus establishing a likely mafic origin of their parent rocks. Because of their age, BIF's are found mostly in continental cratons, and all continents have one:
    • In North America, the aforementioned Vermilion area (47° 49.679'N 92° 14.358'W)
    • In South America, the Guyana shield near Port Kaituma (7° 39.824'N 59° 51.393'W)
    • In Europe, the Voronezh Massif of the Ukranian shield (49° 16.685'N 30° 20.518'E)
    • In Africa, the Liberian shield (6° 48.427'N 10° 18.602'W)
    • In Asia, the state of Karnataka on the Indian craton (15° 3.781'N 76° 35.442'E)
    • In Australia, the Yilgarn craton of Western Australia (26° 35.627'S 118° 29.693'E)
    • In Antarctica, exposures of Mount Rucker on East Antarctic craton (78°11.000′S 162°32.000′E)
    The oldest one (3.8 Ga) is found near Isua, Greenland (60° 20.788'N 45° 26.724'W).
    Various Precambrian shields of Earth
    Most studies conclude that the source of iron for the formations is from hydrothermal vents (black smokers). In a cyclical mechanism, iron was scavenged from the oceanic crust and re-deposited on the ocean floor by hydrothermal fluids. High-temperature hydrothermal alteration of early Archean oceanic crust played an important role in the deposition of the formations. Also playing a role in the construction of the alternating bands was the buildup of siliceous material near the black smokers, which eventually led to submarine landslides that covered the iron-rich layers. Those layers were evened out with the assistance of turbidity and deep ocean density currents, as seen in the following diagram:
    Hypothesized steps to achieve alternating bands of iron-rich & iron-poor layers
    Favored depositional environments for this style of Banded Iron formation mechanism include island arc basins on flanks, back arc basins of convergent plate boundaries, and rifting zones that developed within Archaean cratons. Thus deposition occurred in a shallow marine environment under transgressing/regressing seas, and even possibly on continental shelves of passive margins. 

    Just like every facet of any geological discipline, there is categorization that has emerged for Banded Iron Formations. The two different classes, derived in 1983, are of course named after where they were first studied in some detail. First we have the Superior-type sequences; not hard to figure out where those were discovered and what craton they belong to. They are thought to be deposited in shallow marine environments as largely granular & oolitic iron-formations grading into laminated iron. Deposition was primarily in adjacent basins surrounding the perimeter of uplands that projected above sea level, and seen today the type is relatively unmetamorphosed and undeformed, with widespread uncovered outcrops alongside such members as conglomerate and quartzite rocks. That is indicative of environments that were high energy, but that somewhat buffered the shield BIF's deposited in basins from marginal deformation. Secondly, we have the Algoma-type sequences, named after another part of the Canadian shield not far away from Superior (northern Ontario), and deposited in deep marine environments. Algoma BIF's are highly metamorphosed and deformed, and considered to have parent rocks that were volcanic in origin, thus they reside in extensive greenstone belts where other volcanic entities (ex. pillow lava) are adjacent to them, and their outcrops are more discontinuous and klipped relative to the Superior class.

    That was all the nitty-gritty foundational details of Banded Iron Formations, however the genesis of their unique thin bands is apparently one of the more highly debated topics in current paleogeology. Looking through online journal sources, there seems to be repetition of two major hypotheses as to the precipitation of the iron-rich layers. One approach to precipitation of free iron produced by black smokers involves a downward diffusion of CO2 that intercepts the iron upwelling from deep waters, resulting in a chemical reaction between the compounds and thus preventing iron from reaching shallow waters. This posits diffusion of CO2 as an even process in the Archaean/Proterozoic Earth, or at least for the time given for creation of banded iron formations, so that we observe the iron-rich bands as an expression of a downward diffusion of a finite volume of CO2 interacting with the iron; following that is either the quiescence of hydrothermal vent activity or heightened depositional activity from terrigenous sources or a combination of the two.

    An emerging hypothesis involves anoxygenic phototrophic organisms living beneath the ocean's windmixed surface layer, and these organisms precipitated the free iron provided by black smokers into iron oxide, an explanation for BIF deposition in a stratified ancient ocean at a constricted depth of a few hundred meters. One of the links in the additional info leads to a paper where geochemical analyses has derived a thickness of the layer where these phototrophs must live to both stay below cyanobacteria above and still get enough sunlight penetration. These phototrophs living below a mixed layer inhabited by cyanobacteria could have been responsible for the absence of iron in shallow waters and for Precambrian BIF deposition in a stratified ancient ocean. Take a look:
    diagram of layers involved in precipitation of Fe2O3 by anoxygenic phototrophs
    The phototrophs work hard when the hydrothermal vents pump out the free iron, and thus the paper considers the alternating bands evidence of either hydrothermal quiescence or a seasonality mechanism in the early Earth atmosphere. 

    The several papers I've read tend to focus highly on the quantitative geochemical elements of Banded Iron Formations, and are lacking in observational geological elements. Essentially the linking between the two facets is limited so far, and such a key connection is hopefully what future studies will focus on. As far as I'm concerned, I simply wish to have more first-hand experience with BIF's, but unfortunately for the time being I reside in a geologically young and active area, which is the polar opposite of the geologically old and inactive areas where you find these beautiful swirling rocks.

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