Through The Sandglass

Sunday sands: local Sardinian ingredients

 
What a difference a few kilometres make. Local ingredients are, more often than not, the key to a beach’s composition and, as the geology changes along a coast, so does the sand. The images above are from two beaches around twenty-five kilometres apart on the west coast of Sardinia, and, given the varied geology that is packed into that relatively small island, it’s no surprise that they are so completely different. On the left is sand from the wonderfully named Putzu Idu, made almost entirely of biogenic and limestone fragments – shells, broken shells, sea-urchin bits and pieces, and whatever that is in the centre of the image. Yes, there are a few quartz and volcanic rock grains, but this is definitely a carbonate sand. Further up the coast, the beach at Porto Alabe is totally different. Glittering quartz grains, angular and young, volcanic rock fragments and shiny, polished, orange-brown limestone grains.

Look at the geology and it’s clear why these sands are so different. Porto Alabe is backed by hills of explosive volcanic rocks, formed as the western Mediterranean fractured and fragments, like Sardinia, of the old continents moved into their current positions. Putzu Idu is backed by younger sedimentary rocks, river and coastal deposits, and limestones, apparent in the photograph, some of them travertine, chemically formed from springs.  

  
I wasn’t completely sure that the polished orange-brown grains from Porto Alabe were, indeed, limestone, and so, not having any hydrochloric acid to hand, I poured a little vinegar on them (appropriately, a chardonnay vinegar from Italy). The telltale bubbles soon appeared, some of course from the bits and pieces of shells, but others clearly originating from the grains in question. I’d never looked at such things down a microscope before – it’s really quite captivating. And yes, I have to get a life….

 

[Many thanks to my brother-in-law, Hans, for collecting these sands for me. Beach photos from Panoramio and Travel webshots.] 

Sand and ice 1: deep time and the struggles of life

Catching up on catching up, I finally settled down for an hour or so to attack the pile of journals and magazines that had accumulated during my travels – a forlorn task, since there is always a pile. The two at the top of the stratigraphy just happened to be the recent issues of GSA Today and the Proceedings of the Geologists Association, and they just happened to present me with a pleasing and fascinating convergence on a theme: sand and ice. I had originally thought to combine the two into one post, but each is intriguing enough in its own right to make this a miniseries.

Sand, more often than not, brings to mind images of balmy beaches and baking deserts, and it’s easy to forget how many of the sands of Europe and the eastern coast of North America owe their origin to distinctly more frigid climes: the Ice Ages. New York was built from the glacial sands of Long Island and windblown sands of glacial origin still blow across the landscapes of Europe. My second post in the miniseries will look at reconstructing the landforms directly associated with receding glaciers, but for now let’s look at how the strange processes of freezing beach sands shed light on ancient climates and episodes in the ebb and flow of biodiversity hundreds of millions of years ago.

Over the past couple of hundred years, geologists have done a pretty good job of cataloguing the peculiarities of sedimentary rocks and, through comparison with today’s observable processes, revealing what stories they tell. But even so, oddities and mysteries continue – thank heavens – to show up. I will readily admit that, if were me examining the outcrop in the photo below, I would be mystified. In amongst these orderly beds of sandstone – that I might well have correctly identified, from their character and internal structures, as being from an old shoreline – is a chaotic layer filled with disconnected and jumbled slabs of sand of all different sizes.

 
I might well reach into the lexicon of geo-jargon stored in my brain and declare that these are intraclasts – larger fragments of a sedimentary rock that is itself made of fragments (clasts). But calling it something doesn’t do anything to explain its origin. Intraclasts are often made of mud, sufficiently coherent and solid to be ripped and broken up, perhaps by waves and tides, to be then buried by more mud. But these slabs are made of sand that is as easily crumbled as the sand between and around them – there is little cementing the grains together. But when these slabs were broken up and jumbled about, there must have been something cementing the grains, temporarily solidifying, lithifying them. What was going on here, what could that cement have been?

Cut to the shores of Lake Superior in winter; the image at the head of this post, taken from the cover of November’s GSA Today (full citation below), shows slabs of sand buffeted by the waves – it’s ice that forms the cement and prevents the slabs from disintegrating. The lake itself is not frozen, but freezing of snowmelt and stream water between the sand grains of the beach and dunes results in a material that has a “similar strength to weak Portland cement concrete.” If the freezing takes place only down to  a depth of a few centimetres, or if the deeper sand thaws, then the waves will erode the uncemented sands, undercutting the frozen layer that will then collapse into broken slabs, moved around by the waves and subsequently buried in sand in the next storm. These two images show the correlation of features in one of the old sandstone slabs with their modern equivalents: 

 
During the transition between freezing and thawing, thin films of water around the grains result in the slabs being ductile, bending and draping themselves over irregularities in the surface beneath:

 
The paper from which this story is taken is by Anthony Runkel at the Minnesota Geological Survey and his colleagues at Carleton College and the University of Minnesota, with the title, as shown on the header image, “Tropical shoreline ice in the late Cambrian: Implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event.” The story of the origin of the strange sandstone slabs is fascinating and thoroughly documented: here’s another set of photographs of the phenomenon, ancient and modern, together with the description:

 
Large sandstone intraclasts of the Furongian Jordan Formation, Minnesota, USA, and ice-cemented sand clasts on modern, temperate, fresh-water shoreline of Lake Superior. (A) Intraclasts (up to 1.2 m in length) mantle a scour surface and are overlain by beach swash sands (late diagenetic iron oxide staining accentuates intraclast margins). (B–C) Examples of in situ brecciation (intraclast formation) by undercutting and collapse of hard (frozen) swash-zone sand in the Cambrian Jordan Formation (B) and the modern partially frozen, temperate fresh-water shoreline of Lake Superior (C). (D–E) Imbricated intraclasts in the Jordan Formation (D), and imbrication of frozen clasts along modern shoreline (E).

But the story has far more to it than this. The “Furongian Jordan Formation” in which the sandstone slabs are found is made of sands originally deposited around the shorelines of the old North American continent, Laurentia; the “Furongian” refers to the time division of the Late Cambrian period, 488 to 501 million years ago. At that time, the old core of North America lay around the equator and was oriented roughly ninety degrees differently from today – here’s the reconstruction of the geography and the sediments of those times:

These were critical and challenging times for life. The earlier Cambrian records the great “explosion” of life and biodiversity, but this was not to last. By the time the Jordan sandstones were being deposited, life was struggling, biodiversity had levelled off at best, and there were a whole series of extinction events that decimated the fauna of the ancient continental shelves. Later on, the struggle would be won, as recorded by the “Great Ordovician Biodiversification Event” (otherwise known as “GOBE”).

Exactly what caused the Late Cambrian fluctuations in biodiversity has long been a topic of debate, but the general interpretation has been that this was a period of sustained greenhouse conditions under which temperatures and atmospheric-ocean chemistry were challenging and, on occasion, fatal. But this paper offers a different interpretation. It was not that much earlier, geologically speaking, that the planet had undergone drastic climatic changes, with evidence suggesting a “snowball earth” in which ice may well have essentially covered everything. The Cambrian “explosion” was thanks to warming, but could the climatic instabilities that had plunged the earth into the deep freeze have remained lurking in the system and periodically staged a reprise during the later Cambrian? The evidence that Runkel and his colleagues present is compelling (and there is other support in the geological record that they discuss) – not of another snowball earth, but of dramatic and deadly cold periods in which, even at the equator, freezing conditions swept the continental shelves, rendering conditions there impossible for much of life.

It’s a great story, one that moves from strange slabs of sandstone in Minnesota to the way the earth looked 500 million years ago, from ancient to modern and back again – and provides important fodder for the debate on the early struggles of life on our planet. The stories that sand can tell….

 

[The paper can be found in full, open access, at http://www.geosociety.org/gsatoday/archive/20/11/pdf/i1052-5173-20-11-4.pdf. A couple of further photographs are at  ftp://rock.geosociety.org/pub/reposit/2010/2010290.pdf.

The full citation is: Tropical shoreline ice in the late Cambrian: Implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event

Anthony C. Runkel, Minnesota Geological Survey, Univ. of Minnesota, 2642 University Ave. W, St. Paul, Minnesota 55114-1057, USA, [email protected]; Tyler J. Mackey*, Clinton A. Cowan, Geology Dept., Carleton College, 1 North College St., Northfield, Minnesota 55057-4001, USA; and David L. Fox, Dept. of Geology and Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, Minnesota 55455-0129, USA

*Now at Geology Dept., Univ. of California, One Shields Ave., Davis, California 95616-8605, USA

GSA Today, v. 20, no. 11, doi: 10.1130/GSATG84A.1]

 

 

Sun-day sand

For those of us trapped in the wet-dog-days of autumn grimness, of unremitting gloom, cold, and damp, when the sun recedes in the mind into merely a concept, increasingly difficult to visualise, then a little relief, if only imaginary, is in order. Of course, this image is undeniably photoshopped, but then this is, after all, a creative effort. The sand perhaps naturally carries with it the genetics of a sun-drenched beach; it’s not an extraordinary sand, no exotic minerals, no shapely foraminifera or other organic designs are on display. But under a microscope no sand is really ordinary.

This one is from the Playa de los Muertos on Mexico’s Pacific coast near Puerto Vallarta. It contains quartz, limestone and other grains of rock from the cliffs along the coast and the hinterland, along with the occasional shell fragment.

 
It’s a highly popular resort beach. Its name, “the beach of the dead,” is hardly enticing and there are any number of stories as to the origin of the name – Indian attacks on silver-miners, pirates, smugglers, and graveyards. But we must remember the cultural context: the Dia de los Muertos is a major and cheerful celebratory occasion in Mexico. The name would certainly not bother me, but I’m not much for popular beaches and prefer poking about in the sand to simply lounging – but boy, does this place look good from London in November.

 

[Thanks to Betsy Kimak at Sandrific for this sand.] 

A prominent role for a sandglass

 
I promise that I’ll get back to more serious topics shortly, but I’m still trying to catch up on catching up. Among recent distractions was the fact that on Tuesday I was giving a presentation at a meeting of The Geological Society here in London (in other words, The Geological Society). The talk was on Ralph Bagnold, a personal and scientific hero; I could go on at some length about the man and his achievements and legacy, so the time-consuming challenge in preparation was attempting to do justice to him in thirty minutes.

I had in mind to write a post on one of the recent papers that I have been reading, but this requires a bit more digging and so must be postponed. However, for now, an amusement. The Geological Society resides in some splendour on Piccadilly, its historic “apartments” being in Burlington House, also home of the Royal Academy and several other learned societies. The Society was originally established as a dining club “for the purpose of making geologists acquainted with each other, of stimulating their zeal, of inducing them to adopt one nomenclature, of facilitating the communications of new facts and of ascertaining what is known in their science and what remains to be discovered.” I can only hope that my talk stimulated at least a little zeal….

Across Piccadilly from Burlington House is the illustrious, old, and exorbitant emporium of Fortnum and Mason, whose wares stimulate a zeal in the appetite but a reluctance to open one’s wallet. US readers may be amused to read the following on the store’s history, quoted from their website:

1773 Boston Tea Party inspires American Independence movement

It is not on record who supplied the tea, but it was probably not us: we have never charged extortionate prices and none of our teas do at all well with salt water. Since Independence, though, our American cousins have been among our most loyal customers.

  En route to the GeolSoc, I was walking past Fortnum’s and the Christmas window displays for which they are famous. I was, of course, delighted to see, as a prominent motif in one the extravagant rotating arrangements, a sandglass: my thesis on the ubiquity of sand justified again. The photo at the head of this post also features Burlington House in the reflection.

Sunday sand: new-born grains and an exotic mineral

 
Southwest Greenland. In among the even older rocks is a huge intrusive igneous complex formed more than a billion years ago. Exhumed by erosion to form the dramatic topography of fjords and glacial terrains and the subject of geological scrutiny, these rocks are indeed complex; many different varieties reflect a series of pulses of intrusion and the compositional evolution of the fluids in the original magma chamber, not to mention their chemical interactions with the rocks into which they were intruded, intrusions into intrusions. Exposed not only to geologists but to weathering, the rocks of the Ilímaussaq complex are rotting and disintegrating, shedding their detritus into scree (or talus) slopes along the flanks of the cliffs and, ultimately, into rivers and fjords. Amongst this detritus are, of course, sand-sized grains reflecting the sizes of the original minerals, new-born sands. And, in amongst the many wonderful samples sent to me not long ago by a very generous arenophile colleague in the Netherlands, was a sample of these sands from the area of Kvarnefjeld, a modest mountain within the intrusive complex not far from the town of Narsaq. This area, and the overall location are shown below:

The entire Ilímaussaq complex is made up of exotic rocks with exotic names. Overall, these rocks contain little or no quartz and are thus relatively low in silica and referred to as alkaline; commonly, the dominant minerals are feldspathoids, resembling feldspars but with a different structure and much less silica. In terms of their broad family background, they belong to the group of alkaline igneous rocks termed syenites, but many of their specific compositions and minerals are exotic. There’s an excellent and quite detailed summary of the geology and mineralogy in the Geology of Greenland Survey Bulletin from 2001, and the rock types shown on the geological map include lujavrite, arfvedsonite lujavrite, kakortokite, naujaite, sodalite foyaite, and pulaskite. Give a petrologist a slight variation in composition and off they run conjuring up entirely new and fantastic names, challenges to remember or even spell. In fairness, of course, many of these names derive from the locality at which the rock type is first identified so a degree of cultural and linguistic tolerance is in order, but even so….

Exotic rocks contain exotic minerals, and there are plenty of those to be found in the Ilímaussaq complex, the type locality – the definitive location – for many of them. In among the rock fragment grains in the sand at the top of this post, and embedded in some of those rock fragments, are diamond-shaped minerals: these are the mineral naujakasite, a sodium-iron-manganese-aluminium silicate with the formula Na6 (Fe+2,Mn+2) Al4 Si8 O26. The original specimens came from a small boulder collected in 1897, but the mineral was tentatively identified as a variety of the mica-like mineral, chlorite. It wasn’t until 1933 that it was recognised as a new mineral and was named after the place where it was found. It later became clear that naujakasite occurred widely through rocks of the complex, particularly in the lujavrites, but for a long time this was its only known occurrence in the world. But in 1998 it was found in the Lovozero alkaline complex of Russia’s Kola Peninsula – and today these remain the only two known locations of this mineral. A full description of both can be found in the same Geology of Greenland Survey Bulletin linked above.

Naujakasite itself is of interest for its rarity, but it has no particular value or use. Not so with the rock-type in which it’s found, lujavrite. This is an important source of uranium and, of even greater interest today, rare earth elements. These comprise the fifteen lanthanides and yttrium, and were named rare earths after their original identification in the 18th century in rare minerals. These elements have a wide variety of clever and critical roles in all kinds of high-tech applications and were for a long time extracted from placer sand deposits around the world. Today, however, the market is dominated by China, the minerals extracted from hard-rock ore bodies in Mongolia. Given the economic importance of these elements, this is creating momentum for the search for alternative supplies: the Ilímaussaq complex is one focus of this effort; an example of this, with a good geological description, can be found in the recent bulletin by Greenland Minerals and Energy Ltd. Here’s an example of the occurrence of a black lujavrite from that publication – shedding its sandy detritus into the fjord below.

Exotic rocks, exotic minerals, exotic names, and exotic locations – many thanks, Carla, my arenophile friend, for providing the source of this Sunday sand story. 

A catching-up miscellany

 
I am now returned to dry land and trying to catch up after the frustrations of bandwidth-challenged satellite internet on board the Queen Mary 2 (not that I am in any way complaining about the voyage – it was a really memorable experience). In the early hours of Monday morning we slipped successfully past the Bramble sand bank (see chart above, northern tip of the Isle of Wight at the bottom) in the mouth of the Solent on our way to berthing in Southampton. It had occurred to me that, given that this was the topic of my inaugural blog post almost exactly two years ago, and was where the Queen Elizabeth 2 had run aground on her retirement voyage, it would be entirely appropriate and quite entertaining if we experienced a bump in the night and a repetition of of our predecessor’s experience. This did not happen, but because of storms and berth congestion we docked at a different terminal from that planned. Having remarked on the relative ease and pleasure of this method of travel compared to flying (simple check-in procedures, immigration officials on board to get to that business out of the way in advance of arrival, excellent food), we were to be brought rapidly back to reality; on disembarkation we emerged into what can best be described as a vast, bleak, shed where total chaos abounded. It seemed that a conspiracy of arrivals in Southampton that morning had resulted in 11,000 passengers arriving and trying to continue their journeys. Southampton was gridlocked and the queue for taxis was an hour-and-a-half long. It struck me that this was not the ideal way to welcome visitors to our sceptered isle…. 

So, before recommencing business as usual on Through the Sandglass, I will simply provide a miscellany of news and amusements harvested during my attempt to catch up with events. First of all, from while I was away and talking of vessels running aground in the sand, we had the remarkable adventure of the poorly named British attack submarine, HMS Astute, that ran into the sands of the Scottish coast and had to be ignominiously towed off on the next high tide. Having commented in the past on the fact that the rate at which coastal sand banks morph their topography essentially renders nautical charts outdated as soon as they are printed, I enjoyed the statement that “Some reports suggest that outdated charts could be to blame.”

Back to one of my favourite (if that word can be applied to something infuriating) topics – the sand berms in the Gulf of Mexico. Governor Jindal and his cronies have succeeded in spending $220 million on little more than ten miles of environmentally reckless sand-shifting and they are now announcing a rethink. On November 1 the Governor’s Office announced that:

The Governor provided an update on the $360 million sand berm project that was launched to protect Louisiana’s coastal areas from the oil spill. To date, nearly 17 million cubic yards of sand has been dredged. This includes over 12 million cubic yards of sediment that was dredged from the Mississippi River and placed in the barrier island chains.  To put that in perspective, Louisiana loses a football field of land every 38 minutes or about 13,800 football fields a year.  Thus far, the dredging project has moved enough material with these sand berms to pile one foot of sand on over 12,100 football fields….

Governor Jindal announced an agreement to commit up to $100 million of the remaining sand berm funding to convert the sand berms into Louisiana’s historic barrier islands that have eroded.  The other $40 million in remaining berm funds will be dedicated to berm construction, stabilization, environmental support and other compliance costs associated with the berm work.

So this has now become a “coastal restoration project.” Given the fact that no notice whatsoever has so far been taken of scientific expertise, it’s difficult to have any confidence in these people understanding that “to convert the sand berms into Louisiana’s historic barrier islands” is a far from simple task or their seeking advice from those who know what they are talking about. Nature has recently been reporting from the Geological Society of America’s annual conference in Denver, including a summary of the session added to the program to discuss the status of the aftermath of the oil spill. Rob Young, whose wisdom I have often quoted before, was instrumental in the session:

Young said: “Not only were scientists not involved in planning this massive project, but [the Louisiana governor’s office] was pretty uninterested in scientific feedback.” What’s needed in the future, Young said, is a way to bring scientists to the table very early on to “peer review ideas on the fly”.

Unfortunately, the word “fly” immediately brings to mind an image of this guy taking to the air:

Meanwhile, I was struck by a couple of items from India. The headline “Sand mafia strikes again, assaults Forest officials in Chandigarh periphery” caught my eye: it seems that illegal extraction and smuggling of river sand in the Punjab has become a major and violent criminal problem. And, among the many ways in which President Obama was welcomed on his recent visit was a sand sculpture – somewhat unflattering, I would say, but the sentiment is excellent:

 
Around the blogs (just the initial catch-up – I have a total of 680 unread entries in Geobulletin), for nature’s stunning designs, Pathological Geomorphology has some spectacular satellite imagery of channel patterns near the confluence of the Urmi and Amur Rivers in Russia, and the latest Accretionary Wedge offers the usual entertaining reading, including the piece by its host, Matt Kuchta, on his chunks of trinitite, “the glass made out of fused sand and rock and bits of equipment (casing, wire, etc) that formed in the explosion from the first atomic bomb,” which glow eerily and beautifully in UV light:

Finally, Atlas Obscura has an article on Singing Beach at Manchester-by-the-Sea in Massachusetts; the voice of the sands may be more squeaky than melodious, but there’s another place to add to my list of must-do destinations – once I’ve caught up from my most recent travels.

    

 

Ship's Log: 3 November

Yesterday passed Sable Island and the Sable Bank, one of the graveyards of the Atlantic, gigantic volumes of ever-shape-shifting sands. Currently the Sargasso Sea, south of the Grand Banks, passing the location of the Titanic; speed 23 knots, weather fair but windy,seas moderate – ship’s Brown Bros/Rolls Royce stabilisers functioning well. Ship’s TV carrying video of interesting lecture on sand. Next lecture scheduled Friday, shortly after crossing Mid-Atlantic Ridge. On board satellite internet slow, communication limited. Correction to previous post: this is not a cruise, it should be referred to as a Trans-Atlantic crossing – apologies. Mainbrace still spliceable.