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:
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]