Wednesday, February 10, 2016

Sizing Up Sand a Grain at a Time

The world is a mist. And then the world is
minute and vast and clear. The tide
is higher or lower. He couldn't tell you which.
His beak is focussed; he is preoccupied,

looking for something, something, something.
Poor bird, he is obsessed!
The millions of grains are black, white, tan, and gray
mixed with quartz grains, rose and amethyst.

~ from the poem Sandpiper by Elizabeth Bishop

I am the sandpiper.

So often my search for fossils, large or very small, takes me to sand, from beach sand at the foot of eroding cliffs to the soupçon of sand in a shell.  I suppose it was inevitable that I would become a sand lover, an arenophile, with beach sand the focus of my affection.

Perhaps the best point from which to launch an exploration of the science of sand is geologist Michael Welland’s Sand:  The Never-Ending Story (2009).  Given the riches of this book, it may be the only text one needs.  This is an authoritative account describing the central, though unheralded, roles that sand plays in our lives and the life of the planet.  On a journey, much like that which Welland posits for a grain of sand traveling down the Susquehanna River to the Chesapeake Bay and on to the Atlantic Ocean, the reader is carried easily from the simple and seemingly trivial, such as why your footprints on wet sand briefly have a halo of dry sand, to an exploration of fundamental phenomenon, such as how sand’s behavior mirrors, at times, a solid and, at others (more dangerously) a liquid.

The book has put the lie to my pat response to anyone who, after observing me working with the fossil shells of foraminifera or ostracodes under the microscope, asks, “What are you doing?  Studying sand?”  Until now, I’ve responded, “No, I’m working with microfossils.”  Though in essence true, I now know that this answer begins with a denial that is false.

Sand is defined by the size of its rock and mineral particles, not the composition of those particles.  Particles between the 0.0625 mm to 2 mm are technically considered sand.  So, yes, I am studying sand; those calcium carbonate hard parts left by marine microorganisms are sand.

Though what I have under the scope is not what is typically thought of as sand, most sand does conform to that expectation – some 70 percent of the sand particles on this planet are made of quartz.  Pictured below is a sample of beach sand from Miamogue Point, New York, a spit of land that marks the boundary between Flanders Bay and the Great Peconic Bay, a couple of the bays that separate the North and South Forks at the east end of Long Island.  This particular sample meets sand expectations as it is composed largely of quartz.  Of course, depending upon when and where on the Point one takes the sample, other kinds of minerals and bits of shell will certainly be mixed in.

Welland observes,
Quartz – otherwise known as silica, SiO2 – is the potato, the staple ingredient, of sand cuisine.  Some sands are made of essentially 100 percent quartz . . . .  Other sands contain no quartz at all.  We know that sand is simply a matter of size; therefore anything reasonably hard that presents itself as a sand-sized grain is entitled to form a sand.  And a remarkable variety of things do.  (p. 15)
Sand comes in a seemingly endless array of colors, textures, and, certainly, composition.  That “remarkable variety of things” composing sand often comes down to what’s available.  For instance, Welland notes, “On the Normandy beaches where D-day landings took place, you will find sand-sized fragments of steel.”  (p. 15)  Beach sand often reflects the rock that is available locally.  Material eroding from inland formations may supply the beach.  The ocean may supply the beach, and that’s where things get interesting.
The living world contributes huge amounts of biogenic sand to coasts, beaches, barrier islands, and shoals.  Particularly in regions rich in marine life at every scale, from coral reefs to minute floating organisms, much of the sand is the debris of this activity.  Many of the world’s prized tropical beaches, which are often far from major landmasses, are made up almost entirely of broken pieces of shells, corals, and the other hard parts of marine creatures.  The inhabitants are the only source of the sand.  (p. 18) 
Pictured below is a sample of sand from a beach at Ft. Lauderdale, Florida.  It includes a fair amount of biogenic sand consisting of well-worm fragments of shells and corals.

Sand from the Little Barren River in Kentucky is composed of material eroding from the Fort Payne Formation which is dated to the Late Mississippian epoch (about 331 to 323 million years ago).  Though composed of material that is largely inorganic, the sand contains a few ringers.  Scattered in this Little Barren River sand are distinct circular objects (two show up in the picture below) that offer a biogenic touch.  These are fossilized skeletal segments or ossicles, originally secreted by crinoids, invertebrate animals, to create their exoskeletons.

A wonderful biogenic sand sample comes from the small island of Hatoma, part of the Okinawa prefecture, in Japan.  The island is pictured below in a map from Google Earth.

As seen below, Hatoma (with the marker) is part of a small cluster of islands.

To put the island further into geographic context, Hatoma (with the marker) actually lies closer to Taiwan than to Okinawa.

What makes sand from beaches on Hatoma Island so special is that it is purely biogenic sand.  Consider the sample pictured below; everything, and I mean everything, shown in this picture is recent organic material and nearly all of it consists of the calcium carbonate shells of those single-celled protists called foraminifera (discussed frequently on this blog).

This sand, found on a number of beaches on Japanese islands, is called star sand, presumably because of the star-shapes of several of the most prominent types of foraminifera found in it (see picture above, and discussion below).  There is a sad Japanese folktale, as recounted by Welland, which offers an explanation for how sand in this shape came to be found on one of the islands.  Star sand, so the story goes, is composed of the bodies of babies delivered a long time ago by the Southern Cross and fathered by the Polar Star in an ill-fated effort to bring life to Earth.  This effort angered the Seven-Dragon sea god who engineered the babies’ death; their bodies then washed up on the beach in countless numbers.  In an act of kindness, a local goddess placed some of the bodies in her incense burner so they could rise with the smoke and rejoin their mother in the sky.  (Hence the practice that continues to today of putting some sand in incense burners.)  In another context, I’ve been told that this sand simply brings good luck and, so, people often keep a small sample nearby.  Certainly, hard to reconcile good luck with the melancholy folktale.

The diversity of foraminifera genera and species in my sample of star sand is staggering.  I’ve barely begun to find what’s here and already I feel overwhelmed.  I’ll close this post with a closer look at a few of the different kinds of grains that make up star sand.  (These identifications are made with varying degrees of confidence.)

Baculogypsina sphaerulata

Calcarina spengleri

Neorotalia calcar - showing both sides

Sorites orbiculus

Amphistegina lessonii - showing both sides

Peneroplis planatus - showing both sides

I am the sandpiper searching among grains of sand for . . . grains of sand.

I consulted various sources to aid in the identification of sand star foraminifera species, including the Foraminifera Gallery of the website, and Jean-Pierre Debenay’s A Guide to 1,000 Foraminifera From Southwestern Pacific:  New Caledonia (2012).


Nature Blog Network