Sunday, December 31, 2017

Testing Gravity and Shear Strength ~ Ending the Year on a Humorous Note with a Handful of Pliocene Foram Fossils

No, not that kind of gravity.

Looking back over a decade of posts on this blog, I stumbled upon a minor leitmotif:  humor in science.  On a few occasions I’ve drawn attention to examples of refreshing and engaging humor unexpectedly interjected into serious work by otherwise quite serious scientists.  For instance, in one post, I enthusiastically pointed out that the geologist and micropaleontologist Cesare Emiliani, responsible for some quite important analysis using the chemical composition of the fossil shells of foraminifera to reveal key attributes of ancient climate, found the motivation and time (and, perhaps, the foolhardiness) to pen irreverent and funny articles and letters-to-the-editor for otherwise staid publications.  To wit, in an article that appeared in a 1993 issue of Eos:  Transactions, American Geophysical Union, Emiliani suggested that the burdens and challenges of peer reviewing scientific articles would be immensely relieved if authors peer reviewed their own articles; it was a no-brainer, he argued, because, “being the most knowledgeable person about his or her own work, [the principal investigator] can be expected to be totally objective.”

So it should not have been surprising that, despite my best intentions, I have managed to turn this current post, which is ostensibly about the Deep Sea Drilling Project (DSDP) and some Pliocene fossil foraminifera shells brought up in 1979, into one long buildup to a mildly amusing (at least I find it so) bit of scientific humor.  Still, I think it’s a fitting note on which to end this year.

First, let me be clear that I don't think my pursuit of humor in the hallowed halls of science is misguided.  Rather, such humor is singularly important.  I would point to Anthony Cooper, Earl of Shaftesbury (1671 – 1713), who in his Essay on the Freedom of Wit and Humor quoted an “ancient sage” (the Greek philosopher Gorgias Leontinus) (to be completely obsessive about it, Shaftesbury was, in fact, quoting Aristotle who had quoted Gorgias), as having said that
humor was the only test of gravity, and gravity of humor.  For a subject which would not bear raillery was suspicious; and a jest which would not bear a serious examination was certainly false wit.  (Shaftesbury, in Characteristicks [sic] of Men, Manners, Opinions, Times, 5th edition, 1732, p. 74, punctuation modernized.)
So bear with me as I begin this story.  The DSDP, begun in 1966 when the National Science Foundation and The Regents of the University of California first entered into contract, conducted coring and drilling in the floors of different oceans around the world between 1968 and 1983.  This and two subsequent drilling projects were designed to increase our understanding of the geophysical properties of the ocean floor.  Among its myriad scientific contributions, the DSDP provided strong evidence of continental drift and the constant renewal of the sea floors, crucial support for the theory of plate tectonics.  The initial reports and studies from the DSDP are available on the web.

The workhorse of the DSDP was the marine vessel Glomar Challenger, designed specifically to drill deep into the ocean floor and retrieve core samples for study; the ship was in action from 1968 to 1983.  (The picture of the vessel below is in the public domain and can be found on Wikimedia Commons,)

Each cruise of the Glomar Challenger was called a “Leg” (to my mind, a leg is one part of a whole so I guess that in its 15 year life span the Glomar Challenger was on a single voyage that consisted of 96 legs).  Well into its coring career, the Glomar Challenger began Leg 68 in Curaçao, Dutch Antilles on August 13, 1979.  The initial round of coring on this leg occupied 11 days as the ship drilled a tightly clustered series of holes (Holes 502, 502A, 502B, 502D) in the Caribbean Sea.  On the map below this cluster is approximately at the marker in the Caribbean Sea.  The Glomar Challenger then sailed through the Panama Canal and the shipboard team began coring at the first of three clustered sites in the Pacific Ocean.  These holes were Hole 503 cored from September 6 to 7, Hole 503A - from the September 7 to 11, and Hole 503B - from September 11 to 13.  For a reason that will become clear in a moment, Hole 503A is of particular interest to me and is the one marked in the Pacific Ocean on the map below.  Water depth at this position is 3,672 meters (2.3 miles) and the coring went 235 meters (771 feet) into the ocean floor.  (I drew much of the information provided here from the online reports on the coring during Leg 68.)

Among its objectives, Leg 68 was to test a new “hydraulic piston corer” which had been designed to deal with the problem of extracting cores from soft depositions such as those in the floor of the Pacific Ocean.  Conventional drilling machinery could not maintain the layering of this mushy sediment and had it been used at the 503 holes, whatever wouldn't have been lost at the outset “would most probably have been mashed into a murky paste of limited geological value.”  (Mort La Brecque, Coring Near the Mudline, Mosaic, September/October, 1981.)

La Brecque noted that the sediment in the Caribbean was a compacted mixture of mostly clay and calcium carbonate shells.  "As is usual with such sedimentary rock, it had lost its water content as it aged growing increasingly hard as material accumulated over it."  In contrast, the sediment in the Pacific Ocean floor not only had clay and calcium carbonate shells but also spiny fossils made of silica.  "These maintain their structure and act as jacks to support more porous, softer sediment column[s]."  In geological terms, the shear strength of the floor sediments differed significantly from ocean to ocean and from core depth to core depth.  As I understand it, shear strength is the amount of stress a material can sustain before failing.  The Pacific mush high in the sediment column doesn't have much.

That the new corer succeeded meant that another objective of this leg could be worked toward – dating of the closing of the Isthmus of Panama and analysis of its impact.  The cores that were brought up with the new corer on either side of Panama contained material that ranged in age from the present to some eight million years ago.  One of the initial studies of the planktic (living in the water column) foraminifera shells that were present in the cores drilled from the 502 and 503 cluster of holes showed that the diversity of the planktic foraminifera assemblages in the Pacific and the western Atlantic Oceans remained generally analogous from the eight million year mark until around roughly the beginning of the Pliocene Epoch (this epoch ran from 5.3 to 2.6 million years ago).  (L.D. Keigwin, Jr., Neogene Planktonic Foraminifers from Deep Sea Drilling Project Sites 502 and 503, appearing in Initial Reports of the Deep Sea Drilling Project, Volume LXVIII, 1982)  According to Keigwin, differences in the chronological ranges of selected foraminifera species on either side of Panama in the Pliocene (e.g., Globorotalia pertenuis was found in the Pacific cores until about 3.2 million year ago, but it survived until approximately 2.5 million years ago in the Atlantic) offered “further evidence that the emergent Panama Isthmus was an effective barrier to the migration of planktonic organisms by 3.0 to 3.2 MA [million years ago].”  (p. 276)

The fossil foram shells extracted from the Leg 68 (and all other) cores were carefully picked, sorted and identified, and then mounted on slides.  I know from first hand experience working with similar material from a different drilling project that this is a challenging task.  An example of one of the DSPS foram slides, in this case from core 33 of Hole 503A, is shown below.  The fossils on this slide are from the early Pliocene.  The initial photo shows the entire slide (it’s evident that some of the shells have come loose and now rattle around the slide well).  The photo of cell number 2 shows forams identified as Globorotalia menardii while that of cell 23 shows Globigerinella aequilateralis (as it was identified by the DSDP but is now known as Globigerinella siphonifera).  One of the G. aequilateralis specimens was dyed green at some point, presumably to increase the visibility of some minute attribute for identification or for photographing.

This particular slide is in my personal collection, acquired from a seller on eBay who (if I remember correctly) described its provenance shortly and sweetly as:  “It came from an estate sale.”  One wonders if it should have gone rogue like that given that the DSPS was a federally funded project.  A rock one should not turn over.

Returning for a moment to the problem that the hydraulic piston corer was designed to solve, it is clear that the shear strength of the material being cored is a key attribute and a critical element in the process, presumably dictating how easy it is to drill into the material and likelihood of being able to extract an intact core.  Not surprisingly, shear strength in the material being cored is likely to increase with depth because the deeper material is usually more compacted, but that trend is not always uniform.  Even in my ignorance, I can see how important this attribute of the sea floor at different depths is for the success or failure of any coring effort.

As part of the analysis performed during the coring process, the DSPS team on Leg 68 took what are called “vane shear measurements.”  Based on some online videos and a bit of reading, I’ve come to understand (misunderstand?) that a vane shear test is conducted with an instrument which has several blades at one end.  The extent and ease with which those blades can be turned through material at different depths yield vane shear measurements.

The formal report by the “shipboard scientific party” on Site 503:  Eastern Equatorial Pacific (Initial Reports of the Deep Sea Drilling Project, Volume LXVIII, 1982), provided and discussed shear strength measurements from the coring.  For instance, very low shear strength measurements were found high in the cores, only increasing at about 15 meters (49 feet) where it was about 400 g/cm2.  The maximum shear strength for Hole 503 was found to be 1,686 g/cmreached at 210 meters (689 feet).

And here’s the prize for sticking with me so far.  Supposedly in an effort to be helpful to readers of their report interested in shear strength, the authors penned the following paragraph which repaid me immeasurably for having to wade through otherwise dense, soporific, data-ladened text:
In order to place the vane shear measurements in the proper perspective, shear strengths were determined on several calibration samples.  Each sample was run ten times with the utmost of care.  The shear strength of day-old Jewish rye bread dough (without seeds) was found to be 47.53 g/cm2.  Cream cheese proved to have a strength of 66.13 g/cm2.  Ginger cookie dough had values of 70.26 g/cm2.  A value could not be determined for lime jello, probably because of the large pineapple inclusions interspersed throughout the host material.  Several attempts were made to measure the shear strength of chocolate chip cookie dough, but in each case the cookies were eaten before a measurement could be made.  (p. 171)
On that note, I bid farewell to 2017.

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