Wednesday, April 29, 2020

Our Days Are Longer


Nobody told me there’d be days like these.
Strange days, indeed.
Most peculiar, momma.
~ lyrics from Nobody Told Me,
composed by John Lennon and Yoko Ono


I’d been thinking about time even before I came under a stay-at-home order, and before jokes like the following about quarantine time flashed across the web:
I’ve eaten 14 meals and taken 6 naps and it’s still today.  Are you kidding me? 
2020 is a unique Leap Year.  It has 29 days in February, 300 days in March, and 5 years in April.
This was also before I made and exchanged stop-action movies with my granddaughter.  Such an appropriate medium to work in these days because there is so very little to show for a nearly endless investment of time.

My pre-isolation thoughts about time had been prompted by a research article titled Subdaily-Scale Chemical Variability in a Torreites Sanchezi Rudist Shell:  Implications for Rudist Paleobiology and the Cretaceous Day-Night Cycle (Niels J. de Winter, et al., Paleoceanography and Paleoclimatology, volume 35, 2020).

Such a catchy title.  The text is similarly dense, if not opaque.  This is a narrow, complex, technologically-driven analysis; my understanding of its methodologies and results remains shaky.  Nevertheless, I am enthralled by what I do understand of the conclusions that the authors derived from their study of a single, common, Late Cretaceous mollusk.  Their study of this denizen of marine waters allows de Winter et al. to address aspects of the Earth-Moon system.  As I tried to unpack how they could make such a leap, I came upon a Darwin connection (I find such connections irresistible).  Well, more specifically, a link to George Howard Darwin, Charles' second oldest son, and I turn briefly (very briefly) at the end of this post to George Darwin.

The mollusk at the center of this analysis is a rudist of the species Torreites sanchezi, which lived in the Late Cretaceous during the Campanian Age (84 to 72 million years ago).  Rudists were invertebrate marine animals that lived from the Jurassic through the Late Cretaceous before going extinct in the End-Cretaceous event.  These were successful animals, existing in such numbers that they were the predominant reef-builders during the Cretaceous.  Rudists manipulated the two-valve shell paradigm of mollusks by greatly exaggerating one valve which was anchored to the sea floor and by significantly reducing the other to a small lid on top.  The image below perfectly captures, to my mind, the quintessential facets of rudist morphology.


Though this image turned up in a search for images of rudists and it bears a web address seeming to show it to be part of the collections of the Smithsonian’s National Museum of Natural History, I cannot find it in the Museum’s online collections no matter how creative I get.  I believe this is an image of a Vaccinites rudist (not the rudist at the center of the article, but similarly structured).  (If it is a Smithsonian image, then it’s likely to be in the public domain.  If not, then I will remove it upon request.)

Another nice image of rudists (also specimens of the Vaccinites genus) appears in an entry posted by geologist Mark A. Wilson (The College of Wooster) in the Wooster Geologists Blog (Wooster’s Fossil of the Week:  A Most Unlikely Clam – Rudists From the Upper Cretaceous of the Oman Mountains, January 23, 2011).  (This image has been placed in the public domain by Mark Wilson and is available on Wikimedia Commons.)


More information on rudists can be found on the University California Museum of Paleontology’s website.

Many mollusk species add to their shells on a daily basis, particularly in their first years of life, and so can preserve a detailed physiological and environmental record of their lives.  De Winter and his colleagues capitalized on this attribute and analyzed two longitudinal cross sections from a single, intact rudist shell from the species T. sanchezi, collected in Oman and belonging to the Natural History Museum of Maastricht.  They applied advanced technology to analyze multiple aspects of the layers or laminae laid down, including ratios of different isotopes and trace elements found in the laminae.  These ratios and their changes over time are associated with water temperature and salinity levels, among other environmental attributes.  Given strong evidence of seasonal variation in the data, de Winter and his colleagues were able to group results from laminae into annual clusters and, then, divide them still further into daily and less-than-daily units.

In essence, the researchers reconstructed a record of the day-to-day and the daytime-to-nighttime growth of this specimen over a nine-year span when it lived in the Late Cretaceous.  And, from the extremely detailed data they amassed from this T. sanchezi specimen, de Winter et al. were led to hypothesize or conclude the following:
  • for this rudist, a single lamina is likely to have been laid down daily, giving a composite estimate of approximately 372 days in a year at this juncture in the Late Cretaceous;
  • based on the estimate of 372 days in a Late Cretaceous year, the length of a day was determined to be approximately 23 hours and 31 minutes;
  • further, the length of day estimated in this study allowed the authors to estimate that the Moon was slightly closer to the Earth at that point in the Campanian Age (at an estimated distance of 383,000 km versus 384,000 km today) (more on that below);
  • the growth patterns of the laminae of this rudist specimen appeared to be sensitive to the day-night cycle, probably because this specimen housed a photosynthesizing symbiont which needed sunlight;
  • ratios of certain minerals in these laminae generated estimates of environmental temperatures on a daily and annual basis, suggesting that summertime water surface temperatures were possibly quite high (between 34 to 42 degrees centigrade or 93 to 107 degrees fahrenheit), certainly above what they are today; and
  • the methodologies utilized in this study provided a finely detailed record of growth patterns at a below daily level for this rudist specimen and so, offer "a new, more accurate tool to determine the length of day in Earth’s history” and, further, “potentially allow paleoclimatologists to bridge the gap from climate to weather reconstructions in geological past.”
I find it pretty amazing that, from analysis of the fossil shell of a “lowly” mollusk shell, the authors can speak to the Earth-Moon system.  This is dependent upon the principle of the conservation of angular momentum between a planet and a satellite, a phenomenon that allows one to relate the length of the day on Earth to the distance between the Earth and Moon.  Angular momentum is the product of the mass, speed, and distance to a fixed object; in the Earth-Moon system, this is the sum of the Earth’s angular momentum and that of the Moon, a total largely fixed since the origin of the Moon.  (See, The Story of Earth:  The First 4.5 Billion Years. From Stardust to Living Planet, by Robert M. Hazen, 2013.)  As a consequence of the friction of tidal waters on Earth, its rotation has slowed (lengthening the day), but, in order to conserve angular momentum, the distance between the Earth and Moon has had to increase.  As a result, the moon has slowly, very slowly, been pulling away from the Earth.

As I wrote at the outset, I will end with Sir George Howard Darwin (1845-1912).  It was supposedly said of George that, with his family pedigree, he was "born in the scientific purple."  (As quoted in George Howard Darwin, University of St. Andrews.)  He studied for the bar but spent his professional career as an astronomer.  The understanding of the role of tides in the Earth-Moon system is based in good measure upon the work of George Darwin.  In his The Tides and Kindred Phenomena in the Solar System (1899), a summing up of his work on tides written for a general audience, Darwin stated:
A day is the name for the time in which the earth rotates once, and a month for the time in which the moon revolves once.  Then since tidal friction retards the earth’s rotation and the moon’s revolution, we may state that both the day and the month are being lengthened, and that these results follow from the retardation of the time of high tide.
It must also be noted that the spiral in which the moon moves is an increasing one, so that her distance from the earth also increases.  These are absolutely certain and inevitable results of the mechanical interaction of the two bodies.  (p. 271-272)
Though many of the conclusions Darwin reached regarding such things as the origin of the Moon are no longer supported, his analysis of tidal evolution, reflected in de Winter’s calculations, remains in play.

And, yes, our days are longer.


 
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