Tuesday, December 29, 2015

Winter Solstice & End of the Year Celebrations

The end of the year always brings a bit of reflection. We have now passed the year's winter solstice, which occurred in the Northern Hemisphere on December 22. Our planet Earth is, and all of us living in it are, moving towards perihelion when we will be closest to the Sun. But wait, that makes no sense. How come if we are closer to the sun we are in winter!? Well, that makes more sense for the Southern Hemisphere, than to us in the Northern Hemisphere. This is because the Earth's path of rotation around the sun, which is a year in our human calendars, is not a circle but instead is oblong or ellipsoid. This means that there will be times when the Earth will be closer to the Sun, and others when we are farther from the Sun, as we revolve around in our orbit around the Sun. Come January we will be in full perihelion.

During the winter solstice, the Northern Hemisphere angles away from the sun's direct rays, but the Southern Hemisphere angles towards it. The angling away from the sun gives us winter, whereas is summer in the Southern Hemisphere.

The winter solstice marks the longest night of the year, and the shortest day of the year. Right now we are experiencing earlier sunsets, longer nights, and shorter days. This will continue until de Spring. This natural astronomical shift has been a source of magical mysticism and celebration for millennia.

14th Century "Roman de Fauvel" from the Bibliotheque National de France, Paris).
It depicts a sort of feast, usually associated to the medieval Feast of the Fools.

The solstices are astronomical phenomena. They have been a source of celebration and festivities for many western cultures. Ancient civilizations have long observed these variations, many which celebrated the changes or seasons and erecting monuments to help them predict their approach. Famous examples of these are, of course, Stonehenge in Southern England, and  Machu Picchu in Peru.

Intihuatana stone, in Machu Picchu. A calendar stone atop the ruins of Machu Picchu, rediscovered by Hiram Bingham in 1911.

These festivities we still celebrate today but under different names. The pagan Anglo-Saxons celebrated Modraniht around December 25, other celebrated Yule (more common among the Wiccans of today), and the Romans had Saturnalia. Roman Saturnalia  was a celebration around all the end of the year in honor of the god Saturn. During the medieval period, Saturnalia was known by Christians as the Feast of Fools of Festum Factuorum, and the Yule Log celebrations, many which were altogether later adopted and modified into our modern Christmas.

Yule Log illustration in Robert Chambers 1864 edition of Books of Days, pg. 734. Wikipedia commons.

On that note, happy end of the year festivities, whichever way you may call them. May the new year bring you happiness and health.

Sunday, December 13, 2015

This Was Once the Bottom of the Caribbean Sea

The rocks that make up these low hills are part of the ancient Caribbean sea. In a way, the rocks of this abandoned quarry are a museum, a small window into a shallow sea that divided northwestern Cuba during the Miocene, between 23 and 5 million years ago (fig. 1). This is the Guines formation. Its low rolling hills encase the valleys between Havana and Matanzas, and other parts of northern Cuba. Its rocks are  especially interesting for their richness in fossilized teeth of the extinct Megalodon shark, whales, and with them the many clues they hold about the ancient Caribbean sea world.

Fig. 1: These hard limestones were once the bottom of a warm and shallow sea in the northern Caribbean.

Carcharodon megalodon Agassiz, 1843 is a giant version of the great white shark (Carcharodon carcharias) of our modern oceans, and one of the top predators of its time. It is also one of the most recognized fossils, and one of the most coveted by fossil collectors. In Cuba, fossils of this extinct shark are interestingly common on limestone of the Habana-Matanzas carbonate hills, where local quarrymen frequently find them among whales, dugongs, and other shark fossils (fig. 2) (Iturralde et al., 1996).

Fig. 2: Carcharodon megalodon from the Guines formation. 

Overall the fossilized teeth are superbly preserved. Many can be found together with other teeth, invertebrate fossils, or alone encased in the rock matrix. This same matrix holds microscopic clues to the environment - the paleoecology of the ancient seas where these and other creatures lived - plus clues to how they came to be preserved in such way for millions of years.

On closer inspection, the limestone of the Guines fm. are composed of small microscopic organisms, large mollusks (gastropods and bivalves), sea urchins, and very few corals (fig. 3-6). This type of limestone is called fossiliferous limestones for its abundance of fossils. It is also known as fine-grained, often recrystallized biodetritic limestone because the minerals that make up the rock have changed their structure after many millions of years of exposure to water and other conditions. It is biodetritic because the larger clasts,  were once parts and bits of living organisms, such as the mollusk mentioned above, that became part of the sediment when this was a sea.

Fig. 3. Gastropods and bivalve molds and casts on limestone of the formation.

For instance, fig. 3 shows a rock face where many benthic organisms that were once living within the seafloor are preserved. The half moon structures are casts of bivalve shells, now filled in with minerals. The rounded structures are gastropods and parts of sea urchins. Shark teeth are found in such mix.

Fig. 4: Schizaster cf. guirensis? A regular echinoderm - a sea urchin of this Miocene ancient sea.

The Guines formation dates to the middle-late Miocene and is well over 10 million years old. Over the last 100 years, it has had several descriptions and names. It was once called the Yumuri Limestone by DeGoyler (1918) and Bermudez and Hofftetter (1959), or the Yumuri Formation. Judoley and Furrazola considered this formation just a variation or member (1971). The detailed studies of Iturralde (1969) clearly establish it as a distinct Miocene formation, covering extensive areas in meridional western Cuba (Franco et al, 1992).
(Trivia: The Miocene epoch was named by one of the first geologists Sir Charles Lyell).

Fig. 5: A single scleractinian coral polyp Scolymia cf. cubensis Edwards and Haime, 1849 alongside bivalve clams. The Scolymia is a fossil index of the middle-late Miocene (~ 13 to 5 million years ago).

Under the microscope, and after making a very thin slice of the rock (thin enough to allow light to go through the rock), one can see microscopic shells of tiny water snails called pteropods, miliolid, and amphistaginid foraminifera, within a matrix of the minerals calcite (CaCO3), and dolomite CaMg(CO3)2 which has grown later. In thin section as it is called, the bioclasts have long been eroded and dissolved, leaving behind empty spaces in the shapes of the organisms that were once in the matrix known as ghosts (fig. 5).

Fig. 6: Thin slide showing the ghosts of bivalve shells, a planktic and miliolid foraminifera in the center field of view.
The sparitic crystals suggest recrystallization (see the prismatic crystals inside the ghosts).

The fauna and the minerals suggest that the Guines limestone formed in relatively shallow and warm waters, teeming with bottom living and burrowing lifeforms in its carbonate substrate. The large vertebrate remains indicate the presence of large sharks and whales, and all of them an intricate warm water environment, likely forming open water channel. Such an environment was suggested by previous research Iturralde (1969) and visible in the paleo reconstructions of MacPhee and Iturralde (2000). Figure 7.

Fig. 7: Paleoreconstruction of the Miocene seaway that separated what is now the region of Havana-Matanzas.
Form MacPhee and Iturralde, 2000.  

Fig. 8: A large fragment of bone encased on the limestone. This is large enough to be from a Miocene vertebrate,
maybe even a whale!

But what do the rocks and fossils say about that past? The fossil fauna of the Guines fm. suggests that this given region was once a shallow marine environment, open, with very few reef banks. The frequent fossil shark fossils suggest that this area could have been a shark nursery. I will speculate further and think that it was so because it supported marine mammals and fish to feed these large sharks, and because the basin was warm and confined enough to be a nursery (fig. 7-8). More evidence, as always, is needed.

Stay tuned for more !

Cited Literature

Bermudez, B., and Hofftetter (1959). Stratigraphic Lexicon of Cuba. Lexique Stratigraphique International, 5, Amerique Latine, Fasc. 2c, Cuba et lles Adjacents, 140 pág.

DeGoyler (1918). The geology of Cuban petroleum deposits. American Association of Petroleum Geologists Bulletin, 2: 133-167.

Franco-Alvarez, G.L. et al. (1992). Léxico Estratigráfico de Cuba. Centro de Nacional de Información Geológica, La Habana, 658 pág.

Iturralde-Vinent, M. (1969). El Neogeno de la Provincia de Matanzas, Cuba. Publicacion Especial Instituto Nacional de Recursos Hidraulicos, 7: 3-30.

Iturralde-Vinent, M., G. Hubbell, and R. Rojas Consuegra. 1996. Catalog of Cuban fossil Elasmobranchii (Paleocene-Pliocene) and Paleoceanographic Implications for their Lower-Middle Miocene Ocurrence. Boletin de la Sociedad Jamaicana de Geologia, 31: 7-21.

Judoley and Furrazola (1967). La posición de Cuba en la estructura geológica de la región del Caribe. Tecnológica, 5 (6).

MacPhee, R.D.E., e Iturralde-Vinent, M., (2000). A short history of Greater Antillean land mammals: biogeography, paleogeography, radiations, and extinctions. Tropics, 10 (1): 145-154.