Today is the first full day of the program and we’re off to a great start! After our safety orientation, we’re all excited and ready to learn. The topic of our morning is one that I hold near and dear to my heart: geology. Dr. Anthony Coates is here to teach us about the geologic history of the isthmus of Panama. This is the land that separates the Caribbean Sea (in the Atlantic Ocean) and the Gulf of Panama (in the North Pacific Ocean). It extends from Costa Rica to Colombia, connecting North and South America.
There has been much debate over the dating of the formation event, and the rate of closing of the pre-existing Central American Seaway. It has been proposed that the uplift of the Isthmus of Panama occurred somewhere between 5.5 to 3 million years ago (MYA) during the Middle Pliocene, although some arguments have been made that date it back 12 to 15 MYA.
Regardless of the exact dating, we have determined that the Isthmus formed through convergent plate tectonic activity and the Cocos Plate (subcomponent of the Pacific Plate) subducted underneath the Caribbean Plate. In addition to creating an underwater volcanic arc, the oceanic crust was rising up to (eventually) form land above sea level.
There were two huge geologic results of this event:
- The ‘Great American Biotic Interchange’ took place 2.8 MYA and allowed a major migration of land mammals between the two continents. This played a huge role in biodiversity! Bears, cats, dogs, horses, llamas, pigs, mastodons, camels, and raccoons travelled from North America to South America, and porcupines, armadillos, opossums, sloths, anteaters, and marsupial tigers came up from South American to North America. The outcome was a magnificent exchange of biotic organisms.
- The thermohaline circulation in the ocean works as a conveyor belt to cycle cold, deep, salty water and warm, shallow, freshwater. The formation of the isthmus created an obstruction of oceanic flow changed the dynamics and cycling of surface and deep-water on a global scale. Changes in equatorial currents strongly impacted atmospheric and oceanic circulation and the resulting changes in productivity (i.e., result from oxygenation of the water), and was likely a major driver of evolutionary changes in marine faunas. The Gulf Stream was created and warm, salty ocean water was now forced north to higher latitudes, promoting higher rates of oceanic evaporation and thus greater annual precipitation over eastern Canada, Greenland, and northern Europe (reminder: warmer air can hold more moisture). Eventually, this trend led to the formation of the Northern Hemisphere’s polar ice cap roughly 4 to 2.5 MYA and contributed to our current ice age.
I am so glad that we had this lecture at the beginning of the course, because it will put everything that I observe over the next 21 days into context, allowing me to form a comprehensive framework. I will consider the influence that the formation of the Isthmus of Panama had each time I observe various faunal and floral organisms, piecing together, little by little, the evolutionary history of Panama.
- Coates, A.G. How old is the Isthmus of Panama? 2013 Bulletin of Marine Science, in press
- Coates, A.G., L.S. Collins, M.P. Aubry, and W.A. Berggren. The geology of the Darien, Panama, and the late Miocene-Pliocene collision of the Panama arc with northwestern South America. Geological Society of America, Bulletin 2004;116(11): 1327-1344.
- Coates, A.G., J.B.C. Jackson, L.S. Collins, T.M. Cronin, H.J. Dowsett, L.M. Bybell, P. Jung, and J.A. Obando. Closure of the Isthmus of Panama: the near-shore marine record of Costa Rica and western Panama. Geological Society of America, Bulletin 1992;104:814-828
- O’Dea, A. et al., Formation of the Isthmus of Panama. Sciences Advances 2016;2(8):e1600883