Tuesday, December 1, 2020

The Central American Seaway and Great American Interchange

Learning about South America and its past can be quite enlightening. Was it the land where Lehi landed? Was it always as big as it is now? Was it at one time mostly inundated with water? The first clue is that the land was not connected to Central America.

A significant body of water—referred to as the Central American Seaway—once separated the continents of North and South America, allowing the waters of the Pacific and Atlantic Oceans to mix freely. This Central American Seaway—a free-flowing passage of ocean water from the Pacific Ocean into the Atlantic Ocean—originated when the continental forming took place, and closed by volcanic activity at the time of the crucifixion.

Geologists, of course, have different time frames for these events, but the point is that they agree that seaway and interchange took place, once being open between oceans through the area now known as the Isthmus of Panama. They all agree that the Seaway, closed when Panama uplifted.

During the time of the Seaway, the grouper fish was a single species; however, after the blockage, the fish developed into two entirely separate species: Left: The much smaller Atlantic Grouper; and Right: The much larger Eastern Pacific goliath grouper


This resulted when two plates of the Earth’s crust beneath the surface, were colliding, forcing the Cocos Plate to slide under the Caribbean Plate. The pressure and heat caused by this collision led to the formation of underwater volcanoes, some of which grew large enough to form islands, leading to the direct geologic observation of crustal thickening and submarine deposits in Central America. At the same time, movement of the two tectonic plates were also pushing up the sea floor, eventually forcing some areas above sea level that led to the rising of the Isthmus of Panama and the closing of the Seaway.

This closing allowed a major migration of land mammals between North and South America, which allowed species of mammals such as cats, canids, horses, elephants and camels to migrate from North America to South America, while opossum, armadillo, porcupines and ground sloths made the reverse migration.

This closure of the Central American Seaway had tremendous effects on oceanic circulation in the Atlantic and the biogeography of the adjacent seas, isolating many species and triggering speciation and diversification of tropical and sub-tropical marine fauna. Thus, while the fauna developed as specific creatures, when the seaway closed as the isthmus rose, the species was cut off from one another, and over the years since developed differently. As an example, the goliath grouper grew larger in size in the east Pacific where the nutrient-rich Humboldt Current was available, and the western Atlantic grouper remained smaller in less nutrient waters.

Its emergence affected current flow, salinity, temperature, and primary productivity of the Pacific and the Atlantic and launched marine organisms of the two oceans into independent evolutionary trajectories. As no vicariant—the existence in a separate geographical area and assumed to have originated from a single population that became dispersed by geological events—is better dated than the isthmus, molecular divergence between species pairs on its two coasts (Harilaos Lessios, "The Great American Schism: Divergence of Marine Organisms After the Rise of the Central American Isthmus," vol.39, Annual Review of Ecology, Evolution, and Systematics, Ed. Douglas J. Futuyma, Palo Alto, December 2008, pp63–91).

The flow of currents would not have changed much


During the opening, nutrient-rich water of deep Pacific origin flowed into the Caribbean Sea and the Atlantic Ocean, forming wq deep water mass and isolating South America until the crucifixion. This allowed the evolution of a wholly unique diverse mammalian fauna there; then when it later closed, a faunal exchange with North America ensued, leading to the extinction of many of the native South American forms. This emergence of the Panamanian land bridge permitted the mingling of the long-separate faunas of North and South America. In fact, nearly half of the families and genera now on the South American continent belong to groups that emigrated from North America after this closure (Larry G. Marshall, “Splendid Isolation: The Curious History of South American Mammals,” American Scientist, Vol.76, Yale University Press, New Haven CT., 1980, p266). It might be noted that as the connecting bridge between two vast land masses, the Panamanian biosphere is divided along its length by a mountain range, the isthmus's weather is generally wet on the Atlantic (Caribbean) side but has a clearer division into wet and dry seasons on the Pacific side

Thus the Central American Seaway was closed by the elevation of the Central American Isthmus, which is seen by the evolution of taxa on different sides of the Central American Isthmus along with the different histories of the oceans on either side of the isthmus.

In addition, a closed seaway would not have led to much difference in the Pacific Ocean currents, but would have led to a very different North Atlantic Ocean circulation, impacting the surrounding atmospheric temperatures, which in turn affected the glacial cycle. The one main difference in the Pacific was the strengthening of the Pacific Counter Current that ran west to east between the North and South gyres, which flowed directly across the Pacific and into and through this Central American Seaway.

When the Isthmus rose, the Pacific Counter Current lost most of its force, and the flow of waters to the east changed dramatically


The emergence of the isthmus caused a reflection of the westward-flowing North Equatorial Current northward and enhanced the northward-flowing Gulf Stream. The Pacific coast of South America would have cooled as the input of warm water from the Caribbean was cut off. In addition, the seaway constriction and closure led to decreased biologic productivity caused by decreased current velocity and coastal upwelling and conversely, increased seasonality and primary productivity in the Eastern Equatorial Pacific.

Upwelling—the oceanographic phenomenon that wind-driven motion of dense, cooler, and typically nutrient-rich deep water drives it to the ocean surface, replacing the warmer, usually nutrient-depleted surface water—is caused by the wind, the Coriolis effect and the Ekman Transport. This upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton, whose biomass and presence of cool water in these regions, zones can be identified by cool sea surface temperatures and high concentrations of Chlorophyll-a, which all oxygenic photosynthetic organisms use.

In addition, the inflow of nutrient-rich water of deep Pacific origin into the Caribbean was blocked, so local species had to adapt to an environment of lower productivity (Sreepat Jain and Laurel S. Collins, "Trends in Caribbean Paleoproductivity related to the Neogene closure of the Central American Seaway,". Marine Micropaleontology journal, vol.63, is.1, December 30. 2007, pp57–74). The closure of this seaway led to deep water mass forming in the North Atlantic, and an increase poleward of salt and heat transport, strengthening the North Atlantic thermohaline circulation, which in turn increased moisture supply to Arctic latitudes, contributing to both Arctic continental glaciation and sea ice formation (G. Bartoli, et al., “Final Closure of Panama and the onset of northern hemisphere glaciation,” Earth and Planetary Science Letters, vol.237, is.1, August 2005, pp33–44).

In fact, there is no convincing geological evidence to indicate that South America had a continuous land connection with any other continent until the Bolivar Trough marine barrier disappeared and the Americas were united by the emergence of the Panamanian land bridge (Larry G. Marshall, “Land Mammals and the Great American Interchange,” American Scientist, Vol.76, No.4, July-August 1988, pp380-388).

For those who have difficulty in seeing South America as the island Jacob described, this might convince you that at one time, South America was an isolated land, not connected to Central America—the first step in realizing that South America was a separate land.

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