Cocos/Caribbean plate subduction

The map on the right shows digital elevation land mass of Central America, and sea floor of Cocos, Nazca and Caribbean plates. Moving Northeast at 10 cm/year, the Cocos plate is subducted beneath the Caribbean plate resulting in the volcanism. From this same image Jeff Marshall has noted the boundary between the smooth sea floor associated with the Cocas plate and the rough elevation below sea (Nazca plate) that bounds to the southeast.

Shown lower left is the boundary of the Middle America Trench (purple) off shore of Nicaragua and Costa Rica's Nicoya Peninsula. The Trench is even more dramatic in 3-D from the perspective looking East at the subduction zone (image lower right).

3 MYA: The Isthmus of Panama and Pleistocene Ice Age

An ice age is a generic term used to describe a reduction in Earth temperature, expansion of ice sheets and glaciers. Evidence obtained from chemistry, geology and the paleontological record indicate there have been at least five major periods of ice ages on Earth beginning around 2.4 BYA, 850 MYA, 450 MYA, 350 MYA and the most recent 2.6 MYA.

One trigger for this most recent ice age is considered to be the creation of the Isthmus of Panama. Formation of the isthmus is thought to have begun around 20 MYA. Again for perspective remember that by 20 MYA dinosaurs (having emerged 200 MYA) had already been gone for 45 million years.

The Isthmus of Panama seems to have occurred when land on the western Caribbean plate, now described as Central America, overrode the Cocos plate located in the Pacific Ocean. From the map of plate tectonics (image upper left) the proximity of the Caribbean, Coccos and Nazca plates can be observed along Costa Rica in Central America. During this subduction event the Cocos plate was pushed downward below the Caribbean plate and given exposure to internal heat, becomes molten rock. The building pressures from subductions result in lava and hot gasses that rise underneath the Caribbean plate producing volcanoes.

A map of ocean trenches in the Pacific show the presence of the Middle America Trench (image lower right).

All along the Middle America trench, undersea volcanoes erupted until peaks emerged above sea level. As land mass continued to increase and a connection between North and South America emerged, the Isthmus of Panama was formed.

Prior to formation of this isthmus, warm water currents had run north along northeastern South America through the Caribbean and into the Pacific. With emerging Central America acting as a land bridge, warm northerly flowing ocean stream water was deflected through the Gulf of Mexico reinforcing the Gulf Stream that brings warm waters to North Americas East Coast.

Two conditions are thought necessary for an ice age, cool continental temperatures and air moisture for falling snow. Speculation suggests that by diverting the flow of warm Atlantic waters from the Pacific into the North Atlantic that the Pleistocene ice ages may have been triggered. The Pleistocene ice age generated massive glaciers that covered northern lands and provide much of the recent geological structure observed in Deerfield today.

In terms of adding perspective to a time frame of 3 million years ago, Australopithecus was roaming Africa 2-4 MYA and around 2 MYA Homo erectus is thought to have emerged.

200 MYA: Pangaea breaks apart & New England shows faults

As bedrock folded under Pangaea's formation, other zones that had melted as result of subductions, cooled. Slowly, molten rock cooled into the granite that marks the V-shaped Deerfield valley.

Then, in what would be opposite the motion that caused formation of the super-continent, 200 million years ago, Pangaea began breaking apart. Stress from the super-continental separation created the Eastern Border fault that runs through New Hampshire, Massachusetts and Connecticut. Stress, from this fault zone led to development of a rift valley where sediment was deposited as shown by the image upper right (developed by Richard Little and Will Sillin). Whereas the block diagram of the rift valley was caused by geological events from 200 MYA and shows modern day river systems, it's worth remembering that these river systems are thousands of years old, not millions. Older granite and metamorphic rock underlies the rift and can be observed in far western and eastern Massachusetts, sediment flowed into the rift. This sediment is younger rock, largely red Arkose (sandstone containing feldspar, an aluminum silicate). In Deerfield, red arkose is readily seen along the Pocumtuck Range.

250 MYA: Forming Pangaea - Deerfield's in the Middle

Around 500 million years ago, the super continent Pangaea had yet to form. At that time, geologists believe that what is now the North American shoreline ended near the eastern New York border and almost none of the land that today define as New England was attached to North America. Instead, the land masses of Deerfield and surrounding New England were big islands on the earths crust called exotic terraces. During the collision between the African, European and North American continents these terrace were smashed between North America and Western Africa, forming Pangaea (image top right).

In terms of how we know what we know, Raymo tells interesting stories revealing that when the first reasonably accurate maps of the Atlantic Ocean were drawn in late sixteenth century, people noted that the two sides of ocean continents matched like jigsaw puzzle pieces. Largely from the observing similar rock formations along the eastern coast of South America and the western coast of Africa, the German meteorologist Alfred Wegener proposed in 1915 that all the continents were once united in a supercontinent, Pangaea. Wagener proposed that Pangaea broke apart with continental drift accounting for the present positions. His theories were examined, then rejected by prominent geologists of the time. It was not until much after his death in the 1960s when continental drift and the theory of Pangaea was revived.

Raymo continues the story recounting that the technology of echo sounding (sonar) made possible the mapping of the sea floor that revealed a system of ridges on the ocean basins. Apparently, if it were possible to drain all the water from the oceans, as when emptying a bathtub, the undersea mountain ranges would be the most dramatic of all global ranges. In the Atlantic, the ridge lies exactly in the middle of the ocean basin and at this ridge there is an outflow of heat from the earths interior. The outflow of heat yields seafloor rocks that are youngest at the ridge and oldest near the continental margins. American geologist Harry Hess in 1962 took these observations and developed the theory of sea-floor spreading. Hess maintained that the entire earth crust is cycled through the earth’s interior every several hundred million years. This great looping motion of matter is driven by heat convection in the Earths interior – by the tendency of hotter matter to rise and cooler matter to sink in these great convective loops (image upper left).

As the continents collided forming Pangaea, subduction occurred where one place slid underneath another and forced bedrock upwards, generating the Appalachian Mountains. These mountains, once the size of the great Himalayan ranges today, folded and deformed the deep bedrock into metamorphic rock types gneiss, schist, slate and marble. As these continental collisions and folding bedrock occurred, other subterranean zones that had melted cooled slowly, crystallizing into granite that underlies the Deerfield Basin.

For perspective on these geological events it’s worth considering geological time put into a geological clock (below). Earth began at 12:oo, life at 2:00, photosynthesis at 3:00, oxygen rich atmosphere at 6:00, eukaryotes at 6:30, cambrian explosion at 10:00, Pangaea forming at 11:00, with the arrival of dinosaurs shortly to follow.

3.8 BYA: Life, the Universal Common Ancestor

Who were the first producers on Earth, the last universal common ancestor (luca), the first autotrophs? The ancestor who, based on the fossil and nucleic acid sequence record (Woese), radiated out some 3.8 BYA and since then has left an unbroken chain of life.

Modern day autotrophs (bacteria, plants) produce complex organic compounds (fats, carbohydrates, proteins) from simple inorganic molecules (i.e. water, carbon dioxide) using chemosynthesis or photosynthesis. For example, plants drive the reaction shown below in the forward direction. This converts physical energy from the sun (photons) into chemical energy that becomes stored in the bonds of reduced carbon carbohydrates.

energy + CO2 + H2O <--> O2 + carbohydrates

By contrast, heterotrophs (animals and fungi) are consumers. We heterotrophs drive the reverse reaction, burning carbohydrates, to provide energy while producing carbon dioxide.

Yet when we wonder how the atomic thread once weaved through fabric from nonliving to living systems, some of us at Deerfield might be influenced by our distinguished alum, Professor Woese. Woese's work might be understood by first considering the functional diversity of life today. As mammals, we live by eating other life forms – plant, animal or both. The maples on campus live by fixing carbon dioxide. Microbes that cover nearly every DA surface live by metabolizing nitrogen, methane, sulfur, metals or hydrogen. Citing these couple examples it’s easy to remember life’s extreme diversity. To outwardly express diversity, life today contains variety in our genomes for the unique functions that are common to different organisms. Although life is polymorphic and diverse, it also contains phenomenal similarity. An example of life’s similarity is the molecule that Woese used to map the tree of life called ribosomal ribonucleic acid (rRNA). Nearly unchanged for billions of years, rRNA is so highly conserved because it functions the same in all types of organisms. That is, rRNA is part of the intercellular assembly line on which proteins are manufactured.

As depicted in the image below, it is the highly conserved ribosomes (a ribosome is structurally assembled from rRNA and protein), that functions in protein synthesis (thus under selective pressure to conserve it's sequences) that have made rRNA useful for phylogenic analysis.

Woese concluded that greater differences in rRNA sequence correlates with more distant relations. From his analysis of different organisms, he proposed the three domains of life: archaea, bacteria, and eukaryote. Furthermore, he suggests that on the basis of rRNA gene differences, the three domains of life arose separately from the last universal common ancestor, or common root of the tree of life.

Beyond suggesting that a universal common ancestor existed, some research presently focuses on understanding how such ancestral organisms may have metabolized, replicated and evolved.

Wachtershauser and others have proposed an iron-sulfur world theory, where reaction cascades and catalytic feedbacks (metabolism) drive an early beginning. This iron-sulfur world suggests a pyruvate metabolism where pyruvic acid is synthesized from formic acid in reducing environment. Such a cascade system is seen as attractive, since modern day citric acid cycle, amino acids and sugars employ similar pyruvate based pathways.

Szostak has taken a different approach in the study of common ancestors and early life. By building primitive cells, or protocells, that consist of two main components, a self-replicating genetic polymer and a self-replicating membrane boundary, they look for evidence that they systems will begin to evolve in a Darwinian fashion.

4.0 BYA: Earth before Life

On primitive Earth, chemistry was driven by contact between atoms and molecules. With sufficient energy and proper orientation of molecular collisions, chemistry proceeded to initiate and/or evolve life. To qualify, life requires the following criteria: ability to replicate, metabolize and evolve.

In the 1860's Pasteur was one of the first to address origin of life questions experimentally showing that it could not arise from a closed vessel with nutrient broth. During this period of time Darwin was questioning the origin of life in a warm reducing pond. Haldane took Darwin's idea and coined the term "primordial soup." In 1953 Stanley Miller conducted landmark experiments in prebiotic chemistry assuming that primitive Earth had a reducing environment like the big planets do now (little oxygen since not yet existence of oxygen producing plants or ocean dwelling bacteria). With a flask of early Earth rich molecules (formaldehyde, ammonia, cyanide) Miller subjected the flask to electrical discharge (simulate lightning that provides energy to drive reaction) and famously produced 14 of 20 amino acids found in biological proteins. Eight years later, Joan Oro showed spontaneous formation of adenine, a nitrogenous base found in RNA, DNA and a major energy currency for cell metabolism (see image at bottom of page). Even though an ability to generate some essential-for-life molecules had been documented, some wonder if those events may not have been necessary to jump-start construction of the molecular building blocks required for replicating molecules.

In 1969 a carbonaceous meteorite containing molecular hitchhikers hit Earth that astoundingly contained amino acids, like those found by Miller, and nitrogenous bases, like those made by Oro. One hypothesis: we are stardust. Did the nitrogeneous bases and amino acids arise on Earth, or were they transported from distant reaches of space via meteorites and comets?

Once in place, what conditions were necessary to allow favorable thermodynamics to promote assembly of amino acids and nitrogeneous bases into proteins, RNA and DNA? Were they clays, primordial soups or deep-sea hydrothermal vents? Who knows... But, once assembled how did these molecules acquire catalytic function? In the 1980's, experiments done independently by Cech and Altman ultimately revealed that RNA could act as a biological catalyst, a ribozyme. Since then, catalytic RNA has undergone tremendous investigation. Hammerhead, a catalytic RNA found in organisms as diverse as plant viruses, newts, schistosomes and crickets was initially observed in 2001 by Szostak (image of Hammerhead shown in upper right).

Yet beyond just an ability to replicate, mutate and be naturally selected, life requires metabolism. And metabolism it seems is best contained within membranes or vesicles. So how in the world did chemical thermodynamics manage the assembly of protocells, their assembly and division?

4.5 BYA: Birth of Earth

Earth formed from solar nebula about 4.6 billion years ago by process of accretion (NASA artist impression shown at left). Matter aggregated under force of gravity to form in initially molten Earth. It is believed that at this point in time a number of events were occurring that lead to an increase in temperatures on the already molten Earth. Some of these events included radioactive decay of elements uranium/thorium and compression of earths mass under gravity. Temperatures rising past the melting point of iron, 1500 degrees C, led to the iron catastrophe, the event where iron and other heavy elements sank towards the center and developed Earth's iron rich core.

Using radioactive dating of uranium and thorium from Earth and Moon rocks, it's estimated that formation of Earth took form around 4.5 BYA, at which time it had acquired 99% of it's present mass, it's moon, an iron rich core and some water, present from outgassing. From studies of the lunar surface there were a high frequency of early impact events that would have released heat, melting and re-melting crustal regions.

It is also during this time that Earth out-gassing and liquid water and vapor equilibrium processes could have led to the development of an early reducing Earth atmosphere.

From the biological perspective, the following four properties make water essential for life:

1. cohesion: by virtue of hydrogen bonding

2. moderation of temperature: high specific heats

3. Insulation of bodies of water by floating ice

4. universal solvent