16 Evolution, Geologic History, and the Earth System

Geologic Time and the History of Life

The history of Earth is divided into significant chronological divisions, spanning from the planet's formation approximately $4.5$ billion years ago to the present Holocene Epoch. The Precambrian accounts for the vast majority of Earth's history, covering the time from $4.5 imes 10^9$ years ago to the start of the Cambrian Period approximately $542$ million years ago. Significant benchmarks in the Precambrian include the emergence of the earliest organic structures around $3.5$ to $3.8$ billion years ago and the presence of documented geologic activity through $1$ and $2$ billion years ago. Following the Precambrian, the Phanerozoic Eon is divided into three major eras: the Paleozoic, Mesozoic, and Cenozoic.

The Paleozoic Era contains several critical periods: the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. The transition between the Paleozoic and Mesozoic occurred roughly $251$ million years ago. The Mesozoic Era comprises the Triassic, Jurassic, and Cretaceous periods. The Cenozoic Era, known as the age of recent life, began approximately $65$ million years ago and is divided into the Tertiary and Quaternary periods. The Tertiary Period includes the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs, while the Quaternary Period consists of the Pleistocene and the current Holocene Epochs.

The Great Ordovician Biodiversification Event (GOBE)

The Great Ordovician Biodiversification Event, or G.O.B.E., represents one of the most significant expansions of marine life in history. This event occurred after a notable period of relatively low and steady diversity following the Cambrian explosion. The GOBE is characterized by an incredibly rapid diversification of life forms, even when compared to modern counterparts; for example, the diversity and structure of Rugose corals from this era are frequently compared to those of modern coral systems. Determining the exact trigger for this rapid diversification remains an area of active and ongoing scientific research.

During the 1990s and early 2000s, a popular explanation for the GOBE centered on resource availability. Researchers posited that the primary limiting factor on ecosystems was (and perhaps remains) the availability of resources. In an ecosystem hierarchy, heterotrophs occupy the top levels, feeding on lower trophic levels. At the base of the heterotroph level, organisms consume primary producers. Primary producers require specific inputs to function: environmental energy, which is almost exclusively derived from sunlight, and various nutrients. These essential nutrients include phosphorus ($P$), nitrogen ($N$), silicon ($Si$), iron ($Fe$), and potassium ($K$).

Nutrient Dynamics and the Taconic Orogeny

The availability of nutrients in the global oceans is determined by complex systems of upwelling and recycling, which are governed by ocean currents and thermohaline circulation. Nutrients are introduced into the marine realm through weathering and erosion of continental landmasses, as well as through volcanic eruptions. During the Ordovician, a significant geological event known as the Taconic Orogeny served as a massive nutrient source for the global oceans. This orogeny was the first of three major phases in the building of the Appalachian Mountains.

The Taconic Orogeny was triggered by the closure of the Iapetus Ocean. This process involved the subduction of the Laurentian (ancient North American) oceanic crust, which brought a volcanic island arc into collision with the central coast of Laurentia. This collision and subsequent mountain building occurred from the Middle Ordovician to nearly the end of the Ordovician Period. Specifically, the Timeline of these events is as follows:

• $543$ million years ago: An active volcano is located offshore, while Laurentia features a carbonate bank and oceanic crust.
• $500$ million years ago: The volcanic arc and the pile of sediments scraped off the subducting slab grew larger.
• $440$ million years ago: The collision between the volcanic islands and the ancient continent formed a tall mountain range (the Taconic Mountains). This range has since eroded, leaving its roots exposed in the rolling hills of the Eastern Piedmont.

This tectonic activity had two primary effects on marine productivity: first, the formation of massive mountain ranges increased the surface area available for weathering and erosion, flushing more nutrients into the oceans. Second, the orogeny generated large-scale volcanism; one specific event was so massive it spread an ash layer over most of the paleocontinents. Together, these factors increased the carrying capacity of the global oceans.

Appalachian Mountain Formation and Supercontinent Cycles

The formation of the Appalachian Mountains is a multi-phase process involving several orogenic events related to the assembly and breakup of supercontinents:

• Grenville Orogeny: Occured during the Proterozoic Eon and was involved in the assemblage of the supercontinent Rodinia.
• Taconic Orogeny: Occurred during the Ordovician and early Silurian periods, following the breakup of Rodinia.
• Acadian Orogeny: Occurred during the late Silurian and Devonian periods.
• Alleghanian Orogeny: Occurred during the Carboniferous and early Permian periods, contributing to the assembly of Pangea. The breakup of Pangea followed these events.

The Carbonate-Silicate Cycle and Atmospheric Chemistry

The carbonate-silicate cycle is a crucial mechanism for regulating Earth's long-term climate by cycling carbon between the atmosphere and the lithosphere. The chemical process can be summarized by the following equations: CO_2 + H_2O + CaSiO_3 ightarrow Ca^{2+} + 2HCO_3^- + SiO_2 Ca^{2+} + 2HCO_3^- ightarrow CaCO_3 + CO_2 + H_2O The first part of this cycle utilizes two molecules of carbon dioxide ($CO_2$) to weather silicate rocks (like $CaSiO_3$), while the second part, which involves the formation of carbonate minerals ($CaCO_3$), releases one molecule of $CO_2$ back into the system. Silicates necessary for this cycle are provided through tectonic and weathering processes.

Modern Earth's atmosphere is composed primarily of Nitrogen ($78\%$), Oxygen ($21\%$), and Argon ($1\%$). Water vapor ($H_2O$) is variable, moving between $0\%$ and $4\%$ of the total atmospheric volume. Currently, Carbon Dioxide ($CO_2$) levels sit at approximately $0.43\%$. Despite its relatively low concentration, $CO_2$ exerts an outsized impact on the global climate.

Solar Radiation and Atmospheric Circulation

Solar radiation is the primary energy source for the atmosphere. Sunlight is most intense when it strikes the planet's surface at a perpendicular ($90^\circ$) angle. Consequently, solar radiation is highest at the equator and lowest at the poles. The angle of the sun's rays dictates the concentration of energy: direct impacts concentrate rays into small areas, while oblique angles spread the same energy over larger areas. The Earth's axial tilt is the cause of the seasons, as it changes the angle of solar impact as the Earth orbits the sun. Variations in humidity, elevation, and open water create a heterogeneous system influencing weather and climate.

In an idealized, non-rotating Earth, the atmosphere would function as one large convection center: air would rise at the warm equator and sink at the cold poles. However, the rotation of the Earth creates the Coriolis effect. In the Northern Hemisphere, all currents are deflected to the right, forming clockwise gyres. In the Southern Hemisphere, currents deflect to the left, forming counterclockwise gyres. This effect divides atmospheric circulation into distinct cells:

• Equator to $30^\circ$: Air rises at the low-pressure zone of the equator, cools, and descends at $30^\circ$ North and South. This descending air is cool and dry, creating desert belts on continents and driving the trade winds.
• $30^\circ$ to $60^\circ$: A second cell creates the westerlies, with winds heading in the opposite direction of the trade winds.

The Marine Realm and Ocean Bathymetry

Marine life is heavily influenced by water depth and ocean currents. Most surface ocean currents are driven by wind and are organized into massive gyres within ocean basins. Thermohaline circulation, driven by differences in temperature and salinity, also moves water globally. Tectonic changes can significantly alter these currents; for example, in the Early Pliocene, the Arctic Ocean was more isolated, which affected salinity and the movement of dense water sinking near Iceland compared to the present wind-driven conveyor belts.

The distribution of Earth's surface is described by the hypsographic curve, showing that land covers $29.2\%$ of the surface while oceans cover $70.8\%$. Significant elevations include Mount Everest at $8850\,m$, while the average land elevation is $840\,m$. The average ocean depth is $3729\,m$, with the Mariana Trench reaching a maximum depth of $11,022\,m$.

The continental margin is the transition between continental and oceanic crust and consists of several distinct zones:

• Continental Shelf: A relatively flat zone extending from the shore to the shelf break. It is geologically part of the continent. The average width is $70\,km$ ($43\,miles$) but can extend to $1500\,km$ ($930\,miles$). The average depth of the shelf break is $135\,meters$ ($443\,feet$). Active margins typically have shorter shelves than passive margins.
• Continental Slope: The descent from the shelf into the deep ocean. It features an average slope of $4^\circ$ (ranging from $1^\circ$ to $25^\circ$), which equates to a drop of $7\,feet$ in depth for every $100\,feet$ of horizontal distance. At a $4^\circ$ slope, it takes $33\,miles$ to descend from the shelf break to the average ocean depth.
• Submarine Canyons: Deep, V-shaped valleys with overhanging walls carved into the slope by turbidity currents. They can extend to depths of $3500\,meters$ ($11,500\,feet$).
• Turbidity Currents: These are underwater sediment avalanches moved by gravity. They carry large pieces of rock and deposit their sediment load at the continental rise.
• Continental Rise: The area where sediment from the slope piles up on the deep ocean basin. It is marked by turbidite deposits and represents the transition to oceanic crust.
• Abyssal Plains: Comprising more than $50\%$ of Earth's surface, these are the deepest and flattest parts of the planet. They are formed by the suspension settling of very fine particles that cover irregularities in the oceanic crust. They remain some of the least explored places on Earth.