Neogene

Chapter 15 – The Neogene: The Iceman Cometh

15.1 Introduction to the Neogene

• During the Neogene the configuration of continents and oceans, as well as many mountain ranges and other geologic features, reached their present form
• Climatic cooling continued, and northern hemisphere glaciation occurred
• The evolution and spread of humans took place, possibly under the influence of glacial cycles

15.2 Tectonics and Sedimentation

• Europe, Asia, and Africa
• The Tethyan Seaway continued to close with the ongoing collision of India and Asia, and the crushing of smaller plates between the African plate and southern Europe
• Tectonic closure of the eastern portion of Tethys running through Asia, and the closure of the Straits of Gibraltar (the connection between Tethys and the Atlantic) turned the Tethys into a series of inland lakes by the Late Miocene
• Today, these remnants are the Black, Caspian, and Aral seas, along with the Mediterranean
• Other major Neogene tectonic features include the east African rift valleys and the opening of the Red Sea and Gulf of Aden (all related in a triple junction), and the Zagros Mountains, formed as Arabia collided with Iran
• Central and South America
• In Central America, the remnant of Tethys left between North and South America was closed by the Isthmus of Panama about 3 to 3.5 million years ago
• The Caribbean plate, originally a portion of Pacific Ocean crust was cut off; in the process, what are now the islands of the Greater Antilles moved from the southern end of the North American Corcillera into the Caribbean Sea
• In South America, the Andes continued to rise, shedding sediment eastward to fill the foreland basin (today the Amazon River, with its vast rain forests, occupies this region)
• Atlantic and Gulf Coasts of North America
• Erosion and sedimentation continued into the Paleogene as the Appalachians were again rejuvenated by isostatic uplift
• The Gulf of Mexico likewise served as a gigantic sediment trap for sediment shed during the Laramide Orogeny
• Western North America
• The Rockies underwent simultaneous uplift and erosion; uplift extended into the Colorado Plateau, with downward erosion producing features such as the Grand Canyon
• West Coast of North America
• Subduction continued along the west coast as the North American plate moved over the Farallon plate
• The Columbia River plateau basalts erupted in the Middle Miocene as the North American plate moved over what may be a hotspot
• By Late Miocene and Pliocene the eruptions shifted eastward to form the Snake River Plain and then Yellowstone
• Sediments were also shed from the rising Sierra Nevadas into the central valley of California, forming basins rich in petroleum from the dead remains of plankton
• To the south, subduction also formed the Coast Ranges of California
• Evolution of the San Andreas Fault System
• In the Late Oligocene the portion of the North American plate represented today by central and southern California began to encounter a transform (offset) in the East Pacific Rise spreading center
• Subduction of the Farallon plate ceased, as did volcanism further east
• The Farallon plate split into two plates: the Juan de Fuca (to the north) and the Cocos (to the south)
• Eventually, as the Pacific plate began to encounter the North American plate it was not subducted; instead, the northwest lateral movement of the Pacific plate became dominant
• Thus the western and southwestern portions of California began to move in a northwesterly direction relative to the North American plate; the former transform fault was transformed into the San Andreas Fault, a right lateral strike-slip fault

15.3 How Was the West Widened? Evolution of the Basin and Range

• Background
• The Basin and Range is an extensional tectonic regime, as indicated by a series of north-south trending horst and graben structures
• Hypotheses for the Formation of the Basin and Range
• Early Hypotheses – Early (now rejected) hypotheses included crustal extension caused by the overriding of the East Pacific Rise by the North American plate; another involved the formation of an incipient back-arc basin by the Farallon plate
• Involvement of the San Andreas Fault System – Another early idea was that extension was caused by the right-lateral movement of the San Andreas fault system being transferred to the Basin and Range, stretching it apart
• Volcanism and Uplift – Yet another suggestion was that the crust of the Basin and Range is mechanically weak due to widespread basaltis volcanism in the region
• Mantle Uplift – This hypothesis suggests that the buoyancy of a warm mantle plume might have caused uplift and extension of the crust, perhaps related to the presume Yellowstone mantle plume
• Slab Gap Hypothesis – This is a variation on several of the preceding ideas, that as the crustal block to the west of the Ancestral San Andreas Fault moved northwest, it caused the Basin and Range province to expand
• Gravitational Collapse – A more recent model suggests that the great compression and uplift of previous orogenies caused the ancient mountain ranges of the Basin and Range to collapse on themselves and spread out

15.4 Climate, Ocean Circulation, and Chemistry

• Modern ocean circulation, chemistry, and climate regime also began to be established during the Neogene
• The range of climate in different regions became more extreme during the Neogene in response to glaciation and tectonism
• Ice caps began to grow over the North Pole late in the Neogene (around 2.8 million years ago)
• This follws at least several hundred thousand years after the rise of the Isthmus of Panama, which blocked the flow of warm currents from the Atlantic into the Pacific
• The diversion of water strengthened the Gulf Stream, leading to more vigorous ocean circulation patterns and water masses such as the North Atlantic Deep Water

15.5 The "Ice Ages": Evolution of a Theory

• Background
• Early 19th century natural historians took the presence of glacial erratics and other features to suggest that mountain glaciers had once existed at much lower elevations
• Other geologists maintained that Noah’s flood was responsible for these observations (the “diluvial theory”)
• Lyell initially argued that floating icebergs had deposited debris when sea level was much higher; ultimately he concluded that the evidence for ancient ice sheets was overwhelming, and taccepted the glacial theory
• The Eccentricity of James Croll
• Croll reasoned that Earth warmed and cooled depending on its distance from the Sun (i.e., the eccentricity of its orbit)
• This would account for the advance and retreat of glaciers about every 100,000 years
• At first this seemed to match the record, but a closer look showed that the last glacial cycle ended about 10,000 to 15,00 years ago
• Thus eccentricity alone could not account for the growth and retreat of glaciers
• Precession of the Equinoxes
• Croll looked for another astronomical mechanism and concluded that the phenomenon known as the precession of equinoxes affects the amount of solar radiation received by Earth
• Precession (wobble) has to do with where the seasons occur in Earth’s orbit
• Milutin Milankovitch and Obliquity
• Milankovitch, an astronomer, made a mathematical theory of the climate of Earth, Mars and Venus his life’s work
• He developed a mathematical theory that allowed calculation of the solar flux for any given latitude and season
• Unlike Croll, who had decided that diminished radiation during winter at high latitudes was the critical factor in prompting ice growth, Milankovitch concluded that summer radiation was of more significance
• He also found that the obliquity (tilt) of Earth’s axis strongly affected his calculations; thus there are three separate factors in the Milankovitch theory: eccentricity, obliquity, and precession
• The Milankovitch theory was published in 1924, but fell out of favor because the number of major tillites (four) did not match the predictions of the theory
• Planktonic Foraminifera and the Oxygen Isotope Curve
• As support for the Milankovitch theory waned, studies of the planktonic foraminifera and oxygen isotope records of deep sea cores began
• By the 1960s it was recognized that the oxygen isotope curve primarily indicates ice volume rather than temperature
• Age calibrations using magnetic reversal stratigraphy in the 1970s helped reestablish the link between glacial cycles and astronomical forcing, as predicted by the Milanktvitch theory
• Eccentricity, obliquity and precession (referred to as “Milankovitch cycles”) combine to produce all sorts of gradations of ice advance and retreat, as well as shifts in character of the cycles (i.e., dominated by precession rather than obliquity)

15.6 Neogene Life

• Marine Life
• Diversification of the Modern Fauna continues, although it is unclear if this is real or an effect of better preservation of the fossil record in younger materials
• Some plankton populations have become strongly associated with certain water masses
• Other taxa have become restricted toward lower latitudes because of glaciation and cooling of water masses near the poles
• Land Plants
• Grasses diversified into thousands of species as Earth dried out due to glaciation
• A shift in carbon isotope values during the Late Miocene (6 million years ago) may reflect the expansion of C4 grasses, which extract more of the heavier isotope of carbon during photosynthesis compared to C3 grasses
• The rise of grasses, which contain microscopic phytoliths of opaline silica may have helped supply this biolimiting nutrient to the sea, perhaps contributing to the tremendous Neogene diversification of diatoms
• Terrestrial Vertebrates
• The expansion of grasses has a major effect on terrestrial communities and the diversification of different types of vertebrates, including rodents, frogs, songbirds, and snakes
• The changes in ungulates continued, as horse (and other perissodactyl) diversity declined and artiodactyls diversified
• Mammoths evolved into grazers in the Pleistocene; carnivores (wild dogs, saber-toothed cats, hyenas and bears) preyed upon the herbivores
• Evolution of Humans
• Evolutionary Relationships of Humans – Humans belong to the group of hominoid apes called hominids; significant differences exist between humans and other hominoids
• Hypotheses of Human Origins – Ever since Darwin, various hypothesis have been proposed for the evolution of humans; these are almost constantly being revised or disputer on often fragmentary fossil evidence
• Why Did Hominids Leave Africa? – Various factors possible responsible  for human migration out of Africa include bipedalism, climate change, brain evolution, and evolution of behavior itself,

15.7 Extinction

• Large grazers underwent a significant turnover during the Late Miocene, probably related to a change in vegetation as C4 grasses began to replace C3 taxa
• Species with lower crowned teeth were unable to adapt to the increase in silica content of the C4 grasses
• The Great Fauna Interchange occurred when the Isthmus of Panama  was established as a land bridge in the Pliocene
• Many South American marsupials were replaced by placental mammals
• Mastodons and mammoths died out as recently as 25,000 years ago
• Extinction may have been due to climate change, predation by ancient humans, or both
• All hominid species preceding Homo sapiens have persisted for a few hundred thousand to roughly a million years before becoming extinct for reasons not understood