______________________________________________6
Geology
6.1 Uniformitarianism
6.1.1 The basics of the earth sciences
Aristotle thought the earth had existed eternally. Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War. The Talmudic rabbis, Martin Luther and others used the biblical account to extrapolate back from known history and came up with rather similar estimates for when the earth came into being. The most famous came in 1654, when Archbishop James Ussher of Ireland offered the date of 4004 B.C.
Within decades observation began overtaking such thinking. In the 1660s Nicolas Steno formulated our modern concepts of deposition of horizontal strata. He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil. Robert Hooke, not long after, suggested that the fossil record would form the basis for a chronology that would “far antedate ... even the very pyramids.” The 18th century saw the spread of canal building, which led to the discovery of strata correlated over great distances, and James Hutton’s recognition that unconformities between successive layers implied that deposition had been interrupted by enormously long periods of tilt and erosion. By 1788 Hutton had formulated a theory of cyclic deposition and uplift, with the earth indefinitely old, showing “no vestige of a beginning—no prospect of an end.” Hutton considered the present to be the key to the past, with geologic processes driven by the same forces as those we can see at work today. This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law (which nearly all of us would accept) and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.
That is the background to the intellectual drama being played out in this series of papers. It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains. We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds. Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.
One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists. Most notable is William Thomson, ennobled to become Lord Kelvin in 1892, whose theories make up an entire section of this collection. He was one of the dominant physicists of his time, the Age of Steam. His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Harlow Shapley, who wrote an article in 1919 on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe. Florian Cajori, author of the 1908 article “The Age of the Sun and the Earth,” was a historian of science and, especially, of mathematics, and Ray Lankester, whom he quotes, was a zoologist. H. N. Russell, author of the 1921 article on radioactive dating, was familiar to me for his part in developing the Hetzsprung-Russell diagram for stars, but I was surprised to discover that he was also the Russell of Russell-Saunders coupling, important in atomic structure theory. H. S. Shelton was a philosopher of science, critical (as shown in his contribution, the 1915 article “Sea-Salt and Geologic Time”) of loose thinking and a defender of evolution in debates.
The prologue to the drama is the mid-19th century recognition of the relation between heat and other kinds of energy (see the 1857 article “Source of the Sun’s Heat”). The first act consists in a direct attack, led by Lord Kelvin, on the extreme uniformitarianism of those such as Charles Lyell, who regarded the earth as indefinitely old and who, with great foresight (or great naivety, depending on your point of view: see the third installment of the 1900 “The Age of the Earth” article by W. J. Sollas), assumed that physical processes would eventually be discovered to power the great engine of erosion and uplift.
The second act of the drama sees a prolonged attempt by a new generation of geologists to estimate the age of the earth from observational evidence, to come up with an answer that would satisfy the demands of newly dominant evolutionary thinking, and to reconcile this answer with the constraints imposed by thermodynamics. The third act sees the entry of a newly discovered set of physical laws—that governing radioactivity. Radioactivity offered not only a resolution to the puzzle of the earth’s energy supply but also a chronology independent of questionable geologic assumptions and a depth of time more than adequate for the processes of evolution. Lord Kelvin and his allies used three kinds of argument. The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust. The second referred to such topics as the detailed shape of the earth (bulging slightly at the equator) and the dynamics of the earth-moon system. The third referred to the heat of the sun, particularly the rate at which such heat is being lost, compared with the total amount of energy initially available.
The first argument was completely undermined after considering the amount of heat generated by radioactive decay. The second depended on highly dubious theories of formation of the earth and moon and plays relatively little role in this compilation. The third, which by the end was the most acute, presented a problem that outlasted the controversy itself. Thus, when in 1919 Shapley stated that for him the radiometric timescale was fully established, he acknowledged that there was as yet no explanation for the sun’s energy. (He did not need to wait long. In 1920 Sir Arthur Eddington came up with the answer: the fusion of hydrogen into helium.)
In reply to Lord Kelvin’s attacks, the geologists used two principal lines of reasoning. One referred to the depth of the sediments and the time they would have taken to accumulate; the other referred to the salinity of the oceans, compared with the rate at which rivers are supplying them with sodium salts. In hindsight, both theories were deeply misguided, for similar reasons. They assumed that current rates—of sediment deposition and of salt transport by rivers—were the same as historical rates, despite the evidence they had that our own age is one of atypically high geologic activity. Worse, they measured inputs but ignored outputs. The rock cycle, as we now know, is driven by plate tectonics, with sedimentary material vanishing into subduction zones. And the oceans have long since approached something close to a steady state, with chemical sediments removing dissolved minerals as fast as they arrive. Nevertheless, by the late 19th century the geologists included here had reached a consensus for the age of the earth of around 100 million years. Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more. And we should resist the temptation to blame them for their resistance. Radioactivity was poorly understood. Different methods of measurement (such as the decay of uranium to helium versus its decay to lead) sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature. The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.
It was not until 1926, when (under the influence of Arthur Holmes, whose name recurs throughout this story) the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved. Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling, and laser heating. The resulting knowledge has led to the current understanding that the earth is 4.55 billion years old. (Braterman, n.d.)
6.1.2 Formation of the layers of the earth
Of the rocks exposed on the Earth’s surface, 66 percent (±3.5 percent) are sedimentary and 34 percent are crystalline, at the 95 percent confidence level. Extrusive igneous rocks average about one-fourth of all crystalline rock outcrops, with the highest percentages in Asia and South America. Less than 5 percent of all Precambrian rocks are mapped as sedimentary. The relationship between the geologic age of a sedimentary rock and its outcrop area is lognormal and is well described by a decay curve with a half-life of 130 × 10 6 yr. That is, one-half of all existing sedimentary rocks are younger than Jurassic. Such a short half-life indicates that the rate of sedimentary recycling must be very rapid. (Blatt & Jones, 1975)
Sedimentary rocks form at low temperatures and pressures at the surface of Earth owing to deposition by water, wind, or ice. By contrast, igneous and metamorphic rocks form mainly below Earth's surface where temperatures and pressures may be orders of magnitude higher than those at the surface, although volcanic rocks eventually cool at the surface. These fundamental differences in the origin of rocks lead to differences in physical and chemical characteristics that distinguish one kind of rock from another. Sedimentary rocks are characterized particularly by the presence of layers, although layers are also present in some volcanic and metamorphic rocks, and by distinctive textures and structures. Many sedimentary rocks are also distinguished from igneous and metamorphic rocks by their mineral and chemical compositions and fossil content. Sedimentary rocks cover roughly three-fourths of Earth's surface. They have special genetic significance because their textures, structures, composition, and fossil content reveal the nature of past surface environments and life forms on Earth. Thus, they provide our only available clues to evolution of Earth's landscapes and life forms through time. These characteristics of sedimentary rocks are in themselves reason enough to study sedimentary rocks. In addition, many sedimentary rocks contain minerals and fossil fuels that have economic significance. Petroleum, natural gas, coal, salt, phosphorus, sulfur, iron and other metallic ores, and uranium are examples of some of the extremely important economic products that occur in sedimentary rocks. (Boggs, 2009)
6.2 The Fossil Record for Human Evolution
6.1.1 The Cenozoic
The Cenozoic era, which began about 65 million years ago and continues into the present, is the third documented era in the history of Earth. The current locations of the continents and their modern-day inhabitants, including humans, can be traced to this period.
The era began on a big down note, catching the tail end of the Cretaceous-Paleogene extinction event at the close of the cretaceous period that wiped out the remaining non-avian dinosaurs.
The term Cenozoic first spelled "Kainozoic," was originally used in an 1840 entry in the Penny Cyclopedia encyclopedia in an article written by British geologist John Phillips. The name is derived from the Greek phrase meaning “recent life.”
The Cenozoic era is divided into three periods:
Paleogene period (65-23 million years ago), which consists of the Paleocene, Eocene, and Oligocene epochs).
Neogene period (23-2.6 million years ago), which includes the Miocene and Pliocene epochs).
Quaternary period (2.6 million years ago to the present), consisting of the Pleistocene and Holocene epochs). While it is widely accepted that we are still in the Holocene epoch, some scientists argue that we have entered the Anthropocene epoch. In a 2010 article in the scientific journal Environmental Science & Technology, scientists made the case for a new epoch, blaming humans for causing a drastic shift in conditions. (Zimmerman, 2016)
6.1.2 The earliest Hominins
The early Pliocene African hominoid Ardipithecus ramidus was diagnosed as a having a unique phylogenetic relationship with the Australopithecus + Homo clade based on non-honing canine teeth, a foreshortened cranial base, and postcranial characters related to facultative bipedality. However, pedal and pelvic traits indicating substantial arboreality have raised arguments that this taxon may instead be an example of parallel evolution of human-like traits among apes around the time of the chimpanzee–human split. Here the basicranial morphology of Ar. ramidus for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus is investigated. Besides a relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element. These traits reflect a relative broadening of the central basicranium, a derived condition associated with changes in tympanic shape and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these human-like modifications. The preserved morphology of ARA-VP to 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium, as in Australopithecus and Homo. Reorganization of the central cranial base is among the earliest morphological markers of the Ardipithecus + Australopithecus + Homo clade. (Kimbel, et al., 2014)
Despite a rich African Plio-Pleistocene hominin fossil record, the ancestry of Homo and its relation to earlier australopithecines remain unresolved. Two partial skeletons with an age of 1.95 to 1.78 million years are reported here.. The fossils were encased in cave deposits at the Malapa site in South Africa. The skeletons were found close together and are directly associated with craniodental remains. Together they represent a new species of Australopithecus that is probably descended from Australopithecus africanus. Combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than any other australopith species and thus might help reveal the ancestor of that genus. (Berger, et al., 2010)
Carbon isotope studies of early hominins from southern Africa showed that their diets differed markedly from the diets of extant apes. Only recently, however, has a major influx of isotopic data from eastern Africa allowed for broad taxonomic, temporal, and regional comparisons among hominins. Before 4 Ma, hominins had diets that were dominated by C3 resources and were, in that sense, similar to extant chimpanzees. By about 3.5 Ma, multiple hominin taxa began incorporating 13C-enriched [C4 or crassulacean acid metabolism (CAM)] foods in their diets and had highly variable carbon isotope compositions which are atypical for African mammals. By about 2.5 Ma, Paranthropus in eastern Africa diverged toward C4/CAM specialization and occupied an isotopic niche unknown in catarrhine primates, except in the fossil relations of grass-eating geladas (Theropithecus gelada). At the same time, other taxa (e.g., Australopithecus africanus) continued to have highly mixed and varied C3/C4 diets. Overall, there is a trend toward greater consumption of 13C-enriched foods in early hominins over time, although this trend varies by region. Hominin carbon isotope ratios also increase with postcanine tooth area and mandibular cross-sectional area, which could indicate that these foods played a role in the evolution of australopith masticatory robusticity. The 13C-enriched resources that hominins ate remain unknown and must await additional integration of existing paleodietary proxy data and new research on the distribution, abundance, nutrition, and mechanical properties of C4 (and CAM) plants. (Sponheimer, et al., 2013)
6.1.3 Homo habilis and Homo erectus
The origin of the genus Homo in Africa signals the beginning of the shift from increasingly bipedal apes to primitive, large-brained, stone tool-making, meat-eaters that traveled far and wide. This early part of the human genus is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus. H. habilis is known for retaining primitive features that link it to australopiths and for being the first stone tool makers. Little is known about H. rudolfensis except that it had a relatively large brain and large teeth compared to H. habilis and that it overlapped in time and space with other early Homo. Our understanding of the paleobiology and evolution of the larger-brained H. erectus is enhanced due to its rich fossil record. H. erectus was the first obligate, fully committed biped, and with a body adapted for modern striding locomotion, it was also the first in the human lineage to disperse outside of Africa. The early members of the genus Homo are the first to tip the scale from the more apish side of our evolutionary history toward the more human one. (Dunsworth, 2010)
The Homo habilis OH 62 partial skeleton has played an important, although controversial role in interpretations of early Homo locomotor behavior. Past interpretive problems stemmed from uncertain bone length estimates and comparisons using external bone breadth proportions, which do not clearly distinguish between modern humans and apes. True cross-sectional bone strength measurements of the OH 62 femur and humerus are compared with those of modern humans and chimpanzees, as well as two early H. erectus specimens—KNM-WT 15000 and KNM-ER 1808. The comparative sections include two locations in the femur and two in the humerus in order to encompass the range of possible section positions in the OH 62 specimens. For each combination of section locations, femoral to humeral strength proportions of OH 62 fall below the 95% confidence interval of modern humans, and for most comparisons, within the 95% confidence interval of interval of modern humans, and for most comparisons, within the 95% confidence interval of chimpanzees. In contrast, the two H. erectus specimens both fall within or even above the modern human distributions. This indicates that load distribution between the limbs, and by implication, locomotor behavior, was significantly different in H. habilis from that of H. erectus and modern humans. When considered with other postcranial evidence, the most likely interpretation is that H. habilis, although bipedal when terrestrial, still engaged in frequent arboreal behavior, while H. erectus was a completely committed terrestrial biped. This adds to the evidence that H. habilis (sensu stricto) and H. erectus represent ecologically distinct, parallel lineages during the early Pleistocene. (Ruff, 2009)
The relationship between Homo habilis and early African Homo erectus has been contentious because H. habilis was hypothesized to be an evolutionary stage between Australopithecus and H. erectus, more than a half-century ago. Recent work re-dating key African early Homo localities and the discovery of new fossils in East Africa and Georgia provide the opportunity for a productive re-evaluation of this topic. Here, we test the hypothesis that the cranial sample from East Africa and Georgia represents a single evolutionary lineage of Homo spanning the approximately 1.9–1.5 Mya time period, consisting of specimens attributed to H. habilis and H. erectus. To address issues of small sample sizes in each time period, and uneven representation of cranial data, we developed a novel nonparametric randomization technique based on the variance in an index of pairwise difference from a broad set of fossil comparisons. We fail to reject the hypothesis of a single lineage this period by identifying a strong, time-dependent pattern of variation throughout the sequence. These results suggest the need for a reappraisal of fossil evidence from other regions within this time period and highlight the critical nature of the Plio-Pleistocene boundary for understanding the early evolution of the genus Homo. (Arsdale & Wolpoff, 2013)
6.1.4 Archaic Homo sapiens
Whereas in Europe the transition from Middle to Upper Paleolithic and the replacement of Neanderthal by anatomically modern humans appear to be synchronous events, in Africa this is not the case. Neanderthals as such were not present in Africa, and if the ‘Out of Africa’ model is correct, the ancestors of anatomically modern humans must have made their appearance in a Middle Stone Age context before 100,000 years ago. Subsequently, it seems that they coexisted with Neanderthals for up to 70,000 years in the Near East. If a direct biological correlation can be ruled out, the question arises: what was the impetus for an Upper Paleolithic ‘revolution’ and why should it have taken place at all? (Allsworth-Jones, 1993)
Regional features play a great role in the analysis of the differentiations of Homo erectus and Homo sapiens. However, this poses the question how widespread and variable these features are. In order to examine this with regard to the features commonly seen in China their occurrence and variability were determined in Chinese as well as in African crania of archaic and late Pleistocene/Holocene modern Homo sapiens. Furthermore, some features known from Africa were examined with regard to their occurrence and variability in China. Although the variability might change due to new finds, the present results for some features point to larger morphological spectra in the African than in the Chinese archaic Homo sapiens. It is furthermore remarkable that the early modern Chinese in many features show deviations from the pattern of archaic Homo sapiens of this region and exhibit broader spectra similar to those seen in African archaic and early modern Homo sapiens. (Wu & Brauer, 1993)
6.1.5 Modern Homo sapiens
The fossilized skulls of two adults and one child discovered in the Afar region of eastern Ethiopia have been dated at 160,000 years, making them the oldest known fossils of modern humans, or Homo sapiens.
The skulls, dug up near a village called Herto, fill a major gap in the human fossil record, an era at the dawn of modern humans when the facial features and brain cases we recognize today as human first appeared.
The fossils date precisely from the time when biologists using genes to chart human evolution predicted that a genetic "Eve" lived somewhere in Africa and gave rise to all modern humans.
"We've lacked intermediate fossils between pre-humans and modern humans, between 100,000 and 300,000 years ago, and that's where the Herto fossils fit," said paleoanthropologist Tim White, professor of integrative biology at the University of California, Berkeley, and a co-leader of the team that excavated and analyzed the discovery site. "Now, the fossil record meshes with the molecular evidence."
"With these new crania," he added, "we can now see what our direct ancestors looked like."
The early humans at Herto lived along the shores of a shallow lake created when the Awash River temporarily dammed about 260,000 years ago. The lake contained abundant hippos, crocodiles and catfish, while buffalo roamed the land.
The sediments and volcanic rock in which the fossils were found were dated at between 160,000 and 154,000 years by a combination of two methods. The argon/argon method was used by colleagues in the Berkeley Geochronology Center, led by Paul R. Renne, a UC Berkeley adjunct professor of geology. Wolde Gabriel of Los Alamos National Laboratory and Bill Hart of Miami University in Ohio used the chemistry of the volcanic layers to correlate the dated layers.
The Middle Awash team consists of more than 45 scientists from 14 different countries who specialize in geology, archaeology and paleontology. In this single study area, the team has found fossils dating from the present to more than 6 million years ago, painting a clear picture of human evolution from ape-like ancestors to present-day humans.
"The human fossils from Herto are near the top of a well-calibrated succession of African fossils," White said. "This is clear fossil evidence that our species arose through evolution."
The work was funded by the National Science Foundation and the Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory, in combination with the Hampton Fund for International Initiatives of Miami University and the Japan Society for the Promotion of Science. (Sanders, n.d.)
Southern blot hybridizations of genomic DNA were introduced as a relatively simple fossil–DNA-based approach to classify remains of Neanderthals. When hybridized with genomic DNA of either human or Neanderthal origin, DNA extracted from two Neanderthal finds—the Os parietale, from Warendorf-Neuwarendorf, Germany, and a clavicula, from Krapina, Croatia—was shown to yield hybridization signals that differ by at least a factor of two compared to the signals obtained with the use of fossil DNA of an early Homo sapiens from the Vogelherd cave (Stetten I), Germany. When labeled chimpanzee DNA was used as a probe, Neanderthal however, revealed hybridization signals of similar intensity. Thus, the genome of Neanderthals is expected to differ significantly from the genome of anatomically modern man, because of the contrasting composition of repetitive DNA. These data support the hypothesis that Neanderthals were not ancestors of anatomically modern man. (Scholz, et al., 2000)
Human beings are unusual in many ways but perhaps most strikingly in their unique symbolic form of processing information about the world around them. Although based on a long and essential evolutionary history, the modern human cognitive style is not predicted by that history: it is emergent rather than the product of an incremental process of refinement. Homo sapiens is physically very distinctive and is clearly the result of a significant developmental reorganization with ramifications throughout the skeleton and presumably beyond. It is reasonable to suppose that the neural underpinnings of symbolic thought were acquired in this reorganization. However, the fossil and archeological records indicate that the first anatomically recognizable members of the species substantially predated its first members who behaved in a demonstrably symbolic manner. Thus, while the biological potential for symbolic thinking most likely arose in the morphogenetic event that gave rise to H. sapiens as a distinctive anatomical entity, this new capacity was evidently exaptive, in the sense that it had to await its “discovery” and release through a cultural stimulus. Plausibly, this stimulus was the invention of language. One expression of symbolic reasoning is the adoption of technological change in response to environmental challenges, contrasting with earlier responses that typically involved using existing technologies in new ways. As climates changed at the end of the last Ice Age, the new technophile proclivity was expressed in a shift toward agriculture and sedentary lifestyles: a shift that precipitated a fundamentally new (and potentially self-destructive) relationship with nature. Thus, both of what are arguably the two most radical (and certainly the most fateful) evolutionary innovations in the history of life (symbolic thinking and sedentary lifestyles) were both very recent occurrences, well within the (so far rather short) tenure of H. sapiens. (Tattersall, 2009)
Paleoneurology is an important research field for studies of human evolution. Variations in the size and shape of the endocranium are a useful means of distin- guishing between different hominin species, while brain asymmetry is related to behaviour and cognitive capaci- ties. The evolution of the hominin brain is well documen- ted and substantial literature has been produced on this topic, mostly from studies of endocranial casts, or endo- casts. However, we have only little information about variations in endocranial form, size and shape in fossil anatomically modern Homo sapiens (AMH) and about the evolution of the brain since the emergence of our species. One good illustration of this limited knowledge is that one of the first fossil H. sapiens discovered, in 1868, that is also one of the oldest well-preserved Euro- pean specimen has never been studied in what concerns its endocranial morphology. The first aim of this study was to propose a detailed description of the endocranial anatomy of Cro-Magnon 1, using imaging methodologies, including an original methodology to quantify endocranial asymmetries. The second aim was to compare samples of the fossil and extant AMH in order to document differences in the form, size and shape of the endocasts. A decrease in absolute endocranial size since the Upper Paleolithic was noticeable. Although both extant and older endocrania have the same anatomical layout, we nonetheless found non-allometric differences in the relative size and organization of different parts of the brain. These document previously unknown intraspecific anatomical variations in the H. sapiens brain, demonstrating its plasticity, with some areas (frontal and occipital lobes) having been more subject to variation than others (parietal, temporal or cerebellar lobes). That may be due to constraints to maintain an optimal performance while reducing in size and changing in shape during our recent evolution. (Balzeau, et al., 2013)
The postcranial skeleton of modern Homo sapiens is relatively gracile compared with other hominoids and earlier hominins. This gracility predisposes contemporary humans to osteoporosis and increased fracture risk. Explanations for this gracility include reduced levels of physical activity, the dissipation of load through enlarged joint surfaces, and selection for systemic physiological characteristics that differentiate modern humans from other primates. skeletal remains of four behaviorally diverse recent human populations were considered and a large sample of extant primates to assess variation in trabecular bone structure in the human hip joint. Proximal femur trabecular bone structure was quantified from microCT data for 229 individuals from 31 extant primate taxa and 59 individuals from four distinct archaeological human populations representing sedentary agriculturalists and mobile foragers. Analyses of mass- corrected trabecular bone variables reveal that the forager populations had significantly higher bone volume fraction, thicker trabeculae, and consequently lower relative bone surface area compared with the two agriculturalist groups. There were no significant differences between the agriculturalist and forager populations for trabecular spacing, number, or degree of anisotropy. These results reveal a correspondence between human behavior and bone structure in the proximal femur, indicating that more highly mobile human populations have trabecular bone structure similar to what would be expected for wild nonhuman primates of the same body mass. These results strongly emphasize the importance of physical activity and exercise for bone health and the attenuation of age-related bone loss. (Ryan & Shaw, 2015)
6.2 Plate Tectonics
6.2.1 The formation of the earth’s crust
Simultaneous breakthroughs in our understanding of plate-tectonic processes, depositional systems, subsidence mechanisms, chronostratigraphic, and basin-exploration methods have resulted in rapidly improving actualistic models for sedimentary basins. Basin analysis has become a true science with the development of quantitatively testable models based on modern basins of known plate-tectonic setting. Major subdivisions of basin settings include divergent, convergent, transform, and hybrid; 23 basin categories occur within these settings. Basins are classified according to primary plate-tectonic controls on basin evolution: (1) type of sub-stratum, (2) proximity to plate boundary, and (3) type of nearest plate boundary(s). Sedimentary basins subside primarily owing to (1) attenuation of crust as a result of stretching and erosion, (2) contraction of lithosphere during cooling, and (3) depression of lithosphere by sedimentary and tectonic loads. The first two processes dominate in most divergent settings, whereas the third process dominates in most convergent settings. Intraplate, transform, and hybrid settings experience complex combinations of processes. Several basin types have low preservation potential, as predicted by their susceptibilities to erosion and uplift during orogeny and as confirmed by their scarcity in the very ancient record. Key references concerning actualistic plate-tectonic models for each type of basin form the basis for reviewing the present state of the science. The key references come from many sources, with diverse authorship, including several publications of the Geological Society of America. The further development and refinement of actualistic basin models will lead to improved testable paleotectonic reconstructions. (Ingersoll, 1988)
6.2.2 The mid-oceanic ridges
Whereas the majority of eruptions at oceanic spreading centers produce lavas with relatively homogeneous mid-ocean ridge basalt (MORB) compositions, the formation of tholeiitic andesites and dacites at mid-ocean ridges (MORs) is a petrological enigma. Eruptions of MOR high-silica lavas are typically associated with ridge discontinuities and have produced regionally significant volumes of lava. Andesites and dacites have been observed and sampled at several locations along the global MOR system; these include propagating ridge tips at ridge transform intersections on the Juan de Fuca Ridge and eastern Galapagos spreading center, and at the 98N overlapping spreading center on the East Pacific Rise. Despite the formation of these lavas at various ridges, MOR dacites show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes are controlling their chemistry. Although most geochemical variability in MOR basalts is consistent with low-pressure fractional crystallization of various mantle-derived parental melts, our geochemical data for MOR dacitic glasses suggest that contamination from a seawater-altered component is important in their petrogenesis. MOR dacites are characterized by elevated U, Th, Zr, and Hf, low Nb and Ta concentrations relative to rare earth elements (REE), and Al2O3, K2O, and Cl concentrations that are higher than expected from low-pressure fractional crystallization alone. Petrological modeling of MOR dacites suggests that partial melting and assimilation are both integral to their petrogenesis. Extensive fractional crystallization of a MORB parent combined with partial melting and assimilation of amphibole-bearing altered crust produces a magma with a geochemical signature similar to a MOR dacite. This supports the hypothesis that crustal assimilation is an important process in the formation of highly evolved MOR lavas and may be significant in the generation of evolved MORB in general. Additionally, these processes are likely to be more common in regions of episodic magma supply and enhanced magma crust interaction such as at the ends of ridge segments. (Wanless, Wanless, Perfit, Ridley, & Klein, 2010)
6.2.3 The continental plates
In plate tectonic theory, collision between two continents should quickly terminate because of continental buoyancy. If convergence is to continue, it should do so at a new subduction zone where oceanic crust can be consumed. The protracted continental collisions in the Alps, Zagros, and Himalayas, which have continued to deform continental crust since the early or middle Cenozoic, are therefore anomalies in standard plate tectonic theory. It is commonly held that plates are driven by slab pull, but this does not account for the continuing Tethyan collisions where the descending slab has detached from the subducting continent. These protracted continental collisions are better explained by horizontal traction of the mantle on the base of deep continental roots, dragging the northern and southern continents together along a Tethyan axis of mantle convergence. “Continental undertow” thus resolves the collision anomaly in plate tectonics. (Alvarez, 2010)
6.2.4 Evidence for the positions of the plates
Evolution and biogeography of primates: a new model based on molecular phylogenetics, vicariance and plate tectonics. The ages of the oldest fossils suggest an origin for primates in the Paleocene (∼56 Ma). Fossil-calibrated molecular clock dates give Cretaceous dates (∼80–116 Ma). Both these estimates are minimum dates although they are often ‘transmogrified’ and treated as maximum or absolute dates. Oldest fossils can underestimate ages by tens of millions of years and instead of calibrating the time-course of evolution with a scanty fossil record, the geographical boundaries of the main molecular clades of primates are calibrated here with radiometrically dated tectonic events. This indicates that primates originated when a globally widespread ancestor (early Archonta) differentiated into a northern group (Plesiadapiformes, extinct), a southern group (Primates), and two south-east Asian groups (Dermoptera and Scandentia). The division occurred with the breakup of Pangea in the Early Jurassic and the opening of the central Atlantic (∼185 Ma). Within primates, the strepsirrhines and haplorhines diverged with volcanism and buckling on the Lebombo Monocline, a volcanic rifted margin in south-east Africa (Early Jurassic, ∼180 Ma). Within strepsirrhines, lorises and galagos (Africa and Asia) and lemurs (Madagascar) diverged with the formation of the Mozambique Channel (Middle Jurassic, ∼160 Ma). Within haplorhines, Old World monkeys and New World monkeys diverged with the opening of the Atlantic (Early Cretaceous, ∼130 Ma). The main aspects of primate distribution are interpreted as the result of plate tectonics, phylogeny and vicariance, with some subsequent range expansion leading to secondary overlap. Long-distance, trans-oceanic dispersal events are not necessary. The primate ancestral complex was already widespread globally when seafloor spreading, strike-slip rifting and orogeny fractured and deformed distributions through the Jurassic and Cretaceous, leading to the origin of the modern clades. The model suggests that the topology of the phylogenetic tree reflects a sequence of differentiation in a widespread ancestor rather than a series of dispersal events. (Heads, 2010)
6.3 Dating
6.3.1 Relative dating
The most recent part of the geological timescale presents us with some of the greatest challenges for dating. With the exception of 230Th/234U methods, whose use is restricted to rather specific depositional environments, there is no established geochronometric tool capable of dating more than a fraction of the recent past at a resolution adequate to tackle the environmental issues of this period. Event stratigraphy, the investigation of comparatively rare and abrupt occurrences that leave some trace in the stratigraphic record, has been widely employed as a means of correlation and dating of older geological strata. Yet this approach has frequently been overlooked in efforts to establish chronologies of the recent past. It is ironic, therefore, that because of the acceleration of human activity, stratigraphic events have almost certainly occurred with greater frequency over the last few centuries than at any preceding time in Earth history. Because the history of human-induced events is usually well-established, the markers of such events have immense chronostratigraphic value. They may be employed in circumstances in which radiometric techniques may not be suitable and may offer higher-resolution dates than those associated with conventional dating methods. Dated event horizons may also provide the essential means by which to validate geochronometric analyses of the recent past. Event markers may be divided into those that produce discontinuities in the rock record and those (of much greater value in the terrestrial deposits that are the focus of most investigations of the recent past) that leave some tangible signal in the rocks. These signals may be the result of either natural factors or human-induced processes and may occur in a range of temporal contexts. They may mark the instant of occurrence of a short-lived phenomenon, or they may represent the abrupt disappearance or sudden appearance of some feature. This paper reviews each of these markers, focusing specifically on their application to the chronology of the recent past and the global environmental transformation that has taken place during this time. (Gale, 2009)
6.3.1 Absolute dating
Following the recent development of a new solid-state luminescence dating technique for geochronometry that uses the infrared radio fluorescence (IR-RF) at 1.43 eV (865 nm) of potassium feldspars, an automated radioluminescence (RL) measurement instrument was designed and built. The instrument is based on a commercial Daybreak 1100 automated TL reader system, widely used in thermoluminescence (TL) dating. It was re-designed and highly modified to adapt it to the physical and methodological needs of the IR-RF dating technique and other RL dosimetry applications. This new system holds up to 10 samples, has an integrated bleaching and irradiation unit, and measures the radio fluorescence (RF) (excitation using 10 137 Cs sources, each 5 MBq activity) as well as phosphorescence etc. All technical requirements for the measurement of optically excited luminescence were implemented in order to investigate the defect structure of luminescent materials. Because of the broad wavelength range and the high sensitivity of the photomultiplier detector used, the system is suitable for a great many luminescent materials, natural and synthetic. This source summarizes the technical features and performance criteria of the system. Furthermore, a calibration method and the dosimetric concepts, using the blue RF emission of Al2O3:C at 415 nm for the source dose rate estimation with low calibration errors is described in detail in quoted source. Finally, examples of IR-RF dating results on Quaternary sediments as well as of other RF measurements and their physical interpretation using deferent RF emissions are presented. (Erfurt, Krbetschek, Bortolot, Preusser, & Preusser, 2003)
