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Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
Hill (Twin Research and Human Genetics, Vol. 21, 2018, 84–88) presented a critique of our recently published paper in Cell Reports entitled ‘Large-Scale Cognitive GWAS Meta-Analysis Reveals Tissue-Specific Neural Expression and Potential Nootropic Drug Targets’ (Lam et al., Cell Reports, Vol. 21, 2017, 2597–2613). Specifically, Hill offered several interrelated comments suggesting potential problems with our use of a new analytic method called Multi-Trait Analysis of GWAS (MTAG) (Turley et al., Nature Genetics, Vol. 50, 2018, 229–237). In this brief article, we respond to each of these concerns. Using empirical data, we conclude that our MTAG results do not suffer from ‘inflation in the FDR [false discovery rate]’, as suggested by Hill (Twin Research and Human Genetics, Vol. 21, 2018, 84–88), and are not ‘more relevant to the genetic contributions to education than they are to the genetic contributions to intelligence’.
Ultraviolet (UV) Solar spectral Irradiance (SSI) has been measured from orbit on a regular basis since the beginning of the space age. These observations span four Solar Cycles, and they are crucial for our understanding of the Sun-Earth connection and space weather. SSI at these wavelengths are the main drivers for the upper atmosphere including the production and destruction of ozone in the stratosphere. The instruments that measure UV SSI not only require good preflight calibration, but also need a robust method to maintain that calibration on orbit. We will give an overview of the catalog of current and former UV SSI measurements along with the calibration philosophy of each instrument and an estimation of the uncertainties in the published irradiances.
The Yellow Chat Epthianura crocea is comprised of three disjunct subspecies. Subspecies E. c. macgregori (Capricorn Yellow Chat) is listed as Critically Endangered under the EPBC Act and has a distribution that also appears to be disjunct, with a limited geographic area of less than 7,000 ha. Some populations are threatened by rapid industrial development, and it is important for conservation of the subspecies to determine the extent to which the putative populations are connected. We used 14 microsatellite markers to measure genetic diversity and to determine the extent of gene flow between two disjunct populations at the northern and southern extremes of the subspecies’ range. No significant differences in genetic diversity (number of alleles and heterozygosity) were observed, but clear population structuring was apparent, with obvious differentiation between the northern and southern populations. The most likely explanation for reduced gene flow between the two populations is either the development of a geographic barrier as a consequence of shrinkage of the marine plains associated with the rise in sea levels following the last glacial maxima, or reduced connectivity across the largely unsuitable pasture and forest habitat that now separates the two populations, exacerbated by declining population size and fewer potential emigrants. Regardless of the mechanism, restricted gene flow between these two populations has important consequences for their ongoing conservation. The relative isolation of the smaller southern groups (the Fitzroy River delta and Curtis Island) from the much larger northern group (both sides of the Broad Sound) makes the southern population more vulnerable to local extinction. Conservation efforts should focus on nature refuge agreements with land owners agreeing to maintain favourable grazing management practices in perpetuity, particularly in the northern area where most chats occur. Supplemental exchanges of individuals from northern and southern populations should be explored as a way of increasing genetic diversity and reducing inbreeding.
Historians and social scientists have relied on contemporaneous textual accounts to document African American mobility in the immediate aftermath of emancipation after the Civil War, but they have interpreted them in widely varying ways. Some emphasize large-scale migration across the South, while others suggest that most movements were local and limited. This research tracks the early or “first wave” of African American migrants between 1865 and 1867 within and out of the South in an attempt to map the motion taking place after the war and to document the scale, direction, and intensity of African American mobility in the period between 1865 and 1867. The Freedmen's Bureau records indicate certain kinds of movements within the South, while our census methodology shows that there was more movement out of the South than accounted for in the Freedmen's Bureau labor records or previously accounted for in the historiography. Further, we observe two types of movement: short-term migration based on one-year contracts, perhaps returning to the point of origin, and another movement not always mediated through the Freedmen's Bureau that was more long term, but also subject to the freedperson's return to the point of origin. We seek to chart the process of emancipation over time and across space, detecting spatial patterns on an otherwise highly variable individual experience. No study has used the Freedmen's Bureau labor contracts to trace African American labor movements, and no study has deployed the 1880 individual census data to examine African American migration based on birthplace cohorts.
Whether monozygotic (MZ) and dizygotic (DZ) twins differ from each other in a variety of phenotypes is important for genetic twin modeling and for inferences made from twin studies in general. We analyzed whether there were differences in individual, maternal and paternal education between MZ and DZ twins in a large pooled dataset. Information was gathered on individual education for 218,362 adult twins from 27 twin cohorts (53% females; 39% MZ twins), and on maternal and paternal education for 147,315 and 143,056 twins respectively, from 28 twin cohorts (52% females; 38% MZ twins). Together, we had information on individual or parental education from 42 twin cohorts representing 19 countries. The original education classifications were transformed to education years and analyzed using linear regression models. Overall, MZ males had 0.26 (95% CI [0.21, 0.31]) years and MZ females 0.17 (95% CI [0.12, 0.21]) years longer education than DZ twins. The zygosity difference became smaller in more recent birth cohorts for both males and females. Parental education was somewhat longer for fathers of DZ twins in cohorts born in 1990–1999 (0.16 years, 95% CI [0.08, 0.25]) and 2000 or later (0.11 years, 95% CI [0.00, 0.22]), compared with fathers of MZ twins. The results show that the years of both individual and parental education are largely similar in MZ and DZ twins. We suggest that the socio-economic differences between MZ and DZ twins are so small that inferences based upon genetic modeling of twin data are not affected.
Anomalous aortic origin of the coronary arteries is associated with exercise-induced ischaemia, leading some physicians to restrict exercise in patients with this condition. We sought to determine whether exercise restriction was associated with increasing body mass index over time. From 1998 to 2015, 440 patients ⩽30 years old were enrolled into an inception cohort. Exercise-restriction status was documented in 143 patients. Using linear mixed model repeated-measures regression, factors associated with increasing body mass index z-score over time, including exercise restriction and surgical intervention as time-varying covariates, were investigated. The 143 patients attended 558 clinic visits for which exercise-restriction status was recorded. The mean number of clinic visits per patient was 4, and the median duration of follow-up was 1.7 years (interquartile range (IQR) 0.5–4.4). The median age at first clinic visit was 10.3 years (IQR 7.1–13.9), and 71% (101/143) were males. All patients were alive at their most recent follow-up. At the first clinic visit, 54% (78/143) were exercise restricted, and restriction status changed in 34% (48/143) during follow-up. The median baseline body mass index z-score was 0.2 (IQR 0.3–0.9). In repeated-measures analysis, neither time-related exercise restriction nor its interaction with time was associated with increasing body mass index z-score. Surgical intervention and its interaction with time were associated with decreasing body mass index z-score. Although exercise restriction was not associated with increasing body mass index over time, surgical intervention was associated with decreasing body mass index z-score over time in patients with anomalous aortic origin of the coronary arteries.
We begin the discussions of finite nuclei with some general remarks on the systematics of the nuclear ground state. For even-even nuclei (nuclei with an even number of both protons and neutrons) the nucleons making up the nucleus form pairs containing one spin-up and one spin-down nucleon to yield a net spin of zero and even parity. In Chapter 15 we consider elastic electron scattering from such nuclei as the paradigm for what follows. Odd-even and even-odd nuclei often, but not always, have the same spin as the last unpaired valence nucleon; we shall see examples when discussing elastic magnetic electron scattering and magnetic moments also in Chapter 15. Finally, odd-odd nuclei are somewhat unusual. There exist only four stable nuclei having unpaired protons and neutrons, namely, 2H, 6Li, and 14N with spin-parity 1+, and 10B having 3+. The basic characteristics of the known nuclei are the following: they occupy a region in the NZplane whose central valley runs roughly along the N = Z line at values of A below 40 and then bends towards the region having higher values of N than of Z, as shown in Fig. 13.1. Taking cuts across the valley at either constant Z or at constant N, one climbs out of the valley, on the average moving to less bound nuclei until reaching the so-called drip-lines where nuclei are no longer stable to proton or neutron emission. At the bottom of the valley, where the most stable nuclei reside, one finds the binding energy per nucleon to be relatively constant for nuclei beyond A = 40 at a value ̴ 8.5 MeV per nucleon, as shown in Fig. 13.2 and, as the values of N and Z where stable nuclei exist become very large, this valley of stability narrows and then disappears.
One of the key questions in studies of nuclear systematics is: Do “islands of stability” exist at even higher N, Z-values, the so-called superheavy nuclei? An island of stability, first conjectured by Seaborg in the 1960s, is a collection of heavier isotopes of transuranic elements, expected to be more stable than those closer in atomic number to uranium with radioactive decay half-lives of minutes to days.
The past one hundred years has witnessed enormous advances in human understanding of the physical universe in which we have evolved. For the past fifty years or so, the Standard Model of the subatomic world has been systematically developed to provide the quantum mechanical description of electricity and magnetism, the weak interaction, and the strong force. Symmetry principles, expressed mathematically via group theory, serve as the backbone of the Standard Model. At this time, the Standard Model has passed all tests in the laboratory. Notwithstanding this success, most of the matter available to experimental physicists is in the form of atomic nuclei. The most successful description of nuclei is in terms of the observable protons, neutrons, and other hadronic constituents, and not the fundamental quarks and gluons of the Standard Model. Thus, the professional particle or nuclear physicist should be comfortable in applying the hadronic description of nuclei to understanding the structure and properties of nuclei. Experimentally, lepton scattering has proved to be the cleanest and most effective tool for unraveling the complicated structure of hadrons. Its application over different energies and kinematics to the nucleon, fewbody nuclei, and medium- and heavy-mass nuclei has provided the solid body of precise experimental data on which the Standard Model is built.
In addition, the current understanding of the microcosm described in this book provides answers to many basic questions: How does the Sun shine? What is the origin of the elements? How old is the Earth? Further, it underscores many aspects of modern human civilization, e.g., MRI imaging uses the spin of the proton, nuclear isotopes are essential medical tools, nuclear reactions have powered the Voyager spacecraft since 1977 into interstellar space.
The purpose of the book is to allow the graduate student to understand the foundations and structure of the Standard Model, to apply the Standard Model to understanding the physical world with particular emphasis on nuclei, and to establish the frontiers of current research. There are many outstanding questions that the Standard Model cannot answer. In particular, astrophysical observation strongly supports the existence of dark matter, whose direct detection has thus far remained elusive.
In this chapter, few-body nuclei will be defined as those created through big bang nucleosynthesis, namely lithium, beryllium, and lighter elements. All elements heavier than beryllium and lithium were created much later, by stellar nucleosynthesis in evolving and exploding stars. This is discussed in detail in Chapter 20. In Chapter 11, the groundstate structure and properties of the deuteron have been discussed in detail. Here, the main focus will be on what can be learned from electroproduction from masses A = 2, 3, and 4. In addition, the topics of hypernuclei and fusion will be discussed.
Big bang nucleosynthesis (BBN) began a few minutes after the big bang, when the universe had cooled down sufficiently to allow deuterium nuclei to survive photodisintegration by high-energy photons. At this temperature, nucleosynthesis can take place and protons and neutrons can interact to form deuterium. Most of the deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium. Lithium-7 could also form via the coalescence of one tritium and two deuterium nuclei. The BBN produced the stable nuclei 2H, 3He, 4He, 6Li, and 7Li as well as the radioactive nuclei 3H, 7Be, and 8Be. No elements heavier than beryllium were produced due to the absence of stable nuclei with five or eight nucleons. Note that this bottleneck is overcome in stars by triple collisions of 4He nuclei, producing carbon. However, this process is very slow, taking tens of thousands of years to convert a significant amount of helium to carbon in stars. (See Chapter 20 for further discussion of the Big Bang and astrophysics.)
The theory of BBN predicts that roughly 25% of the mass of the universe consists of helium, with about 0.01% deuterium and smaller quantities of lithium. This prediction depends critically on the density of baryons (neutrons and protons) at the time of nucleosynthesis. The observation that helium is nowhere seen to have an abundance below 23% is strong evidence that the universe went through an early, hot phase. Further support comes from the consistency of the light element abundances for a particular value of baryon density and an independent measurement of this quantity from the anisotropies in the cosmic microwave background (CMB).