[1] Although most paleomagnetic and environmental magnetic papers incorporate a Day plot of the hysteresis parameters Mrs/Ms versus Hcr/Hc, a comprehensive theory covering superparamagnetic (SP), single-domain (SD), pseudo-single-domain (PSD), and multidomain (MD) (titano)magnetites is lacking. There is no consensus on how to quantify grain-size trends within the Day plot, how to distinguish MD from SP trends/mixtures, or whether magnetite, titanomagnetites, and other minerals have distinctive trends by which they might be identified. This paper develops the theory of the Day plot parameters for MD, MD + SD, PSD, and SP + SD grains of titanomagnetite (Fe 3– xTixO4) with compositions x = 0 (TM0 or magnetite) and x = 0.6 (TM60). MD grains have a separate trend that intersects the curve for SD + MD mixtures. SP + SD mixtures generate a variety of trends, depending on the SP grain size. All SP + SD curves lie much above those for MD or SD + MD trends, as has been proposed, but not demonstrated, previously. Data for PSD-size magnetites of many different origins fall along a single trend, but different levels of internal stress shift points for similar grain sizes along the ‘‘master curve.’’ In order to use the Day plot to determine grain size, one must have independent information about the state of internal stress. Theoretical model curves for SD + MD mixtures match the PSD magnetite and TM60 data quite well, although the SD!MD transition region in grain size is much narrower for TM60 than for magnetite. The agreement between PSD data and SD + MD mixing curves implies that PSD behavior is due to superimposed independent SD and MD moments, either in individual or separate grains, and not to exotic micromagnetic structures such as vortices. The theory also matches Mrs and Hc values in mechanical mixtures of very fine and very coarse grains, although nonlinear mixing theory is required to explain some Hcr and Hcr/Hc data. INDEX TERMS: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1533 Geomagnetism and Paleomagnetism: Remagnetization; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism;

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JB000486

Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data

Orogenic gold and geologic time: a global synthesis

Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.

https://science.sciencemag.org/content/sci/334/6059/1091.full.pdf

The Cambrian conundrum: Early divergence and later ecological success in the early history of animals

The role of fossils in dating the tree of life has been misunderstood. Fossils can provide good "minimum" age estimates for branches in the tree, but "maximum" constraints on those ages are poorer. Current debates about which are the "best" fossil dates for calibration move to consideration of the most appropriate constraints on the ages of tree nodes. Because fossil-based dates are constraints, and because molecular evolution is not perfectly clock-like, analysts should use more rather than fewer dates, but there has to be a balance between many genes and few dates versus many dates and few genes. We provide "hard" minimum and "soft" maximum age constraints for 30 divergences among key genome model organisms; these should contribute to better understanding of the dating of the animal tree of life.

/pdf/paleontological-evidence-to-date-the-tree-of-life-5dhwbwgnyr.pdf

Paleontological Evidence to Date the Tree of Life

Human-induced stresses of overfishing, eutrophication, climate change, translocation and habitat modification appear to be promoting jellyfish (pelagic cnidarian and ctenophore) blooms to the detriment of other marine organisms. Mounting evidence suggests that the structure of pelagic ecosystems can change rapidly from one that is dominated by fish (that keep jellyfish in check through competition or predation) to a less desirable gelatinous state, with lasting ecological, economic and social consequences. Management actions needed to stop such changes require tactical coping strategies and longer-term preventative responses based on fundamental and targeted research on this understudied group.

/pdf/the-jellyfish-joyride-causes-consequences-and-management-1c27h0vbp7.pdf

The jellyfish joyride: causes, consequences and management responses to a more gelatinous future

The effect os cooling rate on the intensity of low-field thermoremanent magnetization (TRM) in uniaxial, single-domain magnetite is calculated, using both numerical and analytical techniques. These calculations indicate that when a single-domain assemblage is cooled much more slowly than in a typical laboratory experiment, it can remain magnetically unblocked and in thermal equilibrium with an external field at temperatures significantly lower than the 'laboratory' blocking temperature. Owing to the rapid rise of spontaneous magnetization just below the Curie temperature the TRM can consequently be elevated by several tens of percent over that acquired in the laboratory. The paleointensity obtained from rocks that have cooled in nature over geologic time may therefore be erroneously high, if single-domain particles are the primary carriers of NRM. These calculations indicate that when the ratio of 'natural' to laboratory cooling rates approximately equals 10/sup -11/, as in the case of orogenic bodies that have cooled over several million years, the paleointensity obtained by the Thellier method can be more than 50% too igh, unless the role of cooling rate is taken into account.

The effect of cooling rate on the intensity of weak‐field trm in single‐domain magnetite

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (∼505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

/pdf/exceptionally-preserved-jellyfishes-from-the-middle-cambrian-4fqglxa947.pdf

Exceptionally Preserved Jellyfishes from the Middle Cambrian

The Middle Cambrian Spence Shale Member (Langston Formation) and Wheeler and Marjum Formations of Utah are known to contain a diverse soft-bodied fauna, but important new paleontological material continues to be uncovered from these strata. New specimens of anomalocaridids include the largest and smallest near complete examples yet reported from Utah. New material of stem group arthropods includes two new genera and species of arachnomorphs: Nettapezoura basilika and Dicranocaris guntherorum. Other new arachnomorph material includes a new species of Leanchoilia comparable to L. protogonia Simonetta, 1970; Leanchoilia superlata? Walcott, 1912; Sidneyia Walcott, 1911a; and Mollisonia symmetrica Walcott, 1912. L. protogonia from the Burgess Shale is confirmed as a separate species and is not a composite fossil. The first example of the trilobite Elrathia kingii preserving traces of the appendages is described. In addition, new material of the bivalved arthropods Canadaspis Novozhilov in Orlov, 1960; Branchiocaris Briggs, 1976; Waptia Walcott, 1912; and Isoxys Walcott, 1890 is described.

Middle Cambrian Arthropods from Utah

The fundamental temperature dependence of magnetic domain structure has been recognized for several decades, but its relationship to thermoremanent magnetization (TRM) has been unexplored. As one step toward investigating this possible link, we have studied Bitter patterns on synthetic Ti-rich titanomagnetite (Al0.1Mg0.1Ti0.6Fe2.2O4; Curie point of approximately 75°C) as a function of temperature in a weak field (0.42 Oe), in states of weak field TRM, and after alternating field demagnetization. Two different types of patterns were studied with the following goals: (1) simple Kittel-like patterns of straight walls were studied in order to gain insight into the nature of TRM domain structures and the various possible mechanisms by which particles acquire weak field TRM; (2) mazelike patterns, which occupy regions of high surface stress, were studied in order to estimate the temperature dependence of magnetostriction constant in this material. Results of experiments to study Kittel-like patterns suggest that two different mechanisms, and two resulting species of domain structure, could account for fundamental differences between multidomain and pseudosingle-domain thermoremanence in titanomagnetite. In accordance with traditional models, typical multidomain TRM results from wall pinning in a particle that contains several domains whose volumes are approximately equal to each other. In contrast, pseudosingle-domain TRM is acquired when denucleation of domains and domain walls yields a domain structure with a large associated “intrinsic” moment. The following observations support these conclusions. In a particle with a Kittel-like pattern, the number of domains varies from one TRM experiment to the next. This variation indicates that a grain can acquire TRM in a range of local energy minimum states, rather than in one particular domain configuration of absolute minimum energy. Many such TRM patterns represent the near-ideal local energy minimum domain states, consisting of several domains of approximately equal widths, usually envisioned in models of multidomain thermoremanence. Observations suggest that a particle can maintain such states during cooling if weakly pinned walls adjust readily to demagnetizing fields generated during the denucleation process. In this case, TRM results from small displacements of pinned walls with respect to a state of minimum magnetostatic energy; several observations suggest that blocking of walls at pinning centers in the classical manner occurs mainly after the final nucleation event as the particle cools toward room temperature from the Curie point. In contrast, we have observed examples in which denucleation leaves behind an anomalously large domain where several approximately uniform domains existed prior to denucleation; several smaller domains survive elsewhere in the particle, virtually unaffected by the transition. Evidently, strong wall-pinning forces sometimes prevent surviving walls from responding to the change of internal field which must accompany the denucleation process. It is observed that denucleation can yield single-domain-like states during TRM acquisition. Thus denucleation, rather than complete failure to nucleate during TRM acquisition, is one mechanism for producing single-domain-like states in particles which ordinarily favor walls. Possibly, such states exist by virtue of strong pinning forces at surface defects, into which denucleating domains and domain walls collapse. Particles with an odd number of approximately uniform domains also are observed in TRM states, and such particles also may carry large intrinsic moments owing to domain imbalance; possibly, such domain configurations result from the final denucleation episode during cooling. Thus these observations indicate a strong link between TRM acquisition mechanisms, magnetic domain transitions, and the accessibility of domain states. We hypothesize that denucleation could account for the most desirable properties of TRM associated with pseudosingle-domain titanomagnetite particles, namely, high intensity and high thermal stability. The second class of patterns, mazelike patterns, was studied to estimate the temperature dependence of domain wall energy and magnetostriction constant from the observed temperature dependence of domain width. The temperature dependence of magnetostriction constant obtained in this manner for AMTM60 is similar to that obtained for magnetite in earlier studies.

Magnetic domain patterns observed on synthetic Ti‐rich titanomagnetite as a function of temperature and in states of thermoremanent magnetization

The stable carriers of the palaeomagnetic record in rocks are commonly fine grains of magnetite or titanomagnetite (Fe3–xTixO4) between 1 and 15 µm in diameter. The magnetic properties of these particles are therefore of interest to palaeomagnetists who must rely on them to preserve the Earth's magnetic field record over geological time. We report here domain observations of titanomagnetites in samples from Leg 49 of the Deep Sea Drilling Project. The results indicate that a major contributor to ‘pseudosingle-domain’ (PSD) saturation remanence is single domain remanence of particles larger than the classical single-domain threshold size which fail to nucleate domain walls in this magnetization state.

Susan L. Halgedahl

Papers

The effect of cooling rate on the intensity of weak‐field trm in single‐domain magnetite

Exceptionally Preserved Jellyfishes from the Middle Cambrian

Middle Cambrian Arthropods from Utah

Magnetic domain patterns observed on synthetic Ti‐rich titanomagnetite as a function of temperature and in states of thermoremanent magnetization

Magnetic domain observations of nucleation processes in fine particles of intermediate titanomagnetite