Stepwise Shift Research Articles

Lengthening Atlantic Hurricane Seasons with Earlier Storm Formation Dates Including Implications from 2020

Abstract This paper analyzes the formation dates of the nth storm in a sequence for all named North Atlantic tropical cyclones and assesses whether the intraseasonal length of the Atlantic hurricane season has changed temporally. The record-breaking 2020 season, with 30 named storms, set records for the earliest 3rd TC formation (Cristobal) and from the 6th TC (Fay) onward. Analysis of season length from 1851–2022 identifies only one statistically significant breakpoint detected in the early 1970s, roughly coinciding with the introduction of satellite observations. Since 1970, we also find a trend towards longer North Atlantic hurricane seasons The 1971–2022 trend is robust and statistically significant, whether assessed as the number of days between the first and last storm, or by the distance between intermediate percentiles (e.g., 10th to 90th, 15th to 85th). These increases are mainly associated with storms forming earlier in the calendar year and are best described as an upward trend rather than a stepwise shift between eras. Although a simple trend fits better than a stepwise model, improvement falls short of significance, so we do not formally reject the stepwise hypothesis. If, following this hypothesis, the 1950–2022 period is segmented into a high activity era (HAE1; 1950–1969), a low activity era (LAE; 1970–1994), and second high activity era (HAE2; 1995–2022), the median season length HAE2 is 12 days longer than in the first HAE, but this difference is not statistically significant (p = 0.58) and could be explained by the substantial difference in the observational network. The median season length in both HAE1 and AE2 are significantly longer (by 36 and 48 days, respectively) than the intervening LAE.

Successful delivery of viviparous lantern shark from an artificial uterus and the self-production of lantern shark luciferin.

Our recent success in the long-term maintenance of lantern shark embryos in artificial uterine systems has provided a novel option for the medical treatment of premature embryos for captive viviparous elasmobranchs. The remaining issue with this system is that the embryos cannot survive the abrupt change in the chemical environment from artificial uterine fluid (AUF) to seawater during delivery. To overcome this issue, the present study developed a new protocol for seawater adaptation, which is characterized by a long-term and stepwise shift from AUF to seawater prior to delivery. This protocol was employed successfully, and the specimen survived for more than seven months after delivery, the longest captive record of the species. During the experiment, we unexpectedly detected bioluminescence of the embryonic lantern shark in the artificial uterus. This observation indicates that lantern sharks can produce luciferin, a substance for bioluminescence. This contradicts the recent hypothesis that lantern sharks lack the ability to produce luciferin and use luciferin obtained from food sources.

Determining factors for changes in the ice regime of the Caspian Sea

ABSTRACT This article provides an analysis of the factors driving long-term change in the characteristics of the ice regime of the Northern Caspian Sea using satellite and in situ observation. It assesses the climatic shifts, comparing the linear trend and step change models. We link the stepwise shift in temperatures in the 1980s with the changes in the forms of atmospheric circulation. The decrease in the sum of negative temperatures impacts the length and intensity of winters with the 18–27-day reduction in the freeze period, leading to a shorter period of ice cover and reduced ice thickness by 20–28 cm.

One stimulus-induced two-step photophysical response with high contrast and tunable switching time for dynamic displaying

One stimulus-induced two-step photophysical response, especially with tunable switching time, is a great challenge for organic chromophores. Herein, a polymorphic material 2,7-DCF could undergo in situ two sequential dual-channel responses upon dichloromethane fuming. Both the appearance color and the fluorescence change from red to yellow to deep red with high contrast. The first step corresponds to a fast amorphous-to-crystalline transformation, while the second is a slow solid-state cocrystallization process. Based on single crystal structures and theoretical calculations, such distinct color changes are mainly attributed to conformation twisting and the electron coupling with incorporated solvent molecule through C-H⋅⋅⋅O interaction. Importantly, the second slow photophysical response could be drastically sped up by seeding strategy, or be totally inhibited. Such characteristics pave a way for the potential applications in dynamic anti-counterfeiting and data encryption. Based on the two-step transformation, polymorph 2,7-DCF-a could achieve a successive four-level response to external stimuli. In contrast, polymorph 2,7-DCF-d exhibits a stepwise hypsochromic fluorescence shift over 100 nm. This study would significantly promote the development of stimuli-sensitive systems from “one stimulus, one-step response” to “one stimulus, two or multi-step response”.

Sea ice directs changes in bowhead whale phenology through the Bering Strait

BackgroundClimate change is warming the Arctic faster than the rest of the planet. Shifts in whale migration timing have been linked to climate change in temperate and sub-Arctic regions, and evidence suggests Bering–Chukchi–Beaufort (BCB) bowhead whales (Balaena mysticetus) might be overwintering in the Canadian Beaufort Sea.MethodsWe used an 11-year timeseries (spanning 2009–2021) of BCB bowhead whale presence in the southern Chukchi Sea (inferred from passive acoustic monitoring) to explore relationships between migration timing and sea ice in the Chukchi and Bering Seas.ResultsFall southward migration into the Bering Strait was delayed in years with less mean October Chukchi Sea ice area and earlier in years with greater sea ice area (p = 0.04, r2 = 0.40). Greater mean October–December Bering Sea ice area resulted in longer absences between whales migrating south in the fall and north in the spring (p < 0.01, r2 = 0.85). A stepwise shift after 2012–2013 shows some whales are remaining in southern Chukchi Sea rather than moving through the Bering Strait and into the northwestern Bering Sea for the winter. Spring northward migration into the southern Chukchi Sea was earlier in years with less mean January–March Chukchi Sea ice area and delayed in years with greater sea ice area (p < 0.01, r2 = 0.82).ConclusionsAs sea ice continues to decline, northward spring-time migration could shift earlier or more bowhead whales may overwinter at summer feeding grounds. Changes to bowhead whale migration could increase the overlap with ships and impact Indigenous communities that rely on bowhead whales for nutritional and cultural subsistence.

Photocured Organofunctional Silicon-Based Polymer and Its Y2O3 Nanocomposite as the Luminescence Tracer of Thermal History

Materials with irreversibly changing properties after thermal heating are of great interest as thermal history trackers and are in demand as part of almost any device. Here, a promising thermoreporting material based on an organosilicone nanocomposite is suggested. Samples of photocured poly(1-trimethoxysilylpropyloxymethyl)ethylenoxide (polyGPTMS) and its nanocomposite with a filler of Y2O3 nanoparticles (polyGPTMS–Y2O3) were obtained with UV laser polymerization. The subsequent heat treatment of the samples revealed luminescence, the spectral position of which demonstrated a red shift with temperature increase. Comparative analysis of luminescence and Fourier-transform infrared spectroscopy spectra as well as theoretical modeling were used to characterize the structural peculiarities of the polymer and the nanocomposite. It was found that the luminescence behavior correlates with the color centers formation in the course of thermal treatment. The red shift of luminescence with temperature is associated with the redistribution of the chemical network and changes in the contribution of different color centers. The main effect is connected with an E′ center and oxygen-deficient centers, where the predominant contribution of the latter at high temperatures determines the observed luminescence red shift. The different dynamics of the color centers formation for polyGPTMS and polyGPTMS–Y2O3 results in a correspondingly stepwise and gradual luminescence red shift. The uncovered gradual luminescence red shift with thermal treatment of the polyGPTMS–Y2O3 makes the nanocomposite a promising tracer of thermal history. Using polyGPTMS–Y2O3 nanocomposite as a reporting material allows establishing not only the fact of heating but also the heating temperature.

The influence of the lateral contact point trajectory and the rotation of the monoblock on the impact loads in railway turnouts

A new wheel–rail interaction model for the prediction of turnout impact forces is presented. Although the model is based on the classical “moving irregularity” approach, it is novel in the sense that it accounts for the lateral shift of the wheel–rail contact position during load transfer between the crossing nose and wing rail. This lateral shift increases the mobility of the track due to the rotational response of the monoblock under off-centred excitation. It is shown that the resulting stepwise variation of the wheel and track mobilities upon impact can profoundly influence the impact force time history. It is furthermore shown that these effects are particularly emphasized during the trailing move when the wheel impacts on the off-centred wing rail and much less during the facing move when the impact is absorbed by the crossing nose which is located in the plane of symmetry of the monoblock cross-section. Apart from the stepwise shift upon load transfer the lateral wheel–rail contact point position varies continuously along the longitudinal coordinate of the turnout. Simulations show that these continuous variations have a negligible influence on the impact force time histories and that a simplified model with constant shift gives satisfactory results. A comparison of the calculated crossing nose and wing rail accelerations shows encouraging agreement with field measurements. It is concluded that omission of the rotation leads to an overestimate of the track vibrations. Especially for the trailing move, this overestimate becomes substantial for frequencies above 100Hz.

Synthesis of Distorted Nitrogen-Doped Nanographenes by Partially Oxidative Cyclodehydrogenation Reaction.

A series of partially fused N-doped nanographenes (2-4) are synthesized via the oxidative cyclodehydrogenation of oligoaryl-substituted dibenzo[e,l]pyrene (1), and five, six, and seven new C-C bonds are formed, respectively, implying stepwise C-C bond fusion and extended π-conjugation. Single-crystal X-ray diffraction analysis of compound 4 a revealed that the presence of sterically demanding groups hindered the formation of planar and fully fused nanographene in the oxidative cyclodehydrogenation reaction step. Optical study of compounds 2 to 4 showed that extended π-conjugation leads to a regular stepwise bathochromic shift in the absorption and emission spectra. Furthermore, the HOMO-LUMO gaps of these compounds exhibit a decrease as C-C bond formation proceeds. Thus, the optoelectronic properties of nanographenes are highly dependent on the formation of new C-C bonds in the molecular skeleton.

Analysis and experimental demonstration of temperature step gradients in preparative liquid chromatography

The amount of substance adsorbed on solid surface depends on temperature. Therefore, the migration velocities of the solutes in a chromatographic column can be altered by introducing temperature gradients. Such gradients designed to change retention behaviours can be exploited to improve the separation performances in preparative chromatography. To describe key process features, we used analytical solutions of the equilibrium model with instant stepwise shift of temperature. To achieve a more realistic description, the equilibrium dispersion model was additionally applied to treat finite column efficiencies. The effect of temperature gradients was illustrated experimentally using two identical columns sequentially connected. Temperature of the second column was modulated by thermostats. Wide pulse injections of a single component led to instructive elution profiles in a preliminary investigation. The observations were found to be in qualitative agreement with predictions of the equilibrium dispersion model. Subsequently, the separation of a ternary model mixture was investigated considering a simple two-step temperature gradient. To support the quantitative analysis and to identify suitable switching and cycle times, the temperature dependencies of the Henry constants were determined by short pulse injections. A meaningful variation of the parameters of the temperature gradient is required for adjusting the cycle times, which is the time difference between two consecutive injections that needs to shorten. Decreasing this time is connected with a desirable increase in process productivity. The results achieved revealed that relatively simple to implement stepwise temperature gradients offer an option to improve and fine-tune the performance of repetitive batch chromatography.

Aridification and orbital forcing of eastern African climate during the Plio-Pleistocene

We developed a composite outcrop proxy record of Plio-Pleistocene climate in eastern Africa using the radiometrically-dated sediments of the Omo Group that preserve evidence for early hominin evolution. New stable oxygen isotopic measurements from pedogenic carbonate nodules (δ18OPC, n = 270) are added to the existing database for a total of 759 observations spanning a ~ 3.6 Myr interval from 4.33–0.65 Ma. Linear interpolation of sample positions between tuffaceous marker horizons produced the chronostratigraphy of the δ18OPC record that was correlated to Indo-Pacific marine core sediment records and astronomical solutions of orbital climate forcing. Rainfall/evaporation changes inferred from the δ18OPC record carry orbital eccentricity signals, and the amplitude modulation of short eccentricity (~100 kyr) predicted by the astronomical solutions is evident during some intervals. δ18OPC variations recover many of the ~100 kyr cycles between ~4.3–0.7 Ma, with chronostratigraphic offsets between the Omo Group time series and astronomical solutions usually within the resolution of the radiometric uncertainties, i.e., ≤ 50 kyr; however, missing time at ~1.2 Ma contributes to nearly 150 kyr of offset. These δ18OPC data also indicate a long-term drying trend beginning at 2.11 Ma that correlates to a thicker eastern Indian Ocean mixed layer and upwelling in the eastern Pacific, implying a simultaneous response across the tropics to strengthening Walker Circulation. Published δ13CPC values from the same pedogenic carbonate nodules indicate that C4 vegetation abundances increased beginning at ~2.0 Ma, also suggesting climatic drying. Spectral analysis of the δ13CPC time series provides weaker evidence of orbital forcing as compared to the δ18OPC time series, particularly for long orbital eccentricity (405 kyr) and climatic precession (~20 kyr), suggesting other factors, such as feeding ecology of herbivores or global greenhouse gas, may have contributed to the vegetation patterns. The climatic precession component to the δ18OPC time series increases amplitude through strengthening Walker Circulation, which may signal a change in the intensity of interannual climate events. No indications were observed to suggest that the Plio-Pleistocene Northern Hemisphere glaciation resonated as either climatic cyclicity or an abrupt, stepwise shift in the regional hydroclimate. At major junctures of hominin evolution, including the origins of different genera and the earliest stone tools, environmental variability derived from the orbital forcing of the monsoon appears to have been low. However, we propose that at 2.11–1.66 Ma, directional aridity coupled with interannual climate events presented a new adaptive context where Homo erectus and Acheulian stone tools first appeared in eastern Africa.

Holocene Vegetation and Climate Changes in the Huangqihai Lake Region, Inner Mongolia

AbstractA consensus on Holocene climate variability at the modern northern fringe of the East Asian summer monsoon (EASM) region remains elusive. Here, we present a pollen‐based reconstruction of vegetation history and associated climate variations of a sediment core from Huangqihai Lake, central Inner Mongolia. During 10.7 to 8.8 cal kaBP, typical steppe with small patches of forest dominated the lake area, suggesting a moderately wet climate, followed by ameliorating climatic conditions until 8.0 cal kaBP as deduced by the expansion of forest. Typical steppe recovered the lake area between 8.0 and 7.2 cal kaBP, reflecting a deterioration of climatic conditions; in combination with other proxy records in the study region, we noticed that severe aridity was prevailed in the lake area between 8.0 and 7.6 cal kaBP. During 7.2 to 3.2 cal kaBP, abundant tree pollen indicated dominance of forest‐steppe around the lake, marking regionally wet conditions. A notable absence of broadleaved trees after 5.2 cal kaBP reveals a slight drying trend, and climate deterioration from 4.5 to 4.1 cal kaBP might be linked to the 4.2 ka event. After 3.2 cal kaBP, a transition to steppe was associated with dry conditions in the region. Based on our pollen record and prior paleoclimatic reconstructions in the Huangqihai Lake region, there was a generally‐accepted, stepwise shift to a wet climate during the early Holocene, an overall humid climate from 7.2 to 3.2 cal kaBP, and then severe drought for the rest of the Holocene. Moreover, regional comparisons among pollen records derived from lakes situated in the temperate steppe region suggested a roughly synchronous pattern of vegetation and climate changes during the Holocene and demonstrated an intensified EASM during the middle Holocene.

Generalized MPI Multi-Patch Reconstruction Using Clusters of Similar System Matrices.

The tomographic imaging method magnetic particle imaging (MPI) requires a multi-patch approach for capturing large field of views. This approach consists of a continuous or stepwise spatial shift of a small sub-volume of only few cubic centimeters size, which is scanned using one or multiple excitation fields in the kHz range. Under the assumption of ideal magnetic fields, the MPI system matrix is shift invariant and in turn a single matrix suffices for image reconstruction significantly reducing the calibration time and reconstruction effort. For large field imperfections, however, the method can lead to severe image artifacts. In the present work we generalize the efficient multi-patch reconstruction to work under non-ideal field conditions, where shift invariance holds only approximately for small shifts of the sub-volume. Patches are clustered based on a magnetic-field-based metric such that in each cluster the shift invariance holds in good approximation. The total number of clusters is the main parameter of our method and allows to trade off calibration time and image artifacts. The magnetic-field-based metric allows to perform the clustering without prior knowledge of the system matrices. The developed reconstruction algorithm is evaluated on a multi-patch measurement sequence with 15 patches, where efficient multi-patch reconstruction with a single calibration measurement leads to strong image artifacts. Analysis reveals that calibration measurements can be decreased from 15 to 11 with no visible image artifacts. A further reduction to 9 is possible with only slight degradation in image quality.

Influence of the pumping wavelength on laser properties of Fe2+ ions in ZnSe crystal.

ZnSe:Fe2+ active laser crystal properties at different excitation wavelengths (2.94 and 4.1μm) were investigated, and noticeable variations of the fluorescence spectra shape and their maxima positions, as well as changes in decay times, were observed. A stepwise shift of the laser oscillation wavelength from 4.7μm at 2.94μm pumping to 4.95μm at 4.1μm pumping was achieved at room temperature.

Noise-resistant developmental reproducibility in vertebrate somite formation.

The reproducibility of embryonic development is remarkable, although molecular processes are intrinsically stochastic at the single-cell level. How the multicellular system resists the inevitable noise to acquire developmental reproducibility constitutes a fundamental question in developmental biology. Toward this end, we focused on vertebrate somitogenesis as a representative system, because somites are repeatedly reproduced within a single embryo whereas such reproducibility is lost in segmentation clock gene-deficient embryos. However, the effect of noise on developmental reproducibility has not been fully investigated, because of the technical difficulty in manipulating the noise intensity in experiments. In this study, we developed a computational model of ERK-mediated somitogenesis, in which bistable ERK activity is regulated by an FGF gradient, cell-cell communication, and the segmentation clock, subject to the intrinsic noise. The model simulation generated our previous in vivo observation that the ERK activity was distributed in a step-like gradient in the presomitic mesoderm, and its boundary was posteriorly shifted by the clock in a stepwise manner, leading to regular somite formation. Here, we showed that this somite regularity was robustly maintained against the noise. Removing the clock from the model predicted that the stepwise shift of the ERK activity occurs at irregular timing with irregular distance owing to the noise, resulting in somite size variation. This model prediction was recently confirmed by live imaging of ERK activity in zebrafish embryos. Through theoretical analysis, we presented a mechanism by which the clock reduces the inherent somite irregularity observed in clock-deficient embryos. Therefore, this study indicates a novel role of the segmentation clock in noise-resistant developmental reproducibility.

Palaeoenvironmental implication of grain-size compositions of terrace deposits on the western Chinese Loess Plateau

Sedimentary sequences that developed on river terraces have been widely investigated to reconstruct high-resolution palaeoclimatic changes since the last deglaciation. However, frequent changes in sedimentary facies make palaeoenvironmental interpretation of grain-size variations relatively complicated. In this paper, we employed multiple grain-size parameters to discriminate the sedimentary characteristics of aeolian and fluvial facies in the Dadiwan (DDW) section on the western Chinese Loess Plateau. We found that wind and fluvial dynamics have quite different impacts on the grain-size compositions, with distinctive imprints on the distribution pattern. By using a lognormal distribution fitting approach, two major grain-size components sensitive to aeolian and fluvial processes, respectively, were distinguished from the grain-size compositions of the DDW terrace deposits. The fine grain-size component (GSC2) represents mixing of long-distance aeolian and short-distance fluvial inputs, whilst the coarse grain-size component (GSC3) is mainly transported by wind from short-distance sources. Thus GSC3 can be used to infer the wind intensity. Grain-size variations reveal that the wind intensity experienced a stepwise shift from large-amplitude variations during the last deglaciation to small-amplitude oscillations in the Holocene, corresponding well to climate changes from regional to global context.

Development of phase-shifting device for implementation of three-step interferometric method with arbitrary phase shifts of reference wave

Two basic approaches to the implementation of procedures of a wavefront stepwise and smooth phase shifts in a two-beam optical interferometer are considered. Advantages and lacks of both procedures are analyzed. It is noted that the smooth phase shift procedure removes the oscillations of a phase shifting element (PSE) that occur during the stepwise procedure. A phase-shifting device that is used to realize the three-step phase shifting interferometry (PSI) method with two arbitrary phase shifts of the reference wave within the angular interval (0, π) is developed. This device does not need the calibration procedure, because the three step PSI method allows defining any phase shift angle by calculating the correlation coefficient between two recorded interferograms. The correlation coefficient can be considered as a normalized scalar product of two interferograms that can be represented by centered multidimensional vectors or as a cosine of a phase shift angle between these vectors. Therefore, the developed device is much simpler and cheaper than its calibrated prototypes. It contains a PSE consisting of a piezoelectric transducer (PZT) and a mirror attached to the PZT, and an electronic unit for smooth linear motion of the mirror in the reference beam of the interferometer. Basic characteristics of the developed electronic unit are considered. The operating principle of the phase-shifting device electronic unit is to provide the smooth rising voltage supply initiated a smooth change of the PZT size. Therefore, the mirror attached to the PZT also moves smoothly. The PSE is located in the reference beam of the two-beam interferometer. If the reference beam in the interferometer enters the mirror in the direction perpendicular to the plane of the mirror, the beam wavefront moves to a distance twice that of the mirror. To verify the reliability of the developed phase-shifting device, we have elaborated the experimental setup dedicated for formation, registration and processing of interferograms of test surfaces and based on the Twyman-Green interferometer architecture. In this setup, the three-step PSI method with arbitrary phase shifts of a reference wave is fulfilled by recording of three interferograms during smooth phase shift of the mirror attached to the PZT. Comparative analysis of stepwise and smooth phase shift procedures for interferograms recording in the experimental setup has shown that their modulation transfer functions (MTFs) are similar for small exposures of interferograms. In particular, it is shown that for interferogram’s small exposure times, whose ratio to the voltage increase time in the angular interval (0, π) does not exceed 0.06, the MTF of the interferogram recorded with the smooth phase shift of the reference beam does not differ from the interferogram MTF obtained by stepwise phase shift. Key words: phase shifting interferometry, phase shift, phase shifting element, interferogram, modulation transfer function

Successive stepwise evolution of host layer-stacking framework upon the intercalation of mobile vapor guests within side-chain layers

For the ordered phases of hairy-rod semiconductive poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) sandwiched in between crystalline platelets of hexamethylbenzene, the successive stepwise evolution of layer-stacking framework upon guest intercalation has been studied in this research. The direct consequence of the guest intercalation into side-chain layers is evaluated to cause the lateral shift of thiophene backbones along π–π stacking, resulting in stepwise shift of ultraviolet absorption wavelength. The thermal motions of vapor guests within disordering side-chain layers subsequently cause progressive expansion of host stacking framework. With the increase in side-chain length, thicker layers of disordering side chains in liquid crystals (LCs) accommodate additional vapor guests and larger amplitudes of thermal motions of guests, hence promoting the level of reversible d-spacing change. The mixing between mobile vapor guests and aliphatic side chains is clarified as the mechanism of guest intercalation, which rationalizes successive guest intercalation during heating and the contribution of disordering side-chain layers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017

(Invited) Quasi Phase Transition in a Single File of Water Molecules Encapsulated inside (6,5) Carbon Nanotubes

The confinement of water molecules in one-dimensional channels leads to fascinating new properties which are of critical importance for many processes in biology, geology, catalysis and nanofluidics. Single-walled carbon nanotubes (SWCNTs) are one of the most promising model systems to investigate molecules under confinement. They combined atomically-precise diameters that deliver the necessary level of confinement with a weak interaction of the water molecules with their smooth inner walls. Despite significant efforts, theoretical predictions of the behavior and peculiar properties of a single-file chain of water water molecules (e.g. interesting for ultrafast proton transport) have proven to be very hard to validate experimentally. In this work, we present temperature-dependent (4.2K up to room temperature) photoluminescence (PL) spectra of water-filled and empty chirality-sorted (6,5) SWCNTs. Superimposed on a linear temperature-depenent PL spectral shift of the empty tubes, an additional stepwise PL shift of the water-filled SWCNTs is observed at around 150K. With the empty SWCNTs serving as the ideal reference system, we can assign this observation to temperature-induced changes occuring within the single file molecular chain. Supported by new Molecular Dynamics simulations, we obtain the first experimental evidence of a quasi phase transition of the orientation of the molecular dipoles within such a one-dimensional single file chain of water molecules.

A 20,000-year high-resolution pollen record from Huguangyan Maar Lake in tropical–subtropical South China

A high-resolution pollen record from Huguangyan Maar Lake near the South China Sea spanning the past 20,400yrBP has been presented, aimed to provide an improved understanding of the response of the regional vegetation in tropical–subtropical South China to climatic variations since the late Last Glacial Maximum. The results reveal the strong control of the Asian summer monsoon (ASM) strength on vegetation via orbitally-induced variations in Northern Hemisphere summer solar insolation. The trend of climatic evolution of the region consists of change from a warm temperate–northern subtropical climate (20,400–18,700calyrBP) to a southern subtropical climate (16,500–11,500calyrBP) during the last deglaciation, followed by a stepwise environmental shift from a tropical climate (11,500–8200calyrBP), to a southern subtropical climate (8200–6600calyrBP) and finally to a middle–northern subtropical climate. The Bølling–Allerød warming is characterized by the highest percentages of evergreen Quercus but the lowest percentages of xerophytic Gramineae throughout the record, suggesting intensified warm, wet conditions during 15,000–13,000calyrBP. By contrast, the Younger Dryas is characterized by a marked decrease in subtropical evergreen broad-leaved plants but a significant increase in herbaceous taxa, representing a brief cold reversal during 13,000–12,000calyrBP. The occurrence of two major cooling events at 8.2 and 6.6kyrBP, both of which are characterized by dramatic decreases in tropical evergreen broad-leaved plants, is supposed to be linked to a distinct weakening of the ASM and southward migration of the Intertropical Convergence Zone associated with marked declines in Atlantic Meridional Overturning Circulation. The subsequent brief intervals of high herbaceous pollen percentages but low percentages of tropical taxa correspond to high El Niño/Southern Oscillation (ENSO) activity, supporting a link between the weakening of solar isolation-driven ASM strength and a gradual intensification of ENSO.

Relation between the Macroscopic Pattern of Elephant Ivory and Its Three-Dimensional Micro-Tubular Network

Macroscopic, periodic, dark and bright patterns are observed on sections of elephant tusk, in the dentin part (ivory). The motifs—also called Schreger pattern—vary depending on the orientation in the tusk: on sections perpendicular to the tusk axis, a checkerboard pattern is present whereas on sections longitudinal to it, alternating stripes are observed. This pattern has been used to identify elephant and mammoth ivory in archeological artifacts and informs on the continuous tissue growth mechanisms of tusk. However, its origin, assumed to be related to the 3D structure of empty microtubules surrounded by the ivory matrix has yet to be characterized unequivocally. Based on 2D observations of the ivory microtubules by means of a variety of imaging techniques of three different planes (transverse, longitudinal and tangential to the tusk axis), we show that the dark areas of the macroscopic pattern are due to tubules oblique to the surface whereas bright areas are related to tubules parallel to it. The different microstructures observed in the three planes as well as the 3D data obtained by SR-μCT analysis allow us to propose a 3D model of the microtubule network with helical tubules phase-shifted in the tangential direction. The phase shift is a combination of a continuous phase shift of π every 1 mm with a stepwise phase shift of π/2 every 500 μm. By using 3D modeling, we show how the 3D helical model better represents the experimental microstructure observed in 2D planes compared to previous models in the literature. This brings new information on the origin of the unique Schreger pattern of elephant ivory, crucial for better understanding how archaeological objects were processed and for opening new routes to rethink how biological materials are built.