Compilation|Li Yan
Nature, 16 March 2023, Volume 615 Issue 7952
Nature, March 16, 2023, Vol. 615, Issue 7952
Material science
Photonically active bowtie nanoassemblies with chirality continuum
Photonic active bow tie-shaped nanoassemblies with chiral continuums
▲ 作者：Prashant Kumar, Thi Vo, Minjeong Cha et al.
▲ Link:
https://www.nature.com/articles/s41586-023-05733-1
▲ Abstract:
Here, we demonstrate that nanostructured microparticles with an anisotropic bow tie shape exhibit a chiral continuum and can adjust its angle, spacing, width, thickness, and length. The self-limiting assembly of the bow tie shape makes the geometry presented under different conditions highly reproducible, dimensional monodispersion, and computational predictability.
The bow tie-shaped nanomodules show some strong circular dichroic peaks under the absorption and scattering phenomena. Unlike classical chiral molecules, these particles exhibit continuous chiral measurements that are exponentially correlated with the spectral position of circular dichroic peaks.
Bow tie-shaped particles with variable polarization rotation can print photon-active metasurfaces with spectrally tunable positive and negative polarization characteristics for use in laser detection and ranging equipment.
▲ Abstract：
Here we show that nanostructured microparticles with an anisotropic bowtie shape display chirality continuum and can be made with widely tuneable twist angle, pitch, width, thickness and length. The self-limited assembly of the bowties enables high synthetic reproducibility, size monodispersity and computational predictability of their geometries for different assembly conditions. The bowtie nanoassemblies show several strong circular dichroism peaks originating from absorptive and scattering phenomena. Unlike classical chiral molecules, these particles show a continuum of chirality measures that correlate exponentially with the spectral positions of the circular dichroism peaks. Bowtie particles with variable polarization rotation were used to print photonically active metasurfaces with spectrally tuneable positive or negative polarization signatures for light detection and ranging (LIDAR) devices.
Chemistry
Control of stereogenic oxygen in a helically chiral oxonium ion
Control of stereoisomeric oxygen in helical chiral oxonium ions
▲ 作者：Owen Smith, Mihai V. Popescu, Madeleine J. Hindson et al.
▲ Link:
https://www.nature.com/articles/s41586-023-05719-z
▲ Abstract:
Here, we describe the design, synthesis, and characterization of a helical chiral triarylium oxide ion that prevents the transformation of the oxygen solitary pair by geometric limitation and enables it to function as determinants of the configuration.
We use combinatorial synthesis and quantum computing methods to label their design principles so that structurally stable and room-temperature isolated salts can be produced. To the best of our knowledge, this constitutes the only case in which chiral non-racemic and configuration-stable molecules in which oxygen atoms are single stereoisomeric centers.
▲ Abstract：
Here we describe the design, synthesis and characterization of a helically chiral triaryloxonium ion in which inversion of the oxygen lone pair is prevented through geometric restriction to enable it to function as a determinant of configuration. A combined synthesis and quantum calculation approach delineates design principles that enable configurationally stable and room-temperature isolable salts to be generated. We show that the barrier to inversion is greater than 110?kJ?mol?1 and outline processes for resolution. This constitutes, to our knowledge, the only example of a chiral non-racemic and configurationally stable molecule in which the oxygen atom is the sole stereogenic centre.
Geography
The carbon sink of secondary and degraded humid tropical forests
Carbon sinks in secondary and degraded moist tropical forests
▲ 作者:Viola H. A. Heinrich, Christelle Vancutsem, Ricardo Dalagnol et al.
▲ Link:
https://www.nature.com/articles/s41586-022-05679-w
▲ Abstract:
Here, we quantify recovering above-ground carbon sinks in three major humid tropical regions: Amazon, Borneo, and Central Africa. Based on satellite data, our analysis covers heterogeneous spatial and temporal growth patterns in degraded and secondary forests influenced by key environmental and anthropogenic drivers. In the first 20 years of forest recovery, Borneo's regeneration rate was 45% and 58% higher than in Central Africa and the Amazon, respectively.
This is due to variables such as temperature, moisture deficit and disturbance. We found that between 1984 and 2018, regenerative degraded forests and secondary forests accumulated 107 million tonnes per year, offsetting 26% of the carbon emissions lost by moist tropical forests over the same period.
Therefore, the protection of primary forests is a priority. In addition, we estimate that the potential for future carbon sinks to protect degraded and secondary forests that are being restored in major tropical regions could be 53 million tonnes per year.
▲ Abstract：
Here we quantify the aboveground carbon (AGC) sink of recovering forests across three main continuous tropical humid regions: the Amazon, Borneo and Central Africa. On the basis of satellite data products, our analysis encompasses the heterogeneous spatial and temporal patterns of growth in degraded and secondary forests, influenced by key environmental and anthropogenic drivers. In the first 20?years of recovery, regrowth rates in Borneo were up to 45% and 58% higher than in Central Africa and the Amazon, respectively. This is due to variables such as temperature, water deficit and disturbance regimes. We find that regrowing degraded and secondary forests accumulated 107Tg C year−1(90–130 Tg C year−1) between 1984 and 2018, counterbalancing 26% (21–34%) of carbon emissions from humid tropical forest loss during the same period. Protecting old-growth forests is therefore a priority. Furthermore, we estimate that conserving recovering degraded and secondary forests can have a feasible future carbon sink potential of 53 Tg C year−1(44–62 Tg C year−1) across the main tropical regions studied.
Regime shift in Arctic Ocean sea ice thickness
Changes in sea ice thickness in the Arctic Ocean
▲ 作者:Hiroshi Sumata, Laura de Steur, Dmitry V. Divine, Mats A. Granskog & Sebastian Gerland
▲ Link:
https://www.nature.com/articles/s41586-022-05686-x
▲ Abstract:
Here, we show how the Arctic sea ice sheet changed in 2007 from thicker and irregular to thinner and more morphologically consistent. Ongoing sea ice monitoring in the Frum Strait over the past three decades has revealed this change.
After the change, the ratio of thick ice to irregular ice fell by half and has not recovered to this day. Prior to this transition, Arctic basin sea ice retention time had been shortened twice, first in 2005 and second in 2007.
We demonstrate a simple model describing the stochastic process of dynamic sea ice thickening that explains the observed changes in ice sheet thickness due to reduced retention time. Our study highlights the long-term effects of climate change on Arctic sea ice in the context of reduced ice retention and its links to ocean-sea ice coupling processes in adjacent marginal seas and continental shelves.
▲ Abstract：
Here we show that the Arctic sea ice regime shifted in 2007 from thicker and deformed to thinner and more uniform ice cover. Continuous sea ice monitoring in the Fram Strait over the last three decades revealed the shift. After the shift, the fraction of thick and deformed ice dropped by half and has not recovered to date. The timing of the shift was preceded by a two-step reduction in residence time of sea ice in the Arctic Basin, initiated first in 2005 and followed by 2007. We demonstrate that a simple model describing the stochastic process of dynamic sea ice thickening explains the observed ice thickness changes as a result of the reduced residence time. Our study highlights the long-lasting impact of climate change on the Arctic sea ice through reduced residence time and its connection to the coupled ocean–sea ice processes in the adjacent marginal seas and shelves of the Arctic Ocean.
Geology
Deep, ultra-hot-melting residues as cradles of mantle diamond
Deep ultrathermal melt residues are the cradle of mantle diamonds
▲ 作者：Carl Walsh, Balz S. Kamber & Emma L. Tomlinson
▲ Link:
https://www.nature.com/articles/s41586-022-05690-1
▲ Abstract:
Here, we present the results of thermodynamic and geochemical simulations of magnesia olivine that melts at a sufficiently high temperature to produce cratonic roots. The need for shallow melting and accumulation was eliminated by extremely deep (about 200 km) and extremely high temperature hot melt (≥ 1800°C), and new closed and open systems models reproduce the observed mineral composition of the cratonic mantle.
As observed in the greenstone belt on the surface of the craton's fossil, the magnesium-rich liquid (komatiite) in the simulation develops into an aluminum-rich and titanium-deficient form. The lack of titanium in komatites means that chemical melting (1825°C) such as advanced closed systems is far less common than open system interactions between deep liquids and existing refractory mantles.
▲ Abstract：
Here we present results from thermodynamic and geochemical modelling of melting at sufficiently high temperatures to produce the very magnesian olivine of cratonic roots. The new closed-system and open-system modelling reproduces the observed cratonic mantle mineral compositions by deep (about 200?km) and very hot melting (≥1,800°C), obviating the need for shallow melting and stacking. The modelled highly magnesian liquids (komatiites) evolve to Al-enriched and Ti-depleted forms, as observed in the greenstone belts at the fossil surface of cratons. The paucity of Ti-depleted komatiite implies that advanced closed-system isochemical melting (>1,825°C) was much less common than open-system interaction between deeper liquids and melting of existing refractory mantle.
Weak upper-mantle base revealed by postseismic deformation of a deep earthquake
The upper mantle base is weak as revealed by the deformation after the deep-source earthquake
Written by Sunyoung Park, Jean-Philippe Avouac, Zhongwen Zhan & Adriano Gualandi
▲ Link:
https://www.nature.com/articles/s41586-022-05689-8
▲ Abstract:
Here, we study the viscosity structure of the mantle using post-seismic deformation caused by a deep-source earthquake (about 560 km) located near the bottom of the upper mantle. We successfully detected and understood post-earthquake deformation caused by the 2018 Fiji magnitude 8.2 earthquake. To find viscosity structures that can interpret the detected signal, we model positive viscoelastic relaxation using a series of viscosity structures.
We found that our observations require a relatively thin (about 100 km), low viscosity (1017 to 1018 Pa s) layer at the bottom of the mantle transition zone. Such weak bands explain the flattening and isolation of plates observed in many subduction zones, which are difficult to study in the entire mantle convective system. The low-viscosity layer may be the result of post-spinel transition, weak calcium silicate ore, high water content, or superplasticity formation induced by dehydrated melting.
▲ Abstract：
Here, we study the mantle viscosity structure by using the postseismic deformation following a deep (approximately 560 km) earthquake located near the bottom of the upper mantle. We apply independent component analysis to geodetic time series to successfully detect and extract the postseismic deformation induced by the moment magnitude 8.2, 2018 Fiji earthquake. To search for the viscosity structure that can explain the detected signal, we perform forward viscoelastic relaxation modelling with a range of viscosity structures. We find that our observation requires a relatively thin (approximately 100 km), low-viscosity (1017to 1018Pa s) layer at the bottom of the mantle transition zone. Such a weak zone could explain the slab flattening and orphaning observed in numerous subduction zones, which are otherwise challenging to explain in the whole mantle convection regime. The low-viscosity layer may result from superplasticity induced by the postspinel transition, weak CaSiO3 perovskite, high water content or dehydration melting.