Seawater barium and sulfide removal improved marine habitability for the Cambrian Explosion of early animals.

An increase in atmospheric pO2 has been proposed as a trigger for the Cambrian Explosion at ∼539-514 Ma but the mechanistic linkage remains unclear. To gain insights into marine habitability for the Cambrian Explosion, we analysed excess Ba contents (Baexcess) and isotope compositions (δ138Baexcess) of ∼521-Myr-old metalliferous black shales in South China. The δ138Baexcess values vary within a large range and show a negative logarithmic correlation with Baexcess, suggesting a major (>99%) drawdown of oceanic Ba inventory via barite precipitation. Spatial variations in Baexcess and δ138Baexcess indicate that Ba removal was driven by sulfate availability that was ultimately derived from the upwelling of deep seawaters. Global oceanic oxygenation across the Ediacaran-Cambrian transition may have increased the sulfate reservoir via oxidation of sulfide and concurrently decreased the Ba reservoir by barite precipitation. The removal of both H2S and Ba that are deleterious to animals could have improved marine habitability for early animals.

Photo-Excited Metasurface for Tunable Terahertz Reflective Circular Polarization Conversion and Anomalous Beam Deflection at Two Frequencies Independently.

In this paper, a photo-excited metasurface (MS) based on hybrid patterned photoconductive silicon (Si) structures was proposed in the terahertz (THz) region, which can realize the tunable reflective circular polarization (CP) conversion and beam deflection effect at two frequencies independently. The unit cell of the proposed MS consists of a metal circular-ring (CR), Si ellipse-shaped-patch (ESP) and circular-double-split-ring (CDSR) structure, a middle dielectric substrate, and a bottom metal ground plane. By altering the external infrared-beam pumping power, it is possible to modify the electric conductivity of both the Si ESP and CDSR components. By varying the conductivity of the Si array in this manner, the proposed MS can achieve a reflective CP conversion efficiency that ranges from 0% to 96.6% at a lower frequency of 0.65 THz, and from 0% to 89.3% at a higher frequency of 1.37 THz. Furthermore, the corresponding modulation depth of this MS is as high as 96.6% and 89.3% at two distinct and independent frequencies, respectively. Moreover, at the lower and higher frequencies, the 2π phase shift can also be achieved by respectively rotating the oriented angle (αi) of the Si ESP and CDSR structures. Finally, an MS supercell is constructed for the reflective CP beam deflection, and the efficiency is dynamically tuned from 0% to 99% at the two independent frequencies. Due to its excellent photo-excited response, the proposed MS may find potential applications in active functional THz wavefront devices, such as modulators, switches, and deflectors.

Visible light-induced carbene reactivity of acceptor diazoalkanes: deconstructive difunctionalizations of cyclic ethers with nucleophiles.

A visible light-induced carbene reactivity of acceptor diazoalkanes has been developed for the synthesis of difunctionalized ethers from cyclic ethers and various N/O/S nucleophiles.

Determination of Vanadium Isotope Compositions in Carbonates Using an Fe Coprecipitation Method and MC-ICP-MS.

Vanadium isotope compositions (δ(51V)) in marine carbonates are a potential proxy to trace global redox states of ancient oceans. Although high-precision δ(51V) analyses are available for many geological materials, carbonate-hosted δ(51V) data have not been reported yet due to extremely high matrix elements and low V contents (generally below 10 µg g-1). In this study, we developed an Fe coprecipitation method combined with an Fe column to preconcentrate V from the major matrix elements and subsequent four-step chromatographic procedures to further purify V in carbonates. The δ(51V) values were measured using a sample-standard bracketing method by MC-ICP-MS. The robustness of this method was assessed by analyzing element-doped and matrix-spiked synthetic carbonate solutions containing an in-house δ(51V) standard, USTC-V. The mean δ(51V) value of the synthetic carbonate solutions (0.06 ± 0.08‰; 2SD, n = 33) is in good agreement with the recommended value of the USTC-V relative to the Oxford AA solution (0.07 ± 0.08‰; 2SD, n = 347). In addition, the consistency in the δ(51V) value of the igneous carbonatite standard, COQ-1, which was processed in parallel with the whole purification (-0.48 ± 0.04‰; 2SD, n = 3) and a four-step chromatographic procedure (-0.43 ± 0.08‰; 2SD, n = 3), further validates the robustness of our method. For the first time, we obtained δ(51V) values of four carbonate reference materials: JDo-1, -0.56 ± 0.09‰ (2SD, n = 27); JLs-1, -0.61 ± 0.14‰ (2SD, n = 33); GBW07217a, -0.79 ± 0.09‰ (2SD, n = 6); GBW07214a, -0.51 ± 0.13‰ (2SD, n = 48). The long-term external precision of carbonate-hosted δ(51V) analyses is better than ±0.14‰ (2SD). Our method can be applied to measure carbonate-hosted δ(51V) to trace the evolution in global marine redox states throughout the Earth's history.