Hadal aragonite records venting of stagnant paleoseawater in the hydrated forearc mantle

The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. Circulation of seawater in the mantle rock induces carbonate precipitation within the deep-sea forearc mantle, but the timescale and rates of the circulation are unclear. Here we investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400 m water depth in the Izu–Ogasawara Trench. On the basis of microtextures, strontium–carbon–oxygen isotope geochemistry, and radiocarbon analysis, we found that the aragonite is sourced from seawater that accumulated for more than 42,000 years. Aragonite precipitation is triggered by episodic rupture events that expel the accumulated fluids at 10−2–10−1 m s−1 and which continue for a few decades at most. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior.

Mobile forms of carbon in trees: metabolism and transport

Plants constitute 80% of the biomass on earth, and almost two thirds of this biomass is found in wood. Wood formation is a carbon demanding process and relies on carbon transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here we review the molecules and mechanisms used to transport and allocate carbon in trees. Sucrose is the major form in which carbon is transported, found in the phloem sap of all so far investigated tree species. However, in several tree species sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Further, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular carbon recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C carrying molecules in trees reveals no consistent differences in carbon transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate related environmental factors will not either explain the diversity of carbon transport forms. However, the consideration of C transport mechanisms in relation to tree—rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.

Molecular-scale investigation of the oxidation behavior of chromia-forming alloys in high-temperature CO2

Current and future power systems require chromia-forming alloys compatible with high-temperature CO2. Important questions concerning the mechanisms of oxidation and carburization remain unanswered. Herein we shed light onto these processes by studying the very initial stages of oxidation of Fe22Cr and Fe22Ni22Cr model alloys. Ambient-pressure X-ray photoelectron spectroscopy enabled in situ analysis of the oxidizing surface under 1 mbar of flowing CO2 at temperatures up to 530 °C, while postexposure analyses revealed the structure and composition of the oxidized surface at the near-atomic scale. We found that gas purity played a critical role in the kinetics of the reaction, where high purity CO2 promoted the deposition of carbon and the selective oxidation of Cr. In contrast, no carbon deposition occurred in low purity CO2 and Fe oxidation ensued, thus highlighting the critical role of impurities in defining the early oxidation pathway of the alloy. The Cr-rich oxide formed on Fe22Cr in high purity CO2 was both thicker and more permeable to carbon compared to that formed on Fe22Ni22Cr, where carbon transport appeared to occur by atomic diffusion through the oxide. Alternatively, the Fe-rich oxide formed in low purity CO2 suggested carbon transport by molecular CO2.

Marine Biopolymer Dynamics, Gel Formation, and Carbon Cycling in the Ocean

Much like our own body, our planet is a macroscale dynamic system equipped with a complex set of compartmentalized controls that have made life and evolution possible on earth. Many of these global autoregulatory functions take place in the ocean; paramount among those is its role in global carbon cycling. Understanding the dynamics of organic carbon transport in the ocean remains among the most critical, urgent, and least acknowledged challenges to modern society. Dissolved in seawater is one of the earth’s largest reservoirs of reduced organic carbon, reaching ~700 billion tons. It is composed of a polydisperse collection of marine biopolymers (MBP), that remain in reversible assembled↔dissolved equilibrium forming hydrated networks of marine gels (MG). MGs are among the least understood aspects of marine carbon dynamics. Despite the polymer nature of this gigantic pool of material, polymer physics theory has only recently been applied to study MBP dynamics and gel formation in the ocean. There is a great deal of descriptive phenomenology, rich in classifications, and significant correlations. Still missing, however, is the guide of robust physical theory to figure out the fundamental nature of the supramolecular interactions taking place in seawater that turn out to be critical to understanding carbon transport in the ocean.

Spatial–temporal variations in riverine carbon strongly influenced by local hydrological events in an alpine catchment

Headwater streams drain >70 % of global land areas but are poorly monitored compared with large rivers. The small size and low water buffering capacity of headwater streams may result in a high sensitivity to local hydrological alterations and different carbon transport patterns from large rivers. Furthermore, alpine headwater streams on the “Asian water tower”, i.e., Qinghai–Tibetan Plateau, are heavily affected by thawing of frozen soils in spring as well as monsoonal precipitation in summer, which may present contrasting spatial–temporal variations in carbon transport compared to tropical and temperate streams and strongly influence the export of carbon locked in seasonally frozen soils. To illustrate the unique hydro-biogeochemistry of riverine carbon in Qinghai–Tibetan headwater streams, here we carry out a benchmark investigation on the riverine carbon transport in the Shaliu River (a small alpine river integrating headwater streams) based on annual flux monitoring, sampling at a high spatial resolution in two different seasons and hydrological event monitoring. We show that riverine carbon fluxes in the Shaliu River were dominated by dissolved inorganic carbon, peaking in the summer due to high discharge brought by the monsoon. Combining seasonal sampling along the river and monitoring of soil–river carbon transfer during spring thaw, we also show that both dissolved and particulate forms of riverine carbon increased downstream in the pre-monsoon season due to increasing contribution of organic matter derived from thawed soils along the river. By comparison, riverine carbon fluctuated in the summer, likely associated with sporadic inputs of organic matter supplied by local precipitation events during the monsoon season. Furthermore, using lignin phenol analysis for both riverine organic matter and soils in the basin, we show that the higher acid-to-aldehyde (Ad/Al) ratios of riverine lignin in the monsoon season reflect a larger contribution of topsoil likely via increased surface runoff compared with the pre-monsoon season when soil leachate lignin Ad/Al ratios were closer to those in the subsoil than topsoil solutions. Overall, these findings highlight the unique patterns and strong links of carbon transport in alpine headwater catchments with local hydrological events. Given the projected climate warming on the Qinghai–Tibetan Plateau, thawing of frozen soils and alterations of precipitation regimes may significantly influence the alpine headwater carbon transport, with critical effects on the biogeochemical cycles of the downstream rivers. The alpine headwater catchments may also be utilized as sentinels for climate-induced changes in the hydrological pathways and/or biogeochemistry of the small basin.