Quantifying the topographical structure of rocky and coral seabeds.

Describing the structural complexity of seabeds is of primary importance for a number of geomorphological, hydrodynamical and ecological issues. Aiming to bring a decisive insight on the long-term development of a unified view, the present study reports on a comparative multi-site analysis of high resolution topography surveys in rough nearshore environments. The nine study sites have been selected to cover a wide variety of topographical features, including rocky and coral seabeds. The topography data has been processed to separate roughness and bathymetry-related terrain features, allowing to perform a comprehensive spectral and statistical analysis of each site. A series of roughness metrics have been tested to identify the most relevant estimators of the bottom roughness at each site. The spectral analysis highlights the systematic presence of a self-affine range of variable extension and spectral slope. The standard deviation of the seabed elevation varies from 0.04 to 0.77 m. The statistical and multi-scale analysis performed on the whole set of roughness metrics allows to identify connection between metrics and therefore to propose a reduced set of relevant roughness estimators. A more general emphasis is placed on the need to properly define a unified framework when reconstructing roughness statistics and bathymetry from fine seabed topographical data.

Structure and Chemical Composition of Soil C-Rich Al-Si-Fe Coprecipitates at Nanometer Scale.

Soil carbon stabilization is mainly driven by organo-mineral interactions. Coprecipitates, of organic matter with short-range order minerals, detected through indirect chemical extraction methods, are increasingly recognized as key carbon sequestration phases. Yet the atomic structure of these coprecipitates is still rather conceptual. We used transmission electron microscopy imaging combined with energy-dispersive X-ray and electron energy loss spectroscopy chemical mappings, which enabled direct nanoscale characterization of coprecipitates from Andosols. A comparison with reference synthetic coprecipitates showed that the natural coprecipitates were structured by an amorphous Al, Si, and Fe inorganic skeleton associated with C and were therefore even less organized than short-range order minerals usually described. These amorphous types of coprecipitates resembled previously conceptualized nanosized coprecipitates of inorganic oligomers with organics (nanoCLICs) with heterogeneous elemental proportions (of C, Al, Si, and Fe) at nanoscale. These results mark a new step in the high-resolution imaging of organo-mineral associations, while shedding further light on the mechanisms that control carbon stabilization in soil and more broadly in aquatic colloid, sediment, and extraterrestrial samples.

Hydroclimatic vulnerability of peat carbon in the central Congo Basin.

The forested swamps of the central Congo Basin store approximately 30 billion metric tonnes of carbon in peat1,2. Little is known about the vulnerability of these carbon stocks. Here we investigate this vulnerability using peat cores from a large interfluvial basin in the Republic of the Congo and palaeoenvironmental methods. We find that peat accumulation began at least at 17,500 calibrated years before present (cal. yr BP; taken as AD 1950). Our data show that the peat that accumulated between around 7,500 to around 2,000 cal. yr BP is much more decomposed compared with older and younger peat. Hydrogen isotopes of plant waxes indicate a drying trend, starting at approximately 5,000 cal. yr BP and culminating at approximately 2,000 cal. yr BP, coeval with a decline in dominant swamp forest taxa. The data imply that the drying climate probably resulted in a regional drop in the water table, which triggered peat decomposition, including the loss of peat carbon accumulated prior to the onset of the drier conditions. After approximately 2,000 cal. yr BP, our data show that the drying trend ceased, hydrologic conditions stabilized and peat accumulation resumed. This reversible accumulation-loss-accumulation pattern is consistent with other peat cores across the region, indicating that the carbon stocks of the central Congo peatlands may lie close to a climatically driven drought threshold. Further research should quantify the combination of peatland threshold behaviour and droughts driven by anthropogenic carbon emissions that may trigger this positive carbon cycle feedback in the Earth system.