Continental-scale comparisons of terrestrial carbon sinks estimated from satellite data and ecosystem modeling 1982–1998

A simulation model based on satellite observations of monthly vegetation cover was used to estimate monthly carbon fluxes in terrestrial ecosystems from 1982 to 1998. The NASA–CASA model was driven by vegetation properties derived from the Advanced Very High Resolution Radiometer (AVHRR) and radiative transfer algorithms that were developed for Moderate Resolution Imaging Spectroradiometer (MODIS). For the terrestrial biosphere, predicted net ecosystem production (NEP) flux for atmospheric CO₂ has varied widely between an annual source of −0.9 Pg C per year and a sink of +2.1 Pg C per year. The southern hemisphere tropical zones (SHT, between 0° and 30°S) have a major influence over the predicted global trends in interannual variability of NEP. In contrast, the terrestrial NEP sink for atmospheric CO₂ on the North American (NA) continent has been fairly consistent between +0.2 and +0.3 Pg C per year, except during relatively cool annual periods when continental NEP fluxes are predicted to total to nearly zero. The predicted NEP sink for atmospheric CO₂ over Eurasia (EA) increased notably in the late 1980s and has been fairly consistent between +0.3 and +0.55 Pg C per year since 1988. High correlations can be detected between the El Niño Southern Oscillation (ENSO) and predicted NEP fluxes on the EA continent and for the SHT latitude zones, whereas NEP fluxes for the North American continent as a whole do not correlate strongly with ENSO events over the same time series since 1982. These observations support the hypothesis that regional climate warming has had notable but relatively small-scale impacts on high latitude ecosystem (tundra and boreal) sinks for atmospheric CO₂.     

In situ multi-frequency measurements of magnetic susceptibility as an indicator of planetary regolith maturity

Space weathering is now generally accepted to modify the optical and magnetic properties of airless planetary regoliths such as those on the Moon and Mercury. Under micrometeorite and ion bombardment, ferrous iron in such surfaces is reduced to metallic iron spheres, found in amorphous coatings on almost all exposed regolith grains. The size and number distribution of these particles and their location in the regolith all determine the nature and extent of the optical and magnetic changes. These parameters in turn reflect the formation mechanisms, temperatures, and durations involved in the evolution of the regolith. Studying them in situ is of intrinsic value to understanding the weathering process, and useful for determining the maturity of the regolith and providing supporting data for interpreting remotely sensed mineralogy. Fine-grained metallic iron has a number of properties that make it amenable to magnetic techniques, of which magnetic susceptibility is the simplest and most robust. The magnetic properties of the lunar regolith and laboratory regolith analogues are therefore reviewed and the theoretical basis for the frequency dependence of magnetic susceptibility presented. Proposed here is then an instrument concept using multi-frequency measurements of magnetic susceptibility to confirm the presence of fine-grained magnetic material and attempt to infer its quantity and size distribution. Such an instrument would be invaluable on a future mission to an asteroid, the Moon, Mercury or other airless rocky Solar System body.     

Moderate velocity oblique impact sliding: Production of shocked meteorite textures and palaeomagnetically important metallic spherules in planetary regoliths

We detail the production of metallic spherules in laboratory oblique shock impact experiments, and their applicability (1) to textures in a partly shock‐melted chondritic meteorite and (2) to the occurrence of palaeomagnetically important fine iron or iron alloy particles in the lunar regolith. Samples recovered from 29–44 GPa, 800 ns, experiments revealed melting and textures reminiscent of metallic spherules in the Yanzhuang H‐chondrite, including “dumbbell” forms and other more complex morphologies. Our experiments demonstrate that metallic spherules can be produced via oblique impact sliding at lower velocities (1.85 km s^?1) than are generally assumed in previous work associated with bulk‐shock melting, and that oblique impact sliding is a viable mechanism for producing spherules in shock‐induced veins in moderately shocked meteorites. Significantly, our experiments also produced fine metallic (iron alloy) spherules within the theoretical narrow size range (a few tens of nanometers for slightly ellipsoidal particles) for stable single‐domain (SSD) particles, which are the most important palaeomagnetically, since they can record lunar and planetary magnetic fields over geological time periods. The experiments also produced spherules consistent with superparamagnetic (SP) and multidomain (MD) particle sizes. The fine SSD and SP particles on the lunar surface are currently thought to have been formed predominantly by space weathering processes. Our experiments suggest that oblique shock impact sliding may be a further means of producing the SSD and SP iron or iron alloy particles observed in the lunar regolith, and which are likely to occur in the regoliths of Mercury and other planetary bodies.