RESUMO
Sediments can act as a source or sink of phosphorus (P) for the water column of lakes. In iron (Fe)-rich softwater lakes, redox processes are important contributors to sediment P flux. However, the contribution of redox processes to P flux in hardwater lakes, with high pH and high concentrations of redox-insensitive calcium (Ca) is unknown. Intact sediment cores, collected in different seasons (summer or fall) from a chain of eutrophic hardwater lakes in southeastern Saskatchewan, Canada, were used to quantify sediment P fluxes in laboratory incubations under hypoxic or oxic conditions at temperatures consistent with the season of sample collection. Geochemical analyses determined concentrations of sediment total (TP) and organic P (TPo), organic matter (OM), total Ca and magnesium, and total and extractable manganese, Fe and aluminum. Sediment P pools were determined using sequential fractionation and solution 31P nuclear magnetic resonance spectroscopy. Sediment P fluxes were significantly higher in sediment cores incubated under hypoxic conditions (-24.4 to 28.5 mg P m-2 d-1) than oxic conditions (-60.3 to 14.2 mg P m-2 d-1). There were significant seasonal and lake differences for TP, TPo and cation concentrations, with Ca the dominant cation in all but one lake. Phosphate bound in the redox-sensitive pool was the only sediment P fraction that significantly differed among the lakes (0.10 to 0.18 mg P g-1 d.w.; 9 to 16 % of TP), with an inverse relationship to sediment P flux. Principal component analysis suggests that high concentrations of internally-generated TPo forms and OM in surface sediments play a key role in internal P loading in these lakes. However, sediment Ca appears to have an overriding effect on sediment P, partially masking the impact of redox control on internal P loading in these hardwater prairie lakes.
RESUMO
Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.
RESUMO
Transmission electron microscopy analyses of the polymineralic regolith particle RC-MD01-0025 show microstructural and microchemical characteristics indicative of space weathering on the surface of asteroid Itokawa. The depletion of sulfur and nickel was identified in space weathered rims on troilite and pentlandite minerals. This corresponds to the first report of nickel depletion in samples returned from asteroid Itokawa by the Hayabusa mission. Microstructurally, the sulfide minerals present crystalline rims and the olivine presents both crystalline and amorphous zones in the rim. These results suggest that sulfides might be more resistant to amorphization caused by solar wind irradiation. The space weathering features identified in the regolith particle analyzed here are likely formed via solar wind irradiation. Additionally, the differences in the space weathering features in olivine, pentlandite, and troilite suggest that silicates and sulfides respond differently to the same space weathering conditions in interplanetary space.
