Exposure history of the Sutter's Mill carbonaceous chondrite

The Sutter's Mill (SM) carbonaceous chondrite fell in California on April 22, 2012. The cosmogenic radionuclide data indicate that Sutter's Mill was exposed to cosmic rays for 0.082 ± 0.008 Myr, which is one of the shortest ages for C chondrites, but overlaps with a small cluster at approximately 0.1 Myr. The age is significantly longer than proposed ages that were obtained from cosmogenic noble gas concentrations, which have large uncertainties due to trapped noble gas corrections. The presence of neutron‐capture ⁶⁰Co and ³⁶Cl in SM indicates a minimum preatmospheric radius of approximately 50 cm, and is consistent with a radius of 1–2 m, as derived from the fireball observations. Although a large preatmospheric size was proposed, one fragment (SM18) contains solar cosmic ray–produced short‐lived radionuclides, such as ⁵⁶Co and ⁵¹Cr. This implies that this specimen was less than 2 cm from the preatmospheric surface of Sutter's Mill. Although this conclusion seems surprising, it is consistent with the observation that the meteoroid fragmented high in the atmosphere. The presence of SCR‐produced nuclides is consistent with the high SCR fluxes observed during the last few months before the meteorite's fall, when its orbit was less than 1 AU from the Sun.     

Assessment of the effects of vegetation on soil erosion risk by water: a case of study of the Batta watershed in Tunisia

Soil erosion by water is a serious environmental problem which affects particularly the agriculture of developing countries. Due to specific factors, such as high rainfall intensity, steep slopes and vegetation scarcity, Tunisia is prone to soil erosion. Taking this into account, the main objective of this study was to estimate the soil erosion risk in the Batta watershed in Tunisia using qualitative and quantitative erosion model with remote sensing data and geographic information system (GIS). Moreover, a developed method that deals with evaluating the impact of vegetation on soil erosion by water is also applied. This method used multi-temporal satellite images with seasonal variability and the transformed soil adjusted vegetation index (TSAVI) which is appropriate in arid and semi-arid areas. For both erosion models, the results show that a large area of the Batta watershed is seriously affected by erosion. This potentially high risk is due especially to severe slopes, poor vegetation coverage and high soil erodibility in this watershed. Furthermore, the use of multi-temporal satellite images and vegetation index show that the effect of vegetation is a significant factor to protect the soil against erosion. The risk is especially serious in the summer season, but it decreases with the growth of vegetation cover in spring. Erosion model, combined with a GIS and remote sensing, is an adequate method to evaluate the soil erosion risk by water. The findings can be used by decision makers as a guideline to plan appropriate strategies for soil and water conservation practices.     

Fall,classification, and exposure history of the Mifflin L5 chondrite

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low ²²Ne/²¹Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.     

Quantitative mapping of a precursory seismic quiescence to the Izu–Oshima 1990 (M6.5) earthquake, Japan

A highly significant seismic quiescence with a standard deviate Z = 10.1. corresponding to a 99 per cent confidence level, lasted from 1987.7 up to the 1990 February 20 Izu-Oshima M 6.5 earthquake. The quiescent volume had dimensions of 30 km N-S and 10 km E-W and was centred below 14 km depth. Within the recently upgraded seismograph network of the Earthquake Research Institute (ERI), this main shock was the only one with a magnitude M > 5.8 in the upper 30 km of the crust for which the precursory quiescence hypothesis could be tested. Within a radius of 50 km, and during the observation period (1983.5–1995.9), there were no other 1.5 yr or longer periods of quiescence that were rated Z > 6.5 in the declustered earthquake catalogue, except one that was associated with volcanic activity. The total space-time covered by alarms, including the volcanic one, was less than 1 per cent at the Z = 6.5 level. The rarity of highly significant episodes of quiescence, and the correlation in space and time suggest that a precursory seismic quiescence started 2.5 yr before the Izu-Oshima 1990 earthquake in its source volume and to the north of it, and that it can be recognized with an alarm level of Z = 6.0, generating no false alarms. During the 1.5 yr quiescence window, only 10 earthquakes occurred in the quiet volume, whereas 50 events were expected based on the rate seen at other times. In randomly selected volumes containing 50, 100 and 200 events, the anomaly scored Z = 6.1 to 10.1. On the basis of the data from May 1983 to 1995, there is no highly significant quiescence currently present in the Izu-Oshima area.     

Cosmic-ray-produced53Mn in thirty-one meteorites

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.     

Rupture process of the Miyagi-Oki,Japan, earthquake of June 12, 1978

The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 10²⁷ dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km², respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment $\text{M}{}_0\text{= 1.7 × 10}{}^{27}\text{dyne-cm, slip dislocation}\text{u}\text{= 1.9}\text{m}\text{, Δσ = 95}\text{bar, rupture velocity}\text{ν = 3.2}\text{km s}{}^{\mathrm{?1}}\text{,}\text{rise time}\text{τ = 2}\text{s}$, for the first event; $\text{M}{}_0\text{= 1.4 × 10}{}^{27}\text{dyne-cm}\text{,}\text{u}\text{= 2.4}\text{m}\text{, Δσ = 145}\text{bar}$, for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 10¹⁰ erg cm^?2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.     

The extent of lunar regolith mixing

The activity of solar cosmic-ray-produced⁵³Mn has been measured as a function of depth in the upper 100 g/cm² (～55 cm) of lunar cores 60009–60010 and 12025–12028. Additional samples which supplement our earlier work were analyzed from the Apollo 15 and 16 drill stems. These data, taken in conjunction with our previously published results and the²²Na and²⁶Al data of the Battelle Northwest group, indicate that in at least three of the four cases studied the regolith has been measureably disturbed within the last 10 m.y. In one case gardening to 19 g/cm² is required. Activities measured in the uppermost 2 g/cm² indicate frequent mixing within this depth range. No undisturbed profiles were observed nor were any major discontinuities observed in the profiles. The Monte Carlo gardening model of Arnold has been used to derive profiles for the gardened moon-wide average of⁵³Mn and²⁶Al as a function of depth. The⁵³Mn and²⁶Al experimental results are compared with these theoretical predictions. Agreement is good in several respects, but the calculated depths of disturbance appear to be too low.     

Measurements of 36Cl in Antarctic meteorites and Antarctic ice using a Van de Graaff accelerator

Cosmic-ray produced ³⁶Cl(t_1/2 = 3.0 × 10⁵ years) has been measured in four Antarctic meteorites and one sample of Antarctic ice using a tandem Van de Graaff accelerator as an ultrasensitive mass spectrometer with the extremely low background level of ³⁶Cl/Cl< 2 × 10^?16. Results from this ion counting technique (applied here to extraterrestrial materals for the first time) are used to support a two-stage irradiation model for the Yamato-7301 and Allan Hills-76008 meteorites and to show a long terrestrial age (0.7 ± 0.1 m.y.) for Allan Hills-77002. Yamato-7304 has a terrestrial age of less than 0.1 m.y. The ³⁶Cl content of the Antarctic ice sample from the Yamato Mountain area implies that the age of the ice cap at this site is less than one ³⁶Cl half-life.