Dual-polarized ratio algorithm for retrieving Arctic sea ice concentration from passive microwave brightness temperature

We present a new algorithm for retrieving sea ice concentration from the AMSR-E data, the dual-polarized ratio (DPR) algorithm. The DPR algorithm is developed using vertically and horizontally polarized brightness temperatures at the same channel of 36.5 GHz. It depends on the ratio of dual-polarized emissivity, α, which is determined empirically at about 0.92 by remotely sensed brightness temperature in winter and used for the other seasons as well. The ice concentration retrieved by the DPR is compared with those by the NT2 and ABA algorithms. Since the main difference among these algorithms takes place in marginal ice zones, 17 marginal ice zones are chosen. The retrieved ice concentrations in these zones are examined by the ice concentration obtained by the MODIS data. The mean error, root-mean-square error and mean absolute error of the DPR algorithm are relatively better than those from the other two algorithms. The results of this study illustrate that the DPR algorithm is a more accurate algorithm for retrieving sea ice concentration from the AMSR-E brightness temperature, and can be used for operational purposes.     

Performance evaluation of active wireline heave compensation systems in marine well logging environments

The basic functionality and performance of a new Schlumberger active wireline heave compensation system on the JOIDES Resolution was evaluated during the sea trial and a 3-year period of the IODP Phase II operations. A suite of software programs was developed to enable real-time monitoring of the dynamics of logging tools, and assess the efficiency of wireline heave compensation during downhole operations. The evaluation of the system effectiveness was performed under normal logging conditions as well as during stationary tests. Logging data were analyzed for their overall quality and repeatability, and to assess the reliability of high-resolution data such as formation microscanner (FMS) electrical images. This revealed that the system reduces 65–80 % of displacement or 88–98 % variance of downhole tool motion in stationary mode under heave conditions of ±0.2–1.5 m and water depths of 300–4,500 m in open holes. Under similar water/heave conditions, the compensator system reduces tool displacement by 50–60 %, or 75–84 % variance in downhole tool motion during normal logging operations. Such compensation efficiency (CE) is comparable to previous compensation systems, but using advanced and upgradeable technologies, and provides 50–85 % heave motion and heave variance attenuation. Moreover, logging down/up at low speeds (300–600 m/h) reduces the system’s CE values by 15–20 %, and logging down at higher speeds (1,000–1,200 m/h) eliminates CE values by 55–65 %. Considering the high quality of the logging data collected, it is concluded that the new system can provide an improved level of compensation over previous systems. Also, if practically feasible, future integration of downhole cable dynamics as an input feedback into the current system could further improve its compensation efficiency during logging operations.     

Global mapping of lunar crustal magnetic fields by Lunar Prospector

The Lunar Prospector Electron Reflectometer has obtained the first global map of lunar crustal magnetic fields, revealing that the effects of basin-forming impacts dominate the large-scale distribution of remanent magnetic fields on the Moon. The weakest surface magnetic fields (<0.2 nT) are found within two of the largest and most recent impact basins, Orientale and Imbrium. Conversely, the largest concentrations of strong surface fields (>40 nT) are diametrically opposite to these same basins. This pattern is present though less pronounced for several other post-Nectarian impact basins larger than 500 km in diameter. The reduced strength and clarity of the pattern for older basins may be attributed to: (1) demagnetization from many smaller impacts, which erases antipodal magnetic signatures over time, (2) superposition effects from other large impacts, and (3) variation in the strength of the ambient magnetizing field. The absence of fringing fields stronger than 1 nT around the perimeter of the Imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than ∼¹⁰⁻⁶ G cm³ g⁻¹. This limits the strength of any steady ambient magnetic field to no more than ∼0.1 Oe at the lunar surface while the basin cooled for tens of millions of years following the Imbrium impact 3.8 billion years ago.     

Thermal history analysis by integrated modelling of apatite fission track and vitrinite reflectance data: application to an inverted basin (Buller Coalfield, New Zealand)

Integrated analysis and modelling of apatite fission track with vitrinite reflectance (VR) data allows the timing, magnitude and pattern of Palaeogene subsidence and Neogene inversion to be established for an uplifted and largely denuded basin: the Buller Coalfield, New Zealand. At the time of maximum subsidence in the late Oligocene, the basin consisted of an extensional half graben, bounded to the west by the Kongahu Fault Zone (KFZ), with up to 6 km of upper Eocene to Oligocene section adjacent to it; currently, only a few tens of metres of basal coal measures on basement are preserved on top of a range 800–1000 m above sea level. Integrated modelling of the VR and fission track data show that the deepest parts of the basin were inverted during two Miocene compressional phases (24–19 Ma and 13–8 Ma), and are consistent with a further phase of inversion during the Quaternary that formed the present topography. Palinspastic restoration of the three phases of inversion shows that the basin was not inverted in a simple way: most of the rock uplift/denudation adjacent to the KFZ occurred during the early Miocene phase, and at the same time burial occurred in the south-eastern part of the basin (maximum temperatures were experienced at different times at different places in the basin); during the middle to late Miocene there was broad uplift in the central and eastern parts of the coalfield. Because the timing and magnitude of uplift have been derived from the zone of inversion, they can be compared independently with the timing of unconformity development and rapid subsidence in the adjacent foredeeps, particularly the Westport Trough. For the middle to late Miocene phase of inversion, we show that during the first 1–2 million years of compression, the uplift within the coalfield also involved the margins of the Westport Trough, contributing to unconformity development; subsequently, uplift continued on the inversion structure but the margins of the Westport Trough subsided rapidly. This is explained by a model of stick slip behaviour on the boundary faults, especially for the KFZ. When compression started the fault zone has locked and uplift extends into the basin, whereas subsequently the fault zone unlocks, and the inversion structure overrides the basin margin, thereby loading it and causing subsidence.     

Ages,rare-earth element enrichment,and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island,Hawaii

Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO₂ than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.     

Dynamic character of the primitive to body-centered phase transition in anorthite

We show from elastic neutron diffraction data that anorthite CaAl₂Si₂O₈ (An₁₀₀) undergoes a primitive to body-centered phase transition at T _c =237±1°C. The transition is reversible, and T _c is well defined. Our measurements demonstrate that the I-lattice at the high temperature phase applies to all structural elements; in other words, the time-averaged lattice is exactly body-centered and not just in the space average of An₁₀₀, as concluded earlier by other authors.     

Geochemistry and petrology of dredged basalts from the Bouvet triple junction,South Atlantic

Basalts dredged from the Bouvet triple junction (South Atlantic), from the Mid-Atlantic Ridge near the triple junction, and from a spreading center east of Bouvet Island differ from normal mid-ocean ridge tholeiites by having higher concentrations of K and other large-ion-lithophile elements, higher ${}^{87}\text{Sr}\text{:}{}^{86}\text{Sr}$ ratios, and rare earth element distributions which show relative enrichment in the lighter rare earths. The basalts are more fractionated than typical oceanic tholeiites, however, fractional crystallization does not fully account for their chemical characteristics, and it appears that they were derived from special source materials, contaminated perhaps by a mantle plume rising beneath Bouvet Island.     

Rare earth element geochemistry of the Betts Cove ophiolite,Newfoundland: complexities in ophiolite formation

The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ (i.e., typical for ocean floor basalts) TiO₂ contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO₂ contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO₂ contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.