Petrology of a leucogabbroic interval within basal layered gabbros at North Arm Mountain,Bay of Islands ophiolite

Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An_73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo_76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that 29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that 55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers.     

CREEKS,RUNS, AND HOLLOWS

Three samples of twenty drainage basins were analyzed to test the hypothesis that the generic terms “creeks,'’“runs,'’and “hollows'’connote streams of different size and velocity. Analysis of variance and Scheffé's test indicate the validity of a perceptual hierarchy among generics. It is suggested that other hierarchies may exist among geographically named features.     

Rotating flow over long shallow ridges

Abstract

The flow of a rotating homogeneous, incompressible fluid past a long ridge is investigated. An analysis is presented for flows in which E ? 1, Ro ～ E^½, H/D ～ E⁰, h/D ～ E^½ and cosα ～ E⁰ where E is the Ekman number, Ro the Rossby number, H/D the fluid depth to ridge width ratio, h/D the ridge height to ridge width ratio and α the angle between the free stream flow and a line perpendicular to the ridge axis. The analysis includes effects of the nonlinear inertial terms. Particular examples of a ridge of triangular cross section and a sinusoidal topography are investigated in some detail. Experiments are presented for a triangular ridge which are in good agreement with the theory.     

Linking large-parallax Spitzer–CFHT–VLT astrometry of asteroids

We show that the new ephemeris-space multiple-address-comparison (eMAC) method solves asteroid linking problems despite large parallaxes by applying the method to astrometric asteroid observation sets obtained nearly simultaneously with the Spitzer space telescope, the Canada–France–Hawaii Telescope (CFHT), and European Southern Observatory's Very Large Telescope (VLT). For main-belt asteroids, the parallax between Spitzer and the Earth-based telescopes is approximately one degree which is large as compared to a typical parallax for solely Earth-based telescopes in the arcseconds regime. In the eMAC method, we reduce the initially huge amount of possible linkages between observation sets by comparing samples of ephemerides that have been computed separately for all sets at, say, three common dates. If the non-zero ephemeris probability densities overlap at all common dates, we try to find an orbit solution for these so-called trial linkages. If there exists an orbit which reproduces all the astrometric observations assuming predefined observational errors, we call it a linkage. Known asteroids are independently identified among Spitzer, CFHT, and VLT astrometry, and comparing the identified observations to the linkages found shows that the method found all known correct linkages present in the data. In addition, we also found five previously unpublished linkages between Spitzer astrometry and Earth-based astrometry. Based on our simulations, we found virtually all Spitzer-related linkages between two single-night observation sets, and more than 99.4% of linkages between two single-night observation sets obtained by Earth-based observatories. Virtually all correct linkages consisting of at least three single-night sets were also detected. The results show that large-parallax discovery observations made from a spacecraft can be linked to Earth-based follow-up observations to ensure that the objects are not lost. Furthermore, we compute the heliocentric and Spitzer-centric distances as well as the corresponding solar phase angles at the dates of Spitzer observations. Based on comparisons to simulated geocentric observations, we also show that, for typical nearly-simultaneous observations, the parallax reduces the distance uncertainties by several orders of magnitude.     

Polonium-210: Its vertical oceanic transport by zooplankton metabolic activity

An earlier work based on atmospheric input of ²¹⁰Po to oceanic surface waters, in situ production from its radioactive precursors and its measured equilibrium concentration has shown that ²¹⁰Po is removed from the upper mixed layer at a rapid rate, expressible by a turnover (or mean residence) time of 0.6 years. Since ²¹⁰Po is known to be highly concentrated in numerous marine species, a method was devised to assess the importance of marine zooplankton in removing this radio-nuclide from the surface waters. Measurements of ²¹⁰Po losses from living zooplankton lead to a rapid turnover time of the order of 0.9 years due to zooplankton metabolic activity alone. It is shown experimentally that fecal pellet deposition constitutes a major mechanism in transporting the radionuclide from surface to deeper waters.     

Comparison of streamflow recession between plantations and native forests in small catchments in Central-Southern Chile

In central Chile, many communities rely on water obtained from small catchments in the coastal mountains. Water security for these communities is most vulnerable during the summer dry season and, from 2010 to 2017, rainfall during the dry season was between 20% and 40% below the long-term average. The rate of decrease in stream flow after a rainfall event is a good measure of the risk of flow decreasing below a critical threshold. This risk of low flow can be quantified using a recession coefficient (α) that is the slope of an exponential decay function relating flow to time since rainfall. A mathematical model was used to estimate the recession coefficient (α) for 142 rainstorm events (64 in summer; 78 in winter) in eight monitored catchments between 2008 and 2017. These catchments all have a similar geology and extend from 35 to 39 degrees of latitude south in the coastal range of south-central Chile. A hierarchical cluster analysis was used to test for differences between the mean value of α for different regions and forest types in winter and summer. The value of α did not differ (p < 0.05) between catchments in winter. Some differences were observed during summer and these were attributed to morphological differences between catchments and, in the northernmost catchments, the effect of land cover (native forest and plantation). Moreover, α for catchments with native forest was similar to those with pine plantations, although there was no difference (p < 0.05) between these and Eucalyptus plantations. The recession constant is a well-established method for understanding the effect of climate and disturbance on low flows and baseflows and can enhance local and regional analyses of hydrological processes. Understanding the recession of flow after rainfall in small headwater catchments, especially during summer, is vital for water resources management in areas where the establishment of plantations has occurred in a drying climate.     

Velocity and vorticity measurements of Jupiter's Great Red Spot using automated cloud feature tracking

We have produced mosaics of the Great Red Spot (GRS) using images taken by the Galileo spacecraft in May 2000, and have measured the winds of the GRS using an automated algorithm that does not require manual cloud tracking. Our technique yields a high-density, regular grid of wind velocity vectors that is advantageous over a limited number of scattered wind vectors that result from manual cloud tracking. The high-velocity collar of the GRS is clearly seen from our velocity vector map, and highest wind velocities are measured to be around 170 m s⁻¹. The high resolution of the mosaics has also enabled us to map turbulent eddies inside the chaotic central region of the GRS, similar to those mapped by Sada et al. [Sada, P.V., Beebe, R.F., Conrath, B.J., 1996. Icarus 119, 311-335]. Using the wind velocity measurements, we computed particle trajectories around the GRS as well as maps of relative and absolute vorticities. We have discovered a narrow ring of cyclonic vorticity that surrounds the main anti-cyclonic high-velocity collar. This narrow ring appears to correspond to a ring surrounding the GRS that is bright in 5 μm [Terrile, R.J., Beebe, R.F., 1979. Science 204, 948-951]. It appears that this cyclonic ring is not a transient feature of the GRS, as we have discovered it in a re-analysis of Galileo data taken in 1996 first analyzed by Vasavada et al. [Vasavada, A.R., and 13 colleagues, 1998. Icarus 135, 265-275]. We also calculate how absolute vorticity changes as a function of latitude along a trajectory around the GRS and compare these measurements to similar ones performed by Dowling and Ingersoll [Dowling, T.E., Ingersoll, A.P., 1988. J. Atmos. Sci. 45, 1380-1396] using Voyager data. We show no dramatic evolution in the structure of the GRS since the Voyager era except for additional evidence for a counter-rotating GRS core, an increase in velocity in the main velocity collar, and an overall decrease in the length of the GRS.     

The disappearance of OH from comet P/Encke

Photoelectric observations of Comet P/Encke during its 1980 apparition are combined with other published data to relate molecular production rates to the visual lightcurve. In addition to a substantial asymmetry about perihelion which is already well known, there are shorter-term variations in specific molecules which have not been duplicated by models. The most dramatic of these fluctuations is a rapid decrease by more than a factor of 3 in the production of OH at 0.75 AU preperihelion.     

Density fronts in the vicinity of a long ridge

Laboratory experiments concerning the nature of density fronts in a two-layer fluid in the vicinity of a continuous ridge were conducted. The experiments were carried out in a circular rotating test cell containing an annular ridge of uniform cross-section. The density fronts were established by releasing a lighter fluid contained in a bottomless cylinder in the interior of the region defined by the topography into a heavier fluid occupying the rest of the test cell. The system was also equipped with an oscillating plunger located along the test cell axis to produce simulated tidal currents impinging in the normal direction on the ridge; experiments without and with tidal forcing were conducted. The governing parameters for the physical system considered are the Rossby, temporal Rossby, Burger and Ekman numbers and geometrical parameters. It is found that for both the non-forced and tidally forced experiments the fronts were stabilized by the ridge. The fronts in the simulated tidal currents experiments were found to advect radially outward more rapidly at early times than their non-forced counterparts; at large times, the temporal evolution of the front for these forced experiments approached that of the non-forced experiments. In the region interior to the annular ridge, the motion field is highly baroclinic, while outside this region, the flow response at the forcing frequency is barotropic. Scaling arguments regarding frontal position, viscous decay and barotropic oscillatory flow responses are advanced and supported by experimental observations.