Crystal settling and convection in the Shiant Isles Main Sill

The 168 m-thick Shiant Isles Main Sill is a composite body, dominated by an early, 24 m-thick, picrite sill formed by the intrusion of a highly olivine-phyric magma, and a later 135 m-thick intrusion of olivine-phyric magma that split the earlier picrite into a 22 m-thick lower part and a 2 m-thick upper part, forming the picrodolerite/crinanite unit (PCU). The high crystal load in the early picrite prevented effective settling of the olivine crystals, which retain their initial stratigraphic distribution. In contrast, the position of the most evolved rocks of the PCU at a level ~80% of its total height point to significant accumulation of crystals on the floor, as evident by the high olivine mode at the base of the PCU. Crystal accumulation on the PCU floor occurred in two stages. During the first, most of the crystal load settled to the floor to form a modally and size-sorted accumulation dominated by olivine, leaving only the very smallest olivine grains still in suspension. The second stage is recorded by the coarsening-upwards of individual olivine grains in the picrodolerite, and their amalgamation into clusters which become both larger and better sintered with increasing stratigraphic height. Large clusters of olivine are present at the roof, forming a foreshortened mirror image of the coarsening-upwards component of the floor accumulation. The coarsening-upwards sequence records the growth of olivine crystals while in suspension in a convecting magma, and their aggregation into clusters, followed by settling over a prolonged period (with limited trapping at the roof). As olivine was progressively lost from the convecting magma, crystal accumulation on the (contemporaneous) floor of the PCU was increasingly dominated by plagioclase, most likely forming clusters and aggregates with augite and olivine, both of which form large poikilitic grains in the crinanite. While the PCU is unusual in being underlain by an earlier, still hot, intrusion that would have enhanced any driving force for convection, we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies >100 m thickness.     

Defining Optimal Freshwater Flow for Oyster Production: Effects of Freshet Rate and Magnitude of Change and Duration on Eastern Oysters and <Emphasis Type="Italic">Perkinsus marinus</Emphasis> Infection

In coastal Louisiana, the development of large-scale freshwater diversion projects has led to controversy over their effects on oyster resources. Using controlled laboratory experiments in combination with a field study, we examined the effects of pulsed freshwater events (freshet) of different magnitude, duration, and rate of change on oyster resources. Laboratory and field evidence indicate that low salinity events (<5 psu) decreased Perkinsus marinus infection intensities. Furthermore, when salinity was low (<5 psu), parasite infection intensities continued to decrease even as temperatures exceeded 20°C. At the same time, oyster growth was positively correlated with salinity. To maximize oyster production, data indicate that both low and high salinity events will be necessary.     

The Monitor project: rotation of low-mass stars in NGC 2362 – testing the disc regulation paradigm at 5 Myr

We report on the results of a time-series photometric survey of NGC 2362, carried out using the CTIO 4-m Blanco telescope and Mosaic-II detector as part of the Monitor project. Rotation periods were derived for 271 candidate cluster members over the mass range  0.1 ≲ M /M_⊙≲ 1.2  . The rotation period distributions show a clear mass-dependent morphology, qualitatively similar to that in NGC 2264, as would be expected from the age of this cluster. Using models of angular momentum evolution, we show that angular momentum losses over the ∼1–5 Myr age range appear to be needed in order to reproduce the evolution of the slowest rotators in the sample from the ONC to NGC 2362, as found by many previous studies. By incorporating Spitzer IRAC mid-infrared (mid-IR) measurements, we found that three to four objects showing mid-IR excesses indicative of the presence of circumstellar discs were all slow rotators, as would be expected in the disc regulation paradigm for early pre-main-sequence angular momentum evolution, but this result is not statistically significant at present, given the extremely limited sample size.     

The response of fishes to physicochemical changes in the mangroves of northeast Florida Bay

Historically, large volumes of fresh water from the Everglades reached Florida Bay in the form of overland sheet flow. South Florida's extensive canal system has diverted fresh water from its historic course, resulting in shorter hydroperiods and higher salinities than would have occurred in an unaltered system. The mixing zone between the freshwater Everglades and euryhaline Florida Bay is primarily characterized as a dwarf red mangrove forest. The small, demersal fishes found in this habitat are an important food source for a variety of predators and are excellent bioindicators for both short-term and long-term perturbations in the system. I examine the effect of fluctuating water level, salinity, and temperature on this fish community in order to better understand the impact water diversion has had on the ecotone. Fish were collected at four sites within the ecotone over a t-yr period using a 9-m² drop trap. Principal components analysis was used to generate 10 composite variables (PCs) from a temporal array of 59 physicochemical variables. These composite variables were used in regression analyses to evaluate spatial and temporal changes in the fish community. Regression analysis indicated fish density was significantly related to short-term and long-term changes in water level and with long-term temperature variation (r²=0.50). An ANOVA of density between sites supports the regression results, indicating that sites with longer hydroperiod had higher density than sites with shorter hydroperiod. The impact of changes in density on biomass was reflected by regression analysis, which indicated that increased water level and decreased variability in depth were correlated with higher biomass (r²=0.61). Biomass was also influenced by changes in the salinity regime, presumably through increases in individual fish body size or through a shift in the community toward heavier-bodied fish species. An ANOVA of biomass between sites indicates sites with longer freshwater periods had higher biomass than sites with shorer freshwater periods. The first two axes of a detrended correspondence analysis on community biomass explained 59.2% of the variance in the community and supported the hypothesis that salinity was a primary determinant of community structure. These results indicate historic changes in water deliver could have altered the mangrove fish community, thereby lowering prey availability for higher trophic levels.     

Thermal periodicities in the Venus atmosphere

A search was made for periodic fluctuations in the thermal brightness temperatures recorded by the Pioneer Venus orbiter's infrared radiometer. Data were averaged in 10 × 10° latitude-longitude bins for each of the 72 days the instrument was in operation. This time series of thermal brightness temperatures was then analyzed to determine the amplitude of fluctuations at periods from 2 to 64 days at four levels in the atmosphere (at the cloud tops and at approximately 70, 80, and 90 km). The amplitude of such fluctuations is small at equatorial latitudes and increases to a maximum at 60–70° latitude at most altitudes. The period of the highest amplitude fluctuation is 5.3±0.4 days (at all altitudes) except at 70–80°, where a 2.9-day period which appears to correspond to the polar dipole dominates the cloud-top channel. The amplitude of the periodic fluctuations is a maximum at the cloud tops, decreasing to a minimum at the 80-km channel, and increasing again at the 90-km channel.     

Solar constant secular changes

Our recent model for solar constant secular changes suggests that over extended time intervals, a positive correlation of the solar constant with solar activity results. The positive correlation in this model is not associated with the direct result of active region, which may be in a close energy balance over their lifetimes, but rather is associated with global features - bright global faculae. These exist as both polar and network features on the solar disk. The high latitude faculae enable the solar constant to peak prior to sunspot maximum. Recent solar constant observations support the model's general trend. Using this model, we now calculate a proxy solar constant for: (1) the past four centuries, based upon the sunspot record, (2) the past nine centuries, based upon ¹⁴C observations and their relation to solar activity, and (3) the next decade, based upon a dynamo theory model for the solar cycle. The proxy solar constant data is tabulated as it may be useful for climate modelers studying global climate changes. This could be helpful in disentangling possible solar influences from any anthropogenic changes associated with trace gas increases in the terrestrial atmosphere. An important point for climate modelers to consider in fixing climate model parameters, is that the Sun has been relatively active and thus bright in the latter half of the 20th century, compared with the past few centuries.This paper was presented at the third meeting of the Solar Cycle Workshop, held in Sydney, Australia, January 9–13, 1989.Physics Dept., Univ. of Northern Colorado, U.S.A.     

Short term evolution of coronal hole boundaries

In an examination of the evolution of coronal hole boundaries on a time scale of 1 day, we find that 38% of all the boundaries of coronal holes observed near central meridan passage during the Skylab period shifted in location by >1° heliocentric in 1 day. Of these boundary changes, 70% were on a scale 3 times the average supergranulation cell size. However, large-scale shifts in the boundary locations also occurred, which involved changes in the X-ray emission from these areas of the Sun. X-ray emitting structures on the borders of isolated and evolving holes were less clearly defined than those on the boundaries of well-established, elongated holes. There were generally more changes in the boundaries of the most rapidly evolving holes, but no simple relationship between the amount of change and the rate of hole growth or decay.Skylab Solar Workshop post-doctoral appointee 1975–1976. The Skylab Solar Workshops are sponsored by NASA and NSF and managed by the High Altitude Observatory, National Center for Atmosoheric Research.     

Experiments with scale models of oil collectors for subsea well blowouts — Part II

A previous paper of the same title indicated the feasibility of the collection of oil by an open bottom collector above a blowout with a marine riser above the collector; the whole collection system being driven by gas lift from the blowout gas. That paper was based on small-scale laboratory experiments and it identified the salient dimensionless parameters governing those experiments. This paper describes laboratory experiments on a refinement of the collection system and also describes the results of intermediate scale experiments. The length scale of these experiments was about four times greater than laboratory scale and about one-fourth of full scale. Generally, the intermediate scale results are consistent with the laboratory predictions. Furthermore, two scale-dependent parameters have been identified. The effects of these have been included in an analysis of the results.     

Experiments with scale models of oil collectors for subsea well blowouts

Laboratory experiments on conically shaped oil collectors beneath a marine riser are described. The collection concept involves oil, water and gas entering the collector and being driven into a separating sytem by gas-lift. A parametric study involving various collector shapes, heights above the wellhead, and gas, oil and water flow rates was carried out. The important dimensionless variables were identified and quantified with the conclusion that effective collection is possible if the collector height is sufficiently small and the ratio of water pumped to gas flow is sufficiently large. Increased collector heights can be accommodated at the expense of requiring increased water flows by use of larger diameter risers. The amount of gas required for most efficient oil collection is found to be much less than is expected to come from most blowouts when a single collector and riser system is used. This difficulty can be greatly diminished by the use of a specially designed gas-separating collector which passes most of the gas to the surface through a riser separate from the one which carries the liquids.     

Measurements of the surface flow above round bubble plumes

When a bubble plume exists beneath a free surface, such as after a subsea gas well blowout, a generally horizontal flow occurs in the vicinity of the surface and this flow is influenced by the fact that the surface is free. Two very different theories for such surface flows have been developed in the past and the bases of these theories are reviewed here. The results of measurements of surface flows above plumes of relatively large scale are given. These are compared with both of the existing theories. One theory is found to be accurate at small radii from the plume centre and the other theory is found to be accurate at large radii. The needed boundary condition for the theory that is accurate at large radii is supplied by the results of the theory that is accurate at small radii.