SIMULATING REGIONAL SYSTEMS: A SYSTEM DYNAMICS APPROACH

I.IntroductionTheuseofquantitative111odeIstodevelopUnderstanding,prePareforecastsandexplorepolicyoptionsinthecontextofurbanandroponaldevelpl11entremainsapopularaCtivlty,althouglrathermoresointheacadel11iccol11n1unitythanintl1epIanningprofession.However,thehistoryoftheapplicationoffon11aln1odelstopolicyproblemsdoesnotproducegreatconfidence.objeCtiveevaluationoftheuseofl11odelsinpolicyn1akingiscomplicatedbythefaCtthat,asafonl1ercolleagueoncesaid,con1puter111odeIsarelikecl1ildrenfmostpeopleten…     

Sediment indicators of meromixis: comparison of laminations, diatoms, and sediment chemistry in Brownie Lake, Minneapolis, USA

Meromixis has several powerful effects on lakes, yet there is no single definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in Brownie Lake, Minneapolis, Minnesota, a small eutrophic lake that became meromictic around 1925. The results show that in Brownie the onset of laminations and changes in the iron to manganese ratio most likely occurred before the development of permanently anoxic bottom water and that changes in the diatom assemblage occurred later, most likely only when meromixis was well developed.This is the 11th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Coherent eddies and temperature structure functions for three contrasting surfaces. Part I: Ramp model with finite microfront time

Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field.     

The intermittent vertical heat flux over a spruce forest canopy

Vertical heat and momentum fluxes were measured by the eddy correlation method under near-neutral conditions during both day and night above a spruce forest canopy. Results show that 50% of the heat transported to the spruce canopy during night and away from the canopy during the day occurs in extreme magnitude events, the majority of less than one second duration. Extreme-magnitude events were more frequent and lasted longer during the day than during the night. The distributions of the duration of extreme events in the same direction as the net heat flux and the turbulence intensity for both day and night were similar.During the night, the mean horizontal windspeed was about 1 m s^-1 and the measured coincident transport of momentum and heat accounted for 36% of the total heat flux. The predominant mechanism of forced convection during the extreme nightime heat transport events was excess heat sweeps in which the duration of the event is usually less than 1 s. During the day, the mean horizontal windspeed was about 2 m s ^-1 and the measured coincident transport of momentum and heat accounted for only 16% of the total heat flux. Local free convection was suggested to account for 27% of the total heat flux. The predominant mechanism of mixed convection during the extreme daytime heat transport events is deficit heat inward interactions. During both night and day, about 10% of the total heat transport occurred in extreme events working against the thermal gradient.Now atPurdue University, West Lafayette, IN, U.S.A.     

Occurrence,chemistry, and origin of immiscible silicate glasses in a tholeiitic basalt: A TEM/AEM study

The occurrence and chemistry of immiscible silicate glasses in a tholeiite mesostasis from the Umtanum formation, Washington, were investigated with transmission electron microscopy and analytical electron microscopy (TEM/AEM). TEM observation reveals isolated, dark globules (2.1 micron or less in diameter) randomly distributed in a transparent matrix glass interstitial to plagioclase laths. The globules less than 0.3 micron and larger than 0.8 micron fall beyond the linear relationship defined by the 0.3–0.8 micron globules in a plot of the logarithm of number versus size. Large globules (0.7 micron or larger in diameter) range from homogeneous to heterogeneous in optical properties and chemistry. Homogeneous globules are completely glassy, whereas heterogeneous globules contain crystalline domains. AEM analyses show that the globules have high Si, Fe, Ca, and Ti with subordinate Mg, Al, P, S, Cl, K, and Mn, which gives high normative fa, px, il, and ap. The matrix glass consists dominantly of Si with low Al and minor Na and K, yielding a high normative qz, or, ab, and an.It is proposed that the silicate liquid immiscibility occurs by reaction of network-modifying cations (NMCs) with dominantly chain-like anionic units in the parental melt to form less polymerized, NMC-bearing units and highly polymerized, Si-rich units. The globules nucleated metastably under supercooled conditions, and medium-size globules become either larger or smaller at lower temperatures. Internal nucleation of NMC-rich phases occurred in some larger globules upon cooling.     

Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10¹ km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area.Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m⁻ d⁻¹ and 75 mg m⁻, respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO₃-supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a⁻¹ h⁻¹), high mean α (14 mg C mg Chl a⁻¹ Ein⁻¹ m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters.The results imply a seasonal (June to August) middle shelf production of 150 g C m⁻¹, about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.     

High temperature alteration of Abyssal ultramafics from the Islas Orcadas Fracture Zone,South Atlantic

Ultramafic rocks dredged from the Islas Orcadas Fracture Zone, along the SW Indian Ocean Ridge (6° E and 54° S), show evidence of progressive hydration beginning at temperatures greater than 600° C (and perhaps as high as 900° C) and continuing to less than 50° C. There are two principal types of alteration present in the ultramafic rocks, both of which are the result of hydration reactions. The first type of alteration involves hydration of original clinopyroxene, orthopyroxene and olivine to amphibole, talc, secondary olivine, and serpentine. The second is a vein type of alteration and results in the formation of veins of amphibole, chlorite, talc and serpentine. — The alteration appears to be episodic. The sequence of events suggested by the petrography is: 1) clinopyroxene altering to amphibole; 2) orthopyroxene altering to talc, or talc + olivine; 3) supersolvus hornblende veining; 4) coexisting actinolite + hornblende veining; 5) chlorite, chlorite + actinolite, or chlorite + secondary clinopyroxene veining; 6) talc veining; 7) serpentine veining; and 8) pervasive serpentinization. — The alteration fluid is most likely seawater. It is suggested that the high temperature alterations may reflect seawater circulation into the upper mantle.     

Isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis

Diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios. Interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute HF. The weakened interface breaks when grains are agitated ultrasonically. Subsequent particle size separation is an effective means of concentrating the diagenetic component.