The ilmenite/titano-magnetite assemblage: kinetics of re-equilibration

Compositions of coexisting ilmenite and titano-magnetite commonly have been used to establish the conditions of formation for igneous and metamorphic rocks (Buddington-Lindsley geothermometer/oxybarometer). Temperatures so determined for some rocks are lower than might be expected for magmatic crystallization. These low temperatures are the result of re-equilibration of the Fe-Ti oxides during slow cooling. This process is one in which titano-magnetites are oxidized and ilmenites are reduced upon cooling along a buffer curve or in the presence of a fluid of constant composition. Textural studies indicate that the reduction of the ilmenite-hematite phase may be the slower reaction consequently, the rate-controlling step of the re-equilibration.The kinetics of the reduction reactions in the two-phase region between the joins for the ilmenite-hematite and magnetite-ulvöspinel solid solution series were studied by thermogravimetric analysis at controlledf_O2. With experiments conducted near the Ni/NiO and QFM buffers, annealed ilmenite(ss) synthetic charges were reduced isothermally. The temperature range for the experiments was 900–1250°C, with total gas-flow rates of 0.35 and 1.0 cm/s. Kinetic data were collected by measuring the weight change during each experiment as a function of time. Additionally, a cooling-rate (0.5°C/hr) experiment was conducted near the QFM buffer from 1150 to 985°C starting with an ilmenite-saturated spinel phase.At 1200°C and 0.35 cm/s flow rate, the reaction mechanism is an interfacial process where mass transport of the reacting gases is the rate-controlling step. In isothermal experiments at 1100°C and lower, diffusion of reactants through a solid product layer was the controlling mechanism. The activation energy for this diffusion process is 59±6 kcal/mole. Significant enhancement of diffusion occurs along grain boundaries.Although the individual phases may re-equilibrate internally upon cooling, the Fe-Ti contents must remain unchanged for proper use of this mineral indicator-i.e. no external re-equilibration. It is imperative that the kinetics of the situation be appreciated. These preliminary kinetic experiments indicate that extreme caution must be exercised when applying this ilmenite/titano-magnetite geothermometer/oxybarometer. These phases may only truly “quench” in the parameters of formation in the case of certain hypabyssal and extrusive occurrences     

Effects of a change of terrain height and roughness on a wind profile

We have measured profiles of an onshore wind 200 m upwind and 200 m downwind from an abruptly rising shore using a remote-sensing Doppler lidar anemometer. Data were taken at heights between 4.7 and 66.5 m above sea level. Results show that the onshore velocity vector slopes upward 16.6 to 9.6°, the amount depending on the height of measurement, due to the combined effects of a 1.7-m high bluff shoreline and the frictionally decelerated flow over land. The profile 200 m inland has the expected deceleration at lower levels because of increased surface roughness and implies a velocity vector at 66.5 m height with an upward slope of approximately 18° (2.6 m s^-1 upward component, 8.4 m s^-1 vector magnitude), an acceleration to 0.3 m s^-1 greater than the upwind value, or a combination of both effects. All three options are consistent with mass continuity. The experiment exhibits the usefulness and limitations of a backscatter Doppler lidar for boundary-layer profile measurements in a horizontally inhomogeneous environment.     

The American oyster as a coastal zone pollution monitor: A pilot study

The Condition Index [(dry meat weight)(100)/(internal cavity volume)] has been analyzed and compared in the American oyster,Crassostrea virginica, from two South Carolina estuaries, one of which was considered polluted by coliform bacteria standards. During the warmer months, oysters from the unpolluted habitat showed a consistently and significantly higher Condition Index. Best fit and power function regressions of Condition Index on total coliform bacteria levels conform with previously cited effects of pollutants on oysters. Condition Index droped markedly as the incidence of total coliforms increased toward levels of 100 per 100 ml of water. Our results and the known sensitivity of oysters to a spectrum of pollutants suggest that their condition could be used to monitor waterborne pollution in coastal zone areas.     

Retention of riverine iron in estuaries

Retention of Fe flocs, resulting from the mixing of river water and seawater, was examined in three Maine estuaries. Riverine Fe was found to remain fairly conservative with salinity, implying that the process of floccufation does not necessarily remove Fe from water parcels. Laboratory experiments corroborated the field data by demonstrating that neither gravity nor suspended sediment were very effective in removing flocculated Fe from suspension. However, input of a tannery effluent did appear to result in scavenging of Fe from estuarine waters. Flocculated riverine Fe was found to increase considerably the Fe concentrations of estuarine bottom sediments, with the amount of iron per sediment specific surface area dependent on mean river flow entering an estuary. While no long term retention efficiencies could be calculated for these estuaries, it seems likely that a significant portion of flocculated riverine Fe escapes to shelf waters.     

High precision glacial–interglacial benthic foraminiferal Sr/Ca records from the eastern equatorial Atlantic Ocean and Caribbean Sea

Glacial–interglacial variation in the marine Sr/Ca ratio has important implications for coral Sr thermometry [J.W. Beck et al., Science 257 (1992) 644–647]. A possible variation of 1–3% was proposed based on ocean models [H.M. Stoll and D.P. Schrag, Geochim. Cosmochim. Acta 62 (1998) 1107–1118]. Subsequently, studies have used fossil foraminifera to test this prediction [P.A. Martin et al., Geochem. Geophys. Geosyst. 1 (1999); H.M. Stoll et al., Geochim. Cosmochim. Acta 63 (1999) 3535–3547; H. Elderfield et al., Geochem. Geophys. Geosyst. 1 (2000)]. But whether some component of foraminiferal Sr/Ca variation can be uniquely ascribed to seawater Sr variation is still not clear. To address this question, we developed cleaning and analysis techniques and measured Sr/Ca ratios on individual shells of the modern benthic foraminifer Cibicidoides wuellerstorfi. We showed that different size shells have different Sr/Ca ratios; however, samples with shell sizes of 355–500 μm appear to have normally distributed Sr/Ca ratios (1σ=1.8%). For multi-shell measurements (with estimated errors of 0.12–0.39%), the ratio varied by as much as 7.2±0.5% during the last glaciation for two Caribbean records at the same site and by 3.7±0.5% over the past 40,000 yr for one record from the Sierra Leone Rise in the eastern equatorial Atlantic. The two Caribbean records are very similar indicating that the behavior of shell Sr uptake was identical locally and that the shell Sr/Ca ratio faithfully reflects the local environment. The Atlantic record differs from the Caribbean records by as much as several percent. Thus, the foraminiferal Sr/Ca changes cannot be solely due to changes in seawater Sr/Ca unless the glacial deep ocean had spatial variation in Sr/Ca well in excess of the modern ocean. Certain similarities between the three records do exist. Notably, the rate of change of Sr/Ca is similar between 9 and 0 ka (−0.25%/kyr) and between 25 and 16 ka (+0.16%/kyr). This suggests that during these intervals, benthic foraminiferal Sr/Ca was affected by similar large-scale variables. One of these variables may be the average marine Sr/Ca ratio; however, comparison with model predictions [H.M. Stoll and D.P. Schrag, Geochim. Cosmochim. Acta 62 (1998) 1107–1118] suggests other factors must also be considered. The discrepancies between the two sites may be related to the different water mass histories for the Caribbean and eastern Atlantic. Our results suggest that variation of the seawater Sr budget only partially contributed to C. wuellerstorfi Sr/Ca records, while other significant factors still need to be quantified. At present we cannot confidently determine past seawater Sr/Ca variation from our foraminiferal records.     

On using mixed-layer transport parameterizations with radiometric surface temperature for computing regional scale sensible heat flux

Recent mixed-layer formulations for computing large-scale surface energy fluxes under daytime convective conditions do not require the estimation of surface-layer parameters, such as the roughness lengths for momentum and heat. This greatly simplifies approaches using operational satellite measurements of surface temperature for computing the surface energy balance at regional scales because the surface roughness parameters are not well known for many landscapes. The utility of such mixed-layer formulations is tested using data from several recent multidisciplinary field experiments (HAPEX-MOBILHY, FIFE and Monsoon 90). The results indicate that specific mixed-layer formulations adequately simulate surface sensible heat fluxes in the grassland and shrubland sites. However, use of the original values of proposed empirical coefficients for the forested site yield poor results. This is probably due to the fact that the forested site has significantly different surface geometry and associated distribution of temperature among the surface components (especially the relative importance of soil background temperatures) compared to the other sites. Therefore, the relationship between aerodynamic and radiometric surface temperature may have greatly differed between the forested site and the other locations. However, differences in aerodynamic roughness between the experimental sites were not correlated with changes required in the values of the coefficients. Instead, a two-source model which makes the distinction between aerodynamic and radiative temperature is proposed, as a means to determine which surface properties significantly affect the magnitude of the mixed-layer coefficients.     

Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m^?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m^?2 yr^?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.