MERIS and the red-edge position

The Medium Resolution Imaging Spectrometer (MERIS) is a payload component of Envisat-1. MERIS will be operated over land with a standard 15 band setting acquiring images with a 300 m spatial resolution. The red-edge position (REP) is a promising variable for deriving foliar chlorophyll concentration, which plays an important role in ecosystem processes. The objectives of this paper are: (1) to study which factors effect the REP of vegetation, (2) to study whether this REP can be derived from the MERIS standard band setting and (3) to show what REP represents at the scale of MERIS data. Two different data sets were explored for simulating the REP using MERIS bands: (1) simulated data using reflectance models and (2) airborne reflectance spectra of an agricultural area obtained by the airborne visible-infrared imaging spectrometer (AVIRIS). A “linear method”, assuming a straight slope of the reflectance spectrum around the midpoint of the slope, was a robust method for determining the REP and the MERIS bands at 665, 708.75, 753.75 and 778.75 nm could be used for applying the “linear method” for REP estimation. Results of the translation to the scale of MERIS data were very promising for applying MERIS at, for instance, the ecosystem level.     

Reconsidering the physics of the Chesapeake Bay estuarine turbidity maximum

A series of cruises was carried out in the estuarine turbidity maximum (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late January shifted the salinity structure of the upper Bay towards that of a salt wedge, but most of the massive sediment load delivered by the Susquehanna River appeared to bypass the ETM zone. In contrast, suspended sediments delivered during a flood event in late October were trapped very efficiently in the ETM. The difference in sediment trapping appeared to be due to increases in particle settling speed from January to October, suggesting that the fate of sediments delivered during large events may depend on the season in which they occur. The ETM roughly tracked the limit of salt (defined as the intersection of the 1 psu isohaline with the bottom) throughout the year, but it was often separated significantly from the limit of salt with the direction of separation unrelated to the phase of the tide. This was due to a lag of ETM sediment resuspension and transport behind rapid meteorologically induced or river flow induced motion of the salt limit. Examination of detailed time series of salt, suspended sediment, and velocity collected near the limit of salt, combined with other indications, led to the conclusion that the convergence of the estuarine circulation at the limit of salt is not the primary mechanism of particle trapping in the Chesapeake Bay ETM. This convergence and its associated salinity structure contribute to strong tidal asymmetries in sediment resuspension and transport that collect and maintain a resuspendable pool of rapidly settling particles near the salt limit. Without tidal resuspension and transport, the ETM would either not exist or be greatly weakened. In spite of this repeated resuspension, sedimentation is the ultimate fate of most terrigenous material delivered to the Chesapeake Bay ETM. Sedimentation rates in the ETM channel are at least an order of magnitude greater than on the adjacent shoals, probably due to focusing mechanisms that are poorly understood.     

Transient, longitudinal, sedimentary furrows in the York River subestuary, Chesapeake Bay: Furrow evolution and effects on seabed mixing and sediment transport

Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions. Based on²¹⁰Pb and¹³⁷Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown process which is controlling the meter-scale seabed mixing.     

DETERMINATION OF THE FORMS OF CARBON IN GEOLOGIC MATERIALS

Total carbon was determined in 30 geological reference materials by combustion/IR and combustion/coulometry methods. The combustion/coulometric method does not require empirical calibration and was found to be more precise than the combustion/IR method despite the use of smaller sample sizes. Carbonate carbon was determined by coulometric titration of acid-evolved CO₂. Non-carbonate carbon was calculated as the difference between the total and carbonate carbon contents.     

Geochemistry of the waste isolation pilot plant (WIPP) site, southestern New Mexico, U.S.A.

An extensive geochemical data base, including analyses of major and minor solutes, mineralogical studies of core samples, and isotopic studies of waters, carbonates and sulfates, has been assembled for evaporites and related rocks in the northern Delaware Basin of southeastern New Mexico. These data were compiled for the geological and hydrological characterization of the Waste Isolation Pilot Plant (WIPP), which is excavated in the evaporites of the Salado Formation. These data were evaluated in order: (1) to determine the stability of the evaporite mineralogy over geological time; (2) to compare the aqueous geochemistry with host rock mineralogy; (3) to delineate the nature and timing of water-rock interactions, such as dissolution and recrystallization; (4) to determine the geological and climatic conditions that have governed groundwater recharge. The resulting synthesis of data and current hypotheses concerning the origin, composition and history of waters in the evaporite rocks and related units of the Delaware Basin provides a tentative conceptual model for the behavior of the water-rock system since the deposition of the evaporites in the Permian. Essential components of this model include: (1) widespread Late Triassic/Early Jurassic evaporite recrystallization; (2) accumulation of deep-basin brines isolated from meteoric recharge; (3) evaporite dissolution by meteoric waters flowing in carbonates and sulfates interbedded in the uppermost Permian section and at the basin margin; (4) lateral rather than vertical infiltration of pre-Holocene meteoric waters in the uppermost Permian section; and (5) climatic conditions presently less conducive to recharge than in the Late Pleistocene.     

A comparison of the climatology of a tropospherestratosphere-mesosphere model with observations

Mean fields from a perpetual January simulation of a GCM extending from the surface to 0.01 hPa (near 80 km) are compared to observations. The zonal mean temperature and wind fields correspond quite well with reality; the low stratosphere, especially in the polar night, is too cold, but warmer than in the original version of the model, with an upper boundary at 25 hPa. Mean fields at standard levels show that the major features of the troposphere are represented by the model, but rather over emphasised; the stratospheric winter polar vortex is too strong, too cold, and too barotropic; it resembles an `undisturbed' January rather than the climatology. Differences in the stationary eddy activity between the extended and orginal versions of the model are noted, and used to explain some differences between the two simulations.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute für Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

Body-waveforms and source parameters of some moderate-sized earthquakes near North Island, New Zealand

P-wave first motion and synthetic seismogram analysis of P- and SH-waveforms recorded at teleseismic distances on the WWSSN are used to estimate source parameters of seven of the largest earthquakes (6.1 ≤ m_b ≤ 6.3) that occurred in the vicinity of North Island, New Zealand since 1965. The source parameters of three other (m_b ≥ 6.1) events determined outside of this study are included and considered in the final analysis. Four of the earthquakes occurred at shallow depths (< 20 km), of which three were located within and to the north of North Island. Two of the shallow events show strike-slip and normal focal mechanisms with T-axes oriented in a manner consistent with their location in an area of known back-arc extension. One of the shallow events occurred in northern South Island and shows a reverse-type mechanism indicating horizontal contraction of the crust in an easterly azimuth. Six events occurred at intermediate depths (h = 39 to 195 km) of which five exhibit thrust mechanisms with T-axes consistently oriented near vertical. In the light of previously published plate tectonic models, the near vertical orientation of T-axes of the intermediate-depth events may be used to infer that the southern Kermadec plate boundary immediately north of North Island is not strongly coupled, and hence, not likely capable of producing great earthquakes. A similar inference cannot be made for the section of the Hikurangi Margin adjacent to North Island since the intermediate-depth events considered in this study lie to the north of this segment of the plate boundary.     

VLBI observations of turbulence in the inner solar wind

I discuss the use of Very Long Baseline Interferometer (VLBI) phase scintillations to probe the conditions of plasma turbulence in the solar wind. Specific results from 5.0 and 8.4 GHz observations with the Very Long Baseline Array (VLBA) are shown. There are several advantages of phase scintillation measurements. They are sensitive to fluctuations on scales of hundreds to thousands of kilometers, much larger than those probed by IPS intensity scintillations. In addition, with the frequency versatility of the VLBA one can measure turbulence from the outer corona 5–10R to well past the perihelion approach of the Helios spacecraft. This permits tests of the consistency of radio propagation and direct in-situ measurements of turbulence. Such a comparison is made in the present paper. Special attention is dedicated to measuring the dependence of the normalization coefficient of the density power spectrum,C _N ² on distance from the sun. Our results are consistent with the contention published several years ago by Aaron Roberts, that there is insufficient turbulence close to the sun to account for the heating and acceleration of the solar wind. In addition, an accurate determination of theC _N ² (R) relationship could aid the detection of transients in the solar wind.