Stratigraphic models for microtidal tidal deltas; examples from the Florida Gulf coast

Extensive vibracoring of both flood- and ebb-tidal deltas along the central Gulf Coast of the Florida peninsula reveals a strong overall similarity with subtle distinctions between flood and ebb varieties. Although the coast in question is microtidal, the inlets range from tide-dominated to distinctly wave-dominated. Both types of tidal deltas overlie a muddy sand interpreted to have been deposited in a back-barrier environment. The sharp contact at the base of the tidal delta sequence is typically overlain by a thin shell gravel layer. The ebb-tidal delta sequence is characterized by fine quartz sand with shell gravel in various concentrations; coarse and massive at the margins of the main ebb channel, and finer and imbricated at the marginal flood channels. The flood-tidal deltas are characterized by the same facies but with a small amount of mud. Shelly facies on the channels on flood deltas are not as well developed as on the ebb deltas. The combination of the stratigraphic sequence and the lithofacies make tidal deltas readily identifiable in the ancient record. The differences between flood and ebb varieties are subtle but consistent.     

Water and fine sediment dynamics in transient river plumes in a small, reef-fringed bay, Guam

Fouha Bay is a 400-m-long funnel-shaped, 10-m-deep, coral-fringed embayment on the southwest coast of Guam. It drains a small catchment area (5 km²) of steeply sloping, highly erodible lateritic soils. River floods are short-lived and the sediment load is very large, with suspended sediment concentration (SSC) exceeding 1000 mg l⁻¹. The resulting river plume is about 1 m thick and is pulsing in a series of 1–2 h-long events, with outflow velocity peaking at 0.05 m s⁻¹. Turbulent entrainment results in an oceanic inflow at depth into the bay. As soon as river flow stops, the plume floats passively and takes 5 days to be flushed out of Fouha Bay. The suspended fine sediment flocculates in 5 min and aggregates on ambient transparent exopolymer particles to form muddy marine snow flocs. In calm weather, about 75% of the riverine mud settles out of the river plume into the underlying oceanic water where it forms a transient nepheloid layer. This mud ultimately settles and is trapped in Fouha Bay. Under typhoon-driven, swell waves, the surface plume is at least 7 m thick and bottom entrainment of mud results in SSC exceeding 1000 mg l⁻¹ for several days. It is suggested that successful management of fringing coral reefs adjacent to volcanic islands may not be possible without proper land use management in the surrounding catchment.     

What is a wave-dominated coast?

During the past decade or so, various coasts have been designated as wave-dominated or tide-dominated. Typically there is an association made between coastal morphology and the dominant process that operates on the coast in question. Most authors consider long, smooth, barrier coasts with few inlets and poorly developed ebb deltas as “wave-dominated”. These coasts are associated with microtidal ranges. Conversely, mesotidal coasts tend to develop short, drumstick-shaped barriers with well-developed ebb deltas. They are considered as tide-dominated barriers. Such generalizations may be restricted to coasts with moderate wave energy although this is commonly not stated.

Exceptions to these stated generalizations are so numerous that wave energy and tidal prism must also be included in characterizing coasts. The relative effects of waves and tides are of extreme importance. It is possible to have wave-dominated coasts with virtually any tidal range and it is likewise possible to have tide-dominated coasts even with very small ranges. The overprint of tidal prism will also produce tide-dominated morphology on coasts with microtidal ranges.     

Differences in fine-scale structure and composition of zooplankton between mixed and stratified regions of Georges Bank

Transects were made with the Video Plankton Recorder (VPR) in different water masses on the southern flank of Georges Bank in May 1992. CTD-data, chlorophyll fluorescence, and attenuation were measured simultaneously. Images were recorded at a rate of 60 fields per second, as the instrument was towed at 2 m s⁻¹ (4 knots). Tapes from high and low magnification cameras (imaging volumes of 0.62 ml and 33 ml, respectively) were analyzed with respect to the distribution of copepods and other grazers, as well as invertebrate predators. This paper describes the differences in patterns of occurrence of important zooplankters in well-mixed and stratified waters on the Bank and in Slope Water south of these stations. Planktonic taxa were sampled over the same range of scales as the fluorescence and hydrography, allowing direct visual comparisons of the spatial distributions of these variables. Late copepodites of Calanus finmarchicus were strongly concentrated near the surface in the stratified area, while a dense belt of Limacina sp. and Oikopleura sp. occurred below the pycnocline. Other fragile forms were also found to be dominant. Colonies of hydroid polyps were very abundant at the mixed station, especially deeper in the water column, indicating that they may have been transported up from the bottom. Colonies of the diatom Chaetoceros socialis were abundant in the cold bottom water in the stratified region and also in the mixed area. These colonies may have been in the process of sedimenting out of the water column, as similarly sized and shaped marine snow was abundant in the same area. In the Slope Water, acantharia and Trichodesmium were found in the chlorophyll maximum above the pycnocline. Other important genera encountered were: Pseudocalanus, Oithona, Centropages, Obelia, Pleurobrachia and Sagitta. Taxa that occurred in all three areas were often differently distributed with respect to depth and physical parameters, indicating that vernal stratification is an important structuring factor of plankton populations.     

Flickering, a 10-cps fluctuation within bright auroras

Visually and with image orthicon television systems, a 10 ± 3 cps intensity variation has been observed in bright homogeneous auroras just prior to and during the breakup. It has been observed simultaneously at conjugate locations in the northern and southern hemispheres and at several other locations in the northern hemisphere. This phenomenon is called flickering; it has characteristics which clearly distinguish it from other types of pulsing auroras.     

A Case Study on the Effects of Heterogeneous Soil Moisture on Mesoscale Boundary-Layer Structure in the Southern Great Plains, U.S.A. Part I: Simple Prognostic Model

The atmospheric boundary-layer (ABL) depth was observed by airborne lidar and balloon soundings during the Southern Great Plains 1997 field study (SGP97). This paper is Part I of a two-part case study examining the relationship of surface heterogeneity to observed ABL structure. Part I focuses on observations. During two days (12–13 July 1997) following rain, midday convective ABL depth varied by as much as 1.5 km across 400 km, even with moderate winds. Variability in ABL depth was driven primarily by the spatial variation in surface buoyancy flux as measured from short towers and aircraft within the SGP97 domain. Strong correlation was found between time-integrated buoyancy flux and airborne remotely sensed surface soil moisture for the two case-study days, but only a weak correlation was found between surface energy fluxes and vegetation greenness as measured by satellite. A simple prognostic one-dimensional ABL model was applied to test to what extent the soil moisture spatial heterogeneity explained the variation in north–south ABL depth across the SGP97 domain. The model was able to better predict mean ABL depth and variations on horizontal scales of approximately 100 km using observed soil moisture instead of constant soil moisture. Subsidence, advection, convergence/divergence and spatial variability of temperature inversion strength also contributed to ABL depth variations. In Part II, assimilation of high-resolution soil moisture into a three-dimensional mesoscale model (MM5) is discussed and shown to improve predictions of ABL structure. These results have implications for ABL models and the influence of soil moisture on mesoscale meteorology