Late Holocene estuarine–inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida?

The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sea-level rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped – one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebb-tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf.     

Th sorption and export models in the water column: A review

Over the past few decades, the radioisotope pair of ²³⁸U / ²³⁴Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to ²³⁴Th. It is focused on the models associated with ²³⁴Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C / ²³⁴Th ratios (Buesseler et al.), ²³⁴Th speciation (Santschi et al.) and present and future applications of ²³⁴Th [Waples, J.T., Benitez-Nelson, C.R., Savoye, N., Rutgers van der Loeff, M., Baskaran, M., Gustafsson, Ö., this issue. An Introduction to the application and future use of ²³⁴Th in aquatic systems. Marine Chemistry, FATE special issue]. In this paper, we review current ²³⁴Th scavenging models and discuss the relative importance of the non-steady state and physical terms associated with the most commonly used model to estimate ²³⁴Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1–4 weeks in order to evaluate steady state versus non-steady state estimates of ²³⁴Th export, especially during high flux events (> ca. 800 dpm m^− 2 d^− 1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g., Equatorial Pacific). These suggestions will greatly enhance the application of ²³⁴Th as a tracer of particle dynamics and flux in more complicated regimes.     

Regime shifts and fishery management

Understanding regime shifts is important to management. Optimal allocation of fishery effort can be improved if it were known whether or not the regime was positive or negative. This determination is difficult because a high recruitment, when the stock is at a low level of abundance (or vice versa), may be the indication of the onset of a multidecadal regime shift, or just a chance occurrence. Accordingly, the determination of an increase or decrease in productivity from observations independent of those made directly on fish populations is important.     

On the calculation of the depth of the upper mixed layer in the equatorial ocean

The equation for the turbulence kinetic energy balance as applied to the horizontally-inhomogeneous upper ocean layer encompassing the equatorial zone is analysed. A partial solution of the equation has been derived for the equilibrium conditions. Using the equatorial Atlantic as an example, the prime importance of considering the effect of horizontal heterogeneity in calculating the upper mixed layer thickness is shown.Translated by Vladimir A. Puchkin.     

Late Quaternary Evolution of Amvrakikos Gulf,Western Greece

Amvrakikos Gulf is a Neogene basin, formed during a late extensional tectonic phase within the Plio-Quaternary period. It is a semienclosed embayment, separated from the Ionian Sea by a shallow (< 10 m) channel. The analysis of 3.5-kHz seismic reflection profiles shows that, during the last (Würm) glacial period, the parts of the Gulf that lie at water depths >41 m (below present sea level) were a paleo-lake while the rest were exposed to subaerial erosion. Subsequent offshore depositional sequences accumulated at rates of 1.2–2.3 m/ka.     

Response of the blue mussel Mytilus edulis L. following exposure to PAHs or contaminated sediment

The polycyclic aromatic hydrocarbons fluoranthene and benzo(a)pyrene significantly reduced the feeding rate of mussels. For both compounds the tissue concentration resulting in a 50% reduction of the clearance rate (TEC₅₀) was calculated. At high tissue concentrations both aromatic compounds reduced the tolerance of mussels to aerial exposure, whereas at low tissue concentrations an improved response was noticed. The activities of the antioxidant enzymes Superoxide dismutase (SOD) and catalase were elevated only at low tissue concentrations of fluoranthene and benzo(a)pyrene. At the highest measured tissue concentrations the activity of both enzymes was reduced, possibly due to a narcotic effect. The reproductive success rate of mussels appeared to be affected negatively by the investigated hydrocarbons. The results of a pilot experiment indicate that mussels can be used also for the testing of contaminated sediments.     

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s^-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171.     

Topography and a magnetic analysis of an area south-east of the Azores (36°N, 23°W)

A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m^-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity.