An actualistic perspective into Archean worlds - (cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa.

Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic-physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The 'Nhlazatse Section' includes structures such as 'erosional remnants and pockets', 'multidirected ripple marks', 'polygonal oscillation cracks', and 'gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or 'orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats.

Variability in the edible fraction content of 60Co, 99Tc, 110mAg, 137Cs and 241Am between individual crabs and lobsters from Sellafield (north eastern Irish Sea).

We investigated the variability of anthropogenic radionuclide content of the edible fractions of individual edible crabs (Cancer pagurus L.) and European lobsters (Homarus gammarus L.) caught commercially in the Sellafield offshore area. Sixteen female and 18 male crabs and 20 female and 17 male lobsters were selected from commercial catches made between 25 May and 5 June 1997. Each gender group was selected to be within the known weight range for commercially caught crustacea from the area. Four artificial radionuclides (60Co, 110mAg, 137Cs or 241Am) were detected by gamma-spectrometry. The edible fraction content of these radionuclides between males and females for either species were not statistically significantly different. 99Tc was analysed by chemical separation and beta-counting. 99Tc concentrations in female crabs tended to be higher (172 +/- 205 (16) Bq kg-1(wet); mean +/- standard deviation (n samples)) than those in males (85 +/- 58 (18) Bq kg-1 (wet)), although this was not a statistically significant difference. For both male and female crabs, 99Tc concentrations tended to decrease with increasing whole live weights. For 99Tc in lobsters the picture is less clear. Female lobsters contained more activity (14800 +/- 7400 (20) Bq kg-1 (wet)) than males (7100 +/- 3900 (17) Bq kg-1 (wet)). The results were used to discuss the implications for sampling and monitoring.

Spatial-frequency masking and Birdsall's theorem.

We measured the masking of a spatial 4-cycle/deg sinusoid in the presence of both random and sinusoidal masks. Subjects used a variety of detection strategies, depending on psychophysical technique and familiarity with the mask. Some strategies produce Weber's-law behavior and appear formally equivalent to identification tasks; we hypothesize that these exemplify the operation of Birdsall's theorem. Other strategies produce power-law behavior and are more like the simpler detection task. Our results suggest that criterion change (commonly uncontrolled in masking studies) can produce an unacceptably large bias in the results.

Dispersal pathways for particle-associated pollutants.

Particle-associated pollutants (totaling 10(7) metric tons per year) are introduced into the New York Bight by ocean dumping, estuarine discharge, sewage outfalls, eolian transport, and shipping waste and spillage. Oceanic and estuarine circulation processes dilute and transport the particles by a natural dispersal system that also tends to be highly distributive; particle-associated pollutants apparently seek the same sinks in the Hudson River shelf valley and intracoastal weltlands, regardless of their point of introduction.

The uptake by man of cadmium ingested in crab meat.

Seven male volunteers with normal iron stores ate the brown meat from crabs whose diet had contained radioactive cadmium-115m. Systemic uptake of cadmium by the volunteers, derived from measurements of their residual body radioactivity several weeks after intake, averaged 2.7 +/- 0.9 (se)%, essentially as found by others in studies with extrinsically labelled food.

An inherent nonlinearity in near-threshold contrast detection.

We measured an essentially normal pedestal effect using stationary gaussian targets and slowly moving pedestal gratings. Since these conditions greatly reduce the information provided by the pedestal, we question whether uncertainty about the stimulus can be the main cause of the pedestal effect.

Plutonium redistribution by biological activity in Irish Sea sediments.

As part of a programme investigating the fate of transuranium nuclides discharged to coastal waters we have studied the effects of benthic animals on plutonium redistribution in Irish Sea sediments. From analyses of burrow linings of the large echiuroid Maxmulleria lankesteri, which penetrates to depths exceeding 40 cm, we now show that this animal may have a significant effect on the removal of plutonium to deeper layers in the sea bed.

Spatial frequency masking and Weber's Law.

The threshold masking effect of one simultaneously presented grating upon another was studied as a function of mask contrast and frequency. The masking function typically obeys Weber's Law with method-of-adjustment psychophysics, and typically does not with forced-choice. This apparent discrepancy was studied in some detail. We suggest that thresholds can be set with at least two different criteria, depending upon experimental conditions. When the mask is unfamiliar, it functions as noise and detection occurs at a constant signal/noise ratio, which yields Weber's Law. When the mask is highly familiar, its masking effect is less and obeys a power law. This power-law masking appears to represent an inherent non-linearity of the visual system.

Two-criterion threshold techniques: evidence for separate spatial and temporal mechanisms?

Contrast thresholds were determined for counterphase flickering and drifting spatial gratings using pattern and flicker/motion criteria. In contrast to previous reports, the two criteria yielded contrast sensitivity functions (CSF) of similar form in the counterphase condition. However, moving gratings yielded CSF's of different form for the two criteria. These differences are probably due to eye movements.

An action spectrum for spatial-frequency adaptation.

The mechanism of spatial-frequency adaptation, and channel-bandwidth estimates derived from adaptation, were examined in a series of three experiments. The first two experiments measured adaptation as a function of both the frequency and modulation of the adapt stimulus. The threshold for producing adaptation at different adapt frequencies is considered to be an action spectrum whose half-amplitude bandwidth is found to lie between 1/3 and 2/3 octave. The third experiment employed a novel adaptation paradigm--varying the adapt contrast until the test grating was no longer perceptible--but the results supported those of the earlier experiments. It is concluded that adaptation is probably due to a fatigue-like mechanism, and that a sensitivity measure of adaptation (action spectrum) is probably a better representation of underlying channel tuning than is the more customary response measure.

Bermuda's Southern Aeolianite Reef Tract.

The outer reef on the explored southeast margin of the Bermuda platform is a submerged dune ridge thinly veneered with encrusting organisms. Aeolianites were deposited on what appears to be an older truncated surface, the reef-front terrace, now submerged to a depth of about 18 meters. Underwater examination reveals relict features of probable solutional origin that honeycomb the aeolinite and that have undergone only minor modification by erosion since the last eustatic rise of the sea. Submarine planation, even in this marginal area of reef growth, is a relatively slow process.

Elephant teeth from the atlantic continental shelf.

Teeth of mastodons and mammoths have been recovered by fishermen from at least 40 sites on the continental shelf as deep as 120 meters. Also present are submerged shorelines, peat deposits, lagoonal shells, anz relict sands. Evidently elephants and other large mammals ranged this region during the glacial stage of low sea level of the last 25,000 years.