Mary S. Quinby-Hunt

The whole-rock cerium anomaly: a potential indicator of eustatic sea-level changes in shales of the anoxic facies

Abstract The whole-rock cerium anomaly, tested for outer shelf-upper slope stratigraphic sections from the middle Ordovician through the lower Silurian of Scotland, is proposed as an empirical technique to develop a eustatic 3rd-order or finer-scale sea-level curve. This interval was chosen as it straddles the well-documented Late Ordovician glaciation and can be defined by graptolite zones. The anomaly is calculated from neutron activation analysis of low-carbonate, phosphate-free, fossil-free field-identified shales of the graptolite facies by comparison of the normalized cerium content with the linearized trend of the normalized composition of other rare earth elements in order of atomic number. For sections originally deposited in the main pycnocline below the surface mixed layer, values of the anomaly for a given sample would indicate its position on the redox curves developed for the early Paleozoic by Wilde (1987). Changes in the anomaly that are positive with time would indicate a lowering of sea level as the apparent depth on the redox curve would reflect more oxic conditions. Relative changes negative with time would indicate a rise in sea level as the apparent depth reflects more anoxic conditions. Depending on the vertical sample spacing and the time interval sampled, resolution of the order of 1 m.y. might be achieved. Thus the Vail et al. (1977) curves of the 3rd order (1 to 10 m.y.) or of finer scale could be obtained by this technique with the proper choice of section. Accordingly, for the early through middle Paleozoic when the main pycnocline was anoxic, this geochemical technique could be used to develop eustatic sea-level curves and additionally offer an independent calibration for seismic stratigraphy as well as an indicator of glacial-interglacial climatic sequences or eustatic changes due to fluctuations in global ridge crest volumes.

Iridium Abundances Across the Ordovician-Silurian Stratotype

Chemostratigraphic analyses in the Ordovician-Silurian boundary stratotype section, bracketing a major extinction event in the graptolitic shale section at Dobs Linn, Scotland, show persistently high iridium concentrations of 0.050 to 0.250 parts per billion. There is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years. Iridium correlated with chromium, both elements showing a gradual decrease with time into the middle part of the Lower Silurian. The chromium-iridium ratio averages about 106. Paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event.

Palaeo-oceanography and biogeography in the Tremadoc (Ordovician) Iapetus Ocean and the origin of the chemostratigraphy of Dictyonema flabelliforme black shales

High concentrations of vanadium, molybdenum, uranium, arsenic, antimony with low concentrations of manganese, iron and cobalt heretofore restricted to Dictyonema flabelliforme-bearing Tremadoc black shales in Balto-Scandia, have been found in coeval black shales in the Saint John, New Brunswick area. Prior palaeogeographic reconstructions place these areas about 400 km. apart in high southern latitudes in the Iapetus Ocean, with New Brunswick in proximity to Avalonia (southeastern Newfoundland). These geochemical similarities are not found in coeval Tremadoc black shales of Bolivia, New York, Quebec, Wales, and Belgium. Palaeo-oceanographic reconstructions of Iapetus support the proximity of Balto-Scandia and the Saint John area during the early Tremadoc and Geesx (1981) suggestion that the signature is a feature of eastern Iapetus. Furthermore, first-order modelling of the major surface currents and related primary productivity in the Tremadoc Iapetus Ocean explain the apparent wide latitudinal range of D. flabelliforme (Fortey, 1984) and the anomalous trace metal content of certain black shales of that time. Variations in the elemental content of these black shales is produced by oceanographic and geologic conditions unique to the geographic site. The distinctive Balto-Scandic geochemical signature resulted from the coincidence of anoxic waters transgressing the shelf at latitudes of high organic productivity at the polar Ekman planetary divergence. This produces the conditions for concentrations of V, U, and Mo in the shales. Metal enriched anoxic bottom waters produced by leaching of volcanics or through hydrothermal activity may be the source of the other enhanced signature elements such as As and Sb. The absence of this geochemical signature in younger non-D. flabelliforme Tremadoc and later black shales in Balto-Scandia and other areas suggests that the closing of Iapetus moved the depositional sites into less productive oceanic areas.

Origin of the induced circular polarization in the light scattering from a dinoflagellate

The S11 and S14 scattering matrix elements were measured for light scattered from single dinoflagellates and single irregularly-shaped, alumina particles suspended in a transparent gel. The S14 matrix element indicates the degree of circularly-polarized light induced on incident unpolarized light. The S4 signal from the dinoflagellates was found to be significantly larger than that measured for the alumina particles. The nucleus of the dinofiagellate investigated, Prorocentrum micans, contains about 50 structurally complex, chromosomes with a helical structure. This work provides evidence in support of the hypothesis that the observed S14 signal produced by the light scattering from P. micans is due to the helical nature of their chromosomes.

Immobilization Of Unicellular Marine Organisms For Optical Characterization: A New Method

A new method has been developed to study the light scattering properties of single, immobilized unicellular marine organisms. Cells of the dinoflagellate Prorocentrum micans were fixed1 and immobilized by mixing them into a solution that formed a transparent gel. The residual Rayleigh scattering caused by the gel was effectively eliminated by substituting a second liquid into the gel to match the index of refraction of the gel network, thereby producing a transparent support medium. Scattering measurements of the elements of the Mueller matrix were performed on a single Prorocentrum micans cell using a polarization-modulation scanning nephelometer.2 The angular dependence of Su, and normalized S12, S22, S33, S34, S13, and S14 are presented. The structure in all elements is very rich, cell orientation dependent, and completely reproducible. The measurements also revealed significantly non-zero values for all 16 elements of the scattering matrix and the non-equivalence of cross-diagonal elements. A striking feature of the measurements is the large peak values of S14 for a single immobilized cell. In recent years S14 has generally been reported to be zero for sea water and very small for laboratory systems. Measurements averaged over an ensemble of cells showed lower, but distinctly non-zero values in accord with earlier predictions.3

Effects of wavelength-dependent absorption on the polarization of light scattered from marine Chlorella

This paper investigates the wavelength dependence of the polarization characteristics of light scattered from laboratory cultures of marine Clzlorella. Scattering measurements were obtained using a scanning polarization-modulation nephelometer at wavelengths of 457 and 514 nm. The experimental data are corrected for non-spherical contributions and the resulting curves compared to Mie calculations of coated spheres with a Gaussian size distribution. Although the absorption of Chiorella has been reported to be strongly wavelength-dependent in the blue to green region of the spectrum, the scattering behavior changes very little. To verify the sensitivity of the scattering technique to changes in the imaginary refractive index, measurements were performed on absorbing and non-absorbing suspensions of wellcharacterized, coated copolymer particles. In all cases, the angle-dependent measurements and calculations were compared for four elements of the 16 element Mueller scattering matrix at two wavelengths. In the past, comparison of scattering models and measurements were generally performed for only the total intensity (one element of the scattering matrix). The use of four elements provides a much more stringent test of scattering calculations than those based on a single element. Using this method we are able to infer information about the internal structure and refractive indices of microscopic single cell organisms in vivo.