Paul W. Jewell

Physical Controls on Methane Ebullition from Reservoirs and Lakes

Understanding the nature and extent of methane production and flux in aquatic sediments has important geochemical, geotechnical, and global climate change implications. Quantifying these processes is difficult, because much of the methane flux in shallow sediments occurs via ebullition (bubbling). Direct observation of bubble formation is not possible, and bubbling is episodic and dependent upon a number of factors. Whereas previous studies have correlated methane flux with surface wind intensity, detailed study of Lake Gatun in Panama and Lago Loiza in Puerto Rico suggest that methane flux is more closely correlated with the shear stress in sediments caused by bottom currents. Bottom currents in turn are a complex function of wind, internal pressure gradients, and lake bathymetry. A simple physical model of bottom currents and sediments in these lakes suggests that most methane ebullition originated from the upper 10–20 cm of the sediment column. Our data reaffirm previous studies showing that ebullitive methane flux is minor in water deeper than ∼5 m.

Geochemistry and Paleoceanographic Setting of Central Nevada Bedded Barites

The bedded barite deposits of central Nevada are hosted by rocks of the Roberts Mountains allochthon and constitute the largest barite reserves in North America. Detailed geochemical studies of three barite deposits in the Devonian Slaven Chert indicate that rocks surrounding the barite have elevated concentrations of carbon and phosphorous relative to nonbarite-bearing rocks. Rare earth element data suggest that the barites were deposited in oxygenated seawater far from a spreading center. Ratios of Al, Ti, Fe, and Mn indicate that hydrothermal input to the sediment was minor relative to detrital input. The assembled data suggest a biogenic rather than a hydrothermal origin for the barite deposits. A model is proposed in which the barites were deposited at the

Numerical studies of bottom shear stress and sediment distribution on the Amazon continental shelf

O_{2}-H_{2}S

Controls of Tufa Development in Pleistocene Lake Bonneville, Utah

transition of a coastal upwelling system in the Late Devonian ocean. This model is consistent with recent paleoceanographic analyses of rocks in the Roberts Mountains allochthon, and studies indicating that the Late Devonian marked the transition from an anoxic to an oxic deep ocean.

Geochemistry of the Mississippian Delle Phosphatic Event, Eastern Great Basin, U.S.A.

Output from a one-dimensional, mixed-layer ocean model and a general circulation model suggests a consistent relation between surface-water residence times, large vertical salinity gradients, and anoxic bottom water during transgressive periods in the Cretaceous North American seaway. Model results show that severe storms over the seaway were not effective in mixing oxygen to the sediment-water interface when vertical salinity gradients exceeded 1‰-2‰ and depths were >300 m. At precipitation - evaporation + river runoff ( P - E + R ) rates calculated from the general circulation model (up to 300 cm/yr), conditions favorable to the maintenance of anoxic bottom water would be established within a matter of months. Large vertical salinity gradients at these P - E + R values would form in time periods as short as 1-2 yr. This is considerably shorter than seaway residence times calculated from the Sverdrup relation and from wind-stress curl calculations of atmospheric general circulation models.

Paleoredox Conditions and the Origin of Bedded Barites along the Late Devonian North American Continental Margin

ABSTRACT The relation between bottom shear stress and the distribution of bottom sediments on the Amazon continental shelf has been studied using a three-dimensional, primitive-equation computer model that incorporates the turbulence-closure scheme of Melior and Yamada (1982) for calculating eddy diffusivity and a simple algorithm for computing nonlinear wave-current influences on bottom shear stress. Model results compare reasonably well with salinity data sets for the Amazon plume. Model results on distribution of bottom currents and bottom shear stresses help explain some of the observed sedimentological features of the Amazon continental shelf. High concentrations of suspended sediment in the Amazon River are transported outward over the continental shelf and northward by the North Brazil oastal Current As this sediment settles out of the water column, it forms the prograding, subaqueous delta described by Nittrouer et al. (1986). Accumulation rates are greatest shoreward of the 40-m isobath due to a zone of convergent, cross-shelf residual tidal velocities. Little sediment is deposited in the shallow parts of the shelf, where bottom shear stress exceeds 10 dynes/cm2 over a diurnal tidal cycle. Zones of laminated sand and mud on the Amazon continental shelf coincide with areas of high interseasonal differences in bottom shear stress. Our results suggest that our model may be useful in interpreting sedimentation in ancient sedimentary basins as well.

Geochemistry of the Mercur gold deposit (Utah, U.S.A.)

The nutrient cycles of coastal upwelling zones are studied with simple mass balance models of Ekman transport, longshore transport, surface productivity, and dissolved phosphorous. The models are constrained with data from the Peru, northwest Africa, and Oregon upwelling systems. The onshore-offshore mass balance model agrees with published Ekman transport, surface productivity, and nutrient data as well as hypothesized nutrient f-ratios for highly productive coastal settings. The onshore-offshore model suggests that increased primary productivity in glacial-era coastal upwelling zones was not a linear function of Ekman transport, but instead was probably dependent on the physical and chemical dynamics of a specific setting. In the Peru upwelling system, longshore equatorward surface currents and poleward undercurrents produce positive surface nutrient gradients in the equatorward direction and relatively constant gradients in subsurface waters. Longshore nutrient gradients off northwest Africa are positive in the equatorward direction for both surface and subsurface waters. These observations are consistent with the conceptual model of surface and subsurface currents which are moving toward the equator and continually being upgraded by the offshore flux of nutrients. The northwest Africa and Peru data are not consistent with the longshore nutrient model of Redfield et al. (1963).

The Bonneville Shoreline: Reconsidering Gilbert's Interpretation

Prominent tufa localities along the Provo level (∼14,000 14C yr B.P.) shoreline in Pleistocene Lake Bonneville have been characterized in detail. Three types of tufa are recognized: capping tufa, beachrock, and capping tufa over beachrock. Capping tufa and beachrock are end members of a continuum based on variable clastic content. All three types typically occur on headland environments that had stable substrate and little sediment input. Tufa development correlates with bedrock exposure and landform orientation, which in turn are correlated ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape