Edward W. Bolton

A model for the kinetic control of quartz dissolution and precipitation in porous media flow with spatially variable permeability: Formulation and examples of thermal convection

We present the formulation and model results of kinetically controlled quartz dissolution and precipitation in a two-dimensional heterogeneous permeable medium. The quartz matrix is modeled as a partially occluded spherical close pack, with dissolution and precipitation occurring on the exposed faces of the grains. This formulation yields larger permeabilities and lower surface area to fluid volume ratios in regions of larger grain radii, and thus allows us to investigate the influence of crack-like regions on the flow and silica exchange. We use the kinetic data for quartz dissolution of Rimstidt and Barnes [1980]. Thermal convection results indicate that channelizing of fluid flow in high-permeability zones is enhanced in the transient regime by buoyancy effects arising from the advection of heat. The highly permeable zones are most out of chemical equilibrium, owing to their lower surface areas and to more rapid advection. Porosity changes are most pronounced in regions of high surface area often downstream of high-permeability zones. Oscillatory convection is observed, accompanied by saturation state reversals peripheral to the high-permeability zones. Such internally generated oscillatory regimes provide a mechanism for quartz zonation down to the nanometer scale. When the thermal forcing is strong enough, recurrent plumes emanate from the thermal boundary layers and plunge through the high-permeability zones. When departures from equilibrium become significant, we observe some regions of undersaturated upwelling fluid moving down temperature, and regions of oversaturated downwelling fluid moving up temperature, both cases opposing the conventional wisdom based on equilibrium.

A wavelet analysis of Plio‐Pleistocene climate indicators: A new view of periodicity evolution

Wavelet analysis offers an alternative to Fourier based time-series analysis, and is particularly useful when the amplitudes and periods of dominant cycles are time dependent. We analyze climatic records derived from oxygen isotopic ratios of marine sediment cores with modified Morlet wavelets. We use a normalization of the Morlet wavelets which allows direct correspondence with Fourier analysis. This provides a direct view of the oscillations at various frequencies, and illustrates the nature of the time-dependence of the dominant cycles.

The Weathering of Sedimentary Organic Matter as a Control on Atmospheric O2: II. Theoretical Modeling

To investigate the weathering of sedimentary organic matter and its role in regulating atmospheric oxygen, a theoretical modeling study is presented that addresses the fundamental controls on atmospheric oxygen uptake: erosion rate, organic matter content, and reaction rate. We compare model results with the previous part of this study that analyzed a drill core of black shale from the New Albany formation (Upper Devonian, Clay City, KY) for total and organic carbon, pyrite sulfur, porosity, permeability and specific surface area. As was observed in the field study, the model predicts that the loss of organic matter by oxidative weathering takes place across a reaction “front” where organic carbon content decreases sharply toward the land surface along with pyrite loss. The model is based on kinetic control of reaction of organic matter and pyrite with O2 dissolved in soil water. The downward diffusion of gaseous O2 partitions with dissolved O2 in water films on sediment grains via Henry’s law. Once a weathering profile is developed, the downward migrating O2 reacts with shale organic matter and pyrite. Pyrite reacts faster with O2 than does organic matter (for a given local concentration of oxygen) making the pyrite front generally deeper than the organic matter front. We explore the influence of differing erosion rates, atmospheric O2 concentrations, organic matter contents, porosities, tortuosities, and rates of reaction (that could include possible acceleration due to microbes) on the oxygen consumption. We conclude, based on our modeling, that the erosion rate and the concentration of buried reduced matter, as opposed to the level of atmospheric O2, normally limits the rate of drawdown of atmospheric oxygen. For the vast majority of erosion rates and Phanerozoic oxygen levels, essentially all ancient reduced material is oxidized before reaching the surface. Only in regions of unusually rapid erosion or during very low atmospheric oxygen levels can rates of diffusion of O2 in soils and rates of reaction control O2 drawdown, leading to weathering that is O2-dependent. In this case erosion and rapid reburial of unoxidized organic matter would occur.

Dissolution and precipitation via forced‐flux injection in a porous medium with spatially variable permeability: Kinetic control in two dimensions

We model kinetically controlled dissolution and precipitation in a porous medium. Using quartz as the reactive mineral, our specific focus is on the spatial distribution of flow velocities and on changes in solute concentration and porosity introduced by heterogeneities in the initial permeability field and by differing the solute concentration of the infiltrating fluid. Upon a background permeability we impose isolated “crack-like” zones of high permeability and low surface area to fluid volume ratios. Our two-dimensional modeling of forced-flux injection of oversaturated and undersaturated fluid reveals features inaccessible to previous homogeneous permeability studies. Although realistic velocities can give rise to narrow boundary layers in the solute concentration, disequilibrium is favored in the high-permeability zones yielding plume-like distributions of solute concentration. Aside from the rapid changes in porosity near the injection level, the other regions of rapid porosity change occur just downstream from the crack-like zones, where fluid furthest from equilibrium with respect to quartz encounters regions of higher surface area to fluid volume ratios. Flow rates are significantly enhanced between isolated high-permeability zones, an effect which is even more dramatic for both closer crack spacing and higher permeability contrasts. Undersaturated injection leads to permeability homogenization along the flow direction due to dissolution at the trailing edges of the cracks. Oversaturated injection tends to increase permeability heterogeneities along the flow direction, due to precipitation at the crack trailing edges. We also discuss the scaling properties of flow lines and reaction rates.

Calculation of fluid fluxes in Earth’s crust

Abstract The movement of fluids in the crust and upper mantle not only lead to important mineral reactions but also play an essential role in the geochemical cycling of elements and in controlling global change. Numerous papers have focused on calculation of fluid fluxes driving metamorphic reactions in the earth’s crust. The extent of reaction in nature has been “inverted” to predict the total amount of fluid that was required to drive that much reaction. These models, although based on thermodynamic equilibrium, have extended the earlier concept of water-rock ratio. Any quantitative treatment of the fluid fluxes and the relationship between these fluxes and other variables such as temperature and mineral abundances requires a kinetic model. A simple model is presented that incorporates the essential dynamics of metamorphic processes including both heat flow by conduction and convection as well as fluid flow in and out of a representative volume. Overall mineral reactions can then take place within this rock volume in response to internal and external factors. The production and subsequent expulsion of excess fluids (H2O and CO2) as a result of these reactions leads to increased fluid fluxes, which the model can also handle. Using this kinetic model, the assumption of thermodynamic equilibrium can be tested and forward calculations can compare the numbers “inverted” for total integrated fluid fluxes based on equilibrium with the “actual” integrated fluid fluxes. Other effects such as changes in the temperature field or the presence of dispersion/diffusion can also be readily quantified with this kinetic model. The nontrivial consequences of heterogeneity in natural systems make the kinetic approach much more essential but at the same time much more “invertible” than earlier approaches. Ultimately, the effects of the rates of overall mineral reactions and their interplay with the other kinetic processes taking place in these open systems have to be evaluated to guide us in developing much more powerful and correct ways of extracting fluid velocities from petrologic field data.

Precambrian ``fossil'' Vermiforma is a tectograph

Vermiforma antiqua [Cloud 1976][1], once celebrated as the earliest animal fossil of the United States, is actually a tectonic artifact. The main argument is that the congruence and equal orientation of multiple patterns on the same bedding plane are incompatible with swaying worm bodies or worm burrows. As shown by analog and numerical simulations, these structures can be explained as tracks of particles that broke out from the base of an overlying turbidite and became rolled between beds during bed-to-bed shearing. [1]: #ref-5

Muscular Anatomy of the Podocoryna carnea Hydrorhiza

The muscular anatomy of the athecate hydroid Podocoryna carnea hydrorhiza is elucidated. The polyp-stolon junction is characterized by an opening, here called the chloe, in the otherwise continuous hydrorhizal perisarc. The chloe is elliptical when the polyp first arises, but takes on a more complex outline as multiple stolons anastomose to communicate with that polyp. Surrounding the polyp base are spots, here called anchors, which autofluoresce at the same wavelengths as perisarc and which, like perisarc, contain chitin as assessed by Calcofluor White, Congo Red and wheat germ agglutinin staining. Anchors remain after living tissues are digested using KOH. Collagen IV staining indicates that the mesoglea is pegged to the anchors and rhodamine phallodin staining detects cytoskeletal F-actin fibers of the basal epidermis surrounding the anchors. Longitudinal muscle fibers of the polyp broaden at the polyp base and are inserted into the mesoglea of the underlying stolon, but were neither observed to extend along the stolonal axis nor to attach to the anchors. Circular muscular fibers of the polyp extend into stolons as a dense collection of strands running along the proximal-distal axis of the stolon. These gastrodermal axial muscular fibers extend to the stolon tip. Epidermal cells at the stolon tip and the polyp bud display a regular apical latticework of F-actin staining. A similar meshwork of F-actin staining was found in the extreme basal epidermis of all stolons. Immunohistochemical staining for tubulin revealed nerves at stolon tips, but at no other hydrorhizal locations. These studies bear on the mechanisms by which the stolon tip and polyp bud pulsate, the manner in which the stolon lumen closes, and on the developmental origin of the basal epidermis of the hydrorhiza.

Nutrient Distribution and Absorption in the Colonial Hydroid Podocoryna carnea Is Sequentially Diffusive and Directional

The distribution and absorption of ingested protein was characterized within a colony of Podocoryna carnea when a single polyp was fed. Observations were conducted at multiple spatial and temporal scales at three different stages of colony ontogeny with an artificial food item containing Texas Red conjugated albumin. Food pellets were digested and all tracer absorbed by digestive cells within the first 2–3 hours post-feeding. The preponderance of the label was located in the fed polyp and in a transport-induced diffusion pattern surrounding the fed polyp. After 6 hours post-feeding particulates re-appeared in the gastrovascular system and their absorption increased the area over which the nutrients were distributed, albeit still in a pattern that was centered on the fed polyp. At later intervals, tracer became concentrated in some stolon tips, but not in others, despite the proximity of these stolons either to the fed polyp or to adjacent stolons receiving nutrients. Distribution and absorption of nutrients is sequentially diffusive and directional.

Making Sense of Massive Carbon Isotope Excursions With an Inverse Carbon Cycle Model

The beginning and end of the Proterozoic Eon are marked by extreme variations in carbonate carbon isotope values that have been interpreted to record massive perturbations to the global carbon cycle. The lower Proterozoic contains an extended interval of strata characterized by positive carbonate δ13C values. Conversely, uppermost Proterozoic carbonate strata contain thick intervals with extremely negative δ13C values and multiple large swings in carbonate δ13C. Previous attempts to model these pronounced carbon isotope excursions as shifts in the global marine dissolved inorganic carbon (DIC) reservoir have proved to be problematic, as the direction and magnitude of these positive and negative carbon isotope excursions require unrealistic amounts of either organic carbon burial or organic carbon oxidation, respectively. Here we present a modified global carbon cycle model—coupled with oxygen and sulfur cycle mass balances—that includes a parameterization of the recycling of sedimentary isotope anomalies and allows the extent of organic carbon oxidation to vary as a function of atmospheric oxygen levels. Our model is designed to match carbon isotope records while maintaining redox and mass balance with a given set of initial conditions and carbon cycle parameterizations. Using this approach, we demonstrate that there is a range of plausible biogeochemical perturbations that could induce substantial δ13C excursions in the global marine DIC reservoir. However, we also find that there are multiple, nonunique Earth system states for any observed marine δ13C value.

Model based Paleozoic atmospheric oxygen estimates: a revisit to GEOCARBSULF

Geological redox proxies increasingly point towards low atmospheric oxygen concentrations during the early Paleozoic Era, with a subsequent protracted rise towards present-day levels. However, these proxies currently only provide qualitative estimates of atmospheric O2 levels. Global biogeochemical models, in contrast, are commonly employed to generate quantitative estimates for atmospheric O2 levels through Earths history. Estimates for Paleozoic pO2 generated by GEOCARBSULF, one of the most widely implemented carbon and sulfur cycle models, have historically suggested high atmospheric O2 levels throughout the Paleozoic, in direct contradiction to competing models. In this study, we evaluate whether GEOCARBSULF can predict relatively low Paleozoic O2 levels. We first update GEOCARBSULF by adopting the recent compilation of the δ13C value of marine buried carbonate and replacing the old formulation of the sulfur isotope fractionation factor with empirical sulfur isotope records. Following this we construct various O2 evolution scenarios (with low O2 levels in the early Paleozoic) and examine whether GEOCARBSULF can reproduce these scenarios by varying the weathering/degassing fluxes of carbon and sulfur, or carbonate δ13C. We show that GEOCARBSULF can, in fact, maintain low-O2 (even 1–5% atm) levels through the early Paleozoic by only varying the carbonate δ13C within 2 standard deviation (SD) bounds permitted by the geological record. In addition, it can generate a middle–late Paleozoic rise in O2 concentration, coincident with the diversification of land plants. However, we also argue that tracking atmospheric O2 levels with GEOCARBSULF is highly dependent on carbonate carbon isotope evolution, and more accurate predictions will come from an improved C isotope record.