Marina Lebedeva

Weathering and erosion of fractured bedrock systems

We explore the contribution of fractures (joints) in controlling the rate of weathering advance for a low-porosity rock by using methods of homogenization to create averaged weathering equations. The rate of advance of the weathering front can be expressed as the same rate observed in non-fractured media (or in an individual block) divided by the volume fraction of non-fractured blocks in the fractured parent material. In the model, the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material. The low-porosity rock weathers by reaction-transport processes. As observed in field systems, the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non-fractured system because the volume fraction of blocks is < 1. The increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density, and to the increase in exposure of surface of low-porosity rock to reaction-transport. For constant fracture aperture, the weathering advance rate increases when the fracture spacing decreases. Equations describing weathering advance rate are summarized in the ‘List of selected equations’. If erosion is imposed at a constant rate, the weathering systems with fracture-bounded bedrock blocks attain a steady state. In the erosional transport-limited regime, bedrock blocks no longer emerge at the air-regolith boundary because they weather away. In the weathering-limited (or kinetic) regime, blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate. For convex hillslopes, the block size exposed at the surface increases downslope. This model can explain observations of exhumed rocks weathering in the Luquillo mountains of Puerto Rico. Published 2017. This article is a U.S. Government work and is in the public domain in the USA

Metamorphism of Marbles: Role of Feedbacks between Reaction, Fluid Flow, Pore Pressure and Creep

In many applications of chemical transport modelling to geological problems, it is very important to take into account the changes to the transport properties of the porous medium that will result from chemical reactions driven by the component fluxes which are being modelled. This is particularly true where the reactions involve breakdowr of carbonate minerals, because they produce very large changes in solid volume, but there are many other fluid-rock reactions, involving both precipitation and dissolution, that are capable of perturbing the pattern of flow that originally triggered the reaction. This paper is concerned with the growth of calc-silicate minerals replacing marbles in metamorphism, which we model through the simplest possible metamorphic reaction:

Learning to Read the Chemistry of Regolith to Understand the Critical Zone

Cal + Qtz \rightleftharpoons Wo + C{O_2}

A mathematical model for steady‐state regolith production at constant erosion rate

(17.1) However, our approach is equally appliable a wide range of skarn-forming reactions.

The effect of curvature on weathering rind formation: Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe

In this paper we clarify that our weathering model from 2013 did not explicitly describe weathering of soil moving downhill along hillslopes. In addition, we re-analyze the role of the term that we neglected that describes loss of regolith mass through mineral dissolution. We derive an equation for this term by including lateral flow of water inside the model hill. For the revised hill model, we define a dimensionless parameter that allows estimation of the effect of lateral flow on the steady-state hillslope. This parameter is equal to the ratio of averaged advective flux of dissolved species out of the hill to the rate of total denudation. The parameter also yields a criterion for the existence of a steady-state regolith thickness for systems experiencing unidirectional advection at a constant velocity: for a ridge, the rate of downward flow of water (qy) must be less than the rate of upward movement of rock (E) after normalization by a small parameter, α. This parameter is equal to the equilibrium aqueous concentration divided by the concentration of the reacting mineral in the rock. Alternatively, a steady-state may exist for the case of both vertical and lateral flow in a hill for any value of erosion rate if the Darcy velocities decrease with depth. Subsurface flow systems play an essential role in the existence of both steady-state hillslopes and steady-state regolith thicknesses. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.