Modeling the impact of hydraulic and biogeochemical feedback mechanisms on the morphodynamics of channels incising carbonates

Abstract:

Within bedrock channels incising carbonates and other highly soluble rocks chemical dissolution processes can compose a substantial percentage of total incision. It is unclear whether chemical erosion processes impact the production and migration of knickpoints and, consequently, the relationship between climate, tectonics, and equilibrium channel form. Field observations within cave streams and other channels incising carbonates have demonstrated a variety of internal feedback mechanisms that can control patterns of calcite dissolution. In particular, prior studies have shown that: 1) hydraulic and biogeochemical feedbacks impact dissolved CO₂ concentrations and produce contrasts in dissolution rates along a channel, whereby channel steepening produces downstream reductions in CO₂ due to both increased gas exchange and reduced CO₂ production within coarser sediment; 2) the presence or absence of ventilation within a cave channel can strongly impact the average dissolution rates; and 3) the timing and spatial patterns of cave ventilation can control dissolution rates. However, the long-term implications of these feedback mechanisms on channel morphology remain unclear. We develop a model of channel profile and cross-section evolution that incorporates processes of CO₂ production, gas exchange, and ventilation. We show that equilibrium channel width and slope are both impacted by feedbacks between channel morphology, gas exchange, and CO₂ production. These feedbacks can produce zones of channel steepening and narrowing that are purely chemical in origin. Sudden shifts in cave channel morphology can occur with the onset or termination of cave ventilation. While carbonate channels in nature will evolve by a combination of chemical and mechanical processes, our model suggests that internal feedback mechanisms that impact rates of chemical erosion can drive channels away from standard steady-state morphologies.