Ecohydrologic processes and soil thickness feedbacks control limestone-weathering rates in a karst landscape

Abstract

A B S T R A C TChemical weathering of bedrock plays an essential role in the formation and evolution of Earth's critical zone. Over geologic time, the negative feedback between temperature and chemical weathering rates contributes to the regulation of Earth climate. The challenge of understanding weathering rates and the resulting evolution of critical zone structures lies in complicated interactions and feedbacks among environmental variables, local ecohydrologic processes, and soil thickness, the relative importance of which remains unresolved. We investigate these interactions using a reactive-transport kinetics model, focusing on a low-relief, wetland-dominated karst landscape (Big Cypress National Preserve, South Florida, USA) as a case study. Across a broad range of environmental variables, model simulations highlight primary controls of climate and soil biological respiration, where soil thickness both supplies and limits transport of biologically derived acidity. Consequently, the weathering rate maximum occurs at intermediate soil thickness. The value of the maximum weathering rate and the precise soil thickness at which it occurs depend on several environmental variables, including precipitation regime, soil inundation, vegetation characteristics, and rate of groundwater drainage. Simulations for environmental conditions specific to Big Cypress suggest that wetland depressions in this landscape began to form around beginning of the Holocene with gradual dissolution of limestone bedrock and attendant soil development, highlighting large influence of age-varying soil thickness on weathering rates and consequent landscape development. While climatic variables are often considered most important for chemical weathering, our results indicate that soil thickness and biotic activity are equally important. Weathering rates reflect complex interactions among soil thickness, climate, and local hydrologic and biotic processes, which jointly shape the supply and delivery of chemical reactants, and the resulting trajectories of critical zone and karst landscape development.

Initiation and Development of Wetlands in Southern Florida Karst Landscape Associated With Accumulation of Organic Matter and Vegetation Evolution

“…BP (Table ). Similar mid‐Holocene ages for initiation of the formation of depressions were found based on calcium and phosphorus mass balance estimates in BICY (Chamberlin et al, ) and theoretical estimates of the time necessary to dissolve bedrock under wetlands to a depth of 2 m (Dong et al, ). McDowell et al () reported that peats were deposited right above the limestone in the northern Everglades at ~4,280 BP.…”

“…Exposure at the edge sites would also enhance ventilation of CO 2 produced through remineralization processes. Conversely, much of the CO 2 produced in the centers would be retained and enhance dissolution of the bedrock (Dong et al, ).…”

“…We sought to test hypotheses based on an assumption, evaluated in the project, that LCFAs provide better 14 C age record of wetland development than other organic carbon (OC) sources. Assuming this assumption is correct, we test the hypotheses that (1) landscape patterning developed in the middle to late Holocene, as suggested by Dong et al () and Chamberlin et al (); (2) vegetation changes in the historical landscape were driven by precipitation and local hydrological conditions; (3) soil OM in the center of the domes plays a more important role in bedrock weathering because prolonged inundation reduces CO 2 ventilation and increases carbonic acid production at the water‐bedrock interface compared with the edges.…”

“…The BICY landscape pattern consists of regularly spaced and uniformly sized depressional wetlands, termed “cypress domes,” embedded within a mosaic of pine‐dominated uplands (Figure ). The origins of the observed self‐organizing patterning are not well understood (Dong et al, ; Watts et al, ) but likely relate to interactions between OM burial, remineralization processes creating carbonic acid, and carbonate mineral dissolution and precipitation. Key uncertainties include the age of the extant landforms (e.g., Chamberlin et al, ) and the internal OM dynamics that are thought to control formation of the patterning.…”

Scale‐Dependent Patterning of Wetland Depressions in a Low‐Relief Karst Landscape

“…Wetland depressions across BICY are focal points for organic matter accumulation because of prolonged inundation critical water storage in the dry season. Prolonged inundation and enhanced primary production in the wetland depressions are thought to create conditions favorable for local production of aqueous CO 2 that fuels dissolution of the underlying carbonate bedrock (Dong et al, ). Indeed, the regular arrangement of cypress domes is visually compelling, with domes repeating across the landscape.…”