Carbon cycle driven critical zone evolution in a terrestrial carbonate system

Abstract:

Depressional wetlands are the dominant landform of the limestone geology at Big Cypress National Preserve (BICY) in South Florida. Microbial respiration in wetland sediments forces sediment CO₂ concentrations up to 184,000ppm. CO₂ entering the aqueous phase immediately converts to carbonic acid resulting in excess acidity in sediments and increased limestone dissolution rates. Hydro-geochemical modeling of historic landscape-scale data reveals limestone dissolution an order of magnitude greater than otherwise expected indicating that biologic acidification processes are a major driver for critical zone evolution. Connections between lithology and ecology were explored by measuring sediment chemistry and bedrock physics along transects through the ecological gradient spanning depressional wetland to upland. Sediment calcite (CaCO₃) concentrations varied by an order of magnitude concentrating where microbial communities known to bio-precipitate CaCO₃ thrive. The zones of lowest apparent conductivity (indicative of high-CaCO₃ limestone) occurred below the sediments with the highest CaCO₃ concentrations indicating a correlation between sediment CaCO₃ and limestone physics. This implies biologic CaCO₃ accretions, visible from space, lithify into high-CaCO₃ limestones. Surface water calcium (Ca²⁺) concentrations, a proxy for dissolved limestone, are negatively correlated with sediment CaCO₃ concentrations indicating that surface water may be a Ca²⁺ source for CaCO₃ bio-precipitation. We linked microbial sediment acidification to surface water Ca²⁺ through laboratory manipulations of natural decomposition processes. We found Ca²⁺ was retained in suspension through complexation with dissolved organic carbon (DOC). We determined photolytic degradation of Ca²⁺-DOC complexes does not result in precipitation of CaCO₃ implying Ca²⁺-DOC complexes are likely targeted by microbes in surface waters as a carbon source and CaCO₃ precipitates as a metabolic byproduct. This suggests hydrobiogeochemical carbon cycling plays a significant role on critical zone evolution in terrestrial carbonate systems. Our results are consistent with widespread microbe-driven landscape evolution processes induced through carbon cycling in surface water environments.