Carbon cycle driven critical zone evolution in a terrestrial carbonate system

Wednesday, 26 July 2017: 10:35 AM
Paul Brest West (Munger Conference Center)
Eron Raines1, Todd Osborne1, Sajad Jazayeri2 and Sarah Kruse2, (1)University of Florida, Soil and Water Science, St Augustine, FL, United States, (2)University of South Florida Tampa, Tampa, FL, United States
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.