Calcium carbonate formation below the groundwater table in response to tree transpiration

Tree islands are biogeochemical hotspots that accumulate ions and nutrients in the groundwater and soil through root water uptake and evapotranspiration. These mechanisms lead to high ionic concentrations in tree island groundwater and potential supersaturation conditions with respect to calcite and aragonite. The precipitation of calcium carbonate minerals (caliche) within tree island soils may lead to cementation, providing greater soil stability. Although calcrete layers have been observed at depth in some tree island soils of the Florida Everglades, their method of formation has yet to be discerned. The main objective of this study was to determine if varying hydrologic conditions are conducive to calcium carbonate formation in Everglades tree islands. This investigation was conducted at the Loxahatchee Impoundment Landscape Assessment (LILA), a large physical model of Everglades' tree islands in Florida, USA. Eight tree islands were investigated across four mesocosms for nine years (2014–2023). For the first 4 years, all tree islands were exposed to similar hydrologic conditions as determined by an operational hydrograph with pre-determined seasonal variations in water levels. In the 5th year (2018 to present), the hydrograph was altered such that two mesocosms (containing 4 tree islands) were exposed to more a constrained range in water levels varying about 0.4 m (4.4–4.8 m NGVD29; constrained) on a seasonal basis, while the other two mesocosms (also containing 4 tree islands) continued to experience the larger range in seasonal water levels of about 0.6 m (4.4–5.1 m NGVD29; unconstrained). The geochemical characteristics of groundwater and soils in LILA tree islands were investigated under the two hydrologic regimes. A combination of groundwater saturation indices, soil profiles, scanning electron microscopy, and Electron Dispersive X-ray Spectroscopy were utilized to locate and identify calcium carbonate mineral formation. Groundwaters exhibited persistent supersaturated conditions with respect to calcium carbonate minerals within tree islands subjected to the larger variation in water levels and within tree islands that contained clay. Both calcite and aragonite were found in the tree island soils. Rhombohedral calcite, aragonite needles, and calcium carbonate globules, which may be composed of crystalline or amorphous calcium carbonate, were observed in soils collected 1 m below the mean groundwater table (0.40 m), where shell surfaces, other limestone grains and silica grains served as nucleation sites. The main processes responsible for the calcium carbonate formation include evapotranspiration and seasonal flooding which leads to an accumulation of ions in the groundwater to supersaturation conditions. However, soil composition which influences hydrologic properties and the ease or inhibition of groundwater – surface water interactions also played a role in sustaining supersaturation conditions. The results of this investigation indicate that calcium carbonate can precipitate within modern tree island soils and most likely form under a seasonal fluctuating hydrologic regime where the tree islands are flooded for a period in the wet season.