Mechanistic models for rhizolith formation and their implications for paleoenvironmental reconstructions

Rhizoliths, cylindrical concretions formed primarily by CaCO3 accumulation around plant roots, serve as valuable indicators of past environmental conditions, including hydrology, redox dynamics, and carbon cycling. Despite growing interest in paleo-reconstructions, the lack of quantitative studies on formation mechanisms complicates interpretation. We present "RhizoCalc", the first mechanistic model (deployed in HYDRUS-1D) computing rhizolith formation in CaCO3-containing loess soils, integrating water fluxes, root water uptake, and (Ca)-carbonate chemistry to simulate conditions under which rhizoliths develop. Hydraulic fluxes drive Ca2+ transport (0.13-1 mmol/L) toward the rhizosphere, governed by root water uptake under low (ETo = 0.03 cm/d) and high (ETo = 1 cm/d) flow rates at optimal (ho = -100 cm) and intermediate (ho = -1000 cm) moisture conditions. The simulations show that hydraulic constraints and calcite-induced jamming of the porous medium are key inhibitors of rhizolith growth, distinguishing physical limitations from biogeochemical feedbacks in the rhizosphere. On top of this, our work reveals root encasement and reliquary varieties, linking their physical and biogeochemical mechanisms to rhizolith transformations and offering insights into paleosol hydrology and redox dynamics. Under intermediate soil-water conditions with 1 mmol/L Ca2+, concentric rhizoliths with 0.2-3 cm radii form chrono-sequentially over 1.5-150 years. Each layer preserves CaCO3 constituents (18O, 13C, 44Ca, 46Ca, 48Ca), root-derived biomarkers (e.g., lignin), and clumped isotopes (47), preserving environmental signatures across time into the future. Therefore, this framework conceptualizes each rhizolith as a 'time-capsule' with each successive CaCO3 layer encapsulating a snapshot of vital environmental proxies, providing a window into otherwise inaccessible historic ecosystem dynamics. Refining reconstructions of Earth's paleoclimatic history requires cross-sectional isolation of concentric layers in well-preserved rhizoliths, capturing distinct isotopic bands and their stratigraphy.