From formation to burial: experimental reconstruction of earthworm calcite granule modification

Earthworm-produced calcite granules are a distinct component of pedogenic carbonate that can be preserved in palaeosols, yet the processes that generate their carbonate chemistry and microstructure, as well as the extent to which these attributes are modified after excretion and during early diagenesis, remain poorly constrained. This limits confident identification in the rock record and interpretation of granule-derived carbonate signals within palaeosols and mixed siliciclastic–carbonate successions.

This project uses a staged experimental design to reconstruct the early diagenetic changes of Lumbricus terrestris calcite granules under controlled conditions. In Phase 1, granules are generated in replicated soil microcosms under independently varied organic carbon inputs (C₃ versus C₄ litter) and hydrological regimes (constant moisture versus dry–rewet cycling), producing material with known formation histories. In Phase 2, harvested granules are transferred to continuous-flow reactors supplied with defined Ca–HCO₃ solutions, with and without soil, to isolate post-excretion pedogenic modification processes including carbonate overgrowth, dissolution–reprecipitation, and trace-element redistribution. In Phase 3, granules are heated at 25–60 °C in controlled pore-fluid compositions to simulate shallow burial and low-grade early diagenetic conditions.

A consistent, multi-scale analytical framework is applied to granules recovered from each experimental stage, integrating isotopic, elemental, and microstructural information to assess equilibrium versus disequilibrium precipitation and progressive signal modification. Results obtained to date will be presented and evaluated alongside fossil granules from Miocene palaeosols, providing process-based constraints on the biogenic, pedogenic, and diagenetic contributions to soil-derived carbonates and their implications for carbonate-based paleoenvironmental proxies.