Influence of biotic vs abiotic processes on the genesis and REE distribution of non-marine carbonates

Terrestrial carbonates and particularly spring carbonates are deemed to be good geological repositories of paleoclimate and paleoenvironmental reconstruction (e.g., Brasier, 2011) and for the characterisation of the interaction of biotic and abiotic processes, and they preserve evidence of the velocity of the flow and the chemical composition of the spring water. This study focuses on non-marine carbonates from fossil and active springs from the Bongongo and Ngol areas along the Cameroon Volcanic Line in South-West Cameroon (Bisse et al., 2018). Here, hydrothermal fluids reach the surface giving rise to small thermal springs, with temperatures between 31°C and 49°C, and streams creating waterfalls, terracettes, and barrage carbonate deposits (Bisse et al., 2018). Petrographic analyses of these carbonates revealed that they are made up of stacked laminae of fibrous coarse crystals of low-Mg calcite and laminae of alternate microsparite and micrite. The fibrous coarsely crystalline calcite, often with feather-like fabric, grows from thin layers of micrite and peloids. Filaments of putative microbial origin are preserved within this peloidal micrite. The laminated microsparite and micrite microfacies are characterised by an intricate mesh of hollow filaments of microbial origin. The long feather-like crystals of calcite formed in fast-flowing water where the enhanced CO2 degassing has favoured the precipitation of CaCO3. The laminated micrite and microsparite, on the other hand, are likely formed in ponds where degassing and CO2 removal was lower and the calcite precipitation was fostered by microbial activity. The fast-forming carbonates show higher Ce contents and very low total rare earth elements, revealing a preferential uptake of Ce with respect to other rare earth elements. This would explain the positive or null Ce anomaly in continental spring carbonates elsewhere. The geochemical composition of these carbonates can be used as proxy for the characterisation of fluid/rock interactions between the groundwater and the substratum and for the characterisation of the sources of calcium and other elements that constitute tufa and travertines. The samples from Ngol are characterised by light rare earth element enrichment while those from Bongongo are overall enriched in heavy rare earth elements. Carbonates from both localities have a strong positive Eu anomaly (>4), suggesting a contribution from deep-seated, hydrothermal, crustal fluids in contact with volcanic rocks and the breakdown of plagioclase from the Cameroon Volcanic Line alkali basalts. This study has demonstrated that the strong CO2 degassing, promoted by fast flowing water, resulted in the rapid growth of coarsely crystalline calcite, characterised by overall ∑REE depletion and Ce enrichment. The fast-forming calcite fabrics are therefore characterised by a higher Ce anomaly when compared to the slow forming fine grained, laminated fabrics. It can therefore be concluded that Ce(III) oxidation and scavenging into the coarse carbonates is promoted by fast flowing water and strong CO2 degassing (turbulence). This is an important finding for the interpretation of REE distribution in continental carbonates and provides an alternative explanation for the presence of a positive Ce-anomaly that favours precipitation rates over organic matter contents.