IRON OXIDE CEMENTED NODULES IN THE CRITICAL ZONE: PISOLITHS TO CONCRETIONS

Cemented nodules in the geologic record are remarkably common and comprise important proxies to understand the syndepositional to post-depositional history of fluid flow, mass transfer, and element cycling in Earth’s critical zone. Two end members of spheroidal, iron oxide nodule occurrences are pisoliths and concretions. Out of context, the two types of nodules can look remarkably similar based on the morphology or mineralogy, yet the geologic formation processes can be very different. Thus, the context of the in situ distribution and relation to the host rock lithology and sedimentology is extremely important. Significant controls in pisolith formation are early syngenetic to diagenetic timing of dominantly leaching fluids that mobilize chemical elements, under gravity-driven vadose water conditions. Biological processes may play an important role in pisolith formation. These conditions give rise to random distributions of pisoliths in shallow subsurface soil-related environments. In contrast, formation of concretions can be early (e.g., pyrite with later oxidation) to late diagenetic processes where bleaching fluids mobilize iron with re-precipitation of iron phases, under dominantly diffusion controlled phreatic water conditions. This gives rise to nucleation centers expressed by the self-organized populations in burial conditions.

In both cases, the nucleation phenomena and kinetics are still poorly understood, and the histories can be complex and overprinted by multiple waters and mineralogies. However, these nodules and their formation histories have applications as exploration tools for mineral resources (pisoliths) or migration pathways of hydrocarbon reducing fluids (concretions). Nodules in the critical zone also have analog application to understanding diagenesis in sedimentary deposits of Mars where iron rich waters and records of spherules are recognized at multiple sites.