Iron rich soil nodules occur in many soils and are generally thought to form by chemical processes. However the current study of nodules in soils from a California marine terrace chronosequence revealed morphological features suggesting microbial (fungal) precipitation of iron minerals. Soils on several coastal marine terraces (T1 – T5) were analyzed for nodule abundance, chemistry, mineralogy, and morphology. Nodules were only found in the top meter of the terrace soils. The average weight percent of nodules generally increased with soil age; from T1 (65Ka)-1.4 wt %, to T5 (226Ka)-2.4 wt%. The average iron content of nodules increased with the age of the soil and those nodules which were magnetic have higher Fe2O3 content (37 to 42 wt %) than the non-magnetic nodules (23 to 31 wt %). Iron mineralogy of the nodules was predominantly goethite while a subset of nodules were maghemite.

Nodules consisted of primary mineral grains cemented by iron oxides, clay minerals and fungal hyphae and exhibited no concentric layering. Nodules examined by SEM were found to contain areas of high iron content that include spherical vesicles or coccoidal features approximately 5 microns in diameter. The vesicles were lined with fungal hyphae which also contained or were replaced by iron. The areas of high iron tended to wrap around primary mineral grains, and fungal tunneling of mineral grains was common, especially in the nodules from older terraces. Fungal hyphae occupying tunnels within primary mineral grains also had elevated iron content. This observation supports the hypothesis that the iron is actively precipitated by the fungi. Many thin sectioned nodules contain vesicles in oval structures which have been tentatively correlated to micro-colonial fungi (MCF). Previously, MCF have only been associated with desert varnish.