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Biologic Origin of Iron Nodules in a Marine Terrace Chronosequence, Santa Cruz, California
Author(s) -
Schulz Marjorie S.,
Vivit Davison,
Schulz Charles,
Fitzpatrick John,
White Art
Publication year - 2010
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2009.0144
Subject(s) - chronosequence , hematite , goethite , concretion , terrace (agriculture) , bioturbation , maghemite , iron oxide , mineralogy , geology , soil water , geochemistry , chemistry , sediment , paleontology , soil science , archaeology , history , organic chemistry , adsorption
The distribution, chemistry, and morphology of Fe nodules were studied in a marine terrace soil chronosequence northwest of Santa Cruz, California. The Fe nodules are found at depths <1 m on all terraces. The nodules consisted of soil mineral grains cemented by Fe oxides. The nodules varied in size from 0.5 to 25 mm in diameter. Nodules did not occur in the underlying regolith. The Fe‐oxide mineralogy of the nodules was typically goethite; however, a subset of nodules consisted of maghemite. There was a slight transformation to hematite with time. The abundance of soil Fe nodules increased with terrace age on the five terraces studied (aged 65,000–226,000 yr). Scanning electron microscopy (SEM) revealed Fe‐oxide‐containing fungal hyphae throughout the nodules, including organic structures incorporating fine‐grained Fe oxides. The fine‐grained nature of the Fe oxides was substantiated by Mössbauer spectroscopy. Our microscopic observations led to the hypothesis that the nodules in the Santa Cruz terrace soils are precipitated by fungi, perhaps as a strategy to sequester primary mineral grains for nutrient extraction. The fungal structures are fixed by the seasonal wetting and dry cycles and rounded through bioturbation. The organic structures are compacted by the degradation of fungal C with time.