The carbon isotope composition of organic matter occluded in iron nodules

This study presents δ¹³C and ¹⁴C results for soil organic carbon and carbon occluded by iron nodules from a Quaternary soil profile developed on basalt in western Victoria, Australia. The results suggest that the δ¹³C-value of organic matter in the iron nodules is directly inherited from the surrounding soil profile without isotopic fractionation, and that therefore the δ¹³C-value of organic matter occluded by the iron nodules can be related to the vegetation present during nodule formation. However, ¹⁴C results suggest that iron nodules are not closed systems with respect to organic carbon, and that even chemically resistant immobile particulate carbon (of probable microbial origin) has been added to the nodule carbon pool since formation. Nevertheless, the turnover time for chemically resistant particulate carbon in the iron nodules is at least three times that for the same fraction in the surrounding soil, making iron nodules useful repositories of palaeoenvironmental information, particularly where they have been removed from the active surface soil layer by burial.