Genesis of iron-manganese nodules in red paleosol on the island of Susak : master’s thesis

Fe/Mn-nodule uočene su u sloju crvenog paleotla (OIS 7), članu debele sekvence les-paleotlo na otoku Susku. Istraživanje nodula je provedeno kako bi se utvrdila njihova morfologija, sastav, te mehanizam postanka s ciljem utvrđivanja važnosti redoksimorfnih procesa u crvenom paleotlu. Nakon detaljne mikromorfološke analize primjenom polarizacijskog mikroskopa, izdvojene nodule analizirane su primjenom pretražne elektronske mikroskopije (FESEM) s EDXS-om i rendgenske difrakcije na prahu (XRD). Analizirane Fe/Mn–nodule mogu se podijeliti u tri skupine: 1) crne sjajne nodule, 2) crveno smeđe nodule, 3) konkrecije. Uz Fe/Mn–nodule utvrđena su i magnetična zrna. Nodule su ortične i disortične vrste, a opaženi tipovi nodula su: impregnacijski, tipični, koncentrični, agregatni (dendritični), te mješoviti tip koji pokazuje karakteristike više tipova nodula. Rendgenska difrakcija na prahu pokazala je kako su dominantne mineralne faze u crno sjajnim i crveno smeđim nodulama: goethit i hematit, a sporedne mineralne faze su: ferihidrit, maghemit i birnesit. Fe/Mn– oksidi u konkrecijama se nisu mogli pouzdano utvrditi primjenom rendgenske difrakcije na prahu zbog njihovog niskog udjela i slabog stupnja kristaliniteta. Slabi stupanj kristaliniteta Fe/Mn– oksida u konkrecijama indikator je toga da su one mlađe od crveno smeđih i crno sjajnih nodula, kojima je rendgenskom difrakcijom utvrđen viši stupanj kristaliniteta. Konkrecije su najbrojniji tip nodula u tlu, što ukazuje na to da je ciklička izmjena oksidativnih i reduktivnih Eh/pH uvjeta tijekom i poslije formiranja tla, uslijed fluktuacije razine podzemne vode, dominantni mehanizam postanka nodula u istraženom paleotlu Magnetična zrna nisu in-situ postanka, te su vjerojatno naslijeđena iz lesnog supstrata iz kojeg je većim dijelom nastalo paleotlo.Fe/Mn–nodules had been observed in a layer of red paleosol (OIS 7), a member of thick loess–paleosol sequence on the island of Susak. Investigation of the nodules was done by studying their morphology, content, and mechanisms of formation in order to determine the significance od redoximorfic processes in the host paleosol layer. After detailed micromorphological analysis with polarizing microscope, nodules were analyzed by the means of scanning electron microscopy (FESEM) with EDXS probe and X–ray diffraction, and have been divided in three categories: 1) black shiny nodules, 2) red brown nodules, 3) concretions. Beside the three categories of nodules, presence of magnetic mineral grains was also noted. Observed nodules were of orthic and disorthic origin. Based on their internal fabric, the of types of nodules were recognized: impregnative, concentric, agregate (dendritic), and mixed type that shows the properties of several previously stated types. X–ray diffraction showed that the main phases in black shiny and red brown nodules are goethite and hematite, and secondary phases are ferrihydrite, maghemite and birnessite. Fe/Mn–oxides that build concretions could not be determined by X–ray diffraction because of their low content and cristallinity level. The low cristallinity level of concretions is a sign of their younger age of formation compared to black shiny nodules and red brown nodules, which have higher cristallinity level. Concretions are the most numerous type of nodules in the paleosol, which is an indication that cyclic alternations of oxidative and reductive Eh/pH conditions, caused by fluctuation of underground water level in the paleosol during wet and dry periods, were dominant cause of nodule formation. Magnetic mineral grains haven't formed in–situ, but were probably inherited from the loess substrate from which the paleosol formed