Internal structure and geochemical properties of spheroidal ferromanganese concretions of the Baltic Sea

Research output: ThesisMaster's thesis

Abstract

Ferromanganese concretions or nodules are recent biogeochemical sedimentary formations present in abundant amounts at the fringes of deep basins in the area of the Baltic Sea. The concretions have a distinctive concentric structure, consisting of alternating iron and manganese-rich layers. The iron layers typically host phosphorus, and the concretions also work as traps for many other compounds such as heavy metals and REEs. In general, their dissolution occurs in anoxic conditions whereas precipitation in an oxic environment.

Structural and geochemical analyses were performed to understand the growth and evolution of spheroidal ferromanganese concretions from the Gulf of Finland. Porosity, layer structure and the chemical composition of the concretions were assessed using CT-scanning, EDS, XRF, and LOI analyses. Dissolution experiment in anoxic conditions was also implemented by incubating the deoxygenated samples stored in Anaerogen containers.

The analyses showed that Mn layers host a vast pore network. A qualitative identification of the Fe/Mn ratio of layers with different densities was achieved by CT scanning coupled with EDS element mapping. Diffuse structures and the presence of microbial structures in Mn-rich layers could be affiliated with metal oxide susceptibility to redox reactions mediated by microbial processes. On the other hand, Fe-rich layers are more coherent and dense than the Mn-rich layers. Computed tomography analysis also showed that the pore networks extend from the surface of the concretions to the core. The calculated surface area of the pore network is 2 to 5 times greater than the exterior surface area of the concretions. The large surface area is believed to provide a suitable living environment for Fe and Mn reducing and oxidizing microbes. Both of the metals are susceptible to reductive conditions, Fe seemingly being more vulnerable to dissolution with the presence of a carbon source and sulphate. While oxygen is present, Fe (oxyhydr)oxides are effectively formed. In addition, substantial amounts of P was released during the dissolution experiment which may have importance in the Gulf of Finland.
Translated title of the contributionItämeren pallomaisten rautamangaanikonkreetioiden rakenne ja geokemialliset ominaisuudet
Original languageEnglish
Place of PublicationHelsinki
Publisher
Publication statusPublished - 28 Sep 2015
MoE publication typeG2 Master's thesis, polytechnic Master's thesis

Fields of Science

  • 1171 Geosciences

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