Baltic Sea ferromanganese concretion growth rates and conditions

The transition to renewable energy and the acceleration of technology demand vast amounts of hi-tech metals that are critical in green energy technology. Due to increasing demand for hi-tech metals, rising interest in mining from more unconventional sources, such as the seafloor, is inevitable. Ferromanganese concretions, which are centimeter-scale accumulations of iron and manganese oxides, are common in the Baltic Sea. In addition to iron and manganese, concretions contain hi-tech metals, such as cobalt. The Fe-Mn concretions are important reaction surfaces for diverse microbial communities, and they regulate metal and nutrient cycling in the concretions. Extraction of Fe-Mn concretions from the Baltic Sea could impact the seafloor ecosystem, biogeochemical cycling of elements, concretion growth, and recovery. This study provides information on Baltic Sea Fe-Mn concretion growth rates and conditions in laboratory experiments.

The ferromanganese concretions were collected from the Baltic Sea during May and June 2022 for a 12-week laboratory incubation and metal tracer experiments. Triplicate concretion samples and one abiotic control sample were collected into bottles containing artificial brackish seawater and incubated in the dark at +5 °C in an orbital shaker at 100 rpm to imitate seafloor conditions. Bottles were sampled at the beginning and the end of the 12-week incubation experiment. We assessed the concretion growth with X-ray computed tomography and freshly formed concretion material with a scanning electron microscope. We analyzed the headspace methane concentrations and pH of the incubation solution. We measured phosphorus and metal (Mn, Fe, Co, V, Ni, Zn, Mo) concentrations of the incubation solution with triple quadrupole ICP-MS.

The results provide new information on the growth rates and conditions of Fe-Mn concretions. It was confirmed that concretions grew in laboratory conditions, and new growth was as much as 10 µm in 12 weeks. Headspace methane concentrations decreased in all samples during incubation, but least in abiotic controls, where the microbial activity was eliminated. The microbes living on the surface of concretions utilized methane, indicating that concretions have methanotrophic communities. Incubation solutions’ metal analysis showed that metal concentrations increased more in the abiotic controls than in biotic triplicates after a 12-week incubation, thus metals dissolved from concretions into the incubation solution faster without the activity of microbial communities. We suggest that microbes occupying the concretions have an important role in the concretions’ growth and the factors affecting the accumulation and release processes of metals.

This work was supported by the Finnish Natural Resources Research Foundation and the Research Council of Finland (Fermaid project, grant 332249).