Iron Formations as Palaeoenvironmental Archives

Ancient iron formations - iron and silica-rich chemical sedimentary rocks that formed throughout the Precambrian eons - provide a significant part of the evidence for the modern scientific understanding of palaeoenvironmental conditions in Archaean (4.0–2.5 billion years ago) and Proterozoic (2.5–0.539 billion years ago) times. Despite controversies regarding their formation mechanisms, iron formations are a testament to the influence of the Precambrian biosphere on early ocean chemistry. As many iron formations are pure chemical sediments that reflect the composition of the waters from which they precipitated, they can also serve as nuanced geochemical archives for the study of ancient marine temperatures, redox states, and elemental cycling, if proper care is taken to understand their sedimentological context.

XRF and powder X-ray diffraction analysis of ancient iron slags from the “Sladak Kladenets” and “Malko Dryanovo” archaeological sites, Bulgaria

Using X-ray fluorescence and Powder X-ray diffraction analysis, the chemical and phase composition of ancient iron slags and raw iron ore were investigated. The type of raw ore was identified as self-fluxing. The operating furnace temperature was determined in the range 900–1000 °C. The results obtained are of archaeological importance. They will contribute to the chronological specification of the time of realization of the metallurgical process and the type of used furnaces.

Iron Production at Meroe

The archaeometallurgical remains evident at the Royal City of Meroe represent some of the most extensive evidence for ancient iron production in Africa. Recently, renewed investigations of these remains have generated a number of new insights into Kushite ferrous technology. The duration of iron production at Meroe is now known to span over one thousand years. Currently available data indicates a particularly intensive level of production during the early periods of Kush, while post-Meroitic iron production has been identified at Meroe and at the nearby site of Hamadab. Throughout the long history of iron production in the area, certain aspects of the technological process appear to have remained relatively constant, notably those associated with the acquisition and use of raw materials (iron ore and charcoal fuel). Other facets of the technological practice appear to have changed over time, for example the manufacture of technical ceramics and the design of the furnaces. Such levels of diachronic consistency and variability within technological practice potentially reflect the evolving social, political, and economic circumstances of Kush. Due to the socio-politically embedded nature of technology, interesting insights into broader Kushite history are becoming accessible through greater understandings of Meroe’s iron production remains.

Hydrothermal formation of iron-oxyhydroxide chimney mounds in a shallow semi-enclosed bay at Satsuma Iwo-Jima Island, Kagoshima, Japan

Hydrothermal iron-oxyhydroxide chimney mounds (iron mounds) have been discovered in a fishing port in Nagahama Bay, located on the southwest coast of Satsuma Iwo-Jima Island, south of Kyushu Island, Japan. In the fishing port, uncovered ∼1.0-m-high iron mounds in shallow waters formed under relatively calm conditions. Typically, the fishing port has orange-colored turbid waters that mix with outer ocean waters during high tide. Colloidal iron-oxyhydroxides form due to the oxidation of ferrous iron in hydrothermal waters (pH = 5.5; temperature = 55 °C) as they mix with seawater. The mounds are made of two types of material: hard, dark brown–orange, high-density material; and soft, brownish orange–yellow, low-density material. Computed tomography scans of the harder iron mound material revealed a cabbage-like structure consisting of micropipe structures with diameters of 2–5 mm. These micropipes have relatively hard walls made of iron oxyhydroxides (FeOH) and are identified as discharge pipes. Nucleic acid staining genetic sequencing and scanning electron microscope observations suggest that the mounds formed mainly from bacterial stalks with high concentrations of FeOH colloidal matter. In the harder parts of the mounds, these “fat stalks,” which contain oxyhydroxide colloidal aggregates, are entwined and concentrated. The softer material contains twisted stalk-like structures, which are coated with FeOH colloidal particles. Deoxyribonucleic acid (DNA) examination of the iron mounds revealed the presence of iron-oxidizing bacteria, especially at the mound surface. We estimate that the iron mounds accumulated at a rate of ∼1700 tons/1000 m2/yr. This is an order of magnitude higher than the rate of FeOH sedimentation via chemical precipitation of FeOH colloids within the fishing port. This suggests that biogenic activity, resulting in the production of entwined FeOH stalks, leads to the rapid accumulation of FeOH beds and that biogenic activity within the water mass rich in FeOH colloids is an efficient means of generating thick iron-rich sedimentary sequences. As such, we propose that some ancient iron formations may have also formed through the biogenic production of FeOH stalks rather than solely through chemical sedimentation in a water mass rich in FeOH colloids. It appears that these rapidly forming biogenic FeOH iron mounds, distributed over a wide area of ocean floor, are also relatively protected from erosion and diagenetic alteration (reduction). Previous studies have reported that ancient iron formations were commonly deposited in deeper environments via direct iron oxidation from the water column in a ferruginous ocean. However, there are several hydrothermal vent inflows preserved with FeOH that would have formed appropriate redox boundary conditions in an otherwise anoxic ocean. Under these conditions, iron mound mat-type sedimentary deposits might have formed and been well preserved and affected by early diagenesis where higher heat flow occurred in the Archean ocean. The FeOH mounds in Nagahama Bay provide an example of the iron formation sedimentary environment and important information for estimating the past depositional state of iron formations.