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Biogeochemistry

A 2.5 billion year old story about iron in the ocean, told by a rock

Rasmussen, B., B. Krapez, J.R. Muhling, and A. Suvorova (2015) Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age. GEOLOGY, v. 43(4), pp.  303-306, doi: 10.1130/G36309.1

Introduction

The Precambrian ocean had very little oxygen compared to the modern ocean.  It is thought to have been iron rich, or ‘ferruginous’, and to have had more silica than the modern ocean. Since about 2.45 billion years ago the oceans have had an increase in dissolved oxygen concentration and excess iron has been progressively removed through processes like oxidation and sulfidation.

Constraints on the chemistry of early seawater are defined by information scientists draw from reactive elements, like iron, that are linked to major biogeochemcial cycles and trace metals. The details are preserved in the geologic record as sedimentary rocks with concentrated layers of iron (usually they are orange/red) called banded iron foramations (BIFs) (figure 1).

Figure 1: banded iron formation. Source: http://www.australianminesatlas.gov.au/education/down_under/iron/formed.html

Figure 1: banded iron formation. Source: http://www.australianminesatlas.gov.au/education/down_under/iron/formed.html

In the early ocean, iron entered and existed via natural processes. The source of iron to the ocean is hypothesized to have been basaltic volcanism from which seawater leached iron, resulting in the accumulation of Fe2+ (reactive ferrous iron) in the deep ocean. The primary sink, or loss, of reactive iron is hypothesized to have been ferric oxyhydroxides minerals precipitated by photochemical and biological reactions. Over time ferric oxyhydroxides become dehydrated and are preserved as iron silicates within BIFs and cherts. Iron silicates, however, have been demonstrated, at modern vent systems and in the laboratory, to form other ways too. An alternative explanation for their occurrence is the precipitation of iron silicates from the water column from the rapid reaction of Fe2+ and silica under reducing conditions. Researchers from Australia used their ‘local’ resources to investigate this conundrum over the origin of iron silicates further.

The formation of ferrous silicates rather than ferric oxyhydroxides has implications for the current interpretations of early seawater chemistry, nutrient cycling and trace metal distribution in the early ocean. In the modern ocean, for example, it is important to understand the availabilty of reactive iron because it is linked closely to, and has the abiltiy to limit and enhance, the cycling of major and minor nutrients and trace metals.  Reactive iron availabilty in the early ocean is important to accurately reconstruct for these same reasons, but also to make better understood inferences about the distribution of silica, the timing of enhanced volcanism, and changes in sea level.   Also, if scientists assume that iron was lost to iron hydroxides produced through biologic reactions and light mediated chemical reactions, when it was actually was lost through some other process, than there could be misinterpretations of what biological phenomena were relevant.

Methods

Figure 2: sample and core locations

Figure 2: sample and core locations from Hamersley group on Hamersley Province, Australia

To investigate the origin of early ocean iron silicates more thoroughly, researchers recovered core samples from four sites in the Hamersley group on Hamersley Province, Australia (figure 2); which are aged between 2.63-2.45 billion years old and contain laminated chert beds and BIFs.    It is difficult to simply look at the rocks with a hand lense and draw credible conclusions about their history because metamorphism has distorted and disguised original textures and mineralogic compositions, so careful and strenuous analyses must be completed. Recovered samples were analyzed with a variety of techniques including: high resolution optical and electron microscopy, microanalysis, energy dispersive x-ray spectometry (EDS), and x-ray defraction (XRD).

Results

Figure 3: Micro-images of thin laminated chert beds.

Figure 3: Micro-images of thin laminated chert beds.

Careful investigation lead to the observations that the laminated chert beds are variable in thickness (1-10’s of cm) and the laminae themselves are wavy and parallel and range from .2-2mm (figure 3).  They identified a variety of mineralogic textures including interlocked microcrystaline quartz grains, and elogated, randomly oriented, and variably sized iron oxides and iron silicates within the laminations. In some instances they observed that thick chert laminations graded into thin iron-rich laminations and evidence of density gradients where flocules of mud and nanoparticles where preserved. Researchers speculate that the abundance and shape of nanoparticles have been changed by metamorphism.

Discussion

The observations can be explained by early silica cementation of hyrothermal mud, or in other words, the sediments were solidified in place before being squished.   The appearance of nanoparticles that are ‘floating’ is indicative that the orginal sediment was silicafied before significant compaction, suggesting the cementation occurred during depositional hiatuses. The thin, iron-rich layers formed when cementation was unable to happen before being compacted by deposition.

These conclusions are supported by a number of arguments. First, hydrothermal mud preserved by silica cementation in the modern ocean has similar mineral composition, including an Eu anamoly, and textures as the Precambrian samples. Second, the settling of the nanosilicates, likely covered in absorbed silica, would have prompted cementation on or immediately below the sea floor. Lastly, it can be argued logically, by comparing the modern ocean to the early ocean, that iron silicates would have precipated from the water column. In the modern ocean, iron that enters the water column reacts with oxygen, hydrogen sulfides, and silica to form iron-bearing sulfide, oxide,and silicate minerals. In the old ocean though, there was more volcanism, and a lack of hydrogen sulfide and oxygen sinks, which potentially could have fascillitated the diffusion of the iron off the ridges and further into the oceans. Once enough Fe2+ and silica were released, the precipitation of iron silicates could have occurred spontaneously out of the water column, and settled to the sea floor.

Researchers hypothesize that cementation on the continental margin were episodic events. The conditions would have to be increased mafic volcanism and hydrothermal activity, to provide Fe2+, and high sea level, for limited disturbance from detrial sediment. They believe that the thickly laminated chert beds formed during depositional hiatuses when silica cementation occurred. The thin, iron-rich beds formed when deposition occurred faster than cementation, resulting in compaction.

Importance

Does this new depositional model for BIFs suggest there was an iron age in the Precambrian? If this research yields true it implies that iron silicate precipitation would be an important control on sea water chemistry before the rise of atmospheric oxygen. It just might be true too; there is evidence from the Marble Bar Chert, Pilbara craton, Australia that iron silicates could have precipitated from the water column as long as 3.64 billion years ago.

It is evident that, in light of this research, conclusions about ocean chemistry and biological processes on the early Earth drawn from the idea of ferric (Fe3+) oxyhydroxides may need to be adjusted to account for the possibility of the precipitation of ferrous silicates in seawater.

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