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Throwback Thursday: Looking at a modern lake to study ancient ocean chemistry

The article: Koeksoy A. Sundman J.M. Byrne  R. Lohmayer  B. Planer-Friedrich  I. Halevy  K.O. Konhauser  A. Kappler. Formation of green rust and elemental sulfur in an analogue for oxygenated ferro-euxinic transition zones of Precambrian oceans. Geology (2019) 47 (3): 211-214. https://doi.org/10.1130/G45501.1


The Pre-Cambrian

Today we live in the Holocene, or perhaps the Anthropocene.  Our climate suits human needs, and most places in the ocean are oxygenated, meaning there is oxygen dissolved in the water. This allows water-dwelling organisms to breathe and many bacteria to breakdown waste as it sinks to the bottom of the ocean.

The oceans have not always been oxygenated, though. During the Pre-Cambrian, the time on Earth from its beginning 4.6 billion years ago to about 541 million years ago, much of the ocean was not oxygenated. The exact timing of the oxygenation of the oceans is debated, and it did not happen to all parts of the oceans at the same time. Some parts of the oceans on Earth were oxygenated while others stayed anoxic, meaning they lacked oxygen. The anoxic parts of the ocean could have contained a lot of iron (Fe). Fe comes in different chemicals forms, such as Fe(II), a chemically reduced form of iron. Fe(II) is the form that can dissolve in water, compared to Fe(III), which is oxidized and forms rust. Other parts of the ocean contained a lot of sulfide, the type of sulfur that smells like rotten eggs, making them sulfidic. Scientists believe that the oceans were full of Fe(II) during the Pre-Cambrian, with some sulfidic parts and some oxygenated parts. In places where these different types of ocean waters came into contact, interesting chemical reactions could happen that would leave behind traces in the rock record. This is important to us today because it helps us understand the origin and evolution of life on Earth and can inform our search for life on other planets.

How the oceans looked at the end of the Pre-Cambrian (approximately 550 million years ago). Photo from https://commons.wikimedia.org/wiki/File:Positions_of_ancient_continents,_550_million_years_ago.jpg#filehistory.

A modern day analogue

How do you study chemical interfaces between the oxygenated, ferruginous, and euxinic waters that occurred more than half a billion years ago? One of the ways to do this is to use a modern-day analogue, something that is similar to the Pre-Cambrian oceans. Though it might seem strange, some of the best analogues for these oceans are actually springs and lakes! Koeksoy and colleagues at the University of Tuebingen in Germany studied Arvadi Springs in Switzerland as a Pre-Cambrian analogue. This spring has Fe(II) and sulfides in the waters, which mix oxygen as the spring waters contact air. These conditions, similar to those expected in the Pre-Cambrian, cause minerals to form. The scientists examined these minerals to understand what kind of minerals might have formed in the Pre-Cambrian, which helps us interpret the minerals we find in the rock record.

To determine which minerals formed in Arvadi Springs, Koeksoy et al. used spectroscopy, in this case using X-rays. Minerals and elements in different states, like Fe(II) versus Fe(III), will absorb X-rays differently, resulting in absorption graphs that can be used to identify the types and abundances of minerals present in a sample. Koeksey et al. used this technique on red and white minerals. Though not conclusive, when mineral-minded scientists see red minerals, iron comes to mind, since iron is typically red, like rust. Then they see white, it might indicate sulfur is present.

Seeing red (and green!)

Ferrihydrite precipitating from coal mine drainage in Colorado, USA. Photo from https://commons.wikimedia.org/wiki/File:Ferrihydrite_precipitate.jpg

The red minerals were indeed full of iron, and the scientists found that the most abundant types of iron-bearing minerals were ferrihydrite, lepidocrocite, and green rust. Ferrihydrite is a common mineral found in environments all over the world and even in our own cells! Lepidocrocite is similar to ferrihydrite and is seen as rust on steel pipes. Both of these minerals are stable, meaning they do not change quickly even under relatively stable conditions. In contrast, green rust is very unstable, and does not persist in most environmental conditions.

The white minerals contained sulfur both as elemental sulfur and as sulfate, a type of sulfur compound found in great abundances in seawater. Like with green rust, elemental sulfur often does not stick around very long in many environments, so it is a sign of how different the Avardi Spring, and by extension the Pre-Cambrian ocean chemistries, is to the modern ocean. While only a small portion (less than 0.1%), green rust was also present in the white mineral masses.

Green rust complexed with sulfate. Photo from https://commons.wikimedia.org/wiki/File:Green_rust_from_FeSO4.jpg)

Blast from the past?

Finding so much elemental sulfur and green rust, neither of which would be preserved for billions of years in the rock record, gives insight into ancient ocean chemistry. Green rust might be a precursor to the infamous banded iron formations that are evidence of an early Earth becoming oxygenated. Banded iron formations are rocks which alternate between reddish layers and other, often dark grey or black, layers. These formations are most often found in rocks from 2.4 billion years ago to 1.8 billion years ago. This timing corresponds to when oxygen was being produced and reacting with Fe(II) in the ocean, similar to the what is seen in the Arvadi Spring. Finding green rust in the spring where oxygenated, Fe(II)-rich, and sulfidic waters all come into contact gives us more context for interpreting Banded Iron Formations and how the world might have evolved.

Banded iron formations seen in Australia. Photo from https://commons.wikimedia.org/wiki/File:Banded_iron_formation_Dales_Gorge.jpg




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