The preamble to this review series is here
Halverson, G.P., F. Poitrasson, P.E. Hoffman, A. Nédélec, J.-M. Montel and J. Kirby, 2011, Fe-isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation. Earth and Planetary Science Letters, v. 309, p. 100-112.
The MacKenzie Mountain front in Canada’s NW Territories as seen from the river plain near Norman Wells. Image from Google Earth (Panoramio). The fieldwork for this paper was carried out in these mountains.
Iron is Earth’s most common element by weight. There is so much of it that a compass needle (yes, also the one in your smartphone) lines up with the earth’s magnetic field. We also know that Iron changes form, because we are familiar with rust. Rust is oxidized iron and oxidation happens when Ferrous Iron (Fe2+) is exposed to the atmosphere (rich in oxygen) without being protected by some sort of coating. Under the burning influence of atmospheric oxygen,Ferrous Iron it changes to Ferric Iron (Fe3+) and turns red. A lot of rocks are rust-coloured because of this reason.
Iron also has one unstable isotope: 57Fe. The amount of this isotope varies every so slightly depending on oceanic and atmospheric chemistry. This change can tell us something about conditions in the past, if we know the age of the rocks. This article uses that knowledge to unravel conditions about 700 million years ago.
The authors of this article are Snowball Earth (official site here) experts: their research focuses on testing the hypothesis that earth around this time experienced a global ice age during which continental ice caps covered most of the planet as far as the equator.
Artist impression of Snowball Earth
The most obvious evidence for Snowball Earth is the ubiquitous presence fossilized glacial till in rocks of this age. Till (another word is diamictite) is the chaotic sedimentary mass that forms beneath a continental ice cap: it consists of pulverized rocks and pebbles in hard clay. A dead give-away of a former ice cap over the ocean (such as Antarctica’s Ross Ice shelf today), is the presence of dropstones in marine sediments. Dropstones are rock fragments, eroded from underlying bedrock by the icecap, transported by that same ice cap and subsequently melted out, fallen to the sea floor below and incorporated in the sediment.
The authors of this paper studied a sequence of rocks known as the Rapitan Group in the Mackenzie mountains in the Northwest Territories (latitude 640N). The Rapitan Group is ca. 700 million years old and shows evidence of ice age conditions, such as till and dropstones. When these sediments were deposited, this part of earth was situated at about 180S! Not a latitude where glaciers occur today or in our more recent Ice Ages when they reached as far as about 400 (North or South).
Not only show the rocks of the Rapitan Group evidence of glacial conditions, they are also unusually enriched in Iron.
Left: A dropstone in iron-rich sediments of the 700 million year old Rapitan Group, Mackenzie mountains, NWT (pen for scale below the dropstone; photo from article). Centre: a core with 20,000 year old sediments and dropstones from the Orphan Basin offshore Newfoundland for comparison (vertical scale in centimeters) (Image Source Tripsanas et al, 2007). Right: finely laminated Iron-rich marine mudstones and siltstones from a different interval in the Rapitan Group.
Sedimentary rocks with this much Iron are called ‘Banded Iron Formations’ (BIF). The most widespread BIFs are much older, between 2.4 and 1.8 billion years and they represent the bulk of the world’s iron ore deposits. Those earlier BIFs are thought to have originated biologically: massive Iron fertilization of the world’s oceans through ocean floor vents caused equally massive blooming of cyanobacteria, which were almost the only living organism at the time, fixing the iron in these bacterial sedimentary rocks. In the course of that process, earth’s atmosphere got its first major oxygen injection because cyanobacteria are photosynthesizers (if you think this is one of these completely irrelevant ivory tower types of research, I’m happy to remind you of this ocean iron fertilization controversy).
But the BIFs of the Rapitan Group are different than those much older BIFs. The BIFs of the Rapitan Group were deposited in greater water depths and, unlike the older BIFs, their origin is linked to ice caps (dropstones and the like). Why suddenly all this iron-fixing again after it had ended 1 billion years earlier? Were Snowball Earth conditions the cause of this return of massive iron fixing?
After an immensely complex series of chemical analyses, the interpreted scenario is as follows:
Ice caps covered most of Earth during this time and the remaining ocean waters were cold. Iron was supplied to the oceans by interaction between ocean water and the ocean floor. Because ice covered most of the water surface, remaining ocean waters didn’t circulate or overturn much, so that any deeper ocean waters (just below the thin, oxygenated photic zone) were oxygen-deprived. The underside of the ice caps would nevertheless melt in contact with ocean water and send dropstones and occasional gravity flows (high density currents that hug the bottom) to greater depths, interfingering with the deeper finely laminated silts and muds.
The Iron that was released from the ocean floor precipitated either as a result of fixing by bacteria that thrived in low-oxygen conditions or as a result of abiologic oxidation: the research is inconclusive about which process was responsible.
The lower part of the Rapitan Group sequence is dominated by intervals that show finely laminated iron-rich mudstones and dropstone-dominated intervals. These were deposited and precipitated on the bottom of the sea under oxygen-poor conditions. The 57Fe isotope has low values. Upwards these abruptly make place for tills deposited below a continental ice cap and at this boundary the isotope 57Fe shows a marked increase, which must be explained by an increase in the amount available oxygen.
It’s difficult to interpret element cycling conditions on our planet this long ago because it was such a different place from today: bacteria were the only life forms, the land was barren. And yet the authors have found a modern analogue to the strange conditions that made deposition of the Rapitan Group rocks possible. The modern analogue is Lake Nyos, in Cameroon.
I was at work in the summer of 1986 when news spread that a volcanic eruption in Cameroon had killed nearly 2000 thousand people and a lot of cattle. A few days later (it was the pre-digital age), it turned out that this catastrophe had not been caused by a volcanic eruption. Lake Nyos is a deep crater lake of an inactive volcano. Because it is located in the seasonless tropics, the waters of this lake don’t overturn and hence the lake is anoxic and rich in carbon dioxide below the thin oxygen-rich surface layer. When a small landslide slid into the lake, oxygen-rich surface and anoxic deeper waters suddenly became mixed, allowing previously locked up toxic gases to escape over the crater edge and down the slope of the volcano (carbon dioxide is heavier than air and hugs the ground), suffocating people and cattle in its way. A unique and disturbing disaster (read more about it here).
In order to prevent such a catastrophe from happening again, the lake is now ventilated by a metal pipe through which deep water is pumped to the lake surface. The pipe delivers Ferrous Iron to the surface where it immediately oxidizes. At the anoxic-oxic boundary, the same change in 57Fe isotope is detected as in the Rapitan Group rocks. This observation strengthens the interpretation that conditions changed from anoxic to more oxygenated 700 million years ago.
This is challenging research. Not only does it require travel to the remote Mackenzie Mountains, where you must sample inaccessible sections of rocks and then store and transport the samples without polluting them, you must also carry out very complex chemical analyses. Samples were shipped all over the world to specialized laboratories. The authors go to great lengths to explain all the ifs and buts and potential pitfalls of this kind of work and how they avoided them. Hats off.
This paper is an elegant contribution to understanding the incredibly intricate and complex cycling of elements and nutrients on our planet over time. A small trigger suddenly caused the deposition of massive iron deposits during an utterly exotic global glaciation. Cause for reflection on our role as humans and the potential pitfalls of human intervention in system Earth.
Tripsanas, E.K., D.J.W. Piper and K.A. Jarrett, 2007, Logs of piston cores and interpreted high-resolution profiles, Orphan Basin. Geological Survey of Canada Open File 5299, 339 pages.