Impacts, Ocean Mixing, Iron, and Life

Banded Iron Formation near Timmons in Northern Ontario; 2.7 billion years old. From Laurentian University.

John Slack and William Cannon, two USGS geologists based in Reston, Virginia, have a paper in this month’s Geology that suggests a link between a major asteroid impact and the cessation of banded iron deposition 1.85 billion years ago. Scientists love to correlate impacts with major geological events. We’ve recently seen research suggesting an extraterrestrial impact or impacts 12,900 years ago led to the onset of the Younger Dryas, the last ice age extinction, and the end of the Clovis Culture in North America. (And a skeptical take on that position from RealClimate.org). Though most people accept that the Chicxulub impacter was related to the K-T extinction, there are reasonable contentions that it’s not the sole cause (read the Science Daily article and the abstract).

Map of gravity anomalies in the Chixculub crater. The circular portion is ~180 km across. The white line is the superimposed coast of the Yucatán Peninsula; the white dots are mapped sinkholes. Image from the Geological Society of Canada.

Here’s Slack and Cannon’s abstract:

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O₂ (~1 μM), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ~1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter.

Some background:

Banded Iron Formations (BIFs) are chemical sedimentary deposits composed of thin alternating layers of Fe-rich minerals and silica. The Fe-minerals are usually oxides such as magnetite (Fe₃O₄) or hematite (Fe₂O₃), but they may be Fe-sulfides like pyrite (FeS₂) or Fe-carbonates such as siderite (FeCO₃). The silica is usually microcrystalline or amorphous chert or jasper. They’re found all over the world but most of them are limited to rocks between 3.5 and 1.8 billion years old, from the Archean eon and the Paleoproterozoic era. There’s a group of huge Paleoproterozoic BIFs around Lake Superior in Minnesota and Michigan, many of which are mined for Fe ore.

Some context to understand the Slack and Cannon paper:

• Oxidized, or ferric, iron (Fe³⁺) is relatively insoluble in seawater and will precipitate out. Ferrous iron (Fe²⁺) is more soluble in seawater.
• The ocean was layered until ~1.8 billion years ago, with an anoxic (oxygen-depleted) deep ocean at >600 meter depth and an oxic (relatively oxygen-rich) shallow ocean on the continental shelves.
• Although Superior Province BIFs formed on the continental shelves, the chemical signature, particularly the abundances of rare earth elements, indicates that the source of the Fe was deep ocean hydrothermal vents (Klein, 2005, abstract). Magmatically heated water convects through the newly formed basaltic crust and sequesters Fe, which is then pumped out into the seawater.
• The Fe from the deep oceans remained in solution under anoxic conditions, but came out of solution on shallow continental shelves where the oxic conditions promoted deposition of Fe³⁺-rich sediments, forming the Superior Province BIFs.
• At around 1.8 billion years ago, the oceans became mixed, resulting in some oxygen (suboxic conditions) in the deep oceans. Deep ocean sediments record strong evidence for this mixing: prior to 1.85 billion years ago deepwater chemical sediments were largely sulfide-rich, afterward they were oxide-rich (Slack et al., 2007, abstract only).
• This mixing meant that more Fe was oxidized and precipitated in the deep ocean and so it wasn’t transported in solution up to the continental shelves. Thus ended the Superior-type BIFs.

The Slack and Cannon paper suggests that the Sudbury impact was responsible for stirring the oceans and delivering oxygen-rich material to the deep ocean by underwater landslides. This oxygenated the deep ocean and led to the cessation of BIF formation in the Superior Province and globally. In an upcoming paper, Cannon and others give detailed evidence of an impact-deposited layer of rock that immediately overlies the major Superior BIFs. They tie this to the 1.85 billion year old Sudbury impact ~600 kilometers east of the Superior Province – at 250 kilometers diameter, it’s the second largest known impact event. (Sudbury is a fascinating topic in itself; the impact led to the formation of one of the world’s largest ore deposits. The Geological Society of Canada has nice compilations of geologic maps and cross-sections and other images here and here.)

Geologic Map of the Sudbury impact structure from the Geological Society of Canada. The crater is elongate due to later deformation.

Previously, the ocean mixing event was thought to be related to more gradual processes. Possibly just the incremental accumulation of photosynthesizing organisms, or terrestrial environmental changes. Slack and Cannon make a good case and their models seem sound. Often, the arguments against these correlations come from timing: as people gather more stratigraphic and radiogenic dates stories can fall apart (this is what Keller and others have posited with regard to the K-T extinction, though I remain slightly skeptical). Almost all of the BIF articles I’ve linked to here will affirm how important these mixing and oxygenation events are to the evolution of life, so I expect we’ll hear more about this.

(I found this story through my subscription to Geology, but there were good articles in Science News and Wired Science yesterday.)

Slack, J., & Cannon, W. (2009). Extraterrestrial demise of banded iron formations 1.85 billion years ago Geology, 37 (11), 1011-1014 DOI: 10.1130/G30259A.1