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Rhythmic Rocks Reveal Ancient Climate Change

Andet forskningsprojekt | 31/08/2017

Rocks contain clues to Earth’s history. The clues create a record of what happened here; also way back. The record stretches further back, and is more robust than Greenland ice cores, libraries, or even the Internet. We focus on rocks that were once seafloor and its chemical fossils that reflect life, climate and ecology in sea and air. We extract, purify and measure these chemical fossils. Fortunately for us, the methods to catch and understand these chemical fossils – for a reasonable interpretation of the past environment – has improved the last few decades with new technique. These methods allow us to interpret more of what atmospheric oxygen, algal blooms or lie was about as far back as one, two or even three billion years back. However, some of our methods are still classical, and some are dangerous. The extraction of chemical clues can involve the use of strong and even lethal acids. Acid that dissolves rock also risks dissolving the bones of whoever spills it in the lab. Therefore, any simplification to extract the chemical fossils is welcomed, and a non-destructive ‘gun’ of X-ray is one of them. The ‘gun’ provides – at high accuracy and precision – the chemical components of rocks just as they are. No preparatory dissolution in acid is necessary. Such a remarkable sci-fi gun is now available to us thanks to a contribution from the Carlsberg Foundation.

A Simple Way to Get Some of the Chemical Fossils

The gun that measures the chemical components of rocks is called a handheld XRF, for X-ray fluorescence. The gun is heavy, but certainly possible to hold in the hand. At its tip, a small window is flooded by a beam of X-rays. As such, no hands, or other tissue, should be placed in front of the gun. Against rocks, however, the gun can safely be aimed at its surface. The gun can therefore reveal yet invisible compositions of the rock already in the field. Maybe it reveals a layer of trace metal enrichments, which tell that the conditions were anoxic. Maybe it shows us an iron formation, which many of us are hunting. Or, who knows, perhaps it will give us gold, as a side-effect of our search for small changes in otherwise non-commercial metals.

NordCEE could acquire a handheld XRF thanks to the Carlsberg Foundation. Since we got the gun, we have primarily aimed it at rocks that are 1400 million years old, exposed in today’s China. The rocks deposited on the Southern hemisphere, not too far from the Equator, over a period of a couple of million years. For reasons that remain amazing, the rocks have not been significantly cooked, wedged or crushed during their rest over more than a billion years. In contrast, these rocks remain almost horizontal, with each and every bedding place intact, on the sides of normal busy modern roads. Along these road cuts, the rocks are stunning for a geologist, but when analysed for their chemical composition, they are even more breath taking for geochemists and geobiologists who try to put together the history of life.

'XML Roadcut’. A roadcut north of Bejing shows unit 3 of the Xiamaling Formation. Taken by Don Canfield.

The Xiamaling Formation is part of a sedimentary package deposited on the North China craton. The Xiamaling Formation can be seen in outcrop in several places on the North China Platform and is informally divided into six units that contain significantly different sediments, from chert to carbon-rich shale.

What the rhythmically layered rocks reveal, through its geochemical data that is in part achieved through the handheld gun, are equally rhythmic changes in climate. The climate shifted from two distinctinly different modes. At times, the rain was intense, which led to runoff (water) from the continents that started surface water circulation away from the shores. When surface water leaves off shore, deep water has to upwell instead. These upwelling waters are rich in nutrients, and fuelled algal blooms (primary production). Algal blooms contain carbon that is eventually degraded with the use of oxygen. When algal blooms are particularly intense, the degradation is so massive that the waters become anoxic. Such anoxic conditions, then, produce their own chemical fingerprint, which we can measure in the rocks. At other times, we believe the monsoon zones had moved away from our site. These times were dry, with very little rain, runoff from land, or a reversed kind of ocean circulation where surface water sort of ‘stays in place’. Instead, these waters were more or less stagnant with high evaporation. These conditions are also measurable, although these rocks are much more lean and thin (over the same amount of time).

“The rocks of Xiamaling are some of the most spectacular I have ever come across, and the same goes for the geochemical data and interpretations that has come out of it,” says Don Canfield, Professor at the University of Southern Denmark. 

Number Crunching Reveal Enough Oxygen for Primitive Animals

Math and mathematic models seem challenging to many of us. Not to others. Two of those others (Christian Bjerrum and Don Canfield) are part of our team. Together, they figured out a way to calculate – from the amount of carbon preserved in the now 1400 million years old seafloor – what oxygen concentrations must have been – as a minimum – when and where the deep water was formed (presumably around the poles also back then). 
“The novelty is here that we have applied knowledge about what particular carbon microbes consume at different depths, through which we can say how much oxygen was there to start with,” says Christian Bjerrum, Associate Professor in Geology at Copenhagen University.

Recent high-precision dating of zircon grains, performed at Copenhagen University, refined the age of the Xiamaling Formation to 1384 ± 1.4 Ma (for a volcanic tuff layer located in unit 2) and 1392.2 ± 1.0 Ma (for a volcanic bentonite layer at the top of unit 3) 52 m apart.

These minimum estimates of atmospheric oxygen (a range is presented as several factors, such as sedimentation rate, can differ) are 4-6% of what we have today. This may not sound as much. However, 4-6% of what we have today is higher than simple animals require. These minimum estimates of atmospheric oxygen from about 800 million years before animals diversified then – in theory –  means that the late diversification of animals was not due to too low oxygen.

A fierce battle is ongoing regarding whether or not oxygen concentrations limited animals to diversify on Earth until roughly 90% of its time yet had passed. Our contribution adds to this battle. Still, much remains to be said.

‘XML Unit 4’. In the field, measuring and inspecting unit 4 of the Xiamaling Formation. Unit 4 is laminated with reddish and greenish fine, lean sediments. Taken by Emma Hammarlund.

Relevant Peer-Reviewed Papers

1 ‘The oxic degradation of sedimentary organic matter 1400 Ma constrains atmospheric oxygen levels’
2. ‘Sufficient oxygen for animal respiration 1,400 million years ago’
3. ‘Orbital forcing of climate 1.4 billion years ago’

Other Relevant Papers in the Press

In Weekendavisen 13th of March 2015 ‘Klodens stregkoder noteret i sten’
In Weekendavisen 8th of January 2016 ‘Ilt nok, men hvor er dyrene?’