16 October 2017

South Africa: How We Used the Earth's Magnetic Field to Date Rocks Rich in Dinosaur Fossils

Photo: FunkMonk/Wikipedia
Spinosaurus fossil (file photo).

Covering two thirds of South Africa the Karoo Basin, visually, is a beautiful space. When looking more deeply into its rock layers, like leafing through the pages of a book, one can read about a wealth of palaeoevinromental and biological processes.

The Karoo Basin is an invaluable archive of information over its 120 million year depositional history. Rich in fossils, both plants and animals, the Karoo Basin records crisis periods - mass extinction events - in the distant past when many species became extinct.

So far, there have been five main mass extinction events globally. The biggest, the end-Permian, about 252 million years ago, was the Earth's largest ecological disaster. The Karoo Basin also holds evidence of the third largest mass extinction. This occurred at the end of the Triassic, about 200 million years ago, and heralded the rise of the dinosaurs.

Understanding these climate change events and their impact on biology in the Karoo Basin could influence the way we look at the sixth extinction, which is happening now: the Anthropocene.

Scientists need to know when the ancient extinctions happened and for how long. These events are recorded in layers of rock. So we need to know the age of those rocks. There are certain "geological clocks" which help when dating rocks: a mineral called zircon is one. Fossil pollen and spores are others. But when these are scarce, we need another way of measuring the age of rocks. And the Earth's own magnetic field provides a useful source.

A different technique

My colleagues and I were interested in the age of a specific rock unit in the Karoo Basin: the Elliot Formation. Rocks of the Elliot Formation outcrop in a ring around the Drakensberg Plateau (see figure). The Elliot Formation contains many fossils that shed light on the existence and evolution of dinosaurs in southern Africa.

This is especially interesting as the Formation is thought to span the end-Triassic mass extinction event. However, the age of the Elliot Formation and where this extinction event occurred within its rock layers was debated.

As there are no radiometric dates from zircons for the Elliot Formation, we used the Earth's ancient geomagnetic field as a dating tool. This technique has been used globally on similar aged rocks. Applying it here enabled us to narrow down the age of the Elliot Formation to somewhere between about 213 million and 195 million years old.

These dates may help us to answer broader questions relating to the severity of the end-Triassic mass extinction and the post-extinction recovery period in southern Africa. This time line is particularly useful in measuring the diversity of dinosaurs across the bio-crisis and during a critical time in their evolution.

Magnetic flips

The Earth generates and sustains a magnetic field through the motion of the liquid outer core. Some minerals in rocks are able to record the Earth's magnetic field when they are deposited. Two such minerals, hematite and maghemite, are prevalent in the Elliot Formation. In fact, they lend the Formation a distinct brick-red colour.

Our research has found that minerals within the rocks of the Elliot Formation are able to retain primary magnetisations: they have reliably recorded the Earth's magnetic field at the time of their deposition. That's important because natural processes can cause "overprinting" - wiping out the original magnetic signature.

This method has been used within the Karoo Basin before on older rocks, but it's never guaranteed that rocks will retain their primary magnetic signatures. The fact that the Elliot Formation, largely, didn't fall prey to "overprinting" is what allowed us to record the pattern of the ancient magnetic field.

Pole reversal offers timing tool

The Earth's magnetic field is not constant through time. It "flips" or "reverses" at irregular intervals; on average, every few million years.

When this happens, the magnetic north pole is direct to the geographic south pole and vice versa. Rocks contain alternating layers of north- and south-directed minerals corresponding to every "flip" event. This creates distinct geomagnetic polarity chron(s) - a name to define a specific unit of time during reversals - for any given time period.

By studying the rates and number of these reversals recorded in the Elliot Formation's rocks, we are able to get a more accurate idea of the rocks' age.

The next step in pinpointing the Elliot Formation's relative age was to build its unique magnetic polarity time scale - a log of all the reversal events.

This involved drilling out small samples of rock, using a portable hand-held drill, and orientating them, using a special compass in the field. Thereafter samples were processed in the Paleomag Lab at the University of Johannesburg to recover their unique geomagnetic polarity history.

By doing this, we could build a composite magnetic polarity chronology for the Elliot Formation. We were then able to compare these rocks from South Africa and neighbouring Lesotho to others of a similar time period globally. In so doing, the Elliot Formation records the Earth's magnetic field as it was about 200 million years ago.

We are not the only ones trying to pin down this important rock unit's age. We hope that our work will provide a framework on which to place other kinds of information produced by others in this and related fields.

Disclosure statement

Lara Sciscio receives funding from DST-NRF Centre of Excellence in Palaeosciences (CoE in Palaeosciences).

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