{"id":102993,"date":"2017-10-16T18:47:46","date_gmt":"2017-10-16T16:47:46","guid":{"rendered":"http:\/\/www.myza.co.za\/how-we-used-the-earths-magnetic-field-to-date-rocks-rich-in-dinosaur-fossils-the-conversation\/"},"modified":"2017-10-16T18:47:46","modified_gmt":"2017-10-16T16:47:46","slug":"how-we-used-the-earths-magnetic-field-to-date-rocks-rich-in-dinosaur-fossils-the-conversation","status":"publish","type":"post","link":"https:\/\/www.myza.co.za\/press\/how-we-used-the-earths-magnetic-field-to-date-rocks-rich-in-dinosaur-fossils-the-conversation\/","title":{"rendered":"How we used the Earth’s magnetic field to date rocks rich in dinosaur fossils | The Conversation"},"content":{"rendered":"

How we used the Earth’s magnetic field to date rocks rich in dinosaur fossils<\/a>: <\/p>\n

Barkly Pass, the stratotype for the Elliot Formation. These beautiful rocks hold ancient secrets.<\/span> Lara Sciscio<\/span><\/span><\/figcaption><\/figure>\n

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. <\/p>\n

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 \u2013 mass extinction events \u2013 in the distant past when many species became extinct.<\/p>\n

So far, there have been five main mass extinction events globally. The biggest, the end-Permian, about 252 million years ago, was the Earth\u2019s 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.<\/p>\n

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. <\/p>\n

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 \u201cgeological clocks\u201d 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\u2019s own magnetic field provides a useful source.<\/p>\n

A different technique<\/h2>\n

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. <\/p>\n

\"\"
<\/span> Lara Sciscio<\/span><\/span> <\/figcaption><\/figure>\n

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. <\/p>\n

As there are no radiometric dates from zircons for the Elliot Formation, we used the Earth\u2019s 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. <\/p>\n

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.<\/p>\n

Magnetic flips<\/h2>\n

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\u2019s 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. <\/p>\n

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\u2019s magnetic field at the time of their deposition. That\u2019s important because natural processes can cause \u201coverprinting\u201d \u2013 wiping out the original magnetic signature.<\/p>\n

This method has been used within the Karoo Basin before on older rocks, but it\u2019s never guaranteed that rocks will retain their primary magnetic signatures. The fact that the Elliot Formation, largely, didn\u2019t fall prey to \u201coverprinting\u201d is what allowed us to record the pattern of the ancient magnetic field.<\/p>\n

Pole reversal offers timing tool<\/h2>\n

The Earth\u2019s magnetic field is not constant through time. It \u201cflips\u201d or \u201creverses\u201d at irregular intervals; on average, every few million years.<\/p>\n

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 \u201cflip\u201d event. This creates distinct geomagnetic polarity chron(s) \u2013 a name to define a specific unit of time during reversals \u2013 for any given time period. <\/p>\n

By studying the rates and number of these reversals recorded in the Elliot Formation\u2019s rocks, we are able to get a more accurate idea of the rocks\u2019 age. <\/p>\n

The next step in pinpointing the Elliot Formation\u2019s relative age was to build its unique magnetic polarity time scale \u2013 a log of all the reversal events.<\/p>\n

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. <\/p>\n

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\u2019s magnetic field as it was about 200 million years ago.<\/p>\n

We are not the only ones trying to pin down this important rock unit\u2019s 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.<\/p>\n

\"The <\/p>\n

Lara Sciscio receives funding from DST-NRF Centre of Excellence in Palaeosciences (CoE in Palaeosciences). <\/span><\/em><\/p>\n

Source:<\/strong> The Conversation \u2013 Articles (AFRICA)<\/a><\/small>
\n<\/p>\n

Read more on The Conversation<\/a>.<\/p>\n

Join the fun:<\/strong> Add Your event<\/a> | Submit a Press Release<\/a> | Add your listing<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

How we used the Earth’s magnetic field to date rocks rich in dinosaur fossils: Barkly Pass, the stratotype for the Elliot Formation. These beautiful rocks hold ancient secrets. Lara Sciscio 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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","ngg_post_thumbnail":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4584],"tags":[8890,10963,8880,21708,3995,13336,20166,8234,3015,8778],"class_list":{"0":"post-102993","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-africa-news-2","7":"tag-conversation","8":"tag-date","9":"tag-dinosaur","10":"tag-earths","11":"tag-field","12":"tag-fossils","13":"tag-magnetic","14":"tag-rich","15":"tag-rocks","16":"tag-used","17":"entry"},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/posts\/102993","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/comments?post=102993"}],"version-history":[{"count":0,"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/posts\/102993\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/media?parent=102993"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/categories?post=102993"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.myza.co.za\/press\/wp-json\/wp\/v2\/tags?post=102993"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}