How do we see if the pieces fit?This is a core of sediment taken from the northeastern portion of Au

How do we see if the pieces fit?This is a core of sediment taken from the northeastern portion of Australia’s Northern Territory. This area is filled with sedimentary rocks that formed in the latter half of the Precambrian, starting sometime around 1.5 billion years ago. At the time, this section of the planet was a deep basin that was slowly filling with sediments, something like the Gulf of Mexico today.This section of the core is one part of the Velkerri formation, an alternating sequence of sandy layers and organic-rich shale layers which you can recognize by the alternating black and white pattern. Because the rocks are sedimentary, we can’t easily date exactly when they formed, but we can use cross-cutting relationships to bracket their age. This sequence of sediments is cross-cut by 1.3-billion-year-old igneous rocks, meaning the sediments must be older than that age.Interestingly, in Northern China, there is also a large sequence of Precambrian aged sedimentary rocks including alternating black shale and sand layers. That sequence is also cut by 1.3-billion-year-old igneous rocks. Sorta makes one wonder – might these be the same sedimentary rocks, formed together in the same basin, but separated later by Plate Tectonics? Could they be two puzzle pieces that have rifted apart? How would one see if these puzzle pieces fit together using what we know of the geology of these rocks?There are lots of things we can’t use. You can’t match up individual layers, as like in the modern Gulf of Mexico they might see the same conditions but they could be on opposite sides of the Gulf, with different streams and rivers on each part. You can’t use fossils because these rocks are so old there are no organisms with shells. There are no igneous layers in sequence which could be precisely identified, so you can’t correlate exact layers together. What other ways do we have to see if the pieces fit?An international team including scientists from China, Germany, Australia, and the UK found a neat way to use the layering itself to test this hypothesis. The layers alternate in patterns between dark and light, and it has been proposed from the Chinese basin that the layers are associated with orbital patterns called Milankovitch cycles – patterns created as the Earth’s orbit wobbles around, changing global climate as a consequence. Prior to release of large amounts of greenhouse gases by humans, Milankovitch cycles have been driving the advance and retreat of glaciers on the continents for the past 800,000 years – when the orbit wobbles and it causes increasing solar heating in Canada, the great ice sheets break up and lots of CO2 is released to the atmosphere. Similar cyclicity has been found at many places in the geologic record, so could that match up from one spot to another?The team of scientists recorded the light and dark layers by measuring their magnetic properties; the sandy layers are found to have more sedimentary grains of the mineral magnetite in them and that makes them more susceptible to respond to an external magnetic field in a way that can be measured.The scientists took cores like this from both basins and measured their magnetic susceptibility. They then used a mathematical technique called a Fourier Transform to look for repeating patterns in the data; when they did so, on both units, they found that they could easily recognize 5 different orbital cycles in their data. They were able to see that the thickness of the units varied on scales of 15,000, 20,000, 28,000, 125,000, and 370,000 years – all of these are thought to be within error of the changes in Earth’s orbit occurring in the late Precambrian.So, these units were therefore found to have similar rates of sediment accumulation, similar rock types, and they were responding to the exact same changes in climate associated with orbital wobbles. That’s very difficult to do unless the basins are actually linked and seeing the same changes at the same time.If you imagine the modern Gulf of Mexico was taken and split down the middle, then the two sections were placed on opposite parts of the globe, that’s what these two basins might have looked like. These two basins likely linked up with each other 1.4 billion years ago when they were forming. At that time, there is thought to have been a supercontinent called Nuna that was in the process of breaking up; this basin existing along the edge of that supercontinent and eventually separating fits with the chemistry measured in the rocks.With some effort, two rock sequences from across the globe suggest that they once formed together, and the puzzle pieces can be fit.-JBBImage credit: Northern Territory of Australiahttps://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/85134Original paper:https://doi.org/10.1130/G47587.1 -- source link

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