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Posted by / 07-Aug-2020 11:20

Magnetic dating

The magnetostratigraphic framework, established via correlation with the Geomagnetic Polarity Time Scale 2012, implies growth rates of 4.82 mm/Ma, 4.95 mm/Ma, 4.48 mm/Ma and 11.28 mm/Ma for samples PO-01, SCS-01, SCS-02 and IO-01, respectively.Rock magnetic measurements revealed that the Fe-Mn crust samples from the Pacific Ocean and Indian Ocean were dominated by low coercivity, non-interacting, single-domain (SD) magnetite particles, whereas the South China Sea samples were dominated by SD/pseudo-single-domain (PSD) particles.Cassette tapes or eight-tracks might be the first things that come to mind when thinking about dated magnetic storage, but Bronze Age clay pottery has them both beat.Using information stored in the clay’s magnetic minerals, scientists are developing methods to determine how old these artifacts are when other dating methods come up short.They use these observations to decipher how strong the Earth’s magnetic field was when the pot was first made.The final step is finding at what point in history the preserved strength matches the Earth’s field strength.“They’re like little compasses inside the clay,” Stillinger said.

In addition, Os isotope stratigraphy has been successfully applied to Fe-Mn crusts, which compares the Os isotope ratios of samples with those that define a Cenozoic seawater curve (crusts as old as 70 Ma can be dated).

Due to their continuous and extremely slow growth rate (typically several millimetres per million years), Fe-Mn crusts provide a record of the regional and global long-term environmental variations based on temporal changes in radiogenic isotope geochemistry (e.g., Pb, Sr, Nd, Os, Hf), the stable isotope composition of metals (e.g., Fe, Zn, Ni, Cd, Cu, Tl, Mo), and trace metal element distributions.

Fine-scale analyses are clearly important for correlating the geochemistry of crusts with palaeoceanographic events.

Here, we report the results of a detailed magnetostratigraphic and rock magnetic study of four hydrogenetic Fe-Mn crusts from the Pacific Ocean (PO-01), South China Sea (SCS-01, SCS-02) and Indian Ocean (IO-01).

Two groups of characteristic remanent magnetization directions were defined with nearly antipodal normal and reversed polarities for samples PO-01, SCS-01 and SCS-02, indicating a primary record of the Earth’s magnetic field.

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However, these isotopic techniques are limited by the potential changes in growth rates, the half-lives, and by the difficulty in determining sample thickness, all of which could potentially result in large dating errors.

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