Abstract
Deep-sea hydrogenetic ferromanganese crusts are both potential polymetallic resources and records of long-term environmental changes. For palaeoceanographic studies, it is important to construct a detailed and reliable chronological framework. 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. 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. Multidomain (MD) magnetite may also be present in all samples.
Highlights
Hydrogenetic ferromanganese crusts consist predominantly of seawater-derived ferruginous vernadite and X-ray amorphous Fe oxyhydroxides; they accumulate slowly on hard rock substrates that have been swept clean of sediments for millions of years[1,2,3,4,5,6]
The first-order reversal curves (FORC) diagram for SCS-02 was characterized by two independent closed contours, indicating that minerals with two different coercivities coexisted in the sample (Fig. 2n)
In contrast to SCS-02, SCS-01 showed much lower coercivity (
Summary
Hydrogenetic ferromanganese crusts (hereafter termed Fe-Mn crusts) consist predominantly of seawater-derived ferruginous vernadite (δMnO2) and X-ray amorphous Fe oxyhydroxides; they accumulate slowly on hard rock substrates that have been swept clean of sediments for millions of years[1,2,3,4,5,6]. 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)[6,13] 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. The first successful application of fine-scale magnetostratigraphic determinations on Fe-Mn crusts was reported by Oda et al.[21], who used scanning SQUID microscopy They found that the average growth rate of the Fe-Mn crusts from the Northwest Pacific Ocean was 5.1 ± 0.2 mm/Ma, which was consistent with the results of 10Be/9Be isotopic dating. To remedy the situation and to prepare slices with thicknesses of less than 1.0 mm, we conducted paleomagnetic measurements using a fine saw and a SQUID magnetometer as it is available in a conventional paleomagnetic laboratory
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