Susceptibility as a tool for studying magnetic stratigraphy of marine sediments

Eighty six gravity cores collected from the Pacific Ocean by the Scripps Institution of Oceanography have been logged for magnetic susceptibility using a simple and rapid technique. These logs fall into three types: Type 1 showing several highs and lows, Type 2 with a single-broad-hump, and Type 3 showing nearly constant susceptibility with depth. Type 1 cores are found to be mainly from sediment-trap (trenches) areas which are close to the active volcanoes and the high peaks probably correspond to a slump or deposition of volcanic material; these events occurred between 0.1 and 2.8 million years ago. Type 2 cores are by far the most common, (56 out of 86) and show a maximum deposition of magnetic material (i.e. crest region of the hump) in the range of 0.2 and 1.7 million years. The susceptibility during this period was about a factor of two higher for several cores compared to their respective values during the last 0.1 million years. Oceanwide deposition of volcanic material and/or the atmospherically transported dust rich in magnetic material (cosmic and/or terrestrial) by our planet can account for such an increase. A third possibility may be the change (decrease) in accumulation rates of the sediments during this period. In type 3 cores the susceptibility is almost constant with depth and these are randomly distributed (excluding the sediment trap areas) analogous to the case of type 2 cores. A high deposition rate in these areas can alter type 2 into type 3. It appears that the maximum of type 2 hump can act as a stratigraphic marker since type 2 cores are the most common ones and are widely distributed over the entire Pacific

Anomalous magnetic hysteresis loops and small H<SUB>c1</SUB> values in high T<SUB>c</SUB> superconductors

We have studied the hysteresis loops of RBa2Cu3O7 (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean's model. The anomalies indicate low Hc1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these high Tc, superconductors

PALEOMAGNETISM AND ROCK MAGNETISM OF THE DECCAN TRAPS

Extensive work on the palaeomagnetism of the Deccan traps by several workers has revealed only one reversal (normal-reverse-normal) of the geomagnetic field during the period of eruption of these flows. The scatter in the natural remanent magnetic directions of different flows in a sequence of these traps is rather large and it persists even after magnetic cleaning. Generally this large scatter of directions is attributed to the geomagnetic secular variation during formation of the traps. Recent studies of the magnetic properties of the trap samples indicate that the presence of multidomain magnetite in a lava sequence differs from locality to locality in the Deccan trap province and so the stability of their natural remanent magnetization. While these features can be clearly perceived, an understanding of the same seems to be quite difficult. However, it appears that part of the scatter in directions could be due to the alteration of the magnetic minerals in the Deccan traps and hence it would not be proper to relate the entire scatter to the secular variation

Magnetic techniques for ascertaining the nature of iron oxide grains in basalts

Qualitative techniques are described by which the domain nature and magnetite-maghemite oxidation state of the iron oxides in basalts may be rapidly identified through measurements of hysteresis and susceptibility over a wide temperature range. Detailed studies made on different suites of basalts have revealed that their magnetic properties in most cases can be explained only on the basis of single-domain behaviour. Also it has been found that the observed variations in these properties between basalts are usually best explained by differences in the position the iron oxide minerals in the different samples occupy along the magnetite-maghemite oxidation chain. These observations suggest that the role of titanium usually found in association with iron oxides in basalts is to subdivide the grains physically rather than to form solid solutions of titanomagnetites or titanomaghemites. Some implications of these results to basalt formation and the magnetic anomalies such rocks could cause are discussed. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ARK: https://n2t.net/ark:/88439/y055011 Permalink: https://geophysicsjournal.com/article/111 &nbsp