Susceptibility as a tool for studying magnetic stratigraphy of marine sediments

Abstract

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.