Remember all those plankton organisms in the seafloor sediments. Well, they are not the only things in the muck. There are also mineral grains that are washed into the sea from the land or fall from the atmosphere. Some of those mineral grains are iron-based and therefore magnetic -- like microscopic compass needles.
As the grains drift down to the ocean floor, they align themselves with the Earth's magnetic field. Once they settle into the muck, they are stuck, with each little grain pointing like a compass needle to the Earth's magnetic poles.
Wonderfully, and (still basically) mysteriously, the Earth's magnetic field changes polarity at more or less random intervals; that is, north and south poles reverse. This presumably has to do with complex electric currents in the Earth's metallic core. This happens on average a few times every million years. (A reversal takes several thousands of years to complete.) No reversal has happened in historic times. If a reversal occurred, our compass needles would swing around and point in the opposite direction.
Now back to the cores. The magnetic grains in a sediment core record these reversals, pointing now one way, now the other, as you go down the core into the past, like compass needles frozen in time. The pattern of reversals is the same worldwide, since a reversal affects the entire planet. The magnetic field of those few magnetic minerals in the sediments is of course is very weak, but the JOIDES Resolution has the lab facility to measure the polarity. This strikes me as a remarkable achievement in a steel ship. The first labs designed to do these measurements decades ago were constructed entirely of non-magnetic materials.
Once the pattern of reversals has been determined and calibrated in time (see diagram), the cores are like calendars of Earth history. And, as per the previous post, we are on the way to understanding the ice ages and, hopefully, the causes of climate change.
How is the reversal pattern calibrated in time? That is, in the diagram below (which just shows the past 5 million years), how do we know the times of the reversals? More to come.