A bit more on that diagram from yesterday, showing the magnetic polarity reversals over the past 5 million years (the record can be extended hundreds of millions of years into the past).
Sea-floor sediments are not the only way the planet's magnetic reversals can be observed. Lava that pours out of land volcanoes contains iron-based mineral grains that, while the lava is liquid, align themselves like tiny compass needles to the Earth's magnetic field. When the lava cools, these little "compass needles" are frozen into place, recording the direction and polarity of the Earth's magnetic poles at the time the lava is extruded.
By looking at the frozen magnetism of volcanic rocks geologists can tell where the magnetic poles were in the past with respect to continental volcanoes. As it turns out, the magnetic poles move about a bit, even in historic times, but presumably stay close to the north and south geographic poles -- the axis of the Earth's rotation. The frozen "compass needles" of many volcanoes, taken together, show that the continents -- with their volcanic rocks as passengers -- have drifted across the face of the Earth over the millions of years. Continental drift!
And here's the important thing. Volcanic rocks can be dated in real time by using the half-life decay rates of radioactive isotopes, such as the decay of uranium to lead. When the polarity of volcanic rocks from around the world and their ages are determined, they show the same unique pattern or reversals that we observed yesterday in the ocean sediments. Voila! Now we have a real time calendar for the sediment cores.
By the way, at ocean spreading centers, were volcanic crust is being continuously extruded from below, as for example near Iceland in the mid-Atlantic, the same pattern of reversals can be observed as in the ocean sediment cores -- on both sides of the spreading crust. The Earth's magnetic reversal history is recorded in stereo in the rocks of the oceanic crust!
There is always some uncertainty in using radioactive "clocks" for dating. But in fact, there are many ways of dating the geologic past, including everything from counting tree rings to using the different decay rates of various radioactive elements. Confidence in the geologic time scale comes from comparing the many different "clocks" against each other from as many sites as possible. The geologic timescale is constantly refined as more data is gathered and new dating methods and technologies are evolved. Here on the JR, sediment cores are dated as they come out of the hole.
Mo can pretty much tell the age of a core slice by looking at the microscopic fossil organisms that are her specialty (foraminifera, which of course have evolved new and delightful forms over the millions of years).
So, putting these three posts together, we have a record of changes in the Earth's climate in real time -- ice ages and warm spells over tens or hundreds of millions of years. What causes the changes? Ah, now if we knew that we'd be in a better position to judge the potential impact of human activities on climate. More to come.
(I hope you don't mind these little geology "lessons." It's hard to think about other things while aboard the vessel that has been so instrumental in unraveling past climate history.)