“Exploring Earth’s Past Climate- From Muck”
by Chet Raymo
by Chet Raymo
“How can one tell anything about the Earth's past climate from the compacted muck of the ocean floor? Here’s one way. Oxygen atoms come in basically two kinds - isotopes - oxygen 16, with 8 protons and 8 neutrons, and oxygen 18, with 8 protons and 10 neutrons. About 99.7 percent of naturally occurring oxygen is O16 (I should use superscripts for the 16 and 18, but I won't). O16 is lighter than O18, by a bit more than 10 percent.
Water, H2O, is a molecule with one oxygen atom and two hydrogen atoms. A water molecule with an O16 atom is lighter than a water molecule with an O18 atom. The water vapor in the atmosphere comes mostly from the sea, by evaporation. In effect, the energy of sunlight lifts molecules out of the sea into the air, the same way water in an open pan evaporates in a warm room. O16 based molecules, being lighter, evaporate a bit easier, and are less likely to fall immediately back into the sea. So the ratio of O16 to O18 molecules in the air is slightly more in favor of O16 than in the water, where the heavier molecules are more likely to be left behind.
The water in the air eventually falls as rain or snow onto sea or land. If on land, it flows in rivers back to the sea. So the original mix of O16 to O18 in the sea is maintained. But! If the snow that falls on the land doesn't melt, because the climate is colder, and compacts into ice - glaciers - it doesn't make its way back to the sea and instead piles up on the continent. Which means the prevalence of the heavier isotope O18 in the sea increases slightly but measurably.
Planktonic organisms in the sea build their skeletons out of the atoms in the sea, so the ratio of O16 to O18 in their bodies is the same as in the sea at the time they are alive. They die and their skeletons fall to the sea floor, eventually, over tens of millions of years, building up hundreds of meters of sediment.
The JOIDES Resolution drills into these deep compacted sediments and brings up long "cores," representing millions of years of Earth history. Paleoclimatologists (like Mo her grad students), painstakingly "pick" the microscopic skeletons out of the columns of sediment and use an instrument called a mass spectrometer to vaporize the fossil skeletons and separate their atoms by weight - including O16 and O18, the ratio of which reflects the volume of continental ice at the time the creatures lived. An exquisite continuous record of ice ages of the past! But how does one determine the age of the sediments?
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. Sediment cores are dated as they come out of the hole.
We 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 all this 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.”