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By Root 2095 0

encountered (Chapters 2 and 5). Drilling down into the mud at the bottom of a lake or bog may not strike the rest of us as exciting, but it’s nirvana for a palynologist, because the deeper mud layers were deposited longer ago in the past. Radiocarbon dating of organic materials in a mud sample establishes when that particular layer of mud settled out. Pollen grains from different plant species look different under the microscope, so that the pollen grains in your (you the palynologist’s) mud sample tell you what plants were growing near your lake or bog and were releasing pollen that fell into it in that year. As past climates became colder in Greenland, palynologists find pollen shifting from that of warmth-demanding trees to that of cold-tolerant grasses and sedges. But that same shift in pollen may also mean that the Norse were cutting down trees, and palynologists have found other ways to distinguish those two interpretations of declining tree pollen.

Finally, by far our most detailed information about Greenland climates in the past comes from ice cores. In Greenland’s cold and intermittently wet climate, trees are small, grow only locally, and their timber deteriorates quickly, so we don’t have for Greenland the logs with beautifully preserved tree rings that have enabled archaeologists to reconstruct year-to-year climate changes in the dry U.S. southwestern deserts inhabited by the Anasazi. Instead of tree rings, Greenland archaeologists have the good fortune of being able to study ice rings—or, actually, ice layers. Snow that falls each year on Greenland’s ice cap becomes compressed by the weight of later years of snow into ice. The oxygen in the water that constitutes snow or ice consists of three different isotopes, i.e., three different types of oxygen atoms differing just in atomic weight because of different numbers of uncharged neutrons in the oxygen nucleus. The overwhelmingly prevalent form of natural oxygen (99.8% of the total) is the isotope oxygen-16 (meaning oxygen of atomic weight 16), but there is also a small proportion (0.2%) of oxygen-18, and an even smaller amount of oxygen-17. All three of those isotopes are stable, not radioactive, but they can still be distinguished by an instrument called a mass spectrometer. The warmer the temperature at which snow forms, the higher is the proportion of oxygen-18 in the snow’s oxygen. Hence each year’s summer snow is higher in its proportion of oxygen-18 than the same year’s winter snow. For the same reason, snow oxygen-18 in a given month of a warm year is higher than in the same month of a cold year.

Thus, as you drill down through the Greenland ice cap (something that Greenland-ice-cap-drilling scientists have now done down to a depth of almost two miles) and measure the oxygen-18 proportion as a function of depth, you see the oxygen-18 proportion wiggling up and down as you bore through one year’s summer ice into the preceding winter’s ice and then into the preceding summer’s ice, because of the predictable seasonal changes in temperature. You also find oxygen-18 values to differ among different summers or different winters, because of unpredictable year-to-year fluctuations in temperature. Hence the Greenland ice core yields information similar to what archaeologists studying the Anasazi deduce from tree rings: it tells us each year’s summer temperature and each year’s winter temperature, and as a bonus the thickness of the ice layer between consecutive summers (or between consecutive winters) tells us the amount of precipitation that fell during that year.

There is one other feature of weather about which we can learn from ice cores, but not from tree rings, and that is storminess. Storm winds pick up salt spray from the ocean around Greenland, may blow it far inland over the ice cap, and drop there some of the spray frozen as snow, including the sodium ions in seawater. Onto the ice cap, storm winds also blow atmospheric dust, which originates far away in dry dusty areas of the continents, and that dust is high in calcium ions. Snow formed from pure