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more detailed climate records of the Holocene and the last glacial cycle. They also provide a record of climate in the northern hemisphere, and in the North Atlantic, the region that undoubtedly holds the key to Quaternary glacial cycles. Over roughly to the past 120,000 years, there is an excellent correla- tion between the NGIRP record and marine δ^18 O and Antarctic ice records. The last glacial interval, spanning the period from roughly 110,000 years ago to 14,000 years ago was a time during which, in addition to being colder on average, was also much more variable. In particu- lar, there are cold periods that end with rapid warming on time scales of a few decades. The transition back to cold episodes was much slower. These rapid fluctuations in climate are known as Dansgaard- Oeschager events and can be correlated to δ^18 O variations in high-resolution (i.e., high sedimentation rate) sediment cores from the North Atlantic. As may be seen in Figure 10.23, the warming events cor- relate with isotopic (temperature) maxima in Antarctica, suggesting coupling of climate between the two hemispheres. The cause of these events are still unclear, but changes in the North Atlantic ocean circulation, perhaps triggered by an influx of fresh water, are suspected.
Figure 10.23. Antarctic temperature variation calculated from the EPICA ice core compared to the δ^18 O record from the GRIP ice core from Greenland. From Jouzel et al., (2007).
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A number of other chemical and physical parameters are being or have been measured in these cores. Perhaps the most important finding to date is that cold periods were also dusty periods (again, this had previously been suspected from marine records). Ice formed in glacial intervals has higher concentra- tions of ions such as Ca 2+^ and Na +^ derived from sea salt and calcite and other minerals in soils in arid regions, indicating higher atmospheric dust transport during glacial periods, reflecting conditions that were both dustier and windier. Windier conditions could well result if thermohaline circulation was reduced, as the pole to equator temperature gradient would increase. Atmospheric dust may be an im- portant feedback in the climate cycle: dust can act as nuclei for water condensation, increasing cloud cover and cooling the climate (Walker, 1995). It may also serve as a feedback in another way. In parts of the ocean far from continents wind blown dust is a sig- nificant source of Fe, whose abundance locally limits bio- logical productivity. Increased winds during the last gla- cial period may have fertilized the ocean with Fe, effec- tively turning up the biological pump and drawing down atmospheric CO 2. On the negative side, the abundance of dust and aerosols compromises the record of atmosphere gases such as CO 2 that trapped air bubbles provide in the much cleaner Antarctic ice. It’s easy to understand why, particularly for CO 2. CaCO 3 will react with aerosols in the trapped in the ice such as H 2 SO 4 and HNO 3 (produced from SO 2 and NO 2 released by volcanic eruptions and other natural processes) to produce CO 2. Thus the Greenland ice cores have not be useful in reconstructing changes in at- mospheric CO 2.
Devil’s Hole Vein Calcite Record
Another remarkable isotopic record is that of vein calcite in Devil’s Hole in Nevada. Devil’s Hole is an open fault zone near a major groundwater discharge area in the south- ern basin and range in southwest Nevada (Devil’s Hole is located in the next basin east from Death Valley). The fis- sure is lined with calcite that has precipitated from super- saturated ground water over the past 500,000 years. A 36 cm long core was recovered by SCUBA divers and ana- lyzed by Winograd, et al. (1992). The results are compared with the Vostok and SPECMAP records in Figure 10.24. Ages of the Devil’s Hole core are based 22 U-Th ages de- termined by mass spectrometry. Though the Devil’s Hole record is strongly similar to the SPECMAP record, there are some significant differences. In particular, Winograd et al. (1992) noted that Termination II, the end of the second to the last glacial epoch, in the Devil’s Hole and Vostok records precedes that seen in the SPECMAP record by about 13 kyr (140 kyr vs. 127 kyr). This is an important point because Termination *^ II in the SPECMAP record corresponds with a peak in northern
- (^) The ends of glacial epochs are called Terminations. They are quantitatively defined as the mid-point in the δ^18 O rise
and are numbered successively backward in time.
Figure 10.24. Comparison of Devil’s Hole (DH-11) δ^18 O, marine carbonate (SPECMAP) δ^18 O, and Vostok ice core δD climate records. Dashed lines show the times of Terminations I-V. After Wi- nograd, et al. (1992).
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wind velocity†^ during the glacial maxima just before Termination II would reduce the fractionation re- corded in the Devil’s Hole area. The increase in Devil’s Hole δ^18 O may re- flect this reduced fractionation. A change in ocean-atmosphere circula- tion patterns may have effectively blocked cold Arctic air from reaching the Devil’s Hole area and moderated temperatures there. The controversy surrounding the Devil’s Hole record emphasizes the complexity of factors influencing continental isotopic re- cords and the difficulty in their inter- pretation.
Soils and Paleosols
The concentration of CO 2 dissolved in soil so- lutions is very much higher than in the atmos- phere, reaching 1% by volume. As a result, soil water can become supersaturated with respect to carbonates. In soils where evaporation ex- ceeds precipitation, soil carbonates form. The carbonates form in equilibrium with soil water, but the isotopic composition of soil water tends to be heavier than that of mean annual precipi- tation. There are 2 reasons for this. First, soil water enriched in 18 O relative to meteoric water due to preferential evaporation of isotopically light water molecules. Second, rain (or snow) falling in wetter, cooler seasons in more likely to run off than during warm seasons. Thus there is a strong correlation between δ^18 O in soil carbonate and meteoric water, though soil carbonates tend to be about 5‰ more enriched than expected from the calcite-water fractiona- tion (Figure 10.2 5 ). Because of this correlation, the isotopic composition of soil carbonate may be used as a paleoclimatic indicator. Figure 10.2 6 shows one example of δ^18 O in paleosol carbonates used in this way. The same Pakistani paleosol samples analyzed by Quade et al. (1989) for δ^13 C (Figure 10.07) were also analyzed for δ^18 O. The δ^13 C values recorded a shift toward more positive values at 7 Ma, which apparently reflect the ap- pearance of C 4 grasslands. The δ^18 O shows a shift to more positive values at around 8 Ma, or a million years before the δ^13 C shift. Quade et al. interpreted this as due to an intensification of the Monsoon sys- tem at that time, and interpretation consistent with marine paleontological evidence.
† (^) Both the increased equator-to-pole temperature gradient and increased concentration of dust in ice cores indicate
higher wind speeds at glacial maxima.
Figure 10.27. Relationship between δD and δ^18 O in modern meteoric water and kaolinites. Kaolinites are enriched in (^18) O by about 27‰ and 2 H by about 30‰. After Lawrence and Taylor (1971).
Figure 10.28. δ^18 O in Cretaceous kaolinites from North American compared with contours of δ^18 O (value shown in outline font) of present-day meteoric water. After Lawrence and Meaux (1993).
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Clays, such as kaolinites, are another important constituent of soil. Savin and Epstein (1970) showed that during soil formation, kaolinite and montmorillonite form in approximate equilibrium with mete- oric water so that their δ^18 O values are systematically shifted by +27 ‰ relative the local meteoric wa- ter, while δD are shifted by about 30‰. Thus kaolinites and montmorillonites define a line parallel to the meteoric water line (Figure 10.2 7 ), the so-called kaolinite line. From this observation, Lawrence and Taylor (1972) and Taylor (1974) reasoned that one should be able to deduce the isotopic composition of rain at the time ancient kaolinites formed from their δD values. Since the isotopic composition of pre- cipitation is climate dependent, as we have seen, ancient kaolinites provide another continental paleo- climatic record. Lawrence and Meaux (1993) conclude, however, that most ancient kaolinites have exchanged hydro- gen subsequent to their formation, and therefore a not a good paleoclimatic indicator (this conclusion is, however, controversial). On the other hand, they conclude that oxygen in kaolinite does preserve the original δ^18 O, and that can, with some caution, be used as a paleoclimatic indicator. Figure 10.2 8 compares the δ^18 O of ancient Cretaceous North American kaolinites with the isotopic composition of modern precipitation. If the Cretaceous climate were the same as the present one, the kaolinites should be systematically 27‰ heavier than modern precipitation. For the southeastern US, this is approxi- mately true, but the difference is generally less than 27‰ for other kaolinites, and the difference de- creases northward. This indicates these kaolinites formed in a warmer environment than the present one. Overall, the picture provided by Cretaceous kaolinites confirm what has otherwise be deduced about Cretaceous climate: the Cretaceous climate was generally warmer, and the equator to pole tem- perature gradient was lower.
THE CARBON CYCLE, ISOTOPES, AND CLIMATE
The Short-Term Carbon Cycle and Anthropogenic Impacts
There is considerable reason to believe that the Earth’s climate is linked to atmospheric CO 2 con-
Figure 10.2 9. The Carbon Cycle. Numbers in green show the amount of carbon (in 10^15 grams or giga- tons, Gt) in the atmosphere, oceans, terrestrial biosphere, and soil (including litter, debris, etc.). Fluxes (red) between these reservoirs (arrows) are in Gt/yr. Also shown in the approximate isotopic composi- tion of each reservoir. Magnitudes of reservoirs and fluxes are from Schlesinger (1991), isotopic compo- sitions are from Heimann and Maier-Reimer (1996).
