in solution can be large. Interaction of silica-rich brines with flood runoff may cause relatively sudden supersaturation with respect to amorphoussilica, and thereby lead to inorganic precipitation of chert. If there is rapid mixing of runoff with brine, much silica may remain in the diluted waters. If a strati- fied lake forms, however, biogenic CO, may be retained in the hypolimnion and reduce the pH of the bottom brines; in this manner the bulk of the dissolved silica can be precipitated. Figure 1 shows that a drop in pH from 11.0 to 8.5 can cause precipitation of as much as 3000 ppm SiO., which corresponds to a 1.5-mm-thick layer of chert for each meter of depth of brine. Silica layers that were probably formed by this mechanism have been found in the High Magadi beds, of Pleistocene age, and within the Alkali Valley playa deposits (77). BLAIR F. JoNEs SHIRLEY L, RETTIG U.S. Geological Survey, Washington, D.C, 20242 Hans P. EuGSTER Departinent of Geology, Johns Hopkins University, References and Notes 1. S. N. Davis, Amer. J, Sci. 262, 870 (1964); D. Livingston, U.S. Geol. Surv. Profess. 2. G, W. Morey, R. O. Fournier, J. J. Rowe, J, Geophys. Res. 69, 1995 (1964); R. Siever, 4. 5. 6. Indicators of Pleistocene Glaciation ' Abstract. Selected species of Coccolithaphoridae from recent sediments and midWisconsin glacial sediments of the North Atlantic were examined in an attempt to determine cooling effects. All species showed a definite shift southward during the glacial period. The average shift in this planktonic population was 15 degrees of latitude, with the greatest change in the eastern Atlantic. A paleoisotherm map can be drawn on the basis of the temperature boundaries of coccolithophorids. The species boundaries indicate a possible shift in position of the subtropical gyral to a glacial position roughly parallel to the 33-degree line of latitude. The dramatic fluctuations in Pleistocene climate are recorded in sediments in the Atlantic Ocean (/), but unfortu- nately the means of procuring these data are poorly developed. The only direct technique available at the present time is the use of oxygen isotopes (2). This report deals with a new approach —plotting the migration of biogeographic boundaries for temperaturerestricted species of Coccolithophoridae due to Pleistocene glaciation. Among all the microorganisms that leave fossil records in oceanic sediments, the Coccolithophoridae probably have the greatest potential as paleoclimatic indicators. In addition to their wide geographic distribution and stable mineral skeleton (calcite), these marine Baltimore, Maryland 21218 3. Coccoliths as Paleoclimatic Paper 440 (1963). algae inhabit the upper euphotic zone (3-5) and consequently are under direct climatic control. In living species 80° J, Geol, 1962, 127 (1962). G. J. §, Govett, Bull, Geol, Soc. Amer, 77, 1191 (1966); M. N, A. Peterson and C. C. Von der Bosch, Science 149, 1501 (1965). G. J. §. Govett, Anal. Chem, Acta 25, 69 (1961). Ringbom, Ahlers, Siitonen, ibid. 20, 70° 60° 50° it is possible to correlate biogeographic boundaries with surface water isotherms (4), and this is the basis of my report. The method of attack, being biogeo- graphic, requires the widest possible geographical distribution of core material. This is not easily obtained, for, although the North Atlantic has been the site of intensive sampling, there remain large gaps in the core distribution. A limiting factor is that large areas of the North Atlantic basin are below the carbonate compensation level, with a consequent lack of coccolith flora. Thus the 23 cores sampled (Table 1) are restricted to three linear belts. Two cover the shelf, slope, and rise of both North America and Eu- rope-Africa; the third, the Mid-Atlantic Ridge. Choice of the particular species to be examined requires that two separate cri40° =—30° 20° |O° 78 (1959). A. §. VanDenburgh, Geol. Soc. Amer. Spec. Paper 82 (1964), p. 349. 7. I. 8. Allison and R. S. Mason, Oregon Dept. Geol. Mineral Ind. Short Paper 17 (1947). 8 S. L. Rettig and B. F. Jones, U.S. Geol. Surv. Profess. Paper 501-D (1964), p. 134. >. B. F. Jones, U.S. Geol. Surv. Profess. Paper 502-A (1965), 56 pp. 10. B. H. Baker, Geol. Surv. Kenya Rept. 42 11. 12. 13. 14, 15. 16. 17. 18. 50° (1958). G. Lagerstrom, Acta Chem. Seand. 13, 722 (1989). N. Ingri, ibid., p. 758. J. H. Feth, C. E. Roberson, S. M. Rogers, Geochim. Cosmochim, Acta 22, 75 (1961). R. Wollast, ibid. 31, 635 (1967}. J. A. McKeague and M. G. Cline, Can. J. Soil Sci, 43, 70 (1963). B. F. Jones, Northern Ohio Geol. Soc. Symp. Sait 2nd (Cleveland, 1966}, p. 181. H. P. Eugster, B. F. Jones, R. A, Shepard, unpublished abstract, Geol. Soc, Amer., 1967; H. P. Eugster, unpublished. Authorized by the director, U.S. Geological Survey. Aided by a grant from the Petroleum Research Fund of Amer, Chem. Soc. We thank A. H. Truesdell and A. 8. Van Denbureh for discussion and field assistance in Oregon, O. P. Bricker and R. O. Fournier for comments on the manuscript, and M. D. Edwards for aid with the regression analyses. We also thank the Magadi Soda Co. of Kenya for its cooperation. tw 2 August 1967 1314 ae?” ove 20° Coccolithus petagicus — --—.—— Umbeilosphaera irreguiaris -++-+-+++-«“ Helicosphcera corteri 1O? —| Rhabdosphaera stylifera — — — IO° Syracosphaera puicha + 4+- —- + R= Recent O° G= Glacial Te soe 70° 60° O° 1 30° ee wee ee ee eee ee QO 20°~—s«*I'0? Fig. 1. Species population boundaries for Recent and mid-Wisconsin time. SCIENCE, VOL, 158