Most reports of a correlation between Pleistocene climate and geomagnetic field intensity rely strongly on the assumption that sediment natural remanent magnetic (NRM) intensity provides a record of geomagnetic field strength and is not sensitive to local changes in properties of the sediment. Critical assessment of relevant data presented here and elsewhere from deep-sea sediment cores shows that a pronounced dependence of NRM intensity on sediment composition can occur which implies that this assumption is unlikely to be generally valid. As sediment composition often reflects varying depositional conditions induced by climatic change, the significance of correlations proposed between Pleistocene palaeomagnetism and climatic indicators in deep-sea sediments may be less dramatic than sometimes supposed

AD Chave

BM Cordell

C Amerigian

CE Barton

CGA Harrison

CSM Doake

D Ninkovich

D. V. Kent

DV Kent

EA Johnson

EA Mankinen

G Arrenhius

G Wollin

JD Hays

MR Rampino

NJ Shackelton

RM Cline

RS Sternberg

W Schott

Kent, Dennis V.

Columbia University Academic Commons

©          Nature Publishing Group1982538 The positive correlation between the average geomagnetic activity and the area of the filaments will be discussed else-where10 where we suggest that it arises for two reasons. First, large filaments may produce more extensive disturbances in the solar wind which have a higher probability of intercepting the Earth. Second, the particle and magnetic flux densities within these disturbances may be greater. Whatever the details of the correlation, it is clear that some disappearing filaments signal the occurrence of geomagnetic disturbances. Therefore, as sug-gested by Joselyn and McIntoshs, they should be incorporated into techniques that are used for the prediction of geomagnetic storms. Received 31 December 1981; accepted 29 July 1982. 1. Waldmeier. M. 1. geophys. Res. 51. 537-542 (1946). 2. Leighton. H. l. & Billings. D. E. 1. atmos. terr. Phys. 7,349-350 (1955). 3. Roberts. W. O. & Trotter. D. E. 1. atmos. terr. Phys. 6, 282-283 (1955). 4. Dizer. M. Ann. Geophys. 13,325 (1957). 5. Hansen. R. T. 1. geophys. Res. 64, 23-25 (1959). 6. Bednarova-Novakova. B. 1. geophys. Res. 65, 367 (1960). 7. Oizer. M. Ann. Geophys. 18, 388-389 (1962). 8. Joselyn. J. A. & Mcintosh. P. S. 1. geophys. Res. 86.4555-4564 (1981). 9. Solar Geophysical Data. 1974-1981 (US Department of Commerce. Boulder, Colorado). 10. Wright, C. S. & McNamara, L. F. Solar Phys. (submitted). Apparent correlation of palaeomagnetic intensity and climatic records in deep-sea sediments D. V. Kent Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964, USA Most reports of a correlation between Pleistocene climate and geomagnetic field intensity rely strongly on the assumption that sediment natural remanent magnetic (NRM) intensity provides a record of geomagnetic field strength and is not sensitive to local changes in properties of the sediment. Critical assessment of relevant data presented here and elsewhere1•2 from deep-sea sediment cores shows that a pronounced dependence of NRM intensity on sediment composition can occur which implies that this assumption is unlikely to be generally valid. As sediment composition often reflects varying depositional conditions induced by climatic change, the significance of correlations proposed between Pleistocene palaeomagnetism and climatic indicators in deep-sea sediments may be less dramatic than sometimes supposed. A fundamental relationship between geomagnetic field behaviour and climate change has been suggested, largely on the basis of an apparent correlation in geomagnetic and climatic indices over time scales ranging from historical observatory records of geomagnetic intensity and air temperature3 to the Pleistocene record of palaeomagnetic and palaeoclimatic data in deep-sea sediments4.5 • Periods of low (high) geomagnetic intensity have been proposed to correspond to high (low) air temperature or warm (cold) climate. Suggestions for the under-lying mechanism have included magnetic field interaction with the atmosphere thereby influencing climate6, the distribution of ice affecting core motions and the geomagnetic field 7 , or that there is some common causative agent, for example, variation in the eccentricity of Earth's orbit modulating both climate and the geomagnetic fields-to. A hypothetical magnetic trigger has even been entertained to explain climate change on Marsll. In view of the lack of consensus on a ";,,hle causal mechanism and the complexity of the two syskms involved, it seems worthwhile to examine the nature of the reported empirical correlations to ascertain which lines of inquiry are likely to be productive or if they have so little observational basis that they will probably result in unproductive speculation. In this regard, Sternberg and Damon 12 found no statistically significant global Nature Vol. 299 7 October 1982 relationship between geomagnetic intensity and climate over the period covered by direct observatory records. Correlations between geomagnetism and climate over geo-logical timescales are by necessity based on indirect records such as those obtained in continuously deposited deep-sea sediments. Deep-sea core RCll-120 from the southern Indian Ocean (lat. 48°31'S, long. 79°52'E) has one of the most complete and detailed records of climate change available for the past 250,000 yrl3 and proved suitable for magnetic study. The down-core variation of oxygen isotope composition eSO) of plank-tonic forminifera, carbonate content (CaC03), magnetic sus-ceptibility (k) and the intensity of stable NRM and its inclination direction are compared in Fig. 1. The ISO variation is inter-preted to reflect changes primarily in global ice volume and provides a climatic record which can be correlated world-wide13.14. The remanent inclinations are generally consistent with expected values for a normal polarity dipole field and show little indication of anomalous field behaviour except possibly for a long-term trend which is not considered here. The striking feature in all these data, however, is the close visual correlation of the ISO, CaC03, k and NRM curves. The significance of these correlations bears directly on the question of geomag-netic-climatic relationships. Although carbonate deposition is a complicated process with many controlling factors, a close association between carbonate content and climate has been recognized in many Pleistocene deep-sea pelagic sediments from early stages of research15.16. In core RCll-120, the high inverse correlation between ISO and CaC03 (r = -0.79) indicates that high carbonate contents closely correspond to intervals of low global ice volumes. The same climatic interpretation of carbonate contents can be made c: .S! -;; 34 00 30 40 50 60 70 80 90 '25 '00 075 050 025 000 .E +30' U o· .:: o -,----,----, " Core depth (m) " 10 Fig, 1 Palaeoclimatic, lithological and palaeomagnetic record in deep-sea sediment core RCll-120. Shown from top to bottom are down-core logs of: a, oxygen isotope composition of planktonic foraminifera (S I80) 13; b, calcium carbonate content (% of CaC03, with scale inverted); c, magnetic susceptibility (k); d, intensity of NRM; e, the inclination direction of NRM. The intensity and direction of NRM are after alternating field demagnetization at 100 Oe to remove unstable magnetization component. The S1S0 values are interpreted to reflect primarily global ice volume and the record is subdivided into the stages shown at the top, odd (even) numbered corresponding to interglacial (glacial) intervals; the age of the stage 1/2 boundary is approximately at 9.4 kyr, stage 5/6 boundary at 127kyr and the stage 7/8 boundary at 251 kyr 13. 0 U '" U " oil ~. , 0 2: c:: z ©          Nature Publishing Group1982Nature Vol. 299 7 October 1982 "' ro " 50 60 (,;V\Jv~ ~ ~ : o 70 U 2SV ~V~~ '" 80 U 90 "" " 20 100 o 10 o 4 ~i\fv .~ '0"1 i'}~1 0:: 0 ? 34 678 Core depth (m) Fig. 2 Lithological and palaeomagnetic record in deep-sea sedi-ment core RCll-209. Shown from the top are down-core logs of: a, calcium carbonate content ('Yo CaC03 with scale inverted)22; b, IRM; c, intensity of NRM; d, the declination direction of NRM. The NRM and IRM parameters are after alternating field demag-netization at 1000e. The 1800 shift in declination at 690 cm is interpreted as the Brunhes/Matuyama boundary, age 730 kyr. In this area of the world ocean, high (low) carbonate contents corres-pond to glacial (interglacial) intervals, for example, the carbonate low labelled B3 correlates to oxygen isotope stage 5 in Fig. 1. vi "" u < , 0 ~ 0:: Z for late Pleistocene sediments from the Atlantic, Caribbean, North Pacific as well as other areas of the Southern Ocean '7•'8 . Factors likely to influence the intensity of NRM are the quantity of magnetic material, typically magnetite, in the sedi-ment and the strength of the geomagnetic field at the time the remanence was acquired '9- 21 . The good correlation between NRM intensity and the 180 curve observed here (r = 0.74) may therefore signify a close relationship between geomagnetic field and climate, as has been suggested on the basis of similar comparison, assuming that variations in magnetic mineralogy can be discounted as an important contributing factor. The magnetic susceptibility provides a measure of the concentration of magnetic material and in this core is found to be highly correlated to NRM intensity (r = 0.85). The assumption that NRM intensity is simply related to geomagnetic field strength is therefore not supported. Moreover, although other non-calcareous but non-magnetic components are likely to con-tribute to the total sediment composition, for example biogenic silica and clay minerals, the high inverse correlation between k and CaC03 (r = -0.93) shows that to a good approximation the magnetic material occurs in fixed proportion to the non-carbonate fraction which in turn varies as the complement to the measured carbonate content in these sediments. In such circumstances, variations in magnetic susceptibility can be inter-preted to have climatological significance, in an analogous man-ner to that ascribed to carbonate variations. On the basis of these relationships, the observed correlation between NRM intensity and climate (for example, the 180 record) can be largely accounted for as due to an intermediary lithological effect, that is, decreased carbonate contents during glacials result in increased concentrations of magnetic material which in turn contribute to higher NRM intensities. Absence of positive evidence for a correlation of geomagnetic field and climate change is apparent even using a more representative index of relative geomagnetic field strength, obtained by nor-malizing the NRM intensity of each sample by its magnetic susceptibility. The correlation between this palaeointensity 539 index (NRM/ k) and the 180 climatic curve is negligible (r = 0.04). Another example of how NRM intensity can be more depen-dent on lithology than geomagnetic field is seen in deep-sea core RCll-209 (Jat. 3°39' N, long. 104°4' W). Previous Iitho-, bio- and magnetostratigraphical studies indicate that this core is representative of the Pleistocene sedimentary history of the equatorial Pacific22 . Figure 2 shows a comparison of carbonate content, isothermal remanence (IRM, a parameter like mag-netic susceptibility which is proportional to content of magnetic material) and NRM intensity values for the upper part of the core, down to the level marked by a 180° shift in remanent declination which is correlated to the Brunhes-Matuyama geomagnetic reversal22, now dated at 730,000 yr BP (ref. 23). There is less detail than in core RCll-l20 because of a lower deposition rate but the same kind of correlations can be seen over a longer time interval. High carbonate values in RC11-209 again generally correspond to reduced magnetic mineral con-tents (r = -0.93 for CaC03 versus IRM) which seem to account for reduced sediment magnetizations (r = 0.86 for IRM versus NRM). Even though an oxygen isotope climatic record is not available for core RC11-209, it is well known that high carbon-ate contents correspond to high global ice volumes or glacial intervals in Pleistocene pelagic sediments from the equatorial Pacific'6.22.24, exactly opposite to the phase relationship inter-preted for carbonate variations and climate in southern Indian Ocean core RCll-120. Therefore, whereas high NRM intensities characterize sediments deposited during glacial inter-vals in one region (for example, RCll-120), high NRM intensities are associated with sediments deposited during inter-glacials in another (for example, RCll-209). This is a further indication that the assumption that NRM intensity provides a reliable record of geomagnetic field intensity change is suspect, since geomagnetic intensity and climatic variations are expected to have a similar phase relationship globally. Interestingly, a weak positive correlation (r = 0.52) between NRM/IRM and CaC03 is observed in RC11-209 which can optimistically be related to some geomagnetic-climatic dependency or more likely results from an inadequacy in the normalization by IRM to take into account fully the strong lithological effect on the NRM. I thank J. E. T. Channell and J. D. Hays for critical reading of the manuscript, N. D. Opdyke for providing initial magnetic measurements on these cores, W. Robinson for carbonate determinations on core RCll-120, and NSF grants OCE78-14765 and OCE81-19695 for support of this research. Lamont-Doherty Geological Observatory Contribution No. 3379. Received 18 February; accepted 3 August 1982. I. Amerigian. C. Earth planet. Sci. Lerr. 21, 321-326 (1974). 2. Chave, A. D. & Denham. C. R. Earth planer. Sci. Lerr. 44, 150-152 (1979). 3. Wollin, G., Kukla, G. J., Ericson, D. B., Ryan, W. B. F. & Wollin, J. Nature 242, 34-37 (1973). 4, Wollin. G., Ericson, D. B., Ryan, W. B. F. & Foster. J. H. Earth planet. Sci, Lett. 12, 175-183 (1971). 5. Wollin, G., Ericson, D. B. & Ryan, W. B. F. Nature 232, 549-550 (1971). 6. Harrison, C. G. A. & Prospero, J. M. Nature 250,563-564 (1977). 7. Doake, C. S. M. Nature 267,415-417 (1977). 8. Wollin, G., Ryan, W. B. F., Ericson, D. B. & Foster, J. H. Geophys. Res. Lerr. 4,267-270 (1977). 9. Wollin, G., Ryan, W. B. F. & Ericson, D. B. Earth planer. Sci. Lerr. 41, 395-397 (1978). 10. Rampino, M. R. Geology 7,584-587 (1979). 11. Cordell, B. M. Geophys. Res. Lerr. 7,1065-1068 (1980). 12. Sternberg, R. S. & Damon, P. E. Nature 278,36-38 (1979). 13. Hays, J. D., Imbrie, J. & Shackleton, N. J. Science 194, 1121-1132 (1976). 14. Shackelton, N. 1. & Opdyke, N. D. f. Qual. Res. 3, 39-55 (1973). 15. Schott, W. Mereor 3,43-134 (1935). 16. Arrenhius, G. Swed. Deep·Sea Exped. 1947-1948 Rep. 5, 1-228 (1952). 17. Cline, R. M. & Hays, J. D. (eds) Geol. Soc. Am. Mem. 145 (1976). 18. Kent, D. V., Opdyke, N. D. & Ewing, M. Bull. geol. Soc. Am. 82, 2741-2754 (1971). 19. Johnson, E. A., Murphy, T., & Torreson, O. W. Terr. Magn. atmos. Elect. 53, 349-372 (1948). 20. Kent, D. V. Nature 246,32-34 (1973). 21. Barton, C. E., McElhinny, M. W. & Edwards, D. 1. Geophys. f.R. astr. Soc. 61,355-377 (1980). 22. Hays, 1. D., Saito, T., Opdyke, N. D. & Burckle, L. H. Bull. geol. Soc. Am. 80,1481-1514 (1969). 23. Mankinen, E. A. & Dalrymple, G. B. f. geophys. Res. 84, 615 (1979). 22. Ninkovich, D. & Shackelton, N. J. Earth planer. Sci. Lerr. 27,20-34 (1975). 

Apparent correlation of palaeomagnetic intensity and climatic records in deep-sea sediments

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Apparent correlation of palaeomagnetic intensity and climatic records in deep-sea sediments

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