In this paper we present the results on the relative changes of the geomagnetic field intensity measured in the Tyrrhenian sea core CT85-5 between 23 and 51 ky BP in order to investigate the origin of the enhancement of the cosmogenic isotope 10Be concentration, recently reported in the same core at 35 ky BP

Bonino, G.

Cini Castagnoli, G.

Lehman, B.

Taricco, C.

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IL NUOVO CIMENTO Marzo-Aprile 1998VOL. 21 C, N. 2Cosmogenic isotopes and geomagnetic signalsin a Mediterranean sea sediment at 35 000 y BPG. CINI CASTAGNOLI (1), G. BONINO (1), C. TARICCO (1) and B. LEHMAN (2)(1) Dipartimento di Fisica Generale dell’ Università, Torinoand Istituto di Cosmogeofisica del CNR - C.so Fiume 4, I-10133 Torino, Italy(2) Istituto Nazionale di Geofisica - Roma, Italy(ricevuto il 10 Dicembre 1997; approvato il 22 Dicembre 1997)Summary. — In this paper we present the results on the relative changes of thegeomagnetic field intensity measured in the Tyrrhenian sea core CT85-5 between 23and 51 ky BP in order to investigate the origin of the enhancement of the cosmogenicisotope 10Be concentration, recently reported in the same core at 35 ky BP.PACS 91.25 – Geomagnetism and paleomagnetism.PACS 96.40 – Cosmic rays.1. – IntroductionWe have reported in a recent paper [1] the evidence for an enhanced 10Be depositionat about 35 ky BP in the Mediterranean sea sediment core CT85-5. The peak is similarboth in age and amplitude to the increase in the 10Be concentration observed in Vostokand Dome C ice cores [2]. Be and Zn profiles were also measured in the same sea corein order to enquire whether biogeochemical and sedimentological processes wouldproduce variations similar to those observed in the 10Be concentration. We obtainedclear evidences in favour of the view that the global enhancement of the 10Be flux isrelated to an increase in the production rate and not to a redistribution of the 10Befallout between different latitudes and reservoirs.Changes of the production rate can be related to the cosmic-ray flux variations dueto helio- and geomagnetic modulation effects or due to fluctuations in the primarycosmic-ray flux. In this paper we enquire the possibility that the precise cause of theobserved peak could be a concomitant decrease in the Earth magnetic moment.2. – Experimental dataThe 10Be concentration measurements in the CT85-5 Tyrrhenian core are givenin [1]. In fig. 1a) we show the part of the profile that includes the peak. This peak hasan absolute age estimated to be 3463 ky BP on the basis of its stratigraphic positionG Società Italiana di Fisica 243G. CINI CASTAGNOLI, G. BONINO, C. TARICCO and B. LEHMAN244Fig. 1. – a) 10Be profile from 23 to 51 ky BP. b) Curves for NRM with ARM and x normalizations asVADM calibration using the volcanic data available.between two tephra layers originating from two volcanic eruptions (CampanianIgnimbrite and Citara) dated by K-Ar, 40Ar-39Ar and 14C methods [3-5] (shadowedareas in fig. 1). We briefly discuss here the correction in the time scale, to be applieddue to the discontinuity in the sedimentation rate introduced by the thick CampanianIgnimbrite (CI) layer. To evaluate the actual duration of the deposition of the vitricCOSMOGENIC ISOTOPES AND GEOMAGNETIC SIGNALS ETC. 245tuffs, we consider the dilution effect produced in the 10Be concentration by the volcanicmaterial. The average concentration (see fig. 1a)) over 12 cm (1200 y) before the CI is(1061) Q108 at/g and over 12 cm after the CI is (13.561) Q108 at/g. The meanconcentration value of 11.7 Q108 at/g together with the measured average concentrationof 2 Q108 at/g may be used to evaluate the dilution factor of about 6. A sedimentation rateof 5.9 Q1022 cm/y is therefore used during the CI layers instead of the average value1022 cm/y . The time scale of fig. 1 is constructed according to these results. Theradiocarbon test of this time scale has been also performed [6].In order to obtain a record of the relative changes of the geomagnetic field from thecore CT85-5, we have used the ratio of the Natural Remanent Magnetization (NRM) tothe Anhysteretic Remanent Magnetization (ARM) or to the low-field susceptibility (x)which are both related to the amount of magnetite. This method is classically used forrelative paleointensity studies deduced from marine sediments.We have examined the rock-magnetic characteristics of the core to determinewhether they are rock-magnetically uniform and suitable for estimating relativepaleointensity. Values of NRM, ARM and x were used with hysteresis parameters. TheNRM and the ARM were measured on U-channel with a small access high-resolution3-axes cryogenic magnetometer. In line AF demagnetization was used throughout withdemagnetization steps at 5, 10, 15, 20, 25, 30, 40, 50 and 60 mT for the NRM and at 10,20, 25, 30, 40, 50 and 60 mT for the ARM. Hysteresis parameters and derived para-meters were obtained using an alternating gradient field magnetometer. Forestimating relative paleointensity the core should meet three main requirements:a) homogeneity of the magnetic minerals; b) low-concentration changes; c) homogene-ity of magnetic granulometry [7]. The results of our investigations show that theCT85-5 core is suitable for paleointensity studies (paper in preparation). The curves forNRM with ARM and x normalizations are shown in fig. 1b) as Virtual Axial DipoleMoment (VADM) calibration using the volcanic data available [8]. We can see thatthere is a fairly good agreement between our curves and the volcanic data. Verypronounced minima are observed around 35 ky BP. These values can be correlated withthe Laschamp event which occurred during a low-intensity period around 40 ky. Thisevent has been observed on Massif Central (France) and Iceland lavas and has beendated by different methods at different localities between 33 and 50 ky [9, 10].Our core meets well with all the requirements that are generally asked to makepaleointensity studies and offers the advantage to record paleomagnetic intensity inthe same samples in which the 10Be concentration was measured. The time sequence isthe same for the two records.At this stage of the study we cannot exclude perturbations in the deposition of themagnetic grains due to short-time climatic events. However, the results seem veryinteresting and worthy of further investigation.3. – Discussion of the resultsThe 10Be profile (see fig. 1a)) shows a rise from the average concentration (661)108 at/g at cm 385 up to (1061) Q108 at/g at cm 334, where the CI ash layer startsto disturb the record, diluting 10Be atoms in the volcanic tuffs. At cm 289, where theglass shards disappear, the 10Be concentration rises again reaching its maximum valueof 16 Q108 at/g at cm 280. The relative paleointensity (see fig. 1b)) drops at cm 368 fromG. CINI CASTAGNOLI, G. BONINO, C. TARICCO and B. LEHMAN246the average value and stays fairly constant at this low value until the volcanic ashlayer starts. Thereafter the average value is restored.The ratio 10/641.6 in the 10 Be production rate attained at cm 334 is in goodagreement with the value k4O1.541.6 expected from the concomitant variations of theEarth magnetic moment. However, we may notice: a) that the peak of the 10Beconcentration after the volcanic ash layer is not easily explained by this process andb) that the 10Be increase starts about 18 cm (A1800 y ) before the magnetic momentdrops. These results cannot unambiguously lead to the conclusion that the magnetic-moment variation is the primary origin of the cosmogenic isotope event but certainlysupport the view that the two very exceptional behaviours partly overlap.Further studies are in progress in this core to understand the concurrence of suchinteresting phenomena around 35 ky BP.* * *We thank C. LAJ and K. KISSEL for the collaboration and J. BEER for helpfuldiscussions.R E F E R E N C E S[1] CINI CASTAGNOLI G., ALBRECHT A., BEER J., BONINO G., SHEN CH., CALLEGARI E. andTARICCO C., Geophys. Res. Lett., 22 (1995) 707.[2] RAISBECK G. M., YIOU F., BOURLES D., LORIUS C., JOUZEL J. and BARKOV N. I., Nature, 326(1987) 273.[3] CASSIGNOL C. and GILLOT P. Y., in Numerical Dating in Stratigraphy (Wiley, New York)1982, p. 159.[4] DEINO A. L., SOUTHON J. and ORSI G., Proceedings VIII International Conference onGeocosmochronology, Berkeley, 1994.[5] GILLOT P. Y., Datation par la methode du potassium-argon des roches vulcaniques recentes(Pleistocene et Holocene), Thèse Dr. Sci. Paris-Sud, 1984, p. 249.[6] HAJDAS I., BEER J., BONANI G., BONINO G., CINI CASTAGNOLI G. and TARICCO C., Past andPresent Variability of the Solar-Terrestrial System: Measurement, Data Analysis andTheoretical Models, in Proceedings of the International School of Physics Enrico Fermi,Course CXXXIII, edited by G. CINI CASTAGNOLI and A. PROVENZALE (IOS Press,Amsterdam) 1997, p. 83.[7] KING J. W. et al., J. Geophys. Res., 88 (1983) 5911.[8] TRIC E. et al., Phys. Earth Planet. Int., 85 (1994) 113.[9] GILLOT P. Y., LABEYRIE J., LAJ C., VALLADAS G., GUERIN G., POUPEAU G. and DELIBRIAS G.,Earth Planet. Sci. Lett., 42 (1979) 444.[10] LEVI S., AUDUNSSON H., DUNCAN R.A., KRISTJANSSON L., GILLOT P. Y. and JAKOBSSON S. P.,Earth Planet. Sci. Lett., 96 (1990) 443.

Cosmogenic isotopes and geomagnetic signals in a Mediterranean sea sediment at 35000 y BP

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Cosmogenic isotopes and geomagnetic signals in a Mediterranean sea sediment at 35000 y BP

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