Evidences of precession and obliquity orbital forcing in oxygen-18 isotope composition of Montalbano Jonico Section (Basilicata, southern Italy)

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1 Applied Radiation and Isotopes 52 (2000) 957±964 Evidences of precession and obliquity orbital forcing in oxygen-18 isotope composition of Montalbano Jonico Section (Basilicata, southern Italy) Mauro Brilli a, *, lan Lerche b, Neri Ciaran c, Bruno Turi a a Earth Science Department, University of Rome ``La Sapienza'', P.le A. Moro, Rome, Italy b Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA c Department of Geology and Geophysics, University of Bari, Via E. Orabona 4, Bari, Italy Received 13 October 1998; received in revised form 16 February 1999 Abstract Quantitative signal processing methods have been applied to a d 18 0 pro le for a land-based stratigraphic section, extending from the upper part of lower Pleistocene to the lower part of middle Pleistocene. The section is well exposed with a continuous succession of muds and muddy silts, about 400 m thick, located in the southernmost part of Bradano Trough, near Montalbano Jonico in Basilicata (south Italy). The sampled part of the section is about 240 m thick, in which a foram benthic species (Cassidulina carinata ) is continuously available for oxygen isotope ratio measurements. The aim of the data treatment is to discover how much of the Earth's orbital periodic movements, precession and obliquity, which represent the dominant periodicities in paleoclimatic variations from the base of the Pleistocene until Myr BP, are responsible for the oscillations observed in the oxygen-18 record of the Montalbano Jonico section. A time framework of the section was constructed on the basis of calcareous nannofossil biostratigraphic analyses, preliminary magnetostratigraphic results and oxygen isotope correlation with the record from DSDP s607 (isotope data collected in the NOAA World Data Center). The resulting time-scale extends from 1.15 to 0.74 Myr. Power spectrum analysis was performed on the isotope data to illuminate the most important periodicity components of the Montalbano Jonico record. The periodic components of 41,000 and 21,000 yr are present in this record; the former associated with periodic changes in the tilt of Earth's axis and the latter with periodic changes with the precession of the equinoxes, as predicted by the astronomical theory of ice ages. They are, however, not the most important components of the power spectrum, in which a lower frequency component contains most of the variance. This low-frequency component is centered at a period around 208,000 yr. This periodicity seems not to be attributable to any known astronomical or paleoclimatic phenomenon. An attempt was made to verify if this periodicity was due to the composite e ect of precession and obliquity signals together at di erent frequencies from their forcing frequencies. In order to investigate this e ect, isotope data have been parameterized in terms of a sum of simple functions of precession and obliquity signals with unknown coe cients. The coe cients are estimated from the time series with the assumption that the best coe cients are those which minimize the `noise' i.e. the di erence between the data function and the precession and obliquity functions. Cross-spectra analyses were also performed on the data and the precession signal and on the data and the * Corresponding author /00/$ - see front matter Elsevier Science Ltd. All rights reserved. PII: S (99)

2 958 M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957±964 obliquity signal. The power spectrum of the residual `noise' functions and the cross-spectra demonstrate that precession and obliquity signals are not in phase with the data at their forcing frequencies and so damp. The precession and obliquity signals were then shifted towards lower frequencies at equally spaced lags, the resultant `noise' power spectra were plotted for every combination of lags of precession and obliquity. The results of this data processing demonstrate that it is possible to have a combination of precession and obliquity cyclicities that could be responsible for the signal with 208 kyr periodicity Elsevier Science Ltd. All rights reserved. 1. Introduction Pleistocene climatic variability has been characterized by very large, pseudoperiodic uctuations in global ice volume. The record of ice volume change is clearly shown by changes in the oxygen isotope composition in calcium carbonate of forams shells found in deep sea sediment cores. These continuous records show that the variance spectrum of this indicator of ice volume uctuations is dominated by the contributions at very low frequencies. Periods that dominate these records are 100, 41 and 21 kyr and they are clearly related to the earth's orbital movements: eccentricity of earth's orbit, obliquity of earth's axis and precession of the equinoxes (Pisias and Moore, 1981). In particular, in the late Pleistocene the variations in the isotope data records are dominated by the 100 kyr cycle, while in the early/middle Pleistocene the variations seem to be dominated by the other periodicities. This ideal response of isotope records to the orbital forcing is valid under ideal conditions, veri ed in some deep sediment cores. When these conditions are not satis ed the in uence is not so easy to identify of the orbital forcing in the isotope data records. A land-based section, located in southern Italy, has been studied isotopically. The data record does not show easily interpretable trends. We have attempted to trace the orbital signal component in the data signal. Quantitative signal processing methods have been developed for this purpose. horizons (Ciaran et al., 1996). Moreover, magnetostratigraphic studies and biostratigraphic analyses on nanno ora assemblages have improved the reconstruction of this succession. The overall section is about 400 m thick, the 2. The Montalbano Jonico section In the Montalbano Jonico area, southwestern part of the Bradano Trough, one of the most recent onland marine Quaternary successions, consists of well-preserved coarsening-upward, muddy deposits belonging to the `Argille subappennine' formation. A composite section has been reconstructed by means of selected stratigraphic sections including 9 volcano-clastic layers. The correlation has been made possible through the presence of peculiar fossil assemblages that characterize each volcano-clastic layer; therefore these layers can be used as marker Fig. 1. Lithostratigraphic age plot of the composite section from Montalbano Jonico.

3 M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957± lower 150 m are characterized by hemipelagic muds and muddy silts and include the lowest volcanoclastic layers (V1±V4). The upper part of the succession, about 70 m thick, consists of silts and sands in which the remaining ve volcano-clastic layers are included. These sediments represent the topmost regressive part of a marine cycle. Paraconformably overlying this sequence are shelf sands and conglomerates that outcrop for a thickness of about 10 m. A continental, sandy, conglomeratic body covers these marine deposits (Ciaran et al., 1996) (Fig. 1). Calcareous nannofossil biostratigraphy (Marino, 1996), according to the zonation of Rio et al. (1990), allows one to refer this succession (from base to top) to the top of the large Gephyrocapsa zone, to the small Gephyrocapsa zone and to the Pseudoemiliana Lacunosa zone, respectively. In the composite section, the positions of these biozonal boundaries are in agreement with the locations of the Jaramillo subchron and the Matuyarna±Brunhes boundary, recognized by the magnetostratigraphic studies (Fig. 1). On the basis of these studies it has been possible to refer the succession to the upper part of the lower Pleistocene and to the lower part of the middle Pleistocene. 3. Isotope analyses In order to integrate the stratigraphic framework, that portion of the succession from some meters below the volcano-clastic layer V1 to several meters above the volcano-clastic layer V4 (230 m thick) has been investigated from the isotopic point of view. The selected interval contains all the marked magnetostratigraphic events. Oxygen-18 isotopic analyses have been carried out on a foram benthic species (Cassidulina carinata ), continuously available along the succession. Samples were analyzed every meter (Fig. 2). Fig. 2. Oxygen isotope record (d 18 0) with time of the composite Montalbano Jonico section.

4 960 M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957±964 Standard techniques for oxygen isotopic analysis were used. Around 50±60 individuals of the species Cassidulina carinata were picked from each sample, washed in oxygen peroxide and distilled water and then cleaned in an ultrasonic bath to remove ne fraction contamination. The CO 2 was extracted from the carbonate reaction with 100% orthophosphoric acid at 728C and separated by a series of two freezing transfer steps. The CO 2 was analyzed in an on-line 252 Finnigan Mat mass spectrometer. All the data are referred to PDB standard and expressed in the usual d notation (Fig. 2). if the obliquity and/or precession orbital forcing could be responsible for the 208 kyr, as well as the 41 and 21 kyr perodicities. The investigation asks if the composite e ect of the obliquity and precession signals, extending over the same time interval as the isotope record, could combine to give a signal response at low frequencies where the variance of the data spectrum is concentrated. Using precession and obliquity signals extending from 1,160,000 yr to 744,000 yr BP (from Laskar, 4. The age model To develop an age model of the Montalbano Jonico section a correlation has been attempted between the section and several isotope records from oceanic and Mediterranean cores. The correlation of the oxygen-18 isotope pro le with an oxygen isotope record from DSDP s607 (downloaded from the NOAA World Data Center Internet site and utilized as a reference curve, most of which has been published by Ruddiman et al. (1989)), corroborated by tie-points from the biostratigraphic and magnetostratigraphic measurements and provided a precise time framework of the Montalbano Jonico section. The resulting time-scale extends from 1.16 to 0.74 Myr (Fig. 1). 5. Data treatment In order to investigate the cyclic characteristics of the isotope data, a power spectrum analysis was performed. The power spectrum of the data (Fig. 6a) shows that the most signi cant peak is at very low frequencies. It is more instructive to transform the frequency to a period and, because the relation between frequency ( f ) and period (T )isf =1/T, the frequency of the observed peak that represents most of the variance of the power spectrum corresponds to a period of 208,000 yr. To all appearances, this periodicity cannot be attributed to any known astronomical or paleoclimatic phenomenon. The spectrum of the data contains not only the peak at 208 kyr but also other signi cant spectral peaks, centered at periods of 41, 21 and 15 kyr. The 41 kyr period and the 21 kyr period could be associated, respectively, with periodic changes in the tilt of the earth's axis and with the precession of the equinoxes, as predicted by the astronomical theory of ice ages (Imbrie and Imbrie, 1979). The principal purpose of this paper is to determine Fig. 3. Time and power spectral plots of the precession and obliquity reference signals obtained from NOAA: (a) precession signal with time; (b) precession signal power spectrum with frequency; (c) obliquity signal with time; (d) obliquity signal power spectrum with frequency.

5 1990), we have three records for the same time interval (isotope data, precession and obliquity reference records). It is possible to calculate a function of the data, D(t ), parameterized in terms of a sum of the precession, S p (t ) and obliquity, S o (t ), signals with unknown coe cients a and b and residual component I(t ), where I(t ) is that part of the data not due to precession and obliquity forcing; consequently I(t ) is considered as `noise'. We can write M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957± D t ˆaS o t bs p t I t : 1 In this equation, the following assumptions have been made:. linear addition of precession and obliquity signals is permitted;. the precession and obliquity records, utilized in the following development, are appropriate for the time interval considered;. the timescale of the isotope record is correct. To determine how much of the precession and the obliquity signals could be responsible for the data signals, the coe cients a and b have to be estimated by minimizing the di erence function D t as o t bs p t ˆI t 2 Fig. 5. Cross-phase spectra with frequency of the isotopic signal together with: (a) obliquity; (b) precession. for all measurement times t i (i =1,..., N ). A least squares control function, L, is given by: L ˆ XN D t i as o t i bs p t i 2 ˆ XN I t i 2 iˆ1 iˆ1 3 which, when minimized with respect to a and ˆ X D i as oi bs pi S oi ˆ X D i S pi a X S 2 oi b X S pi S oi ˆ ˆ X D i as oi bs pi S oi ˆ X D i S pi a X S oi S pi b X S 2 pi ˆ 0 5. From Eqs. (4) and (5), the two coe cients a and b can be calculated as a ˆ X X S 2 pi Di S oi X X D i S pi Spi S oi X X S 2 pi S 2 oi X S pi S oi 2 6 b ˆ X X S 2 oi Di S pi X X D i S oi Spi S oi X X S 2 pi S 2 oi X S pi S oi 2 7 Fig. 4. Cross-coherency spectra with frequency of the isotopic signal together with: (a) obliquity; (b) precession. where the index i, denoting each time t i, has been omitted for clarity of presentation. Then the `best t' a and b values can be used to calculate I(t ) from:

6 962 D t as o t bs p t ˆI t : M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957±964 8 Fig. 7. Power spectra with frequency of: (a) the original d 18 0 isotopic data and the minimum least squares `noise' after using the best shifting frequencies for the precession and obliquity signals; (b) the shifted component of obliquity which contributes to the observations; (c) the shifted component of precession which contributes to the data. Note the dominance of precession in accounting for the d 18 0 observations at 208 kyr. In practice it is more useful to operate in frequency space and to display the functions in terms of frequency, translating from the time domain to the frequency domain. The graphs in 3(a)±(d) show, respectively, precession and obliquity signals in time and their power spectra, while Figs. 6(a) and (b) show a corresponding presentation of the data, D(o ) and of I(o ) calculated from Eq. (8), using the coe cients calculated from Eqs. (6) and (7). Cross-spectral analyses were also performed: (i) on the data and the precession signal (Fig. 4a) and (ii) on the data and the obliquity signal (Fig. 4b). The crosscoherency spectra show the highest coherence at the precession and obliquity forcing frequencies (Figs. 4a and b). More interestingly, the cross-phase spectrum (Figs. 5a and b) shows that the precession and obliquity are in phase with the data signals at very low frequencies but not at their forcing frequencies. This behavior is also shown by the power spectrum of the `I' function calculated from Eq. (8) in which, at the precession and obliquity forcing frequencies, l(o ) assumes negative values. Evidently, the lags in the responses of precession and obliquity cause damping of the isotope signals at the forcing frequencies. 6. Frequency shifting analysis Fig. 6. Power spectra with frequency of (a) the original d 18 0 isotopic data from the Montalbano Jonico; (b) the residual `noise', I; (c) overlay of (a) and (b). It is possible that a combination of the precession and obliquity signals at di erent frequencies from their forcing frequencies could be responsible for the data signal. In order to investigate this e ect, the precession and obliquity signals were shifted towards lower fre-

7 M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957± Fig. 8. Variation of `noise' power at all combinations of frequency lags (O p for precession, O o for obliquity) indicating a relatively good determination of O p but a relatively poor estimate of O o (units of the O-values are cycles/kyr). quencies at equally spaced lags, the `noise' power spectra plotted for every combination of lags of precession and obliquity (Figs. 7a,b). Thus one minimizes D o as o o no o bs p o mo p ˆI o over all frequencies o, where n =1, 2,..., N; m = 1,2,..., M. A three-dimensional graph has been constructed (Fig. 8) that shows the variation of P i I o i 2 for every combinations of O o and O p. Fig. 8 illustrates that there is greater resolution in determining O p than O o, which has various minimal valleys. But, in any event, the minimum point on Fig. 8 is located in a region where the contribution of obliquity to the low frequency isotopic signal is almost negligible, while the contribution of the precession is due mainly to the shifted peak, originally centered at 19 kyr, while the e ect of the 23 kyr peak is negligible. 9 The results of this data processing demonstrate that it is possible to have a combination of precession and obliquity cyclicities that could be responsible for the signal with 208 kyr periodicity. A more detailed investigation could be carried out on the obliquity contribution in order to increase its resolution and determine if a better re ned estimate would, or could, contribute more to the 208 kyr signal in the isotope data. But it would appear that the precession component is the dominant contribution irrespective of any such re ned estimate. Acknowledgements The authors wish to thank Professor D. F. Williams for the courtesies a orded M. Brilli during a visit to the University of South Carolina; C.N.R. `Centro di Studio per il Quaternio e 1'Evoluzione Ambientale' for making available the facilities of the Mass Spectrometry Laboratory and Mr. Marco Mola for his assistance with the analytical work. 7. Conclusions References Ciaran, N., D'Alessandro, A., Loiacono, F., Marino, M., A new stratigraphical section for the marine Quaternary in Italy. Palaeopelagos 6, 361±370. Imbrie, J., Imbrie, K.P., Ice Ages: Solving the Mystery. Harvard University Press, Cambridge, MA. Laskar, J., The chaotic motion of the solar system: a

8 964 M. Brilli et al. / Applied Radiation and Isotopes 52 (2000) 957±964 numerical estimate of the chaotic zones. Icarus 88, 266± 291. Marino, M., Quantitative calcareous nannofossil biostratigraphy of the lower±middle Pleistocene Montalbano Jonico section (southern Italy). Palaeopelagos 6, 534±541. Pisias, N.G., Moore Jr, T.C., The evolution of Pleistocene climate: a time series approach. Earth Planet. Sci. Lett. 52, 450±458. Rio, D., Ra, I., Villa, G., Pliocene±Pleistocene calcareous nannofossil distribution patterns in the western Mediterranean. In: Kastens, K., Mascle, J. (Eds.), Proc. O.D.P. Scienti c Results, 107, pp. 513±533. Ruddiman, W.F., Raymo, M.E., Martinson, D.G., Clement, B.M., Backman, J., Pleistocene evolution: northern hemisphere ice sheets and north Atlantic Ocean. Paleoceanography 4, 353±412.