Milankovitch Cyclicity and Rock-Magnetic Signatures of Palaeoclimatic Change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy

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Cretaceous Research (1999) 20, 189–214 Article No. cres.1999.0145, available online at http://www.idealibrary.com on Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy

Helmut Mayer1 and Erwin Appel

Institut fu¨r Geologie und Pala¨ontologie, Abteilung Geophysik, Eberhard-Karls-Universita¨t Tu¨bingen, Sigwartstr. 10, 72076 Tu¨bingen, Germany. 1Present address: Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, USA; email: [email protected]; also at: Geomathematik, Fachbereich VI Geographie/Geowissenschaften, Universita¨t Trier, 54286 Trier, Germany

Revised manuscript accepted 21 October 1998

Detailed cyclostratigraphic analyses of the Valanginian to Hauterivian part of the Biancone Formation, a pelagic nannofossil limestone in the Southern Alps of Italy, were carried out. The Cismon section in the Belluno Trough near Feltre and the Pra da Stua section on the Trento Plateau near Avio were studied. Carbonate content, magnetic susceptibility and natural remanent magnetization were measured on densely spaced samples from Cismon. The first two properties vary in a cyclic fashion in this pelagic limestone section and are almost perfectly negatively correlated, while cyclicity in natural remanent magnetization is only vaguely indicated. Quantitative time-series analysis is critical in cyclic stratigraphy. The geostatistical method of cova functions (a generalization of the cross-variogram) which has proven to be the most versatile and robust time-series-analysis method is applied. Cova functions can be calculated from unevenly and non-correspondingly spaced time series without any preprocessing. This method also retains relatively more of the signal when noise and extreme outliers obscure the picture. The periodicities detected in the Cismon time series fall in the range of Milankovitch cycles. Cycle periods of 45 cm, 80 cm and 180 cm likely correspond to dominant precession, obliquity and eccentricity cycles. Owing to the inaccuracy of the Cretaceous time scale, periods cannot be matched exactly, but cycle ratios are extremely close to expected ratios so that Milankovitch climate cycles could be positively identified in this Early Cretaceous section. In the Pra da Stua section bedding thickness was measured and analyzed quantitatively. A cycle period of 55 cm is dominant in this data set, while periods of 115 cm and 170 cm are only vaguely indicated, although bedding in the sampled interval visually appears cyclic and even hierarchically structured. It can be expected that densely spaced measurements of sedimentary properties such as susceptibility and carbonate content will reveal the cyclicity much better. This identification of Milankovitch cyclicity in the pelagic Biancone Formation has important consequences for our understanding of the climate system in the past. These results demonstrate that orbital forcing was effective enough to create palaeoclimatic cycles even in the Cretaceous warm, equable, ice-free climate state. Magnetic susceptibility proved to be a reliable proxy for carbonate content reflecting palaeoproductivity cycles in this pelagic setting. 1999 Academic Press

K W: Milankovitch cycles; rock magnetism; palaeoclimate; carbonate content; susceptibility; Biancone Formation; Valanginian; Hauterivian; Southern Alps.

1. Introduction the Mesozoic sequence of the Southern Alps oﬀered The focus of this study is on sedimentary-parameter most favourable conditions. The Valanginian to variations through the sections studied, their analysis Hauterivian portion of the Cismon section was as stratigraphic time-series and their palaeoclimatic studied in detail. Rock-magnetic results from Cismon interpretation. The role of rock-magnetic parameters and Pra da Stua are presented. The cyclostratigraphy in this context is emphasized. The study area was of the Cismon section is investigated utilizing suscep- selected based on the following criteria: continuity of tibility and carbonate-content ﬂuctuations. For the sedimentation, uniformity of facies, lack of tectonic Pra da Stua section bedding-thickness measurements and metamorphic overprint, low degree of diagenetic are analyzed. Quantitative time-series-analysis is alteration and quality of exposure. In all these respects applied to the evaluation of these geologic time series.

0195–6671/99/020189+26 $30.00/0 1999 Academic Press 190 H. Mayer and E. Appel

fossil ooze was deposited at the basins in depths of several thousand metres (Bosellini & Winterer, 1975). The Trento Plateau also received a reduced thickness of this pelagic sediment. The resulting limestone is generally known as the Maiolica Formation or locally as the Biancone Formation. In the Belluno Trough, where the Cismon section is situated, the Biancone Formation extends stratigraphically from latest Tithonian/Berriasian to Aptian (Weissert, 1981). In general the Maiolica/Biancone is characterized by abundant slumps and similar synsedimentary deformation features (Weissert, 1981). Pelagic conditions Figure 1. Location map for Cismon and Pra da Stua prevailed through the Cretaceous into the Eocene sections in northern Italy. Palaeogeographic domains of when terrigenous flysch was deposited in response to the Southern Alps are shown: diagonal ruling— Lombardian Basin; horizontal ruling—Trento Plateau; the onset of Alpidic deformation. The tectonic defor- stippled pattern—Belluno Trough; vertical ruling— mation of the Southern Alps during the Alpidic orog- Friuli Shelf (boundaries after Gaetani, 1975, and eny produced gentle large-scale folds, thrust faults Weissert, 1981). and transcurrent faults (e.g., van Bemmelen, 1966; Doglioni & Bosellini, 1987). During the Neogene the Venetian Alps in particular, i.e., that part of the The palaeoclimatic significance of Milankovitch cycles Southern Alps where the Cismon section is located, and rock-magnetic parameters in the Cretaceous is have been deformed into a fold-and-thrust belt of discussed. This paper contains overview sections to coherent thrust blocks with little internal deformation provide some background information about the (cf., Doglioni, 1992). Overall, the Alpidic defor- various fields of research tied together here. mation of the Southern Alps was relatively mild compared to that of the Western and Eastern Alps.

2. Geological setting Stratigraphic framework Evolution of the Southern Alps The development of the stratigraphic sequence of the The sections studied are located in the Southern Southern Alps (Figure 2) since the Jurassic was as Alps of northern Italy (Figure 1), which represent a follows. On the Trento Plateau peritidal conditions tectonically inverted, former passive continental prevailed through the Liassic. During the Middle margin. Jurassic a thin layer of red nodular limestone (Rosso Extensional tectonism began in the Early Jurassic Ammonitico Inferiore) covered the platform reflecting with the rifting and opening of the Piemonte-Ligurian its drowning by suddenly increased subsidence. Tethys Ocean. On its southern end a block-faulted Pelagic conditions continued with the deposition of extended continental margin formed, whose palaeo- cherty aptychus limestones (Oxfordian Fonzaso relief is clearly reflected in Jurassic facies distributions Formation) and the Kimmeridgian Rosso Ammo- in the Southern Alps (Aubouin, 1963; Bernoulli & nitico Superiore (Bosellini et al., 1981). Starting with Jenkyns, 1974; Gaetani, 1975; Winterer & Bosellini, the Tithonian the white nannofossil-lime ooze of the 1981). From west to east, the alternating palaeogeo- Biancone Formation covered the area and accumu- graphic basins and swells are the Lombardian Basin, lated slowly through the Early Cretaceous and Trento Plateau, Belluno Trough and Friuli Shelf Cenomanian, when it was replaced by the Scaglia (Figure 1). Rossa, reddish pelagic limestones and marls extending Jurassic breccias and slump deposits as well as across the Cretaceous/Tertiary boundary (Premoli thickness contrasts along the boundaries between Silva & Luterbacher, 1966). Near the western margin these blocks are evidence for synsedimentary normal of the Trento Plateau the varicoloured marls and faulting (Bernoulli, 1964; Bosellini et al., 1981). Dur- marly limestones of the Scaglia Variegata are inter- ing the latest Jurassic and Early Cretaceous the entire calated between Biancone and Scaglia Rossa, similar realm deepened, and differential movements between to the situation in the Lombardian Basin (Cita, 1964). the blocks diminished so that a blanket of pelagic In the Eocene the Scaglia Rossa grades into the marls nannofossil-lime ooze draped over the pre-existing of the Scaglia Cinerea. Volcanic layers are common in relief which was gradually levelled out. This nanno- these Palaeogene beds. However, during the Eocene a Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 191

This study is restricted to the Lower Cretaceous Biancone Formation of the Trento Plateau and the Belluno Trough, which corresponds to the Maiolica of the Lombardian Basin to the west and to the Maiolica of the Umbrian-Marchean Basin in the Apennines. Sedimentologic and lithostratigraphic investigations of the South Alpine Maiolica/Biancone Formation were carried out by Weissert (1981), Barberis et al. (1992) and Bersezio (1993). Since the chronostratigraphy, biostratigraphy, magnetostratigraphy and geochronology of the Early Cretaceous stages covered are still active fields of research with many unresolved problems, a number of conflicting correlations are in use. The chronostratigraphy of the Early Cretaceous is, continuing from the Jurassic, based on ammonite zones. These are well studied in stratotypes in southeastern France (Cotillon et al., 1984; Bulot et al., 1992). However, as ammonites are rather rare in most portions of the pelagic sequence of the South Alpine and Apenninic basins, microfossils and nannofossils have become the most useful groups for biostratigraphy there. In the Upper Tithonian to Lower Aptian Maiolica of the Apennines, Micarelli et al. (1977) established a microfossil zonation based on nannofossils, calpionellids and foraminifera. Similar efforts were undertaken in the Southern Alps by Channell et al. (1979) who included magnetostratigraphy. The foraminiferal zonation goes back to Sigal (1977), while the calpi- Figure 2. Schematic stratigraphic succession of Mesozoic onellid zonation was established by Allemann et al. strata in the Southern Alps (Dolomites/Venetian Alps (1971). The first calcareous-nannofossil zonation of sector). Special facies on the western Trento Plateau on the left (after Gwinner, 1971, and Bosellini et al., Thierstein (1971) was continuously refined (cf., 1981). Bralower, 1987; Bralower et al., 1989; Bergen, 1994). Nevertheless the correlation to stage boundaries and magnetic-polarity chrons is fraught with problems drastic change in the depositional conditions occurred (cf., Rawson, 1983; Cooper, 1984; Remane, 1986; with the onset of flysch sedimentation which was Ma´rton, 1986). A number of investigations in the followed by molasse from the Miocene onwards. The Cretaceous of the Southern Alps have contributed to neighbouring Belluno Trough features a very similar an improvement of these correlations (e.g., Channell but thicker and more complete sequence (Praturlon & et al., 1979, 1987; Channell & Medizza, 1981; Sirna, 1975). The main differences are in the Lower Bralower, 1987; Channell & Grandesso, 1987; Erba & and Middle Jurassic. The plateau margin migrated Quadrio, 1987; Channell & Erba, 1992). successively west during the Early and Middle Jurassic A new definition of Cretaceous stage boundaries is as individual fault blocks subsided (Sarti et al., 1992). currently being developed. The boundaries of interest The Oxfordian Fonzaso Formation, which occurs to this study have recently been proposed as follows only in patches on the Trento Plateau, is coherently at the Second International Symposium on Creta- developed in the Belluno Trough. Starting with the ceous Stage Boundaries, Bruxelles, 8–16 September, Rosso Ammonitico Superiore in the Kimmeridgian 1995 (J. Erbacher, personal communication 1995; the relief between the former swell and trough had Mutterlose, 1995): the Berriasian/Valanginian bound- been greatly reduced. The Biancone and the remain- ary at the base of the Calpionellites darderi Zone (very der of the sequence are almost identically developed in preliminary), the Valanginian/Hauterivian boundary both areas apart from thickness differences and the at the first occurrence of the ammonite genus Acanth- lack of volcanism in the Belluno Trough. odiscus corresponding to the last occurrence of the 192 H. Mayer and E. Appel nannofossil Tubodiscus verenae and a positive carbon- landmark volume titled ‘‘Symposium on Cyclic isotope (ä13C) excursion, and the Hauterivian/ Sedimentation’’ (Merriam, 1966) by the Kansas Barremian boundary at the first occurrence of the Geological Survey. ammonite species Spitidiscus hugii. Recently Cecca Duff & Walton (1962) made an effort to assess the et al. (1994) and Channell et al. (1995a) proposed cyclicity of sedimentation quantitatively by the appli- revisions to the correlations between biostratigraphic cation of statistical methods, which was later zones, magnetic chrons and the Hauterivian and expanded by Duff et al. (1967). Indeed, this is the Barremian stage boundaries based on new ammonite only way to document true cyclicity convincingly, finds. Earlier, Channell et al. (1993) had attributed since geologists tend to see more order in nature than reduced intensities of magnetization in the upper there actually may be, as Zeller (1966) suggested in Valanginian, where the positive ä13C excursion oc- his provocative article ‘Cycles and psychology’. curs, to increased magnetite dissolution by reduction Various types of cyclic succession have been recog- which in turn was induced by increased carbon burial. nized. In a succession of more than two lithologies According to the new set of magnetobiostratigraphic (e.g., a, b, c) cycles can be symmetric (a-b-c-b- correlations proposed by Channell et al. (1995a) and a-b-c. . .) or asymmetric (a-b-c-a-b-c. . .). Such cycles by Erba et al. (1995) the Valanginian would comprise are common in clastic or mixed carbonate-clastic magnetic chrons CM15N through CM11R, while the sequences, marine, lacustrine or fluvial as in the Hauterivian would extend from CM11N through Carboniferous coal measures where cyclic sedimen- CM4. This means that the sections described tation was studied early on. They also occur in here would belong entirely to the Hauterivian, since shallow-water carbonate platforms with subtidal, in- they could be correlated to chrons CM10N-CM8 tertidal and supratidal beds (e.g., Read et al., 1986). (Cismon) and CM10-CM5 (Pra da Stua), respect- In hemipelagic and pelagic limestone-marl alter- ively (Mayer, 1996, 1997). However, until these rec- nations, where the focus of research in cyclic sedimen- ommendations are formally accepted, it is preferred to tation has shifted to since the 1970s (e.g., Dean et al., designate the sections of this study as ‘Valanginian to 1978; Volat et al., 1980; Einsele, 1982; Cotillon, Hauterivian’, which is also consistent with the low 1984; Einsele & Ricken, 1991; Huang et al., 1993), magnetization intensities encountered (see below). generally only two lithologies are involved (a-b-a-b-a- b. . .), but redox cycles may be superimposed on the carbonate cycles resulting in the occurrence of inter- Sedimentary cyclicity vening black-shale beds. In many cases the limestone Apparent stratigraphic cycles in sedimentary rock member of the couplet is thicker than the correspond- sequences are common throughout the stratigraphic ing marl or shale member, but the opposite situation record, occur in a variety of depositional environ- also occurs (Einsele, 1982). Cyclic fluctuations of ments and have caught the attention of geologists carbonate content can even be detected in mon- early on (see Gilbert, 1895, 1900). Weller (1930) otonous limestone sequences, thus providing a con- recognized the widespread repetition of characteristic tinuously defined lithologic parameter which lends sedimentary cycles in the Pennsylvanian (Upper itself to direct numerical analysis. Carboniferous) of the North American midconti- In the case of clastic cycles and of most platform- nent consisting of a given sequence of lithologies. carbonate cycles their primary depositional origin is From then on research on sedimentary cyclicity obvious and goes unchallenged. This is not the case concentrated on the transgressive/regressive cycles for limestone-marl alternations and carbonate cycles in the coal-bearing Carboniferous deposits in North in limestones. A purely diagenetic origin or a domi- America (Weller, 1966) as well as in Britain nant diagenetic modification has been postulated for (Robertson, 1952). Starting with Schwarzacher’s this type of cycle by several authors (e.g., Sujkowski, (1947, 1954) pioneering observations on periodic 1958; Hallam, 1964, 1986, 1987; Eder, 1982; changes in bedding thickness of the Dachsteinkalk of Ricken, 1985, 1986; Ricken & Eder, 1991; Thierstein Lofer in the Triassic of the Northern Calcareous Alps, & Roth, 1991). On the other hand, many studies Austria (in particular the hierarchical structure with demonstrated that rhythmic diagenetic unmixing bundling of five smaller cycles in one large cycle), from a perfectly homogeneous primary marl sediment continued by Fischer’s (1966) sophisticated elabor- is extremely implausible (cf., Arthur et al., 1984; ation on the Lofer cyclothems, this type of cyclicity in Fischer, 1986; Ogg et al., 1987; Weedon, 1987) and sedimentary rocks began to attract more and more that at least a small primary difference in carbonate interest of the geologic community. This increased content or structure must have existed which may interest became manifest in the publication of a have been more or less modified by diagenesis. Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 193

Bioturbation is another factor perturbing primary Section description sedimentary signals (e.g., Guinasso & Schink, 1975) and could potentially obliterate cyclicity. For Two sections in the Lower Cretaceous nannofossil example, Berger & Heath (1968) demonstrated that limestone of the Biancone Formation, Cismon and the effective vertical displacement of particles is 9 cm Pra da Stua, were studied. for a typical homogeneous-layer thickness of 4 cm in deep-sea settings. This is a considerable aberration if Cismon. Geographically, the Cismon section is located we look for first or last appearances of microfossils for in the valley of the river Cismon. Palaeogeographi- biostratigraphic purposes. However, if we look for cally, it belongs to the western part of the Belluno cyclicity using quasi-continuous physical or chemical Trough (Figure 1). The Valanginian to Hauterivian properties, the effect of bioturbation is much less part of the section, which was sampled, is exposed in severe, it acts like a moving-window averaging filter the steep walls along the old road around the tunnel and thus reduces the highest attainable resolution ‘‘Pala della Lerla’’ (46.1518)N/11.9063)E), 15 km (e.g., Trauth, 1995). It does not distort periodicities northwest of the city of Feltre on the road (SS 50) in the Milankovitch band at typical pelagic sedimen- leading to Passo Rolle (Figure 3a). tation rates, but may decrease the amplitudes of peaks An almost continuous succession through the in power spectra (Dalfes et al., 1984; Pestiaux & Cretaceous and into the Tertiary is accessible in Berger, 1984). roadcuts and cliffs along the river. The Biancone Additionally, the effect of variable sedimentation Formation extends here from the higher Tithonian rates on cyclic signals in the sedimentary record needs into the lowermost Aptian. Above that a varicoloured to be considered. The magnitude of sedimentation marly interval follows, the Scaglia Variegata which rate seems to have little effect except for the obvious resembles the Umbrian Scisti a Fucoidi and extends situation of high-amplitude high-frequency signals at through the Cenomanian. A hiatus is present in the too low a sedimentation rate. In this case the signal Aptian to Albian (Channell et al., 1979). The remain- cannot be resolved in the sedimentary record (Morley der of the sequence consists of Scaglia Rossa. Previous & Shackleton, 1984). Very strong changes of sedimen- studies dealt with the sedimentology (Weissert, 1981), tation rate through one section will obviously the magnetic stratigraphy at lower resolution and obliterate a time-periodic signal, so that it may be biostratigraphy of the Hauterivian to Campanian por- unrecoverable in extreme cases. However, minor fluc- tion (Channell et al., 1979) and Maastrichtian to tuations of sedimentation rate do not obliterate the Eocene portion (Channell & Medizza, 1981), and periodicities severely, so that the resulting distortions with the stable-isotope stratigraphy of the Aptian/ may be eliminated by mathematical procedures Albian portion (Weissert et al., 1985). Claps et al. (Schiffelbein & Dorman, 1986; Park & Herbert, 1987; (1991) performed cyclostratigraphic time-series Trauth, 1995). analysis on the Cenomanian Scaglia Variegata at Another subject of controversy regarding sedimen- Cismon independent of our cyclostratigraphy in the tary cycles is the question whether the cycles are Biancone (Mayer, 1993, 1994). A new high- created by environmental oscillations within the depo- resolution magnetostratigraphy was established sitional system entirely driven by sedimentation itself recently (Mayer, 1995, 1996, 1997). (‘autocycles’) or whether the cyclic sedimentation is The lower part of the section is particularly well driven by oscillations of some driving force outside the suited for stratigraphic studies, because sedimentation depositional system (‘allocycles’), possibly of global was continuous, the succession is not disturbed scale. Note the slightly generalized usage of the terms by slumping, and faulting produced only small off- ‘autocycle’ and ‘allocycle’ compared to Beerbower’s sets which are readily restored. Unlike higher up in (1966) original definition; it portrays the widely the section bedding is not so clearly developed adopted usage of these terms more exactly (cf., in the Valanginian part as to allow unambigu- Ginsburg, 1971; Fischer, 1986; Osleger, 1991). ous bedding thickness measurements which have Schwarzacher (1993b) demonstrated with the help of mostly been used to date for cyclostratigraphic numerical simulations that the influential autocycle studies in Cretaceous pelagic carbonates (e.g., model of Ginsburg (1971) for carbonate platforms Fischer, 1980; Schwarzacher & Fischer, 1982; Fischer cannot work. & Schwarzacher, 1984; Herbert, 1992). This less In the following study the apparent cyclicity of clearcut bedding is due to only small-amplitude the limestone-marl type in the Lower Cretaceous variations of an overall high carbonate content of Biancone Formation of the Southern Alps is c. 90%. At Cismon, the Biancone Formation consists investigated. of white to light grey pelagic nannofossil limestones 194 H. Mayer and E. Appel a Additional deformation is restricted to gentle warping, monoclinal folding and dragfolding next to faults. These normal faults form a graben and some staircase blocks in the outcrop. The offsets are on the order of one to two metres and were restored by matching characteristic bed sequences across faults in order to piece together a continuous stratigraphic section. The possibility of undetected hiatuses cannot absolutely be ruled out, but the absence of physical indications and biostratigraphic evidence for hiatuses minimizes the probability of their presence. For the reasons given above, thickness measurements were not suited for quantitative cyclostratigraphic analysis. Therefore, 31 m of the section were sampled with a hand-held gasoline-powered rock drill to obtain cores for b palaeomagnetic work and material for the analysis of physical properties and geochemistry. For the cyclostratigraphic investigation the lower part of the section was sampled in great detail at an average interval of 5.6 cm (151 oriented cores in 8.45 m). Based on field observations, this portion was selected as representative for the entire 145 m of Biancone at Cismon.

Pra da Stua. The section Pra da Stua is located on the east flank of Monte Baldo along the road between Avio and San Valentino just above the reservoir Pra da Stua (45.8883)N/11.2058)E, 11 km from Avio) (Figure 1) and was called ‘‘Valle Aviana’’ by Cita (1964) and by Weissert et al. (1985). Palaeogeo- Figure 3. Outcrop of Early Cretaceous Biancone For- mation. (a) Around road tunnel Pala della Lerla, 15 km graphically, this section is situated on the Trento NW of Feltre along SS 50 in the Cismon valley. Normal Plateau, near its western margin (Figure 1). The faults display only small offsets. Bedding planes are Biancone Formation gradually develops through a spaced from 5 to 20 cm and bundled into major units narrow transitional zone from the underlying Rosso 80 to 100 cm thick. (b) Opposite the reservoir Pra da Ammonitico Superiore. It extends from the Upper Stua, 11 km W of Avio on the east flank of Monte Baldo, level 2600 cm to 2800 cm (Scale with centi- Tithonian into the Barremian and is unconformably metre subdivisions on the left and inch subdivisions on overlain by the Albian to Cenomanian Scaglia Vari- the right is located in lower centre of photograph). egata. The Biancone here consists of white to drab limestones which are well bedded, with intercalations of brown to black chert nodules in average intervals of with occasional bands of dark grey to black chert 50 cm (Figure 3b). Numerous joints, mostly perpen- nodules (Figure 3a). dicular to bedding, some curved, dissect the rock. Bedding planes recur every 5 to 20 cm and are Stylolites in various orientations are common. The grouped in major units of c. 80 to 100 cm thickness beds generally strike 20) and dip 15) to the NW. Only (see Figure 3a). Bedding is generally well developed, one normal fault produces an offset of 80 cm in the but somewhat undulatory and often paralleled by entire section. Slumping is absent except for a narrow stylolitic seams resembling ‘‘pseudobedding’’ in the interval with phacoidal structures at metre 18. Pre- sense of Alvarez et al. (1985). But slight lithologic vious work on this section was restricted to biostratig- changes up-section prove that the observed bedding is raphy (Cita, 1964) and to oxygen- and carbon-isotope parallel to the primary sedimentary stratification. The stratigraphy (Weissert et al., 1985). Weissert et al. sampled section is free of slumps. The general bed- (1985, p. 537) also mention an unsuccessful attempt ding attitude is 110)/10)NE (strike/dip) in the lower at magnetostratigraphy. A preliminary magneto- part of the section and 85)/17)NW in the upper stratigraphy correlating the Biancone Formation at part. Several minor normal faults disrupt the section. Pra da Stua to polarity chrons CM10 to CM5 was Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 195 published recently (Mayer, 1996, 1997). For cyclostratigraphic analysis bedding thickness was measured in the outcrop in an interval where bedding appeared particularly cyclic and even hierarchically structured (level 2500 cm to level 2900 cm, CM5, Hauterivian; Figure 3b).

3. Rock magnetism In the course of magnetostratigraphic studies of the Cismon and Pra da Stua sections (Mayer, 1995, 1996, 1997) rock-magnetic experiments were carried out to obtain some information on the magnetic- mineral content of the rocks studied. Previous studies had revealed that the dominant magnetic mineral in the Maiolica/Biancone limestone is magnetite in the Southern Alps (Channell et al., 1979, 1992, 1993; Channell & Erba, 1992) as well as in the Umbrian Apennines (Lowrie et al., 1980a, b; Lowrie & Alvarez, 1984; Lowrie & Channell, 1984). The results of rock-magnetic experiments performed on samples from the Cismon and Pra da Stua sections are in agreement with these earlier studies. During acquisition experiments of isothermal remanent magnetization (IRM) using an ASC Scientific IM-10 impulse magnetizer Biancone samples from Cismon and Pra da Stua almost reach saturation around 0.2–0.3 T (Figure 4), which indicates the dominance of magnetite, while high-coercivity minerals like haematite contribute only little to the remanence. Hysteresis loops were obtained from ten samples distributed through the entire lower Cismon section by experiments on an Alternating Gradient Force Magnetometer (Princeton Measurements Corporation) (Figure 5). The majority of these samples showed a dominant diamagnetic behaviour with a small ferrimagnetic component; some have a stronger ferrimagnetic character with some diamagnetic contribution. The dominant diamagnetism is due to the low content of ferrimagnetic and paramagnetic minerals of the samples and not surprising for rock consisting of 90% calcite. Only one sample was domi- Figure 4. Acquisition curves of isothermal remanent mag- nated by paramagnetism with a small ferrimagnetic netization (IRM) for typical Biancone samples. (a) component, while another sample had a stronger ferri- Cismon specimens ca109, ca150.5, ca248.5 and ca365. magnetic component with a paramagnetic contri- The dominant remanence carrier is magnetite, with a bution. The paramagnetism must be attributed to a small contribution of a high-coercivity mineral, prob- different clay-mineral content, since the non-carbonate ably haematite. (b) Pra da Stua specimens ps000B, ps800B, ps900B and ps1800B. The dominant rema- fraction is not higher in these samples compared to nence carrier is magnetite, with a small contribution of diamagnetic samples. These same two samples also a high-coercivity mineral, probably haemtite. have by far the highest initial susceptibility values of all samples. Furthermore, these high susceptibility values do not correspond to low carbonate content, whereas After correction for the dominant diamagnetic or in the remainder of the section these two variables are paramagnetic slope, hysteresis properties of the very tightly negatively correlated. ferrimagnetic component were evaluated. Ratios of 196 H. Mayer and E. Appel saturation remanent magnetism over saturation magnetite of pseudo-single-domain state. The Cismon netization (Mr/Ms) and of remanent coercive force Biancone samples do not have ‘wasp-waisted’ hyster- over coercive force (Hcr/Hc) were computed and plot- esis loops as are characteristic for remagnetized ted in a diagram of Mr/Ms versus Hcr/Hc (after Day limestones (Jackson, 1990). In a comparative study et al., 1977)(Figure 5e) which allows for the assess- of hysteresis parameters of unremagnetized and re- ment of the domain state of the magnetic carrier magnetized limestones Channell & McCabe (1994) mineral, if only one phase in a narrow grain-size range examined a large number of pelagic limestone samples is present. Coercivity ratios are consistent with mag- from various Mesozoic formations of Italy. All white Maiolica samples fell in the pseudo-single-domain field and had normal hysteresis loops, whereas pink and red limestone samples from other formations fell outside the pseudo-single-domain field and had ‘wasp-waisted’ hysteresis loops, characterizing them as remagnetized. The results of hysteresis-loop analysis of Cismon Biancone samples presented here strongly suggest that these rocks carry a ‘primary’ magnetization. A sudden increase in susceptibility and intensity observed during thermal demagnetization at temperatures between 350) C and 500) C(Mayer, 1996, 1997) may be attributed to the experiment-induced oxidation of pyrite or other iron sulfides to magnetite and maghemite or to other mineralogical changes in Cismon and Pra da Stua samples. This change is stronger and more common in rocks from Cismon than in rocks from Pra da Stua. Volume-susceptibility e values range from 5#10"6 to 30#10"6 (SI units) at Cismon with the exception of the aforementioned two samples reaching about 60#10"6. At Pra da Stua susceptibility values are between 11#10"6 and 87#10"6 (SI units).

4. Cyclostratigraphy Introduction During the late 1970s and early 1980s renewed interest in the causes of the Pleistocene ice ages led to a ﬁrm veriﬁcation of the Milankovitch theory of palaeoclimates (e.g., Hays et al., 1976; Berger, 1977, 1978a; Imbrie et al., 1984; see below). Stimulated by this

Figure 5. Hysteresis analysis of some Cismon samples. (a)–(d): Hysteresis loops for: (a) Sample ca239 displaying dominant diamagnetism at high fields with a small ferrimagnetic component. (b) Same as (a) corrected for diamagnetic slope. (c) Sample ca438 displaying dominant paramagnetism at high fields with a small ferrimagnetic component. (d) Sample ca446.5 dominated by ferrimagnetic component (small paramagnetic slope removed). (e) Hysteresis-properties diagram (after Day et al., 1977) for ten samples distributed through the lower Cismon section. SD=single-domain field; PSD=pseudo-single-domain field; MD=multi-domain field (see text for explanation). Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 197 success, more and more stratigraphers and sedimentologists concentrated their efforts on the detection of Milankovitch cycles in the older record, so that cyclic stratigraphy or cyclostratigraphy (Fischer et al., 1990) emerged as a distinct sub-discipline of geology incor- porating a variety of techniques from related fields and developing new methodologies of its own. This development over the last decade can be traced in the volumes edited or introduced by Einsele & Seilacher (1982), Berger et al. (1984), Arthur & Garrison (1986), Smith (1989), Einsele et al. (1991), Fischer & Bottjer (1991) and de Boer & Smith (1994).An increasingly important aspect of Milankovitch cycles is their usefulness for geochronology, not for absolute dating, but for accurate estimation of the time spans represented by cyclic sequences (House, 1985), in particular when combined with magnetostratigraphy in the same section (Mayer, 1994, 1996, 1997, in press). A central role in cyclostratigraphy is played by quantitative time-series-analysis methods, their applicability and value for the detection of characteristic cycle periods. Important contributions to this field were made by Schwarzacher (1964, 1975, 1987a, b, Figure 6. Variations of carbonate content (left) and magnetic susceptibility (right) in the lower Cismon section. 1989, 1991; Schwarzacher & Fischer, 1982), Wigley Cyclic fluctuations are well developed in both variables, (1976), Weedon (1986, 1989, 1991), Park & Herbert but in opposite directions. The negative correlation (1987), and Hinnov & Goldhammer (1991). between the two is very tight except for two narrow In the following, cyclic fluctuations of magnetic intervals at level 460 cm and level 610 cm, where susceptibility and carbonate content through the uncorrelated susceptibility spikes (indicated by the dashed line) attributed to magnetic alteration occur. Cismon section will be described and analyzed quantitatively. To that extent, the powerful geostatistical clearly contains rhythmic oscillations (Figure 6) which time-series-analysis method of Herzfeld (1990) will be are almost perfectly negatively correlated with the applied in order to gain information about the oscillations in magnetic susceptibility, with the excep- potential cyclicity of the time series. tion of two susceptibility spikes at c. 450 and 610 cm, which do not correspond to carbonate lows. Variations in magnetic susceptibility and carbonate content The numerical correlation is strongly affected by these mismatches in two very narrow intervals: the The magnetic susceptibility of the samples from correlation coefficient jumps from "0.36, when all Cismon was measured on a Kappabridge KLY-2 points are included, to "0.90, when only those four (Agico, Brno, Czech Republic; formerly Geofyzika points are excluded which form the major susceptibil- Brno). Each sample was measured five times, and the ity spike around 450 cm. When only one more point is mean of these values is reported after volume normal- removed (the one constituting the minor spike at ization. The Biancone limestone is characterized by 610 cm), the correlation coefficient becomes "0.93 very low magnetic susceptibility (5 to 30 10"6 di- # (Figure 7). This excellent correlation between carbon- mensionless SI units, with the exception of two ate content and susceptibility indicates that magnetic samples reaching c.60 10"6). The susceptibility # susceptibility can be used as an approximate measure variations through the section appear to follow a cyclic of variations in carbonate content, if care is taken that pattern (Figure 6) the analysis of which is presented extreme values not correlated to opposite extremes in further down. carbonate content are excluded (Mayer, 1993; Mayer Through the entire lower section (8.45 m) a split of & Appel, 1995). each drilled sample has been analyzed for carbonate content by coulometry. The amplitudes of the vari- Time-series analysis ations in carbonate content are relatively small, all samples fall between 87 and 97% CaCO3. However, With the increasing degree of quantification sought in the curve of carbonate content through the section geology today more and more geologists apply various 198 H. Mayer and E. Appel

summarized here, because they form the basis for the ensuing interpretations. The detection of periodicities in stratigraphic sequences requires the application of quantitative time-series-analysis methods. These methods, however, differ in their approach to the problem and in their suitability for different types of data sets. The adaptive-multitaper technique requires evenly spaced samples. So, the data have to be interpolated first if they are unevenly spaced, as in our example and in most geologic applications. Several filters are applied and two inversions from the time domain to the frequency domain and back are involved. All these Figure 7. Cross-correlograms of the carbonate and suscep- steps may introduce rounding errors and thus lead tibility series from the Cismon section. Dotted line: away from the original data. Correlation analysis also carbonate and complete susceptibility series. Dashed line: carbonate series and susceptibility series with main requires evenly spaced data in the time series. Linear spike at level 460 cm removed. Solid line: carbonate interpolation and cubic-spline interpolation were series and susceptibility series with main spike at level compared in this case and were found to give very 460 cm and minor spike at level 610 cm removed. similar results. Linear interpolation was preferred, because it did not even out the magnitudes of the time-series-analysis methods to geological data (see peaks in the correlograms as much as cubic-spline Merriam, 1967). In particular, time-series analysis interpolation. Cova functions can be applied directly seems to be suited for the study of stratigraphic data, to the raw data, because they can accomodate because the stratigraphic succession of sedimentary unevenly and non-correspondingly spaced series. This rocks basically records the passing of (geologic) time. is a considerable advantage over other methods, since Ideally, stratigraphic thickness can be converted there is less room for introducing errors through directly to length of time. The validity of this approach preprocessing steps. Therefore, cova functions were depends on the following condition: well-dated layers selected as the analysis method for this study. Before at the bottom and top of the section must be present. the results are presented, a short overview of the In order to measure time by thickness in subsections method is given. between two well-dated layers the additional con- Based on the cross-variogram which allows the ditions of continuous sedimentation and constant quantification of the covariation of two series sampled sedimentation rate must be fulfilled. Unfortunately, in unevenly spaced corresponding locations, Herzfeld these conditions are very rarely exactly met anywhere (1990) formulated the generalized case, extending the in the sedimentary record. Kominz & Bond (1990) applicability to unevenly and non-correspondingly therefore developed an alternative method which con- spaced data series, and termed it cova function. siders different accumulation rates for the different The variogram is the structure function commonly lithologies comprising a cycle. But we can find, on the applied in geostatistics, the theory of regionalized other hand, sedimentary sections where the ideal case variables (Journel & Huijbregts, 1978). For a stochas- is approximated to a high degree, so that time-series- tic process Z(x), the variogram (sometimes also called analysis methods can be applied to stratigraphic data semivariogram) is defined as sets with limited uncertainty in such cases. These conditions are met in the selected interval of the Cismon section. The sedimentary time series obtained will be described in detail. The main focus of an where E denotes the expectation and h the distance earlier paper was on the value of time-series analysis in between two points. The (auto-)covariance function is cyclic stratigraphy and its application (Mayer, 1993). also a function defined in the lag domain, whereas the For this matter, a detailed comparison of three differ- power-spectral density is defined in the frequency ent time-series-analysis methods was carried out: one domain. If the stochastic process Z(x) is stationary, popular spectral technique (adaptive multitaper) was then the variogram is related to the (auto-)covariance compared with a basic statistical method (auto-/cross- function, denoted cov(h), by correlation) and a versatile geostatistical approach (cova functions). The results of this analysis will be Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 199

(cf. Journel & Huijbregts, 1978, p. 40f), and the preprocessing steps. The cova function defined in relationship to the power-spectral density is stated by Herzfeld (1990) avoids all preprocessing and still the Wiener-Kinchin Theorem: the power spectrum permits the investigation of the variability of two data equals the Fourier transform of the (auto-)covariance series that may be spaced unevenly and non- function (cf. Herzfeld, 1992). Although it is theoreti- correspondingly. The cova function is based on the cally possible to transform the variogram and the concept of the cross-variogram (cf. Eqn. 4). covariance function into the spectrum and back, there To handle unevenly spaced samples the calculation are differences in practice. Variogram estimation has of cova functions is carried out in distance classes proven useful in the geosciences because of its robust- which are multiples of a unit lag or step size. Analysis ness and simplicity as a function in the lag domain. of non-correspondingly sampled series presents the Noise in geophysical data series, for instance, and additional problem that matching pairs of points in small errors in the spacing of sampling positions affect the two series need to be found. This is achieved by the power spectrum much more than a function in the introducing a variable tolerance: given a pair (y11, y12) lag domain. Uneven spacing of experimental data is in time series 1 in a particular distance class, a pair usually handled by calculating the covariance function (y21, y22) in time series 2 in the same distance class is and the variogram in distance classes, that is, a pair of used in the multiplication in the formula correspond- points (x1,x2) is used in the multiplication in the ing to (Eqn. 4), if y11 and y21 are at most the value of experimental formula tolerance apart in time (distance). Both step size and tolerance can be chosen by the user to fit particular data sets. Herzfeld (1990) provides an extensive set of simulated example time series and resulting cova if the distance D(x ,x ) is in a distance class [h+ä, functions which demonstrate the robustness and gen- 1 2 eral applicability of the method. The particular prop- h"ä], where 2ä is the size of the distance class. An experimental spectrum can be calculated in distance erties that characterize cova functions in contrast to all classes too, but the usual technique is to use an spectral methods are: their direct applicability to the interpolation and resample at evenly spaced locations. original data without any preprocessing and the fact Such an intermediate step may distort the signal and that residual cova functions are defined for non- is not needed in lag functions. Furthermore, the stationary time series. Of course, the method is also variogram exists in situations where the covariance applicable to evenly and correspondingly spaced time function and the spectrum may not exist because of series. In the case of correspondingly spaced series the trends in the data. To compare two data sets the cova function reduces to the cross-variogram if toler- corresponding bivariate functions are used. For a ance is set equal zero. Two series of different variables can be compared (cova function) or one series with bivariate stochastic process Z(x)=[Z1(x), Z2(x)] the cross-variogram is defined as itself (auto-cova function). To make this possible two copies of the same series are concatenated in order to provide long enough overlapping sequences. Spectral- analysis methods and correlation analysis require equally spaced data points, whereas cova functions Relationships to the cross-covariance function and and cross-variograms are not subject to this limi- the spectrum hold, similar to the one-dimensional tation. The advantage of cova functions over cross- case. Uneven spacing of experimental data is handled variograms in our case is that bad data can be left out by calculation in distance classes, analogous to the in one series (susceptibility) while all the data can be one-dimensional case. Two geophysical data sets, retained in the other series (carbonate), because however, may not be spaced correspondingly. With cova functions are defined for non-correspondingly data from different survey or measurement techniques spaced series. the corresponding spacing (assumed in most process- In the following, the results of the application of ing software) is rarely the case. To date, the usual cova-function analysis to the carbonate and suscepti- practical approach to compare two variably spaced bility series obtained from the Cismon section are data sets is to preprocess at least one of them in presented. At first, only thickness periods are order to match the spacing, commonly by linear or analyzed. How these might be related to time cubic-spline interpolation. Although there is a large periods is discussed later together with associated number of software packages for comparing two time time-scale-accuracy and -resolution problems. series, most routines are restricted to equally spaced Auto-cova functions were calculated for the Cismon data. Valuable information is lost in the required carbonate series (150 samples, Figure 8a) and the 200 H. Mayer and E. Appel

Cismon susceptibility series (144 samples, Figure 8b). Cismon section can be confirmed at thicknesses of Residual and ordinary (auto-)cova functions were approximately 45 cm, 80 cm, and 180 cm. calculated in each case. Cova functions were com- Natural remanent magnetization (NRM) was ana- puted comparing the carbonate curve with the suscep- lyzed as another sedimentary parameter in the section tibility curve (Figure 8c). Cova values close to zero (Figure 9). This time series shows some potentially indicate the highest degrees of covariation. Therefore, cyclic undulations similar to the susceptibility series as one has to look for minima of the absolute cova value in we would expect. However, the regularity and struc- plots of cova value versus distance in order to detect ture of variations is much less clear in this parameter potential cycle periods. If we look at the calculated and is further obscured by several large spikes. Cova- cova functions in this way, we find that major minima function analysis of the NRM series indicates poten- occur in the auto-cova function of the Cismon car- tial cyclicity at periods of 50 cm and 105 cm (Figure bonate series at distances of 80 cm, 160 cm and 9b). However, these data are not good enough for 265 cm. Minor minima occur at 120 cm and 175 cm conclusive extraction of cycle periods. They can only (Figure 8a). These numbers are valid for a step size of be interpreted together with the susceptibility and 5.0 cm and a tolerance of 0.0 cm. A step size close to carbonate-content data from the same section. the average spacing of the data points in the time In the Pra da Stua section we measured bedding series gives the best results (Mayer, 1993). In the case thickness in an interval that visually appeared cyclic of the carbonate series (average spacing 5.63 cm) and even hierarchically structured (level 2500 cm to 5.0 cm is chosen as the preferred step size. level 2900 cm; see Figure 3b). Bedding thickness was The auto-cova function for the susceptibility series measured in the outcrop and then resampled in regu- (Figure 8b) looks much like the one for carbonate lar intervals of 5 cm to produce a stratigraphic series (Figure 8a): the three most prominent minima are at (Figure 10a). Visual inspection of this time series 85 cm, 160 cm and 265 cm. Secondary minima are suggests the presence of a subtle cyclicity on the order located at 45 cm and 180 cm. If we now analyze the of half a metre. Cova-function analysis reveals perio- covariation of the two variables carbonate content and dicities of 55 cm, 115 cm and 170 cm (Figure 10b). susceptibility, we get negative cova values because the However, only the 55-cm cycle can confidently be two variables are negatively correlated. So, in this plot extracted, while the others are only vaguely indicated. we need to look for actual maxima (being minima of In the following, we concentrate on the susceptibility the absolute value) to identify cycle periods. We find and carbonate series from Cismon which produced that again we get three prominent minima (of the unequivocal evidence for periodic cyclicity. absolute value) at 85 cm, 160 cm and 265 cm. Sec- Once thickness periods have been established, the ondary minima (of the absolute value) are found at problem arises as to how they convert to time periods. distances of 45 cm, 120 cm and 175 cm (Figure 8c). To approach this problem the correlation of the new For all the series mentioned, residual cova functions Cismon magnetic-polarity sequence (Mayer, 1996, as well as ordinary cova functions were calculated, and 1997) with the marine magnetic anomalies of the it was found that these are essentially the same for M-sequence in the version of Harland et al. (1982) each case except for a small deviation at large lags. was used to estimate time (Figure 11). The geo- This demonstrates that the time series do not underly chronologic scales are rather poorly constrained in this a pronounced trend. However, a small trend to higher time interval. There are only two widely accepted carbonate values and lower susceptibility values reliable radiometric calibration points, namely at the up-section can be detected in the original curves (see Oxfordian/Kimmeridgian boundary [156 million Figure 6), which is confirmed by the small deviation years ago (Ma)] and the Barremian/Aptian boundary between ordinary and residual cova functions at large (119 Ma) (Kent & Gradstein, 1985; Ogg, 1995). In lags. between these tie points all ages for chronostrati- Cova functions produce essentially the same major graphic levels and magnetic reversals are based on cycle period for the Cismon carbonate and suscepti- linear interpolation assuming constant sedimentation bility profiles and their cross-comparison at a thick- rate and constant seafloor-spreading rate, in our ness period of 80 cm. The prominent periods of example over a time span of 37 million years (m.y.). 160 cm and 265 cm must be interpreted as multiples. As these assumptions have to be regarded as rather Cova functions reveal additional significant periods at unrealistic, in particular for time periods that long, 45 cm and 180 cm. The resolution is limited by the there is considerable potential for improvement of the sample spacing in the original time series and the time scale by new methods and approaches, such as accordingly chosen step size (here, 5.0 cm). In synop- cyclostratigraphic calibration (Mayer, 1994, in press). sis, we can conclude that apparent cycle periods in the Other radiometric time scales (e.g., Odin et al., 1982; Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 201 a a

Figure 9. Variations of natural-remanent-magnetization (NRM) intensity through the Cismon section. (a) Stratigraphic plot. (b) Auto-cova functions. Step size is 5.0 cm, tolerance is zero.

Harland et al., 1982, 1990; Hallam et al., 1985) have Figure 8. Cova functions for the carbonate and suscepti- incorporated more radiometric dates in this time bility series from the Cismon section. Step size is interval, but these dates are discussed quite controver- 5.0 cm, tolerance is zero. (a) Auto-cova functions for the carbonate series. (b) Auto-cova functions for the sus- sially (see Harland, 1983; Hallam et al., 1985; Odin, ceptibility series. (c) Cova functions for the carbonate 1985). Not surprisingly a compilation of a number of and susceptibility series. diﬀerent time scales revealed substantial disagreement 202 H. Mayer and E. Appel a

Figure 11. Correlation of the Cismon-polarity sequence to the geomagnetic-polarity-time scale of Harland et al. (1982) which is based on the Hawaiian marine- magnetic-anomaly lineations of Larson & Hilde (1975). Biostratigraphic information is taken from Channell et al. (1979). b on the duration of the magnetic-polarity chrons ident- iﬁed at Cismon ranging from a minimal 0.704 m.y. to a maximal 1.583 m.y. (Table 1). The 8.79 m of cumulative thickness from the Cismon section which were correlated with polarity subchrons were divided by the respective duration from each of the time scales used in order to obtain average sedimentation rates. In this calculation those subchrons only partially represented in the section and the sampling gap are excluded. The resulting rates range from 5.55 m/m.y. to 12.49 m/m.y. with a mean of 7.72 m/m.y. (Table 1). These rates appear a little low if compared to typical accumulation rates for calcareous ooze in deep-sea environments amounting to tens of metres per million years (Berger, 1974), but are within the possible Figure 10. Variations of bedding thickness through the Pra range, albeit at the very low end. Compaction can da Stua section. (a) Stratigraphic plot. (b) Auto-cova only account for a small fraction of the discrepancy. functions. Step size is 5.0 cm, tolerance is zero. Nevertheless, using these sedimentation rates the Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 203

Table 1. Comparison of sedimentation rates calculated from the cumulative thickness of magnetic subchrons identiﬁed at Cismon (partially represented subchrons and sampling gap not included) and their cumulative duration according to diﬀerent time scales.

Thickness Duration Sedimentation rate [m] [m.y.] [m/m.y.] Time scale

8.79 0.704 12.49 van Hinte (1976) 1.017 8.64 Lowrie (1982) 1.583 5.55 Harland et al. (1982) 1.556 5.65 Hallam et al. (1985) 1.390 6.32 Kent & Gradstein (1985) 1.170 7.51 Lowrie & Ogg (1986) 1.038 8.47 Haq et al. (1988) 1.100 7.99 Harland et al. (1990) 1.248 7.04 Gradstein et al. (1994) 1.160 7.58 Channell et al. (1995b) 7.72 mean

thickness periods from the time-series analyses were say that the periods are of the right order of magni- converted to time periods. The resulting cycle periods tude, but we cannot get any closer to the true cycle vary considerably depending on the time scale used, periods by this conventional method. In this situation, but all the periods calculated fall in the range expected adifferent approach is needed. Even if we consider the for Milankovitch cycles (Table 2), although they seem case that the numerical ages are far off, the ratios too long for the expected Milankovitch cycle periods between the sedimentary cycles at Cismon still match of 19 thousand years (kyr) and 23 kyr (precession), the expected ratios for present-day Milankovitch 41 kyr (obliquity) and 95 kyr (eccentricity). Consider- cycles very closely. The same is true if we consider ing the poorly constrained time scales in this time periods of these astronomical parameters as calculated interval which leave room for large errors, we can only back for the mid-Cretaceous by Berger et al. (1992) (18.5 kyr and 22.3 kyr for precession, 38.8 kyr for obliquity and an unchanged 95 kyr for eccentricity at Table 2. Conversion of observed thickness periods at 100 Ma) (see Tables 3–5). Based on this extremely Cismon to time periods for cycle analysis using selected sedimentation rates from Table 1. close match of period ratios we can confidently as- sume that the observed cycles represent Milankovitch Corresponding time period [kyr] cycles. Then we can use the known periods of the using sedimentation rate Thickness period latter to convert the observed thickness periods of [cm] min. mean max. the former into time periods. In order to explore the effects of the uncertainty in the observed cycle periods on the match, ratios were calculated for the possible 180 324 233 144 80 144 104 64 range of thickness periods determined by time-series 45 81 58 36 analysis (Table 5). Milankovitch-cycle periods for the present, 100 Ma and 150 Ma were taken from Berger

Table 3. Milankovitch periods for selected times in geologic history (after Berger et al., 1992).

Cycle periods [kyr] Time Precession 1 Precession 2 Average of p.1 and p.2 Obliquity Eccentricity

Present 19 23 21 41 95 100 Ma 18.5 22.3 20.4 38.8 95 125 Ma* 18.35 22.1 20.23 38.25 95 150 Ma 18.2 21.9 20.05 37.7 95

*Values for 125 Ma were obtained by linear interpolation between values for 100 Ma and 150 Ma. 204 H. Mayer and E. Appel

Table 4. Ratios between Milankovitch cycle periods for selected times in geologic history calculated from values in Table 3.

Cycle ratios Time Avg. prec.:Obliquity Avg. prec.:Eccentricity Obliquity:Eccentricity

Present 0.512 0.221 0.432 100 Ma 0.526 0.215 0.408 125 Ma 0.529 0.213 0.403 150 Ma 0.532 0.211 0.397

Table 5. Ratios between sedimentary cycle periods good. This cyclostratigraphically derived time infor- detected at Cismon encompassing the uncertainty range mation was recently applied to the estimation of the (see text). duration of Early Cretaceous magnetic-polarity zones Cycle set and to an improvement of the time scale (Mayer, a:b:c ratio ratio ratio 1994, 1996, 1997, in press). [cm] a:b a:c b:c

45:80:175 0.563 0.257 0.457 5. Palaeoclimatic significance 45:85:180 0.529 0.25 0.472 45:85:175 0.529 0.257 0.486 The stratigraphic, sedimentologic and magnetic data 45:80:180 0.563 0.25 0.444 obtained in the course of this study lend themselves to further interpretation in terms of palaeoclimatic significance. In this respect, the present investigation contributes to a number of palaeoclimatic issues, et al. (1992), and values for 125 Ma, which is about in particular to the question of orbital forcing in the time of sediment deposition at the studied section, pre-Pleistocene times, climatic variability in the were obtained by linear interpolation (Table 3). Early Cretaceous and magnetic records of climatic Ratios between these Milankovitch-cycle periods were change. also computed (Table 4). Comparing Table 5 with Table 4, we can see that the best match is obtained Cretaceous climate using cycles of 45 cm, 80 cm and 180 cm and present- day Milankovitch periods. This set of sedimentary The Cretaceous is generally considered a time period cycles also emerged as the best solution after appli- of much warmer than present temperatures and a cation of different time-series-analysis methods much reduced equator-to-pole temperature contrast (Mayer, 1993). These cycles appear thick (almost (Frakes, 1979; Barron, 1983; Crowley, 1983; Hallam, twice as thick) compared to Milankovitch cycles in 1985); its climate is thus characterized as warm and the Cretaceous of the Apennines. This implies that the equable (Barron, 1983). This has been inferred from accumulation rate is correspondingly higher in the traditional palaeontological and geological data Biancone, which is supported by lithologic evidence. such as the geographic distribution of certain palaeo- In the Apennines limestone-marl cycles were studied ecologically significant biological communities and with a much greater variation in carbonate content palaeoclimatically significant sedimentary deposits and thus accumulation rate between members of the (e.g., evaporites, coal, tillites, desert sandstones) as couplets. In the lower Biancone of the Southern Alps, well as from stable-isotope temperature proxies. How- on the other hand, we found carbonate variations of a ever, new oxygen-isotope data from the Cenomanian much lower amplitude at a generally high carbonate have been interpreted recently in terms of tempera- content (87 to 97%) resulting in a higher overall tures not significantly warmer or colder than at accumulation rate. This also explains the more con- present near the equator, but still warmer near the sistent cycle and bundle pattern we found. It is poles (Sellwood et al., 1994). These new data and surprising that the match with Milankovitch periods their interpretation have been received with some as calculated for the Early Cretaceous is not quite as scepticism regarding their global significance (Barron, good. The reason for this is unclear, but the difference 1994). Earlier, Kemper (1983) had argued for sub- is actually very small and the match is still extremely stantial climatic changes during the Cretaceous with Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 205 two major cold periods in the Valanginian and late theory was not readily accepted by many, because its Aptian to early Albian bracketing a pronounced warm verification was impossible at the time due to the period. On the other hand, Weissert (1991) has de- absence of suitable dating and correlation methods duced veritable greenhouse conditions for the Val- and because the insolation variations seemed too anginian and Aptian from sediment-distribution small to produce such a dramatic climate change as patterns in the Tethyan and North Atlantic realms between glacials and interglacials. This holds particu- and from positive carbon-isotope excursions. The larly for the 100 kyr eccentricity cycle (Ruddiman & Aptian ä13C peak is split by a trough, which is Wright, 1987). It was not until the mid-1960s that interpreted as representing a cold period or even a new evidence for the validity of the Milankovitch possible glacial period by Weissert & Lini (1991). The theory started to increase (Emiliani & Geiss, 1959; late Valanginian to early Hauterivian ä13C peak re- Emiliani, 1966; van den Heuvel, 1966; Broecker et al., flects accelerated global carbon cycling attributed to 1968; Mesolella et al., 1969; Broecker & van Donk, an elevated atmospheric CO2 level and thus repre- 1970), although Emiliani (1955) had broken the sents a first episode of Cretaceous greenhouse climate ground much earlier by developing the oxygen-isotope (Lini et al., 1992). A global cooling trend at the end of technique for determination of palaeotemperatures. the Cretaceous is well documented (cf. Crowley, The rapid development in the 1960s and 1970s was 1983). However, except for some indications of minor due to improvements in oxygen-isotope stratigraphy, glaciation, solid evidence for massive glacial condi- magnetostratigraphy and radiometric dating. A decis- tions at any time during the Cretaceous or the entire ive breakthrough towards verification of the astro- Mesozoic, for that matter, has never been presented. nomical theory was achieved when Hays et al. (1976) It is therefore interesting to investigate whether orbital matched the oxygen-isotope variations in a first forcing was effective enough to be transmitted into Pleistocene Pacific sediment core to the theoretically the stratigraphic record through a climate system so predicted insolation curve. This work was later cor- different from the Quaternary glacial regime. roborated by new data from a number of additional cores (Imbrie et al., 1984). It became apparent that the subtle changes in insolation can be greatly en- Milankovitch theory of palaeoclimates hanced within the complex climate system with The astronomical theory of palaeoclimates after its global energy redistribution by atmospheric and Milankovitch (1930, 1941) relates climatic change to oceanic circulation. A number of models have been variations of the amount of solar energy received by advanced postulating nonlinear responses (Imbrie & the Earth as a consequence of quasi-periodic changes Imbrie, 1980; Berger & Guiot, 1981; Imbrie et al., in celestial geometry (see Imbrie & Imbrie, 1979, and 1993; Liu, 1995), feedback mechanisms involving the Berger, 1988, for reviews). Of particular importance albedo of growing polar ice caps (Suarez & Held, are variations in the eccentricity of the Earth’s orbit 1976; Weertman, 1976; Le Treut & Ghil, 1983; (with dominant periods of roughly 400 kyr and Pollard, 1983) or the effects of oceanic circulation

100 kyr), in the obliquity of the Earth’s rotational (Ruddiman & McIntyre, 1981) and atmospheric CO2 axis relative to the orbital plane (with characteristic content (Shackleton & Pisias, 1985). periods of 54 kyr and 41 kyr) and in the precession For the obliquity and precession cycles Imbrie et al. of the equinox or more exactly the eccentricity- (1992) established a direct linear link between insol- modulated precession expressed by the precessional ation and climate fluctuations. The strength of the index of Berger (1976, 1978b) (with periods of 23 kyr 100 kyr eccentricity cycle in the records, however, is and 19 kyr). The obliquity cycle is more effective in still somewhat enigmatic and has to be explained high latitudes, whereas the low latitudes are domi- by nonlinear response models (Imbrie et al., 1993). nated by the precessional cycle. Although the exact mechanisms are still not com- The invocation of astronomical variations as causal pletely resolved, the role of orbital variations as the factors of climate change, in particular for the Pleisto- driving force of glaciation cycles is now generally cene ice ages, goes back to Adhe´mar (1842; fide accepted (Berger, 1992; Imbrie, 1992). It is much less Imbrie & Imbrie, 1979). Croll (1875) developed these clear whether orbital forcing was as effective as in the ideas into a coherent astronomical climate theory. The Quaternary during earlier periods of geologic history, exact mathematical formulation of orbital variations particularly during times of ice-free climate conditions and the calculation of an insolation curve through like the Cretaceous. However, since the astronomical time which could be matched to the Pleistocene cycles have been stable through the Phanerozoic glacial-interglacial cycles (Ko¨ppen & Wegener, 1924) except for a small gradual lengthening of some periods was achieved by Milankovitch (1930, 1941). The (obliquity and climatic precession) owing to the 206 H. Mayer and E. Appel changing Earth-Moon distance (Berger, 1989; Berger particles can sensitively reflect environmental changes et al., 1989, 1992; Berger & Loutre, 1994), it is not in the depositional realm or its catchment (Thompson unreasonable to look for a record of Milankovitch & Oldfield, 1986) which are in many cases attribu- cycles from pre-Pleistocene periods (cf. Fischer, 1986; table to climate. The best results of this new approach Kelly & Cubitt, 1993; Schwarzacher, 1993a). to palaeoclimatology have been achieved in aeolian, Several studies reported evidence for Milankovitch lacustrine and pelagic settings (Reynolds & King, cyclicity also in older sedimentary rocks: in the 1995). It has to be pointed out here that rock- Tertiary (e.g., Schwarzacher, 1987c; Hilgen, 1991), magnetic parameters by themselves have little palaeo- Upper Cretaceous (e.g., Schwarzacher & Fischer, climatic meaning. But if calibrated with the help of 1982; Hart, 1987; Cottle, 1990), Jurassic (e.g., palaeontological or isotopic climate indicators for the House, 1985; Weedon, 1986), Triassic (e.g., specific environment studied, they provide the long, Goldhammer et al., 1987) and even in the Pre- continuous and areally extended records needed cambrian (e.g., Grotzinger, 1986). These examples for useful palaeoclimatic reconstruction, which tra- indicate that climate changes probably followed the ditional climate indicators cannot provide because same astronomical cycles through the Earth’s history they are generally not continuously distributed even at times of different climate states. through sections and their determination is much more Milankovitch cycles can be expected to be recorded time-consuming. Thus, the value of environmental- in certain sensitive depositional systems, where peri- magnetic methods lies in the fact that they are in most odic climatic changes result in environmental changes, cases fast, easily applicable, non-destructive and in- which in turn are reflected in the composition or other expensive. Parameters indicative of the concentration properties of the sediments deposited. Climatic of magnetic particles as well as those indicative of changes attributable to variations in insolation can magnetic mineralogy and grain size have proven useful. include the growth and decay of continental ice sheets Of the first group, susceptibility and intensity of natural as in the Quaternary, the latitudinal migration of remanent magnetization are the most important. Hints humid-arid belts, varying precipitation volumes due to on the magnetic mineralogy and grain size of rocks are a changing land-sea temperature contrast (Barron usually obtained from parameters derived from de- et al., 1985), and switches between monsoonal and magnetization behaviour, from the acquisition of iso- zonal wind systems (Prell & Kutzbach, 1987). A most thermal remanent magnetization, from the temperature important effect of insolation-driven variations in lati- and frequency dependence of susceptibility and from tudinal temperature contrast are changes between hysteresis properties, among others. vigorous and sluggish oceanic-circulation regimes. In the Biancone Formation of the Cismon section Vigorous circulation leads to intensified upwelling we have no drastic lithologic changes which could which triggers increased productivity (e.g., Berger, indicate fundamental climatic changes. Accordingly, 1974). Sluggish circulation, on the other hand, pro- the rock-magnetic parameters indicative of magnetic motes stratification of the water mass leading to mineralogy do not show significant variations through reduced bottom oxygenation and productivity. All the section. IRM-acquisition curves (Figure 4) and these changes are potentially recognizable in the s-ratios (IRM acquired at backfield of 0.3 T relative to sedimentary record. Of course, oceanic circulation is the IRM acquired at 1.2 T) (Figure 12) indicate that also influenced by changes in the configuration of the dominant carrier of remanence is magnetite continents and in submarine morphology. However, throughout the section with a small contribution from the plate-tectonic processes responsible for these haematite. The same is true for the Pra da Stua changes are not cyclic (reversible) in this sense and section. However, at three levels in the Cismon sec- operate on longer terms than orbitally driven climate tion (Figure 12) lower s-ratios occur, which must be change. attributed to a higher concentration of high-coercivity minerals, probably haematite. A very subtle trend to decreasing s-ratios up-section is recognizable, accen- Magnetic records of climatic change tuated by the three abnormally low values, which also It has been established mainly during the last two become lower the higher the level examined in the decades that magnetic properties of sediments and section. These features may reflect a trend to slightly sedimentary rocks can provide a detailed record of more oxidized conditions, but this is not sufficiently palaeoclimatic change in stratigraphic sections and documented by these data. In this study a detailed drill cores (King & Channell, 1991; Verosub & record of susceptibility variations through the Cismon Roberts, 1995; Reynolds & King, 1995). Variations in section was obtained which is discussed in the the concentration, type and grain size of magnetic following section. Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 207

Fischer, 1986; Ricken, 1986): The variations can occur in biogenic carbonate production, carbonate dissolution and/or input of detrital material. The end members of this system are referred to as productivity cycles, dissolution cycles and dilution cycles, respect- ively. Of course, any combination of these variables can occur to complicate the record. In practice, a variety of palaeontological and geochemical methods is required to track down the dominant variable for a particular formation. However, at the root of all these different cycles there are climatic variations (Fischer & Schwarzacher, 1984). So, if we are interested in periodicities of climate change, the exact mechanism of recording that change is only of secondary interest as long as it can be established that stratigraphic cycles are primary sedimentary and not purely diagenetic features. No geochemical analyses other than carbonate content have been performed on samples from the lower Cismon section. However, comparison with other Cretaceous sections in the Southern Alps and Umbrian Apennines (Weissert et al., 1985; Herbert & Fischer, 1986; Herbert et al., 1986; Premoli Silva et al., 1989; Bersezio, 1993) demonstrates the dominance of variations in primary productivity in this part of the Tethys Ocean during Early to mid-Cretaceous times (Mayer, 1992). For the Cretaceous of the Figure 12. Variations of the s-ratio (isothermal remanent Western Interior of North America, where previously magnetization (IRM) acquired at backfield of 0.3 T dilution had been favoured (cf. Gilbert, 1895; Fischer, relative to the IRM acquired at 1.2 T) indicate variations in magnetic mineralogy through the Cismon 1980; Arthur et al., 1984; Bottjer et al., 1986), Eicher section. & Diner (1991) also made a case for the prevalence of productivity cycles. A small but recognizable trend up-section is apparent in both sedimentary-parameter curves. Susceptibility tends to decrease, whereas car- Evaluation of sedimentary cycles bonate content tends to increase (Figure 6). After the Much more conclusive regarding palaeoclimatic foregoing demonstration of Milankovitch cyclicity interpretation than the other rock-magnetic par- in this record, the trend may be the expression of ameters are the variations in susceptibility which are long-term climatic change. clearly cyclical through the section. A hierarchy of cycles could be identified and matched to 6. Conclusions Milankovitch cycles. The excellent negative correlation between susceptibility and carbonate content Magnetic susceptibility and carbonate content were verifies that the susceptibility signal reflects the measured on densely spaced samples from the concentration of the non-carbonate fraction, hence a Valanginian to Hauterivian part of the Cismon section primary depositional feature (Figure 6). The NRM and analyzed as geologic time series. The variations of curve also shows some undulation through the both parameters through the section are truly cyclic as section, but does not reveal the cycles so clearly revealed by quantitative time-series analysis. A hier- (Figure 9). archy of cycles with periods at c. 45 cm, 80 cm and The dominant initial-susceptibility signal in marine 180 cm was calculated. The geostatistical cova func- limestones is carried by the (detrital, authigenic or tions proved to be the most versatile and robust of the biogenic) non-carbonate fraction of the rock (cf. compared methods for the analysis of this kind of Lowrie & Heller, 1982). To explain cyclic variations geological data. Natural remanent magnetization in of carbonate content, granted they are primary, three samples from Cismon and bedding-thickness vari- variables have to be considered (Einsele, 1982; ations in the Pra da Stua section were also analyzed. 208 H. Mayer and E. Appel

Cyclicity of the same periods is indicated in these References parameters, but not nearly as clearly and conclusively Adhe´mar, J. A. 1842. Re´volutions de la mer, de´luges pe´riodiques as in the susceptibility and carbonate-content vari- (privately published, Paris; 2nd ed. 1860). ations. Susceptibility is tightly correlated negatively Allemann, F., Catalano, R., Fares, F. & Remane, J. 1971. Stan- dard calpionellid zonation (Upper Tithonian–Valanginian) of to carbonate content so that it can be utilized as a the Western Mediterranean Province. In Proceedings of the II proxy for the latter in this palaeoenvironmental set- Planktonic Conference Roma 1970 (ed. Farinacci, A.), vol. 2, ting. The carbonate cycles represent productivity pp. 1337–1340 (Edizioni Tecnoscienza, Roma). Alvarez, W., Colacicchi, R. & Montanari, A. 1985. Synsedimentary cycles which can be attributed to climatically driven slides and bedding formation in Apennine pelagic limestones. shifts in oceanic circulation. 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