The long-term fluctuation of the earth's climate is assumed to be tuned by variations in solar insolation which is controlled by orbital parameters (Milankovitch frequencies). The sedimentary record of the last few 100 ka has shown that climatic changes cause sea-level fluctuations affecting the patterns of sedimentation. By analogy, cyclic sedimentation patterns in ancient sediments are often considered to be orbitally tuned as well. A famous example of this hypothesis is the regular stratal pattern in the Middle Triassic Latemar carbonate platform in the Western Dolomites (Italy). The interpretation of the cycles as being orbitally tuned has mainly been based on graphic space-time and spectral analyses of the cyclic succession (Hinnov & Goldhammer, 1991), which suggest a distinct hierarchy in the pattern. A frequency of 100 ka is obtained in the spectrum, if a 20 ka sedimentation interval is assigned to each highest-order unit (both periods occur in the Milankovitch spectrum). The ~600 units counted for the 700 m thick platform carbonate succession at Latemar (Goldhammer et al., 1987) would thus yield a duration of at least 12 Ma for the accumulation of the cyclic platform portion. In spite of the fact that a duration of 12 Ma exceeds estimates based on recent time- scales, this interpretation has been widely accepted.
This contribution aims at testing the interpretation of orbitally tuned sedimentation by means of absolute age determinations. This is based on the multiple occurrence of volcaniclastic intercalations in age-equivalent pelagic limestones (Buchenstein Beds), which show geometrical relations with the carbonate platform (see Fig. 1). Moreover, the occurrence of age-diagnostic fossils (ammonoids and bivalves) in the Buchenstein Beds as well as in the Latemar platform carbonates allows detailed correlation (Brack & Rieber, 1993). The short time span to be determined requires very high resolution in dating. We have dated zircons from volcaniclastic horizons by U-Pb single-grain techniques (Oberli et al., 1989) achieving a precision of 0.2-0.3% (2s error) for each dated layer. Individual crystals which were free of inheritance were treated by air-abrasion techniques in order to minimize the effects of post-depositional loss of radiogenic Pb. The error ellipses of the zircons for an individual layer typically form a tightly concordant cluster. This age of crystallisation is considered to closely approximate of the age of deposition of the volcaniclastic layer.
Zircon grains have been dated from five layers with good stratigraphical control and wide distribution at different locations in the Southern Alps. Three of the age results are of particular importance for the evaluation of the Latemar cycles as they bracket the cyclic sequence. A volcaniclastic layer which is stratigraphically close to the base of the cyclic succession gives an age of 241.2 +0.8/-0.6 Ma (95% c.l.). A layer in the middle portion of the Buchenstein Beds is dated at 238.8 +0.5/-0.2 Ma and a bed from the uppermost Buchenstein Beds, which may be slightly younger than the top of the cyclic sequence, yields an age of 238.0 +0.8/-0.6 Ma. These age results define an interval of less than 4.6 Ma (applying max. error bounds) between the oldest and the youngest of the dated layers (see Fig. 1).
Our results cast serious doubt on the hypothesis of an orbital tuning model for the Latemar cycles. If truly periodic, the periods of sea level fluctuations would be substantially shorter than those known from the Milankovitch spectrum. Preliminary results of spectral analysis performed on the same succession suggest that the sequence may not even be periodic and that shallow marine carbonate environments may not be suitable for recording orbital frequencies. This has to be tested at additional carbonate platform sites.
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Hinnov, L. A. & Goldhammer, R. K., J. Sed. Petr. 61, 1173-1193 (1991).
Oberli, F., Fischer, H. & Meier, M,. Abstr. 28th Int. Geol. Congress, Washington D.C. 2, 536-537 (1989).