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.
