Paleoenvironmental Clues from Milankovitch Cyclicity and Rock-Magnetism in the Early Cretaceous Pelagic Realm of the Southern Alps, Italy

In the Southern Alps of Italy a relatively continuous record of the Valanginian to Hauterivian is preserved in the Maiolica/Biancone Formation. The focus of this study is on sedimentary-parameter variations through well exposed, tectonically undisturbed sections, their analysis as stratigraphic time series and their paleoclimatic and paleoenvironmental interpretation. Through time-series analysis characteristic Milankovitch cycles were detected. Parameters studied include carbonate content, bedding thickness, magnetic susceptibility, natural remanent magnetization, coercivity ratios, demagnetization behavior, and acquisition of isothermal remanent magnetization. Rock-magnetic parameters by themselves have little paleoclimatic or paleoenvironmental meaning. But if calibrated with the help of paleontological or isotopic climate indicators for the specific environment studied, they provide the long, continuous and areally extended records needed for useful paleoenvironmental reconstruction. Thus, the value of environmental-magnetic methods lies in the fact that they are fast, easily applicable, non-destructive, and inexpensive. In the Biancone Formation an excellent negative correlation of magnetic susceptibility with carbonate content verifies that the susceptibility signal reflects a primary depositional feature. Susceptibility may be used as a proxy for carbonate content with high confidence in this pelagic setting. The carbonate cycles represent primary productivity cycles which can be attributed to climatically driven shifts in oceanic circulation in this Tethyan basin. Evidence for Milankovitch cycles in the Early Cretaceous suggests that orbital forcing was as effective then as it has been during the Quaternary, although the climatic setting was quite different with warm, equable and ice-free conditions in contrast to the Quaternary ice-age climate. Orbitally forced and climatically driven environmental change was strong enough even in the Cretaceous climate state to leave a clear mark in the sensitive pelagic record, although no polar ice caps existed, which play a dominant role in climate models for the Quaternary.