Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments

Abstract

Bedded pelagic limestones of early to mid Cretaceous age may record cyclic climate variability driven by changes in the Earth's orbit (“Milankovitch cycles”). If properly calibrated, the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy. The lower Cretaceous Maiolica Formation of northern and central Italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper M-series marine anomalies. The magnetic reversals divide measured sections into 0.5–2 Myr increments in which sedimentation rate (assuming continous deposition) can be measured to 15–25% accuracy. Paleomagnetically estimated sedimentation rates over a number of polarity zones establish, with one exception, a narrow range in periods of the carbonate cycles. Carbonate couplets have an estimated mean period of 23.5 kyr, and prominent modulations in bedding thickness (“bundling”) have a mean period of 117 kyr (Kent and Gradstein 1985 time scale). The close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing.Comparison of measured sections from different locations shows that bed-to-bed correlations are possible regionally, cosistent with the Milankovitch hypothesis. Episodes of Barremian “black shale” deposition occur in the troughs of the short eccentricity cycle, as roughly one half of the carbonate-marl couplet, or about 10 kyr duration. It is possible to estimate both the timing (relative to the top of reversed chron M-0) and duration of a period of unusual organic carbon-rich deposition (“Selli Horizon”) in the early part of the Cretaceous Long Normal Magnetic Polarity Interval by tracing the sedimentary cycles up from the Barremian strata.