Anoxic events, productivity rhythms, and the orbital signature in a Mid‐Cretaceous deep‐sea sequence from central Italy

Abstract

Albian pelagic sediments from the Umbrian Apennines exhibit rhythmic bedding in outcrop. High temporal resolution (∼ 4‐kyr spacing) sampling of carbonate, Si, and Al along a 1.6‐m.y. interval of core demonstrates that changes in accumulation rate of calcareous and siliceous microorganisms were controlled by insolation cycles. Optical densitometry logging correlates variations of geochemical parameters to episodes of deep‐sea anoxia, indicated by numerous black, laminated zones (“black shales”). Harmonic analysis of data shows oscillations at three frequencies, which correspond closely in estimated duration and ratio to the modern eccentricity and precessional orbital cycles. As in Pleistocene marine sequences, the eccentricity (approximately 100 kyr) term dominates the sedimentary variance in the Umbrian cycles. Anoxic episodes occur in phase with a limestone‐marl repetition and reflect minima of the precessional (21 kyr) cycle. The covariance of carbonate and silica, with estimates of foraminiferal abundances, suggest that carbonate cycles are the result of changes in surface productivity of calcareous organisms rather than an effect of dissolution or diagenesis. Recognition of orbital cyclicity provides a tool for making quantitative estimates of these changes. Productivity during anoxic events was uniformly low (0.4– 0.5 g/cm2/kyr carbonate, 0– .05 g/cm2/kyr silica), but it rose to high levels during oxygenated periods (1– 2.5 g/cm2/kyr carbonate, 0.1– 0.3 g/cm2/kyr silica). Vertical overturning and organic carbon flux decreased during anoxic episodes, but decreased oxygenation of the deep water led to enhanced preservation of the organic matter delivered. The degree of paleoceanographic variability indicated by this study requires that accepted notions of climate stability and oceanic quiescence during nonglacial times be reassessed.