ALBERT

Description: The pelagic Marne a Fucoidi Formation from the Umbria–Marche Basin (Central Italy) is one of the best-preserved deep-marine archives of the Aptian–Albian. Frequent changes in lithology, from black to green and red marlstones and limestones, record fluctuating oceanographic and climatic conditions during the deposition of these sediments. Here the lithological changes in the Albian of the Marne a Fucoidi Formation are studied with unprecedented resolution using X-ray flourescence core scanning measurements of major and minor element abundances, as well as statistical analyses of calcareous nannofossil indices. The results obtained with these two independent methodologies are in good agreement and allow for improving the understanding of the peculiar palaeoceanographic conditions of the western Tethys during the Albian. Black shale-marlstone couplets were driven by orbitally forced changes in physical oceanography, resulting in oxygen depleted bottom water at times of decreased mixing of shallow water masses. On the other hand, red marlstones represent episodes of enhanced oxygenation at the bottom, probably due to the influence of deep waters from the eastern Tethys. Overall, the Marne a Fucoidi Formation is the expression of highly variable oceanographic conditions related to an unstable Albian circulation mode. This article is protected by copyright. All rights reserved.

Description: We established a new high-resolution carbonate carbon isotope record of the Albian interval of the Marne a Fucoidi Formation (Central Apennines, Italy), which was deposited on the southern margin of the western Tethys Ocean. Bulk carbonate sampled with 10–15 cm spacing was used for the construction of a continuous carbon isotope curve through the Albian stage. Spectral analyses reveal prominent 400 kyr cyclicity in the δ13C curve, which correlates with Milankovitch long eccentricity changes. Cycles occurring in our record resemble those observed in several Cenozoic δ13C records, suggesting that a link between orbital forcing and carbon cycling existed also under mid-Cretaceous greenhouse conditions. Based on comparisons with Cenozoic eccentricity-carbon cycle links we hypothesize that 400 kyr cycles in the mid-Cretaceous were related to a fluctuating monsoonal regime, coupled with an unstable oceanic structure, which made the oceanic carbon reservoir sensitive to orbital variations. In the Tethys these oceanographic conditions lasted until the Late Albian, and then were replaced by a more stable circulation mode, less sensitive to orbital forcing.

Description: ABSTRACT Isotopic mass-balance models are employed here to study the response of carbon-isotope composition (δ 13 C) of the ocean-atmosphere system to amplitude-modulated perturbations on Milankovitch time scales. We identify a systematic phase distortion, which is inherent to a leakage of power from the carrier precessional signal to the modulating eccentricity terms in the global carbon cycle. The origin is partly analogous to the simple cumulative effect in sinusoidal signals, reflecting the residence time of carbon in the ocean-atmosphere reservoir. The details of origin and practical implications are, however, different. In amplitude-modulated signals, the deformation is manifested as a lag of the 405-kyr eccentricity cycle behind amplitude modulation (AM) of the short (~100-kyr) eccentricity cycle. Importantly, the phase of AM remains stable during the carbon-cycle transfer, thus providing a reference framework against which to evaluate distortion of the 405-kyr term. The phase relationships can help to (1) identify depositional and diagenetic signatures in δ 13 C, and (2) interpret the pathways of astronomical signal through the climate system. The approach is illustrated by case studies of Albian and Oligocene records using a new computational tool EPNOSE. Analogous phase distortions occur in other components of the carbon cycle including atmospheric CO 2 levels; hence, to fully understand the causal relationships on astronomical time scales, paleoclimate models may need to incorporate realistic, amplitude-modulated insolation instead of monochromatic sinusoidal approximations. Finally, detection of the lagged δ 13 C response can help to reduce uncertainties in astrochronological age models that are tuned to the 405-kyr cycle.