Precessional and half-precessional climate forcing of Mid-Devonian monsoon-like dynamics

D De Vleeschouwer,M Crucifix,A C Da Silva,F Boulvain,P Claeys

doi:10.5194/cp-8-337-2012

Abstract

Abstract. A Devonian magnetic susceptibility (MS) record obtained on limestones ranging from the Uppermost-Eifelian to the Lower-Givetian and located on the southern border of the Dinant Synclinorium in Belgium was selected for time-series analysis. In these carbonate ramp and platform deposits, spectral analyses highlight persistent, high-frequency cycles in both the MS-signal and the microfacies curve, reflecting environmental and climate changes. These metre-scale variations in the MS-signal are interpreted as changes in the flux of magnetic minerals towards the marine system, most likely controlled by monsoon rainfall-intensity. By combining chrono- and biostratigraphic information with theoretical knowledge of sedimentation rates in different depositional environments, these cycles are interpreted as astronomically driven and dominated by precession. It is hypothesized that during precession minima (longitude of the perihelion ≈270°) the trans-equatorial pressure gradient reaches a maximum and intensifies the northeasterly monsoonal circulation. The consequent increase in moisture transport towards the continent leads to enhanced precipitation and runoff, which, in turn, induces a higher flux of detrital material – including magnetic minerals responsible for the MS-signal – towards the marine system. Moreover, this unique high-resolution climate signal reveals half-precessional cycles. These cycles suggest that during precession maxima (longitude of the perihelion ≈90°) southeasterly monsoonal circulation strengthened due to an increased inter-hemispheric pressure gradient.

Highlights

It is only since the late seventies that variations in the Earth’s orbit were accepted as the pacemaker of the Quaternary ice ages (Hays et al, 1976)
The Devonian is a key period in the evolution of life on Earth with, for example, the fish-tetrapod transition occurring during the early Middle-Devonian
The main objective of this study is to reveal the finger print of these cycles in the section, in order to obtain a better insight into the response to astronomical forcing of an extremely hot Greenhouse climate

Summary

Introduction

It is only since the late seventies that variations in the Earth’s orbit were accepted as the pacemaker of the Quaternary ice ages (Hays et al, 1976). Greenhouse world, without any major ice sheets, icerelated feedback mechanisms and nonlinearities do no longer apply. Other mechanisms must explain the amplification of subtle variations in the Earth’s orbital parameters into global climate and environmental changes. The Greenhouse Devonian Period (418–361 Ma; Kaufmann, 2006) is characterised by climate boundary conditions that differ completely from those of the Quaternary: atmospheric CO2 concentration was up to 10 times higher compared to today’s value (Berner, 2006), continents were concentrated in the Southern Hemisphere (SH), global temperatures and sealevel were high and vast shallow epicontinental seas allowed reefs to develop at high latitudes, reaching 65◦ N–55◦ S during the Middle-Devonian (Copper, 2002). The Devonian is a key period in the evolution of life on Earth with, for example, the fish-tetrapod transition occurring during the early Middle-Devonian

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Results