Diagenetic versus detrital origin of the magnetic susceptibility variations in some carbonate Frasnian–Famennian boundary sections from Northern Africa and Western Europe: implications for paleoenvironmental reconstructions

Abstract

To provide a new insight into the diagenetic versus detrital origin of the magnetic susceptibility variations in ancient carbonate sequences, a study was conducted within four Frasnian-Famennian platform carbonate sections from Germany, France and Morocco. The study includes along-section magnetic susceptibility and carbonate content measurements complemented by analyses of magnetic hysteresis parameters, inorganic geochemistry and clay mineralogy. Our results show that the magnetic susceptibility evolution is dominantly controlled by the variations in the concentration of low-coercivity ferromagnetic magnetite grains and, to a lesser extent, of paramagnetic clays. In more detail, hysteresis ratios suggest the coexistence of two magnetite populations with significantly different grain size: (1) a dominantly coarsegrained detrital fraction including a mixture of multidomain and single-domain particles (2) an authigenic finegrained fraction composed of a mixture of single-domain and superparamagnetic particles. Despite a diagenetic imprint on the clay assemblages, no relationship is established between magnetic susceptibility and illite crystallinity, therefore discarding a noticeable distortion of primary within-section magnetic susceptibility evolution. The overall inherited character of the magnetic susceptibility fluctuations is corroborated by a significant correlation of magnetic susceptibility with terrigenous proxies (Zr, Th). The poorer correlation of magnetic susceptibility with the Fe content is consistent with the existence of a very fine-grained authigenic magnetite component that possibly induces a global magnetic susceptibility increase at the section scale, but no distortion of the within-section evolution. The magnetic susceptibility curves presented here provide a general record of climate-driven detrital influx and carbonate productivity through Frasnian-Lower Famennian times.

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

“…In the studied section, large-scale MS variations can be ascribed to relative sea-level changes (Mabille and Boulvain, 2007;Da Silva et al, 2009). On a shorter time-scale, climate, through the intensity of wind and precipitation, plays a decisive role in respectively determining the eolian and riverine flux of magnetic minerals to the marine system (Ellwood et al, 2000;Hladil et al, 2006;Riquier et al, 2010) as it was, in this case, the Rheic Ocean (Fig. 1).…”

“…The paleolatitude combined with the proximity of large continental areas (as well Euramerica as Gondwana) play an important role in determining the potential for an intense monsoonal circulation over Euramerica (Witzke, 1990). Given the significant influence of astronomical forcing on modern monsoonal systems, it is highly probable that also during the Devonian period, Milanković cycles influenced the climatic factors controlling the detrital inputs to the Rhenohercynian basin (i.e., wind and precipitation) and/or of carbonate productivity, both affecting the MS signal (Riquier et al, 2010).…”

Palaeoclimatic event at the Lochkovian-Pragian boundary recorded in magnetic susceptibility and gamma-ray spectrometry (Prague Synclinorium, Czech Republic)

“…However, there are also various factors which play against this generalization (e.g., Hladil et al 2006Hladil et al , 2009Da Silva et al 2009;Vacek et al 2010). The MS signal may be strongly modified by secondary processes such as magnetite neomorphism during diagenesis or secondary remagnetization (see e.g., Riquier et al 2010), which certainly devalues interpretations that the primary MS record reflects depositional conditions and processes. Vacek et al (2010) and Koptíková et al (2010b), who studied analogous facies from this stratigraphic level in other sections in the Prague Synclinorium, ascribed the major effect on the MS signal to changing amounts of various paramagnetic minerals (pyroxenes, amphiboles, micas, chlorite, a.o.)…”

Cyclostratigraphic calibration of the Frasnian (Late Devonian) time scale (western Alberta, Canada)

“…In the studied section, large-scale magnetic susceptibility variations can be ascribed to relative sea-level changes and millionyear-scale climate changes due to, for example, continental drift or biosphere innovations (Algeo and Scheckler, 1998;da Silva et al, 2009;Mabille and Boulvain, 2007;Day, 2008, 2010). On a shorter time scale, climate, through the intensity of wind and precipitation, plays a decisive role in determining the respective eolian and riverine fl ux of magnetic minerals to the marine system (Hladil et al, 2006;Riquier et al, 2010), in this case, the western Canadian sedimentary basin (Fig. 1).…”