Magnetic susceptibility as a high-resolution correlation tool and as a climatic proxy in Paleozoic rocks - Merits and pitfalls: Examples from the Devonian in Belgium

Abstract

Low-field magnetic susceptibility (χin) measurements are quick and sensitive enabling the creation of high-resolution records; making χin an attractive correlation tool and a proxy for paleoclimate and paleoenvironments. In geologically young material – foremost in Cenozoic sediments – χin belongs to the geoscientist's toolkit. However, χin is a convolved signal and may reflect other processes than the often implicitly inferred depositional conditions. Diagenesis, remagnetization and low-grade metamorphism, can potentially obscure the original, depositional, χin signal. This aspect is particularly important when interpreting χin records from Paleozoic rocks.

Here, we review data obtained from a large sample collection of Middle to Upper Devonian sections in Belgium. Comparison of χin trends with paleoenvironmental indicators (facies) and with detrital input proxies (Zr, Th, Ti, Al) allowed to assess the persistence of depositional trends. Furthermore, the χin signal was deconvolved into its dominant mineralogical contributions with the help of magnetic-property analysis.

The main results are pointing to a magnetic signal dominated by fine-grained magnetite, of which paleomagnetic analysis indicated a formation during Carboniferous remagnetization. This prompts a potentially strong influence of post-depositional processes and it complicates the interpretation of χin records in terms of depositional environments. However, in most of the sections, there is a relatively good relationship between χin trends and facies evolution and between χin and geochemical proxies for detrital inputs. This indicates that the newly formed magnetite grains would at least partly remain where they are formed, and this allows a relative preservation of the original signal, despite the strong influence of diagenesis. Two sections show a stronger impact of diagenesis, where for about half of these sections, the primary, depositional information is lost.

The Eifelian–Givetian Monts de Baileux section was selected for time-series analysis of the χin series. The Average Spectral Misfit (ASM) method is applied to explicitly evaluate the null hypothesis of no orbital signal and in this section, there is 99.05% chance that the MS signal is reflecting an orbital imprint.