Early diagenesis of magnetic minerals in marine transitional environments: geochemical signatures of hydrodynamic forcing

Abstract

The low-field magnetic susceptibility ( χ) of surficial sediments from the Ria de Pontevedra (NW Spain) shows a characteristic, hydrodynamically driven textural control, where diamagnetic sandy bioclastic carbonates occur toward the coastline, leaving the finer muddy sediments to be deposited in the central and deeper areas of the ria. Three gravity cores were collected from the inner (core 1), middle (core 2) and outer (core 3) zones of the clayey-rich central axis of the ria, from which the magnetic properties, geochemical characteristics and hydrodynamic regime of the uppermost 90 cm of the sediment record were reconstructed. χ profiles have a distinctive pattern that is controlled by depth. Isothermal remanent magnetization (IRM) acquisition curves facilitated the identification of three well-defined zones with depth, depending on their degree of magnetic saturation, and thus of their dominant magnetic behaviour: magnetite-like, goethite-like and greigite-like. This zonation is interpreted as the result of mineralogical changes during early diagenesis. The relative depths of each zone are mostly dependent on distance to the open sea, with the magnetite-like zones being thicker in the outermost part of the ria. Toward the inner part of the ria, the three zones become shallower and thinner. In the innermost areas, only greigite-like coercivities are observed. Good correlation between magnetic and geochemical parameters such as χ vs. 2Fe/S, or H cr vs. C/S, demonstrates that magnetic property variations with depth are diagenetically controlled, and that these parameters can be used as magnetic proxies for early diagenesis. Similar significant positive correlations were found between χ of the clay fraction and modelled wave heights, and between other magnetic and hydrodynamic parameters, providing magnetic evidence of cryptic wave climate forcing of the diagenetic pathway. The combined analysis of the magnetic, geochemical and hydrodynamic data ultimately indicates that the sediment distribution and subsequent diagenetic pathway in this type of transitional environment mostly depend on the local distribution of wave energy and water depth, rather than on estuarine-like circulation processes.