A kinetic model that explains the dependence of magnetic susceptibility of sediment on grain size and organic matter content in transitional marine environments. Testing case studies in estuarine-like environments of NW Iberia
Abstract
The wide use of magnetic proxies to study pollution, sedimentological processes, and environmental and paleoclimatic changes is currently limited by the lack of transference functions that closely correlate with the unmeasurable variables. Among them, magnetic susceptibility (MS) is the oldest and most popular, but have yet to live up to its expectations. This paper explores and quantifies how MS values of surficial sediments in transitional environments depends on grain size and on what can be said about the spatial distribution of hydrodynamic forces and the potential modulation of MS by sediment and organic matter provenances. The concentration of (oxyhydr)oxides in sands (d50 > 63 microns) is primarily controlled by their degree of dilution in the diamagnetic framework, which is larger for coarser grainsizes. In contrast, the concentration of (oxyhydr)oxides in muddy sediments is controlled by their dissolution rate during very early diagenesis, which is controlled by their content in organic matter (TOC), inversely dependent of grainsize. The balance between both components results in the study area in sands of d50 = 68 microns displaying the maximum MS values. The influence of organic matter on the dissolution of magnetite in surficial sediments can be quantified using a simple kinetic model. The model reveals the existence of a negative exponential relationship between magnetic susceptibility and grain size, that depends on the TOC of the fine-grained fraction. The model accurately predicts that a TOC increase of 0.35% results in a 50% reduction in the concentration of magnetite in the sediments of the Ría the Muros. We have also encountered this relationship not universal in this form, as its quantification is strongly modulated by coarse sediment mineralogy, TOC lability and by other factors such as wave climate, depth, and sediment oxygenation. Better understanding and quantification of the role that TOC, hydrodynamics, and changes in the geochemical environment have on the surface and downcore evolution of magnetic properties is useful to correctly interpret magnetic properties as proxies for pollutants, sedimentological processes, and environmental and paleoclimatic changes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMGP13A..07R
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 1505 Biogenic magnetic minerals;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1512 Environmental magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM