Particle Size Specific Magnetic Properties
The magnetic properties of sediments are largely a reflection of sediment transport processes, sediment sources, and sediment diagenesis. The OSU P-mag Lab has a number of ongoing projects that aims to better understand the relative importance of these factors through use of particle size specific magnetic measurements. For example, Rob Hatfield and Joe Stoner have found that magnetic coercivity (or ‘grain-size’) changes in Northern North Atlantic sediments is largely influenced by sediment source by studying the magnetic properties of only the silt sized fraction (Figures 1 and 2). In this case, magnetic minerals from Icelandic silt are generally included in silicate minerals and appear ‘finer’ than their physical particle size, while magnetic minerals from Greenlandic silt are generally free particles and are magnetically coarser. This observation has been applied to better understand the response of the Greenland Ice Sheet during past warm interglacials (see Hatfield et al., 2016, Earth and Planetary Science Letters).
Insight to Paleoenvironments
Magnetic minerals in marine and lacustrine sediments are often sensitive indicators of changes in past environmental conditions. One way is as a tracer for sediment source. For example, in 2015 Jason Dorfman, Joe Stoner, and colleagues published a study in Quaternery Science Reviews documenting down core changes in the magnetic mineral assemblages at Burial Lake in Arctic Alaska. They found, using the S-Ratio parameter which is sensitive to the concentration of hematite and other high coercivity magnetic minerals, that the lake recorded the timing and magnitude of regional dust deposition.
Figure 1: Backscatter electron images of river silts collected from Greenland (top) and Iceland (bottom). Fe-Ti oxides appear as the brighter regions due to their higher atomic numbers and are typically free magnetites in Greenlandic samples and included (titano-)magnetites in Icelandic samples. Rob Hatfield and colleagues in 2017 demonstrated how this difference can be used to better understand sediment source contributions to Northern North Atlantic sediments in their paper in Earth and Planetary Science Letters.
Figure 2: Day plot (after Day et al., 1977) of terrestrially derived silt (3-63 µm) from Greenland (green data) and Iceland (black data) to visualize the variation in magnetic grains size with source. Note that Icelandic silts possess relatively similar fine Psuedo-Single Domain (PSD) magnetic grain sizes. In contrast Greenland silts plot in a region consistent with coarser magnetic grain-size, likely reflecting differences observed using electron microscopy in Figure 1. All data from Rob Hatfield’s 2013 study in Earth and Planetary Science Letters.