Discrimination of Terrestrial Source Materials to the Northern North Atlantic Using Particle Size Specific Magnetic Measurements and Electron Microscopy.

Abstract:

We investigate the magnetic properties of different terrestrial sediment fractions (sand, silt, and clay) from Iceland and Greenland as major sediment sources to the northern North Atlantic (NNA). Magnetic susceptibility (MS) and hysteresis data have previously shown to be strongly particle size dependent with silt (3-63µm) important for hosting the ferrimagnetic fraction and discriminating source. Here we expand upon these data with more fundamental observations including low temperature remanence, low and high temperature MS, and electron microscopy. All Iceland fractions lack a Verwey transition (Tv) and MS decreases gradually on heating between 100-500°C, consistent with (TM60) titanomagnetite. Frequency dependent MS (fd%; 1-998 Hz) of ~8% across all Iceland fractions implies significant SP grain populations within the average Day plot determined PSD grain size. Homogeneity in magnetic grain size across all Icelandic fractions implies a disconnect with physical grain size that is visualized in electron backscatter images as fine Fe-rich fragments are included within larger host grains. In contrast Greenlandic silt and sand possess a strong Tv and MS values that fall steeply between 560-580°C on heating, consistent with magnetite. Greenlandic ferrimagnetic fragments within the silt and sand size fractions exist as discrete particles and average magnetic grain size scales with physical grain size; the sand fraction is dominated by MD grains and silts are coarse PSD in size. While finer PSD clays are indistinguishable from all Iceland fractions on a Day plot SP contributions are lower and the Tv is more pronounced in Greenland clay. These new magnetic mineralogy, magnetic grain size, and electron microscopy measurements expand the differentiation of source and grain size of NNA source materials, and further highlight the necessity for grain-size specific magnetic measurements to isolate source from physical grain size variation in bulk marine sediment cores.