Abstract
Nondestructive magnetic detection of tephra layers in ice cores will be an important method to identify and correlate stratigraphic horizons of ice bearing volcanic ash particles. Volcanic ash particles were extracted from tephra-bearing ice samples collected from Nansen Ice Field south of the Sor Rondane Mountains, Antarctica. Particles are fresh glassy volcanic ash with diameters of ~50 μm, and chemical composition of the matrix glass belongs to a low-K basaltic andesite group, ranging from SiO2 60–62 wt% and K2O 0.40–0.50 wt%. Considering the grain size of ash particles and chemical composition of volcanic glass, the ash in tephra-bearing ice samples might be originated from the South Sandwich Islands located 2800 km northwest of the sampling sites. Correlations on major element concentrations with tephra layers associated with South Sandwich Islands in EPICA-Dome C, Vostok, and Dome Fuji ice cores show high similarity. Rock magnetic experiments show that the magnetic mineral is pseudo-single-domain titanomagnetite with ulvospinel content of 0.2–0.35 mixed with single-domain to superparamagnetic (titano)magnetite. Small blocks of the tephra-bering ice were measured with a SQUID gradiometer at 1-mm intervals with a spatial resolution of ~3 mm. With DC magnetic field of 25 mT, magnetic signal could be enhanced and detected for all the samples including the one with invisible amount of tephra particles. In order to simulate a thin ash layer in ice core, volcanic ash particles extracted from the tephra-bearing ice were used to fabricate a thin ash layer, which were subsequently magnetized, measured with the gradiometer. The noise level for Z axis gradiometer was about 0.6 pT. Detection limit for a half-cylinder with 29 mm radius and a thickness of 1 mm uniformly magnetized in X axis direction is ~9 × 10−5 A/m, which could be improved down to ~2 × 10−6 A/m by reducing the sensor-to-sample distance to 0.5 mm.
Highlights
The importance of ice core records to reconstruct climate in Late Pleistocene to Holocene is increasing for the past decades (e.g., Jouzel 2013)
Sulfur dioxide is extensively emitted at the time of volcanic eruptions as a major component of volcanic gases (e.g., Mori and Kato 2013), which is further transformed into sulfate by oxidation in the atmosphere (e.g., Kroll et al 2015)
High-sensitivity nondestructive magnetic detection of ash layers in ice cores could be an important method to identify stratigraphic horizons of volcanic activities for synchronization combined with electrical conductivity peaks related to sulfate supplied at the time of volcanic eruptions (e.g., Parrenin et al 2012)
Summary
The importance of ice core records to reconstruct climate in Late Pleistocene to Holocene is increasing for the past decades (e.g., Jouzel 2013). High-sensitivity nondestructive magnetic detection of ash layers in ice cores could be an important method to identify stratigraphic horizons of volcanic activities for synchronization combined with electrical conductivity peaks related to sulfate supplied at the time of volcanic eruptions (e.g., Parrenin et al 2012). We report on geochemistry and rock magnetic properties of volcanic ash particles extracted from tephra-bearing ice block samples collected from Nansen Ice Field, Antarctica. The volcanic ash particle distributions may have not been modified significantly by the effect of ablation at the time of deposition or exposure to the surface because the tephra-bearing ice layer could be traced continuously more than several kilometers and could be traced down into the ice at an angle with equal thickness (see Fig. 1d).
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