Geomagnetic field inclinations for the past 400 kyr from the 1-km core of the Hawaii Scientific Drilling Project

摘要

A volcanic record of geomagnetic field inclination for the past ∼400 kyr at Hilo, Hawaii, has been obtained from the 941.5 m of core recovered by the Hawaii Scientific Drilling Project. The analysis of 195 lava flows reveals six instances of near-zero inclination and two instances of fully negative inclination (reverse polarity) within an otherwise normal-polarity core. In particular, flow unit 23 (∼178 m depth) records a horizontal inclination and may be associated with the Laschamp event; flow units 40 and 42 (∼260 m depth) record negative inclinations and are close in age to the Blake event; and flow unit 55 (∼320 m depth) records a negative inclination with a relative declination change of ∼75° with respect to the overlying flow and is probably the Jamaica/Biwa I/Pringle Falls event. The five instances of shallow inclination found below 400 m depth appear to have resulted from long-term secular variation as they are part of inclination swings between ∼0° and ∼60° with a periodicity of ∼10–50 kyr. In contrast, the inclination shifts at ∼178 m and ∼320 m depths significantly deviate from long-term trends, suggesting the existence of at least two independent processes producing time variations of the geomagnetic field. The secular variation has a mean of 30.9° (α_(95) = 2.27°), which is significantly shallower than the expected dipole mean of 36°. The dispersion (σ = 12.5°) agrees with global paleosecular variation data for 0–5 Ma and secular variation models.