Global relative paleointensity & regional paleosecular variation : high resolution signals in New Zealand marine sediments

Robbins, Jessie Louise.

Advancement in understanding paleosecular variation and its driving mechanisms is hindered by the current paucity of paleomagnetic records from the Southern Hemisphere. To date, the record from New Zealand is limited, and dominated by data from volcanic rocks, with a distinct lack of continuous paleointensity data. Seven cores from the Waipaoa Basin, Bay of Plenty and northern Chatham Rise have been subjected to Alternating Field demagnetisation to derive records of paleosecular variation (PSV) and relative paleointensity (RPI) of Earth's magnetic field over historic and millennial timescales. Artificial remanence parameters saturation ARM and saturation IRM are used to assess the level of compliance with magnetic uniformity criteria of King et al. (1983), which is prerequisite for deriving relative paleointensity from sediments. Tephrochronology and 210Pb isotope analyses have provided independent sedimentation rates for some cores. The Laschamp (41 ka) and Mono Lake (34.6 ka) geomagnetic excursion events are identified in piston core JPC95, from the Bay of Plenty. The Laschamp excursion is manifest as the lowest (7% of maximum) intensity between ~ 47 ka and the present day, accompanied by an abrupt (~ 1ky) shallowing of inclination by 45°. The path of the dipole field during the Laschamp excursion is constrained longitudinally between 87.44° E and 99. 77° E; and the magnitude of the latitudinal deviation from geographic north estimated to be 80°-110°. Both excursions and other millennial-scale features of the JPC95 RPI record are manifest in the GLOPIS-75 paleointensity stack of Laj et al. (2004); and in the numerous records compiled to produce the stack. Such correspondence lends testimony to the fidelity of the RPI records, and to the global and thus dipolar nature of the millennial scale RPI signal; whilst validating the use of sARM as a normalising parameter, and demonstrating the potential of New Zealand marine sediments to provide reliable records of RPI back to ~ 47 ka. Identification and correlation of features with time constants similar to the shortest associated with the dipole field (600-700 years; Hulot & Le Mouel, 1994) suggests that the JPC95 record approaches the maximum resolution of the dipole RPI signal. Cores from the Waipaoa Basin provide higher resolution, recording continuous PSV and RPI variability on a decadal scale back to 300 yrs B.P. More records of comparable resolution, with independent age control, will constrain the spatial extent of this signal and assess its potential application as a high-resolution correlation tool. Available records of paleosecular vanat10n are not globally correlative over the period investigated (0-47 ka). Features of the JPC95 PSV record are identified in records from the South Atlantic, but not in Northern Hemisphere records of comparable temporal resolution. This implies manifestation of a regional, non-dipole signal that is hemispheric in extent, and persistent hemispheric asymmetry in conditions at the core-mantle boundary over the time frame explored here. An inclination anomaly of -8.1° from core JPC95 is greater than previously estimated for New Zealand, but is consistent with the proposition of Elmelah et al. (2001), of a negative inclination anomaly characterising the southwest Pacific. Alternatively, the negative inclination anomaly, together with the far-sided virtual geomagnetic poles from core JPC95 constitute evidence for a northward displacement in the axial dipole, as modelled by Wilson (1971).

204 p. : ill. (some col.), maps ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2006Robbins

Open Access