Paleomagnetic records of the last two million years of climate and oceanographic change in the New Zealand sector of the South Pacific

Abstract

A high standing oceanic island in the mid-latitudes of the South Pacific, New Zealand is well situated to record far-field effects of global climate change. A 1.8 Ma paleomagnetic and environmental magnetic record from the Challenger Plateau, Tasman Sea, is presented here.

Continuous sub-samples (u-channels) were collected from three long (up to ~40 m) giant piston cores (MD06-2987, -2988 and -2989), as well as shorter cores (TAN0712-14, -15, -17, -23, -27), within the global westerly system in front of New Zealand's Southern Alps.

Magnetic grain size in the cores was determined from anhysteretic remanent magnetism (κARM) and volume magnetic susceptibility measured on u-channel samples using a pass-through cryogenic magnetometer with an in-line Bartington MS2C susceptibility bridge and solenoid in a shielded room (150 nT). Magnetic mineralogy was investigated using temperature-dependent susceptibility, hysteresis parameters and elemental analysis. Age control was achieved through radiocarbon ages, and correlation of the paleomagnetic inclination and relative paleointensity records (derived from NRM/saturationARM; RPIarm) to radiometrically dated geomagnetic excursions. An inclination anomaly and RPI minima in TAN0712-14 at 29.92 cal ka BP (2σ age range: 29.36 : 30.5 cal ka BP) is inferred to record the Mono Lake excursion and falls within the error range of the published radiometric age, supporting a relatively rapid lock-in of the paleomagnetic signal.

MD06-2987, -2988 and -2989 make up a relative paleointensity (RPIarm) stack---West Coast South Island 1800-ka or WCSI-1800---and contain fifteen Brunhes and Matuyama excursions. Correlation with other stacks and the presence of coeval paleointensity minima support it as a South Pacific representation of Earth's geomagnetic field.

Influxes of fine magnetic minerals in cores MD06-2988 and -2989 are inferred to be a clay content (ice cap glaciation) proxy and correlate with cold stadials (MIS 58, 52, 38, 36, 20 and 6). The magnetic grain-size record of MD06-2989 has spectral power at both obliquity and precession frequencies.

Magnetic grain-size fining in MD06-2987 corresponds to Antarctic warm events (A6-A1) and implies circum-Antarctic waxing and waning in bottom water production on millennial timescales during the last interglacial. Higher frequency fining events correlate to millennial-scale warm Antarctic Isotopic Maxima events (AIM) 7-3. Termination I between 12.5 and 11.75 cal ka BP is defined by a monotonic increase in κARM, through the Younger Dryas (YD) chronozone and mirroring the EPICA ice core δ18O transition out of the Antarctic Cold Reversal.

A simple phenomenological model is derived from the positive linear relationship (r = 0.86; n = 21; p = 1 x 10-6) of κARM and water depth on the continental shelf. Hydrodynamic sorting is inferred to be the primary process. Rock magnetic paleobathymetry has potential to provide quantitative constraints on estimates of water depth changes in, for example, shallow glacimarine settings.

Together, the cores record most of the past two million years of oceanographic and climatic change at these sites, including tight connections between cooling/warming in New Zealand and Antarctica on timescales as short as 1000 years. This implies climate signals are rapidly communicated through both the ocean and atmosphere.