Secular variation of the Earth's magnetic field in the South West Pacific

• Main Author: Stark, Florian
• Other Authors: Hill, Mimi. ; Shaw, John. ; Holme, Richard
• Published: University of Liverpool 2011
• Subjects: 577
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.559407

The dearth of archaeomagnetic intensity data from the southern hemisphere is a limiting factor in evaluating models of global geomagnetic field evolution during the Holocene. High quality microwave archaeointensity data were obtained from 106 individual ceramic fragments (33 archaeological contexts) from Fiji, Vanuatu, New Caledonia, Bismarck Archipelago and Solomon Islands in the SW Pacific. Complementary Thelliertype experiments, corrected for anisotropy give good agreement with the microwave results. Magnetic mineralogical investigations were performed using a Variable Field Translation Balance (VFTB). Single Curie temperatures between 460°C to 580°C were typically found for most of the island regions indicating an impure to pure magnetite phase. The remaining samples revealed a second TC around 350°C, which was primarily found in samples from the Island of Efate. SEM and EDX investigations have been carried out on selected samples and identified some large titanomagnetite grains (> 50 µm) which were probably introduced by adding beach or river sand to the clay matrix during the manufacturing of the pottery. The newly established archaeointensity curve covers a time interval from 1000 BC to 1750 AD and suggests a more distinctive geomagnetic field strength variation during the last three millennia than those predicted by current global models. The majority of the new data prior to 250 AD exhibit significantly lower intensity than predicted by current global field models (CALS3k.3 and ARCH3k) for the region, with an apparent intensity minimum at 250 BC reaching as low as 50% of the present-day field strength. Between 400 AD and 1500 AD, the data are broadly consistent with the global field models but with a 20% higher field between 1200 and 1400 AD. Two significant increases of the geomagnetic field strength between 200 AD and 400 AD as well as 1200 AD and 1400 AD correlate with intensity peaks found in French data sets and hint at an occurrence of archaeomagnetic jerks in the SW Pacific. Strong climatic variations around 1300 AD in the SW Pacific seem to fit to the second intensity peak and could support the hypothesis that archaeomagnetic jerks influence the climate. Nevertheless more data are needed for an unambiguous identification of such geomagnetic events and their correlation with the climate. For the first time in the SW Pacific this study has attempted to provide chronological information using archaeomagnetic dating. Three poorly dated archaeological sites on the Santa Cruz Islands, which are an important link between Near and Remote Oceania, were investigated. Similar archaeointensity results found from sites RF-2 and RF-6 on Reef Island contrast with a significantly higher value obtained from Nendö (SZ-8) potsherds suggesting a different age for this site. The SW Pacific archaeointensity results highlighted the potential of a new and independent dating tool that can be used by archaeologists to solve problems that cannot be addressed using conventional dating methods, such as the timing of the advance of the Lapita peoples across Oceania.