Holocene Relative Paleointensity Record From the St. Lawrence Estuary, Eastern Canada: Millennial to Centennial-scale Variability of the Global Scale Geomagnetic Field

Two long piston cores (MD99-2220: Lat. 48°38.32N, Long. 68°37.93W, water depth 320 m, length 51.6 m and MD99-2221: Lat. 48°10.60N, Long. 69°30.35W, water depth 212 m, length 31.0 m) were raised from the St. Lawrence Estuary, Eastern Canada because of the expanded Holocene sediment sequence this location provides. A u-channel based paleomagnetic study, augmented by AMS ¹⁴C dates, rock-magnetic and sedimentological data, indicates that these sediments provide a geomagnetic directional paleomagnetic secular variation (PSV) and relative paleointensity (RPI) proxy records for the last ~8500 cal BP, with sedimentation rates varying from ~1.2 to 4.2 m/kyr in the postglacial sediments. Paleomagnetic data are characterized by a strong and generally stable single component magnetization that is carried by pseudo-single domain (PSD) magnetite for cores 2220 and 2221 along with a less stable component carried by a coarser grain magnetic assemblage in core 2221. Component inclinations, as determined by principal component analysis, vary about the expected inclination for the sites latitude, with maximum angular deviation (MAD) values generally lower than 2° and 6° for cores MD99-2220 and -2221 postglacial sediments, respectively. Concentration dependent magnetic parameters vary by less than one order of magnitude. Isothermal remanent magnetization (IRM) normalization of the natural remanent magnetization (NRM) intensity is not coherent with its normalizer nor with any other environmentally sensitive rock-magnetic parameters at significant frequencies observed in the power spectrum of core MD99-2220 NRM/IRM record. Therefore, we interpret the NRM/IRM record from core MD99-2220 as a relative paleointensity (RPI) proxy that reflects changes in the strength of the Earth's geomagnetic field. This new RPI record compares favorably with RPI records from North America and Europe at millennial and even some centennial timescales. Comparisons between core MD99-2220 RPI proxy with the ¹⁰Be flux record from the Greenland Summit (GISP2) ice core and a smoothed ¹⁴C production rate record suggest that geomagnetic modulation, rather than solar variability, may control the millennial and some centennial scale variability within cosmogenic isotope records. And in turn, these correlations imply that core MD99-2220 RPI record reflects changes in global scale geomagnetic field at these timescales.