Paleomagnetic secular variation and revised chronostratigraphy of Bering Sea (IODP expedition 323) deep-sea sediments (MIS 5)

Abstract

This study presents new high-resolution, full-vector paleomagnetic secular variation (PSV) records from IODP Expedition 323 Sites U1339, U1343, U1344, and U1345 in the Bering Sea (51°N-60°N) during Marine Isotope Stage 5 (71–130 ka). The chronology of the records has been determined by oxygen isotope stratigraphy with an age uncertainty of ∼±2000 years. The data come from shipboard paleomagnetic measurements of deep-sea sediments with accumulation rates of 26–54 cm/ky. The sample interval in the composite records is 5 cm with a time resolution of ±100–200 years. The composite PSV records for each site were developed by correlation and analysis of 3–4 paleomagnetic records from individual holes at each site. The composite records are a combination of the best correlatable PSV data from each site. We have been able to significantly correlate the PSV records from all four sites. We have identified 56 inclination features (highs/lows) and 54 declination features (east/west extremes) in all the records. Relative paleointensity has been determined by normalizing the sediment natural remanence, after 20-mT af demagnetization, to magnetic susceptibility. This normalization process is complicated by environmental variability, thus our relative paleointensity records are limited to large-scale correlations among the four sites. We correlated our relative paleointensity records to the global PISO-1500 paleointensity record that is itself dated by oxygen isotope stratigraphy. We can identify 6 key intensity highs/lows in our composite records and the PISO-1500 record that provide additional dating isochrons for our records. There are three short magnetic field excursions recorded with replication at these four sites – The youngest excursion occurred at 100 ± 2 ka and is associated with the Fram Strait Excursion. Two older excursions occurred at 116 ± 2 ka and 119.5 ± 2 ka. We associate both of these with the Blake Event. All three of the excursions have very short time intervals (∼200–400 yrs) with somewhat out-of-phase inclination and declination variability. All three of these are Class I excursions. We have studied the statistical characteristics our composite records by calculating the 3ky and 9ky averaged inclinations, declinations, unit vectors, and relative paleointensity over 71–130 ka after removing all excursional directions. The averaged inclinations and declinations are consistent among the four sites. The averaged angular dispersion is bimodal in its overall pattern. Most of the time angular dispersion is low (∼±10°–12°) except for three shorter intervals that have significantly higher angular dispersion (∼±24–30°) and lower paleointensities. The high angular dispersion intervals are associated with the three short excursional intervals. The timing and pattern of interrelated high angular dispersion, low paleointensity, and excursions is synchronous with the PSV pattern recorded during MIS 5 in the central North Atlantic Ocean. We think this defines a distinctive low-intensity/low-energy state for the geomagnetic field.