Paleomagnetic secular variation and revised chronostratigraphy of Bering sea (IODP Ex. 323) deep-sea sediments (MIS 1–4)

Abstract

This study develops composite full-vector paleomagnetic secular variation (PSV) records from IODP Ex. 323 Sites U1339, U1343, U1344, and U1345 in the Bering Sea (51°N-60°N). These PSV records cover the last 71 ka (Marine Isotope Stages (MIS) 1–4). This is the first time that long PSV records have been recovered from such high latitudes. The records have been dated by oxygen isotope records and paleointensity correlations. The records have an age uncertainty of ∼1000 years. The paleomagnetic data come from shipboard paleomagnetic studies of deep-sea sediments with sediment accumulation rates of 20–40 cm/ky. The sample interval is 5 cm, with a time resolution of ±120–200 years. We have identified 66 correlatable inclination features and 59 declination features in all four sites. The relative paleointensity has been estimated by normalizing the natural remanence (after 20 mT af demagnetization) by magnetic susceptibility. This normalization process is strongly biased by environmental variability and so our relative paleointensity records are limited to correlating variability among the four sites. We can identify 8 key, dated paleointensity highs/lows in our composite records. There are two magnetic field excursions recorded reproducibly at these four sites – the Laschamp Excursion (∼41 ka) and the Norwegian-Greenland Sea Excursion (∼61 ka). Both occur quickly, in 1000 years or less, are associated with the lowest paleointensities, and show no significant evidence for pre- or post-excursion anomalous PSV. Both of these are Class I excursions. There are no excursional directions associated with the time interval of the Mono Lake Excursion (∼34 ka). We have carried out a statistical study using our PSV records to assess 3ky and 9ky average variability over the last 71 ky after removing all excursional directions. The averaged inclinations and declinations have correlatable variability, mostly on ∼104 yr time scales. We see significant variation in averaged angular dispersion that appears to be bimodal. Most time is spent with low angular dispersion (∼10°–12°) with shorter time intervals with significantly higher angular dispersion (∼24–30°). This study suggests that excursions, high angular dispersion, and low paleointensities occur together. This may define a distinctive low-intensity (or low-energy) state to the geomagnetic field.