Rock magnetic variability of quaternary deep-sea sediments from the Bering Sea and their environmental implications

Abstract

IODP Ex. 323 shipboard rock magnetic measurements of Quaternary deep-sea sediments from the Bering Sea identified a dramatic bimodal character to the sediments, alternating between sediments with strong natural magnetic remanence (NRM) and magnetic susceptibility (chi) and those with order-of-magnitude lower values. We now generally associate the high-magnetic-intensity sediments with interglacial/interstadial times and the low-magnetic-intensity intervals are generally associated with glacial/stadial conditions. This pattern can be largely correlated among all seven IODP Ex. 323 sites. We have now completed more detailed rock magnetic measurements on selected u-channeled sediments from these sites. U-channel rock magnetic measurements indicate that the high-intensity sediments contain relatively coarser magnetic grains (sand/silt) associated with coarser siliciclastic sediments while the low-intensity sediments contain finer magnetic grains (silt) associated with finer siliciclastic sediments. We associate the coarser magnetic grains and overall coarser clastic sediments with warmer intervals when more open water conditions permit sediment flux from the continental shelves. The finer magnetic grains and associated finer clastic sediment are largely derived from sediment reworking and redeposition associated with slope processes and deep-sea contour currents when ice cover was more permanent. We have corroborated the grain size variability with magnetic hysteresis measurements and clastic grain size analysis. The clastic grain size distributions of the coarser versus finer grained sediments are significantly different; coarser grained sediments have a broad grain size distribution with 50–60 μm mean grain size, while finer grained sediments have a much more narrow grain size distribution with 15–20 μm mean grain size. The finer grain size distribution is consistent in range and mean grain size to North Atlantic deep-sea sediment deposited as drift deposits by contourite deposition (Heezen and Ruddiman, 1966; Johnson et al., 1988). The dominant magnetic mineral in all sediments is detrital magnetite. Early sediment diagenesis plays a minor role in the overall rock magnetic variability of the Bering Sea deep-sea sediments due to the overall large clastic grain size. The magnetic variability that we see in the Quaternary Bering Sea sediments appears to be comparable to other studies from this region. VanLaningham et al. (2009) attribute deposition of the Meiji drift, directly south of the deep-water exit from the Bering Sea, to sediments derived from the Bering Sea. They see a bimodal distribution in the types of sediments that are deposited with younger arc rocks during interglacials and older continental rocks during glacials. That is consistent with our coarser Interglacial magnetic sediments derived primarily from shelf rocks rich in recent volcanics versus deeper Bering Sea sediments associated with reworking of older Bering Sea sediments during the glacials. We also note the strong similarity in timing of strong versus weak magnetic intensity sediments of Lake Elgygytgyn, about 1000 km to the NW of the Bering Sea, Both seem to be controlled by the degree of intermittent open water conditions, more so in the interglacials and very little in the glacials, but operating on a finer scale than simple glacial/interglacial cycling.