Change in the bottom water composition and lithological sediment provenance of the Bering Sea during the Pleistocene : Insights from neodymium isotopes and NGR-based K, Th and U concentrations

Abstract

The Bering Sea is an important location to understand global climate system because of its high sensitivity to advance/retreat of sea ice and continental ice sheet. Enhanced sea ice formation may induce the formation of the North Pacific Intermediate/Deep Water (NPIW/NPDW) in the Bering Sea, and the development of continental ice sheet in Alaska can potentially determine the provenance of the Bering Sea sediments. To examine the relationships between NPIW/NPDW formation, lithological sediment provenance and ice dynamics responding to glacial/interglacial cycles, here I present authigenic and detrital neodymium (Nd) isotopes and NGR-based K, Th and U concentration of marine sediments in the Bering Sea obtained during the Integrated Ocean Drilling Program (IODP) Expedition 323. Authigenic Nd isotopes is a quasi-conservative water mass tracer, and detrital Nd isotopes and NGR-based K, Th and U concentrations can be used as indicators of lithological sediment provenance. To acquire reliable authigenic Nd isotopes without contamination from labile volcanic components in sediments, Nd isotopes and elemental concentrations of six different authigenic fractions are compared each other. Reconstructed authigenic Nd isotopes (ɛNd,AUTH) at site U1345 on the Bering Slope over ~520 kyr showed average value of -3.3 ± 0.9 (1σ, n = 98) with large temporal fluctuations, which can be interpreted either as episodic occurrences of the NPIW formation or boundary exchange between seawater and sediments. Likewise, ɛNd,AUTH records at site U1343 over ~2.4 Myr showed average value of -3.0 ± 1.1 (1σ, n = 155) with large temporal variation. Only one NPDW formation is attributed to this variation, and boundary exchange between seawater and sediments is the most likely reason for ɛNd,AUTH variation. The detrital Nd isotopes (ɛNd,DET) at site U1343 showed average of -6.6 ± 1.5 (1σ, n = 70) with temporal variation, indicating change in lithological sediment provenance. Overall, Alaska is a dominant source over the Aleutian Arc at site U1343, and outstanding unradiogenic ɛNd,DET peaks can result from meltwater discharge from Alaska. Considering the repetitive occurrence of unradiogenic ɛNd,DET peaks after 2.4 Ma, extensive glaciation have probably developed in Alaska since at least 2.4 Myr ago. Spatial and temporal variation in terrigenous sediment provenance of the Bering Sea sediment are partly shown in NGR-based K/Th, K/U and Th/U ratios at sites U1341 and U1342 on the Bowers Ridge and sites U1343 and U1345 on the Bering Slope. The Bowers Ridge sediments showed distinct K/Th, K/U and Th/U ratios from the Bering Slope sediments. However, heavy mineral sorting and/or change in paleo-oceanic redox conditions can disturb primary signals from parent rocks, and therefore K/Th, K/U and Th/U ratios are only used as a preliminary result to trace lithological sediment provenance in the Bering Sea.