Change in the Nd isotopic composition of the bottom water and detrital sediments on the Bering Slope over the last 500 kyrs with implications for the formation of the North Pacific Intermediate Water

Abstract:

The Bering Sea is a potential location for the formation of the North Pacific Intermediate/Deep Water (NPIW/NPDW) and may play an important role in the global heat distribution. We reconstructed the neodymium isotopic ratio (ε_Nd) of authigenic Fe-Mn oxide coatings and detrital sediments on the Bering Slope (IODP Expedition 323 site U1345; water depth 1008 m) over the last 500 kyrs. The ε_Nd is a quasi-conservative water mass tracer. We compared three different leaching techniques to assure that authigenic signals are captured without contamination from terrigenous sources: (1) leaching (3 hours) with 0.02 M hydroxylamine hydrochloride (HH) in 25% buffered acetic acid after decarbonation; sediment/solution (v/v) > 10, (2) leaching (1 hour) with 0.02 M HH in 25% buffered acetic acid without decarbonation; sediment/solution ~ 1, and (3) leaching (1 hour) with 0.005 M HH in 1.5% buffered acetic acid-0.003 M Na-EDTA without decarbonation; sediment/solution > 40. The low Al concentrations and less radiogenic ε_Ndvalues indicated that method (2) is the most appropriate leaching process.

The average ε_Nd of the authigenic fraction over the last 500 kyrs is -3.3 ± 0.9 (1σ, n=38), with large temporal fluctuations. The ε_Nd of authigenic and detrital fractions are well correlated (r² ~ 0.66), suggesting that the bottom water composition in the Bering Sea was governed by terrigenous inflow from surrounding areas. Radiogenic ε_Nd peaks (up to -1.9) seem to be influenced by radiogenic water inflow from the the Kamchatka or Aluetian arcs. The high bulk density and low b* values imply higher terrigenous versus biological contribution and enhanced sea ice formation. Subsequent brine formation would have triggered sinking of radiogenic surface water, forming the NPIW. On the other hand, non-radiogenic ε_Nd troughs (down to -5.3) are observed at times of low bulk density and high b* values. We presume higher biological productivity which is supported by the high opal content at these intervals (Kanematsu et al., 2013). Sea level rise and boundary exchange with terrigenous sediment derived from N. America is a likely mechanism.

This work was supported by the Basic Science Research Program through the NRF funded by Ministry of Science, ICT and Future Planning (No. 2014 0498836)