Tracing the Nd isotope evolution of North Atlantic Deep and Intermediate Waters in the western North Atlantic since the Last Glacial Maximum from Blake Ridge sediments

Gutjahr, M., Frank, M., Stirling, C.H, Keigwin, L.D. and Halliday, A.N. (2008) Tracing the Nd isotope evolution of North Atlantic Deep and Intermediate Waters in the western North Atlantic since the Last Glacial Maximum from Blake Ridge sediments. Earth and Planetary Science Letters, 266 (1-2), 61-77. (doi:10.1016/j.epsl.2007.10.037).

Abstract

A high-resolution authigenic Nd isotope record has been extracted from the Fe–Mn oxyhydroxide fraction of drift sediments along the Blake Ridge in the North Atlantic. These sediments facilitate reconstruction of the timing and extent of major hydrographic changes in the western North Atlantic since the Last Glacial Maximum (LGM). This is one of the few locations where sediments were deposited in the major flow path of the Western Boundary Undercurrent (WBUC), which transports North Atlantic Deep Water (NADW) southward at the present day. The hydrodynamic setting, however, also causes problems. Authigenic Nd isotope compositions similar to the typical present-day NADW ?Nd value of ? 13.5 ± 0.5 were only extracted from sediments located within the main water body of the WBUC coinciding with the highest along slope current velocity below 3200 m water depth. Above this depth the authigenic Nd-isotopic composition is more radiogenic than measured in a nearby seawater profile and appears to be influenced by downslope and lateral sediment redistribution. Our data suggest that these radiogenic signals were formed at shallow depths in Florida current waters, compromising the recorded ambient deep water Nd isotope signal in the Blake Ridge Fe–Mn oxyhydroxide coatings from intermediate depths during the Holocene and the deglaciation.

The unradiogenic Nd-isotopic composition typical of present-day NADW is not detectable along the Blake Ridge for any water depth during the LGM. Unlike the deglacial and Holocene sections, the intermediate core from 1790 m water depth did not experience significant sediment focusing during the LGM, in accord with the higher current velocities at this depth, suggesting that at this site an ambient LGM bottom water Nd isotope signal was recorded. Assuming this to be correct, our results indicate that the ?Nd of the shallower glacial equivalent of NADW, the Glacial North Atlantic Intermediate Water (GNAIW) may have been as radiogenic as ? 9.7 ± 0.4. Since the authigenic Nd isotope compositions of the Holocene and the deglacial sections of the intermediate depth sediment core were biased towards a shallow water signal, this first determination of a GNAIW ?Nd for the LGM will have to be corroborated by results from other locations and archives.

The LGM and deglacial sediments below 3400 m water depth bear no evidence of an ambient deep water ?Nd as unradiogenic as ? 13.5. Although the deep core sites also experienced enhanced degrees of sediment focusing before the Younger Dryas, the ?Nd values of between ? 11 and ? 10 are more readily explained in terms of increased presence of Southern Source Waters. If this is the case, the change to Nd-isotopic compositions that reflect a modern circulation pattern, including the presence of Lower NADW, only occurred after the Younger Dryas.

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