Date of Award

12-15-2014

Document Type

Open Access Thesis

Department

Marine Science

First Advisor

Howie Scher

Abstract

The capacity of the neodymium (Nd) 143Nd/144Nd ratio to trace modern ocean circulation accurately allows for the assumption that the ratio can also accurately trace paleo ocean circulation. Therefore, a complete understanding of Nd cycling and its effect on the isotopic composition in the oceans is vital. Traditionally, Nd isotopic composition (Nd IC) is considered a conservative property in the open ocean, while coastal and margin environments tend to be dictated by nonconservative exchange processes. However, Nd concentrations ([Nd]) in the open ocean typically display a nutrient like profile, increasing with depth. Such a discrepancy between Nd IC and [Nd] has been referred to as the ‘Nd-Paradox’. In this study, Nd IC and [Nd] profiles are presented along the oceanographic section known as Line W, observing the interaction between the poleward flowing Gulf Stream (GS) and the equatorward Deep Western Boundary Current (DWBC) in accordance with the international GEOTRACES program. Surface Nd IC along the transect suggest mixing between three end members passing through the study area; fluvial (Nd IC ~ -11.03) from the margin, GS (Nd IC ~ 9.1) from the Gulf of Mexico and Caribbean, and subtropical mode water (STMW) (Nd IC ~ -9.5) in the open ocean. Just below the Surface mixed layer a prominent STMW signature, flows from the east through to the subsurface GS. Deeper along the margin, the extremely nonradiogenic Upper Labrador Seawater component to the DWBC remains relatively unchanged as it flows equatorward with a predominant -14.87 Nd IC. Classic Labrador Seawater also suggests a relatively unaltered Nd IC, reporting -13.5. However, two over flow water masses, Denmark Strait overflow water and Iceland Scotland overflow water do not conform to conservative mixing. Throughout the study area, [Nd] shows a surface maximum, consistent with lithogenic deposition. Between ~1000 and 2000 m, concentrations remain relatively consistent, coinciding with Nd IC, as a result of increased horizontal advection. Below 2000 m, [Nd] increase while Nd IC remains relatively consistent, indicative of reversible scavenging onto and off of sinking particles. Resuspended sediment in a benthic nepheloid layer produces an inverse relationship between Nd IC and [Nd], likely resulting from lithogenic material with a nonradiogenic Nd IC deposited to the seafloor.

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