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Seminar Details

The Nd-composition of Atlantic Ocean water masses

Date

04/06/2015

Lecturers

Dr. Alison Hartman - The Hebrew University of Jerusalem and IUI

Abstract

Ocean circulation plays an integral part in a multitude of Earth’s processes including the transfer of heat and nutrients across the globe. Additionally, its role in initiating and/or responding to global climate change is thought to be significant though poorly constrained. One tool used to further understand the influence of changes in ocean circulation during climate transitions is paleocirculation records developed from deep sea cores. These records paint a picture of how ocean circulation changed throughout time and are composed of an array of elements and isotopes extracted from different sediment archives. Neodymium (Nd) isotopes have been applied to paleocirculation because of their variability in major water masses that are important for ocean circulation, and their ability to be preserved in deep sea sediments. Nd isotope records in the Southern Ocean are used to investigate changes in Atlantic Meridional Overturning Circulation (AMOC). Records indicate a decrease in AMOC during the Last Glacial Maximum and for previous glacial stages. Interglacial or warm periods are defined by increased AMOC intensity of comparable magnitude to modern circulation. Nd isotopes (εNd) in the modern ocean can be used to monitor biogeochemical cycles and environmental perturbations such as dust or freshwater inputs. In the North Atlantic Ocean, seawater samples were collected along GEOTRACES cruise transect GA03 from Lisbon, Portugal to Mauritania to Woods Hole, USA and analyzed for Nd-composition. The major water masses sampled as part of this cruise are Mediterranean Outflow Water, Antarctic Intermediate Water, North Atlantic Deep Water and Antarctic Bottom Water. Additional features sampled are near shore and open ocean stations, the Saharan dust plume, an expansive oxygen minimum zone near the African coast, nepheloid layers and a Mid-Atlantic Ocean Ridge hydrothermal site. Each sample was tested for conservative behavior of both εNd and [Nd]. Results from this work show that εNd is predominately conservative at deep depths at open ocean stations in the transect, and is sensitive to small changes in water mass end-member Nd-compositions. This finding has important implications for the way end-members are defined in paleoceanographic Nd studies. Hydrothermal inputs are shown to have a small influence on the isotopic composition of adjacent waters but cause no change in water mass εNd compositions. However, an expansive nepheloid layer in the deep western North Atlantic does influence seawater εNd. [Nd] does not show conservative behavior but rather follows the “Nd-Paradox” such that concentrations increase with depth across the basin and exceed predicted [Nd] values.

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