The neodymium composition of Atlantic Ocean water masses: implications for the past and present

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 the geographic variability of these isotopes in seawater and their ability to be preserved in deep sea sediments. Nd isotope records have been extracted from Fe-Mn crusts, leachates of sediment coatings, fish debris and foraminifera dissolutions to investigate changes in circulation at both deep and shallow ocean depths. Several of these records have been developed to investigate changes in the amount of northern vs. southern sourced waters in the South Atlantic Ocean. The advancement of northern sourced waters into the South Atlantic and Southern Ocean is an important branch of the global ocean circulation system known as Atlantic Meridional Overturning Circulation (AMOC). In order to further investigate changes of AMOC in the South Atlantic, we have developed a Nd isotope record from Cape Basin core TN057-6 for the last ~400 kyr. In agreement with the literature, the developed Nd record shows a decrease in AMOC during the Last Glacial Maximum and for previous glacial stages. Interglacial or warm periods defined by increased AMOC of comparable magnitude to modern circulation. These findings are summarized in the first two chapters of this thesis. The potential for Nd isotopes (εNd) as a water mass tracer is dependent on a thorough understanding of Nd cycling within the water column. The use of Nd isotopes in the modern ocean is also a valuable tool for investigating biogeochemical cycles and environmental perturbations such as dust or freshwater inputs. The distribution of εNd within the oceans suggests quasi-conservative behavior, traces water masses and shows correlations with both salinity and silicate. However, one observation known as the "Nd-paradox" suggests there are some poorly constrained sources and sinks of Nd in the ocean. The "Nd-paradox" refers to an apparent decoupling of Nd isotopes and Nd concentration ([Nd]) within the water column. In order to explain such features and the Nd cycle as a whole, it is essential to expand the database of Nd seawater data. As part of the GEOTRACES initiative, there have been a growing number of studies to measure seawater Nd-composition. The last two chapters of this thesis focus on the Nd-composition of seawater samples collected along GEOTRACES cruise transect GA03 from Lisbon, Portugal to Cape Verde Islands to Woods Hole, USA. 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, nepheloid layers and a Mid-Atlantic Ocean Ridge hydrothermal site. For each sample we calculate a predicted Nd-composition based on water mass mixing. By comparing the predicted and measured Nd-composition, we are able to investigate how Nd deviates from conservative behavior. Results from this work show that εNd is predominately conservative at deep depths at open ocean stations 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 no influence on the isotopic composition of nearby 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