Project: Collaborative Research: The benthic influence on North Atlantic deep water eNd signatures

NSF Award Abstract

The speeds and patterns of deep water currents, collectively known as thermohaline circulation, impact the global distribution of heat and chemicals including dissolved oxygen and carbon dioxide. Modern thermohaline circulation has been described as a "conveyor belt," wherein deep water is formed in the North Atlantic and near Antarctica, then moves throughout the rest of the global oceans. Changes in the location and strength of deep water formation have dramatic impact on both the ocean and on Earth’s climate. However, direct measurement of circulation from Earth’s past is not possible and measuring modern ocean circulation is difficult, because of the inaccessibility of the deep ocean and the vast spatial scales involved, geoscientists must rely on geochemical tracers to understand deep circulation. These tracers allow geoscientists to understand changes in ocean circulation from both modern ocean water and ancient waters recorded in ocean sediments, and thus inform models that predict future change. Observations that the isotopic ratio of the dissolved trace metal neodymium (143Nd and 144Nd) appears to mimic modern deep ocean circulation has meant that this ratio is considered one such circulation tracer. Unfortunately, it is not yet understood why this ratio ostensibly mirrors deep ocean circulation, nor is the established view on the marine geochemical cycle of neodymium completely consistent with observations.

To resolve these inconsistencies, the researchers hypothesize that the sediments at the seafloor are a major source of neodymium to the ocean; a proposal in contrast to existing element budgets which consider the sediments to be primarily a sink removing neodymium from the ocean. To test this idea, the investigators will sample sediments, the pore water they contain, and the overlying ocean water from several sites in the North Atlantic, measuring a suite of elements and isotopes in all these samples. This research will contribute to better understand the geochemistry of neodymium and its isotopes in the North Atlantic, one of the regions critical to understanding ocean circulation. Constraining the major controls on neodymium in the ocean is significant to the understanding of ocean-climate interactions as different mechanisms can lead to very different interpretations of the neodymium record of past and present deep water circulation. This improved understanding will result in more accurate interpretations of new and existing data with respect to changes in deep ocean circulation through time and its impact on climate.

Neodymium (Nd) is one of 14 rare earth elements (REEs) frequently used to investigate environmental processes. In addition to its use as part of the REE series, the isotope ratio of neodymium (143Nd/144Nd; eNd) is arguably the most promising tracer of past ocean circulation, and is also heavily invested in the GEOTRACES project for the modern ocean. Unfortunately, many observational and theoretical studies indicate that the mechanistic understanding of both of these tracers has considerable problems, leading to potentially erroneous interpretations. The research team prior efforts have concluded that a benthic source of REEs to the oceans exerts a primary control over the distribution of REEs and eNd in deep waters. To date this work has been conducted in the Pacific Ocean, but in this project the investigators will test the hypothesis in the North Atlantic, a region critical for thermohaline circulation. This project will explore fundamental aspects of the geochemical cycle of marine REEs and Nd isotopes. Thus, this work has transformative implications on the understanding and application of the REEs and Nd isotope data in both the modern and ancient oceans. The investigators make the specific and (perhaps) counterintuitive prediction that benthic fluxes of the REEs will be greater in the North Atlantic than those measured in the Pacific. If proven correct, these results will transform the interpretations of a large suite of existing data and provide a more accurate mechanistic understanding of what information these elements provide about modern and past ocean circulation. Even if proven incorrect, the proposed measurements will provide an important contrast to those made in the Pacific and will offer insight into the global geochemical cycling of these elements and their isotopes. Beyond the use and interpretation of neodymium, the work will further the understanding of the importance of the benthic environment on marine trace metal cycling in a general sense (e.g., for iron, copper, zinc, chromium, etc.)

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.