RECONSTRUCTION OF OCEAN CIRCULATION BASED ON NEODYMIUM ISOTOPIC COMPOSITION: Potential Limitations and Application to the Mid-Pleistocene Transition

• Published in: Oceanography (Washington, D.C.) D.C.), 2020-06, Vol.33 (2), p.80-87
• Main Authors: Tachikawa, Kazuyo, Rapuc, William, Dubois-Dauphin, Quentin, Guihou, Abel, Skonieczny, Charlotte
• Format: Article
• Language: English
• Subjects: Carbon; Carbon cycle; Chemical composition; Circulation; Cryosphere; Deep water; Hydrography; Isotopes; Neodymium; Ocean circulation; Pleistocene; Seawater; SPECIAL ISSUE ON PALEOCEANOGRAPHY: LESSONS FOR A CHANGING WORLD; Tracers; Water circulation
• ISSN: 1042-8275; 2377-617X
• DOI: 10.5760/oceanog.2020.205

As the ocean is Earth’s largest reservoir of carbon, its circulation strongly influences the global carbon cycle. The neodymium (Nd) isotopic composition (143Nd/144Nd or εNd) of seawater has been used as a tracer for ocean circulation. We revisit the capacity of this tracer using compiled modern seawater data sets and recent data (≤10,000 years, 10 kyr) extracted from the sedimentary record. Empirical equations that predict seawater εNd values from hydrography parameters can be used to evaluate possible biases in Nd isotopic ratios. The good overall agreement between measured seawater and predicted εNd values confirms the usefulness of Nd isotopic composition as a tracer of large-scale deepwater circulation in many parts of the modern ocean. Offsets observed between the sedimentary record and predicted values in certain oceanic regions can be partly explained by the contribution of porewaterderived Nd to sedimentary authigenic fractions. We use Nd isotopic composition to study a major climate transition in the middle Pleistocene called the “900 ka event,” which is characterized by a major perturbation in ocean carbon chemistry. All available reconstructed seawater εNd data indicate an increase in isotopic composition at the 900 ka event relative to the present value in the eastern Atlantic Ocean. This shift cannot be explained solely by more active formation of southern-sourced water that has a higher εNd value than the northern-sourced water. We suggest that a reduction in the Atlantic meridional overturning circulation and/or changes in Nd sources to the North Atlantic were the main cause(s) of the change in εNd observed during the evolution of the Northern Hemisphere cryosphere.