Reconstruction of Ocean Circulation Based on Neodymium Isotopic Composition: Potential Limitations and Application to the Mid-Pleistocene Transition

As the ocean is Earth's largest reservoir of carbon, its circulation strongly influences the global carbon cycle. The neodymium (Nd) isotopic composition (Nd-143/Nd-144 or epsilon(Nd)) 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 epsilon(Nd) values from hydrography parameters can be used to evaluate possible biases in Nd isotopic ratios. The good overall agreement between measured seawater and predicted epsilon(Nd) 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 porewater-derived 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 epsilon(Nd) 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 epsilon(Nd) 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 epsilon(Nd) observed during the evolution of the Northern Hemisphere cryosphere.