The Relationship between Atlantic Overturning and Climate in the Pleistocene

Abstract:

Changes in Atlantic Meridional Overturning Circulation play an important role in modulating global climate by controlling northward heat transport in the surface ocean and carbon storage in the deep ocean. We present a new high resolution 1.2 Myr record of neodymium isotopes (ε_Nd) - a proxy for water mass mixing - measured on foraminifera and fish debris from site ODP 929 [6.0°N, 43.7°W, 4356 m] on the Ceara Rise in the western equatorial Atlantic Ocean. This record reveals a fundamental step-change in the nature of glacial Atlantic overturning across the Mid-Pleistocene Transition as well as providing new insight into the relationship between ocean circulation and greenhouse gas forcing during the period known as the “lukewarm” interglacials.

Comparison with benthic foraminiferal carbon isotopes from the same core reveals periods of significant decoupling between ε_Nd and δ¹³C, demonstrating that deep Atlantic water mass mixing proportions and nutrient chemistry can vary independently of one another. In contrast, comparison of the ε_Nd record with benthic foraminiferal oxygen isotopes reveals a tight coupling, exhibiting the control of Northern Hemisphere climate on both ice volume and Atlantic overturning. The high resolution of the records allows cross spectral analysis of the phasing between authigenic ε_Nd and both benthic foraminiferal δ¹³C and δ¹⁸O. This reveals that the different proxy records are coherent at time periods of 100-, 40- and 23-kyr which correlate with orbital forcing. However, the changes in each variable at these periods are not always in phase, indicating that the proxies exhibit different temporal responses to climatic forcings.