A 1.5 Ma history of Atlantic Meridional Overturning Circulation from Nd isotopes

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. However, the proportion of northern- and southern- sourced water masses filling the deep Atlantic during glacial conditions is not adequately constrained. We present new glacial Nd isotope measurements from cores throughout the Atlantic to examine whether there is an ocean-wide coherency to changes in deep water mass sourcing. The resulting profile reveals that, whilst less dominant than during interglacial conditions, northern-sourced waters were a substantial component of the deep glacial Atlantic. Comparison to benthic carbon isotopes shows that the low δ¹³C values seen in the deep North Atlantic at the LGM (Curry & Oppo, 2005) may in part be attributed to redissolution of organic matter, representing a sink of CO₂ under glacial conditions.

This finding is corroborated by a new 1.5 Ma record of neodymium isotopes measured on uncleaned planktic foraminifera and fish debris from ODP Site 929 [6.0 °N, 43.7 °W, 4356 m] on the Ceara Rise in the western equatorial Atlantic Ocean. This record shows other glacial-interglacial circulation reorganizations were similar to the last deglaciation, including that which occurred during the Mid-Pleistocene Transition as defined by Elderfield et al. (Elderfield et al., 2012). The Nd isotope shifts broadly correlate with benthic foraminiferal δ¹³C and δ¹⁸O from the same core (Bickert et al., 1997), however periods of significant decoupling between δ¹³C and ε_Nd, particularly during glacial periods, indicate that deep Atlantic water mass mixing proportions and nutrient chemistry can vary independently of one another.

References:

Curry & Oppo, Paleoceanography 20, 1017 (2005)

Bickert et al., Proceedings of the ODP, Scientific Results 154, 239 (1997)

Elderfield et al., Science 337, 704 (2012)