Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr

- Shoaled upper boundary and invariant lower boundary of NACW in glacial SE Atlantic.
- Persistent propagation of NACW to the upper Pacific over the last 200 kyr.
- Reduced glacial NACW export from the Atlantic into the rest of the global ocean.

Knowledge of the geometry and strength of the deep overturning circulation is central to the understanding of past climate variability on glacial-interglacial (G-I) timescales. In this study, neodymium (Nd) isotopic ratios on planktonic foraminifera are used to reconstruct the water mass source and mixing history of intermediate and deep water in the Southeast (SE) Atlantic and Southwest (SW) Pacific over the past 200 kyr from five sediment cores. Nd isotopes from a depth transect of cores in the SE Atlantic displayed a stronger geochemical gradient around 3.5 km at the LGM, with higher εNd values of ∼−6.0 below that boundary than those of ∼−7.8 above. In contrast, a similar εNd value (∼−9.5) is observed at both the intermediate and abyssal depths in the Holocene. The glacial upper ocean is 1.8 εNd units less radiogenic than the lower ocean, reflecting an increase in the amount of North Atlantic Component Water (NACW) in the upper SE Atlantic. A coherent Nd isotope change was observed in the SE Atlantic and SW Pacific intermediate and deep water with major excursion to more radiogenic εNd values during Marine Isotope Stage (MIS) 2, 4 and 6. This suggests the flux of NACW to the Southern Ocean was reduced during cold marine stages, possibly accompanied by changes in the water mass geometry. The constant 2 εNd-unit offset between intermediate water in the SE Atlantic and SW Pacific suggests the persistent propagation of less radiogenic NACW to the upper Pacific Ocean over the last glacial cycle. Moreover, the larger glacial vertical εNd gradients between the intermediate and deep waters in both the SE Atlantic and SW Pacific indicate a decreased proportion of NACW entrainment into the deep oceans, consistent with studies proposing reduced mixing between the upper and lower meridional overturning cells.